Christel Vidaller
email: christel.vidaller@gmail.com

Thierry Dutoit
email: thierry.dutoit@univ-avignon.fr

Ecosystem services in conventional farming systems. A review

Keywords: Conventional agriculture, Ordinary nature, ES indicators, Ecological indicators, Biodiversity

Ecosystems worldwide are being disrupted under increased pressure from human activities. Nevertheless, most conservation studies and restoration efforts have so far focused on ecosystems of high heritage value related to their species diversity and/or the rarity of their habitats and/or species. However, "ordinary nature" (all the everyday, non-spectacular and non-protected landscapes, species and ecosystems) is also threatened by ecosystem disruption, which could affect major ecological functions and thus the supply of ecosystem services (ES). Conventional farming systems, which are mostly composed of agroecosystems characterised by ordinary nature, are already known to deliver some ES (e.g. pollination, carbon sequestration). Nevertheless, no systematic search has been done yet to determine which ES are identified and how there are studied in these conventional farming systems. We thus performed a first systematic evidence map (review of key articles to show, at a glance, the points that have been most studied, and highlight key gaps in the evidence base) to identify characterisation and measurement of ES provided by conventional agroecosystems from the review of 189 key international scientific articles. We excluded production for economic benefit, as this topic is already well documented in conventional agriculture systems. We found that most studies identified nutrient cycling (49.7%), carbon sequestration (46.6%), pest reduction (48.1%) or pollination (36.5%) as ES supplied by conventional agroecosystems. Correlations were also found between spatial scale and ES studied: for example, carbon storage was determined more at agricultural plot scale, while cultural services were determined more at landscape level. Our map also yielded 74 frequently used ES indicators, 50 of which are easily measurable and operational indicators of significant ES. Afterwards, one challenge that could be addressed in further studies is to determine for each indicator the range of measurement values that should be considered positive for ES provision, which is likely ecosystem-dependent.

1 Introduction

Human activities have strongly modified ecosystems and biodiversity since the Neolithic Age [START_REF] Diamond | Evolution, consequences and future of plant and animal domestication[END_REF][START_REF] Skoglund | Origins and genetic legacy of neolithic farmers and hunter-gatherers in Europe[END_REF], with the beginning of the Industrial Age marking an intensification of ecosystem disruption [START_REF] Lewis | Defining the Anthropocene[END_REF]. Over-exploitation of natural resources, consumption of fossil fuels, urbanisation, industrialisation and agricultural intensification all led to a greater human impact on all ecosystems. Today, human activities are known to have altered about one-third to one-half of Earth's land surface [START_REF] Vitousek | Human Domination of Earth's Ecosystems[END_REF]. Moreover, we now face global environmental change, a major threat to biodiversity and human well-being leading to for example a loss of habitats and species [START_REF] Lewis | Defining the Anthropocene[END_REF][START_REF] Scholes | Summary for policymakers of the assessment report on land degradation and restoration of the Intergovernmental Science-Policy[END_REF]. Such alterations could impact major ecological functions, thereby affecting the supply of ecosystem services (ES) long before global extinction (Millennium Ecosystem Assessment 2005).

ES are the benefits people obtain from ecosystems (Millenium Ecosystem Assessment 2005). These include provisioning services such as food and water; regulating services such as flood and disease control; cultural services such as spiritual, recreational and cultural benefits; and supporting services such as nutrient cycling that maintain the conditions for life on Earth [START_REF] Costanza R, D'arge | The value of the world's ecosystem services and natural capital[END_REF]Millennium Ecosystem Assessment 2005). The Millennium Ecosystem Assessment (2005), which is the first synthesis of the state of knowledge on the connection between sustainable use of ecosystems and human well-being commissioned for by the United Nations Secretary-General Kofi Annan, showed that 60% of the examined ES are being degraded or used unsustainably. Several authors have thus already called for the inclusion of ES in conservation plans [START_REF] Singh | Balancing the approaches of environmental conservation by considering ecosystem services as well as biodiversity[END_REF][START_REF] Balvanera | Applying community structure analysis to ecosystem function: examples from pollination and carbon storage[END_REF][START_REF] Chan | Conservation planning for ecosystem services[END_REF][START_REF] Egoh | Integrating ecosystem services into conservation assessments: a review[END_REF].

However, until recently, conservation studies and restoration efforts have so far focused on preserving biodiversity in ecosystems whose species-richness and/or rarity of habitats and/or species gives them high heritage value such as pristine wetlands and species rich grasslands [START_REF] Scott | Gap analysis: a geographic approach to protection of biological diversity[END_REF][START_REF] Devictor | Can common species benefit from protected areas?[END_REF][START_REF] Inger | Common European birds are declining rapidly while less abundant species' numbers are rising[END_REF]. Less attention has been paid to more common species or habitats, despite their greater importance in terms of area, ecosystem function and service provision [START_REF] Grime | Benefits of plant diversity to ecosystems: immediate, filter and founder effects[END_REF][START_REF] Geider | Primary productivity of planet earth: biological determinants and physical constraints in terrestrial and aquatic habitats[END_REF][START_REF] Gaston | Commonness, population depletion and conservation biology[END_REF][START_REF] Gaston | Common Ecology[END_REF]. Moreover, "ordinary nature" too, defined only in opposition to "extraordinary nature", which is "nature" composed of common species and habitats and characterized with low ecological complexity [START_REF] Pellegrin | Dossier : La fabrique de la compensation écologique : controverses et pratiques -Une définition opérationnelle de la nature ordinaire adaptée à la compensation écologique. Le cas contrasté des régions Centre[END_REF], and related ES are also impacted by environmental change and ecosystem disruption. They are, for example, threatened by rapid changes in genetic diversity, specialisation losses and species range shifts (e.g. [START_REF] Inger | Common European birds are declining rapidly while less abundant species' numbers are rising[END_REF][START_REF] Radchuk | Adaptive responses of animals to climate change are most likely insufficient[END_REF][START_REF] Scholes | Summary for policymakers of the assessment report on land degradation and restoration of the Intergovernmental Science-Policy[END_REF].

Ordinary nature is largely composed of agricultural land (e.g. intensive crops, species-poor and artificial pasture), which currently covers 37.4% of the total land surface of Earth (FAOSTAT 2016;[START_REF] Leff | Geographic distribution of major crops across the world[END_REF]. Agroecosystems not only produce food but also have the potential to generate several ES and obtain ES from non-agricultural land. Both generated and obtained ES are determined by the management decisions and agricultural practices implemented at field scale [START_REF] Tscharntke | Landscape perspectives on agricultural intensification and biodiversityecosystem service management[END_REF][START_REF] Dale | Measures of the effects of agricultural practices on ecosystem services[END_REF][START_REF] Swinton | Ecosystem services and agriculture: Cultivating agricultural ecosystems for diverse benefits[END_REF]. While several studies have interestingly reviewed ES provided by non-conventional agricultural systems such as, for example, organic (e.g. [START_REF] Sandhu | Organic agriculture and ecosystem services[END_REF] or conservation [START_REF] Palm | Conservation agriculture and ecosystem services: An overview[END_REF]) agriculture, no systematic search focusing only on ES identified in conventional agricultural systems (Fig. 1), which is a substantial form of farming management, has been found in the international scientific literature.

The Millennium Ecosystem Assessment synthesis (2005) forecasts an increase in agriculture-related biodiversity loss and ES degradation during the first half of this century, mainly due to habitat loss through development. Because common species and habitat contribute disproportionately to ecosystem biomass and functions [START_REF] Gaston | Commonness, population depletion and conservation biology[END_REF][START_REF] Gaston | Common Ecology[END_REF], there is an urgent need to also consider and deepen the knowledge on ES provided by conventional agriculture. This would allow the development of a holistic and more sustainable approach in conservation and restoration studies, which takes into account both rare and common components of nature. Therefore, the first step is to identify provided ES, their characteristics and indicators.

Currently, ES are mainly measured through a biophysical approach [START_REF] Dale | Measures of the effects of agricultural practices on ecosystem services[END_REF]. In ES characterisation studies, [START_REF] Seppelt | A quantitative review of ecosystem service studies: approaches, shortcomings and the road ahead[END_REF] demonstrated a lack of consistent methodology and particularly stressed the importance of biophysical realism for ecosystem data and models. To ensure that ES are properly identified and measured, target components of biodiversity and ecosystems need to be defined. In addition, appropriate indicators and their characteristics, such as the method of measurement, need to be identified, along with appropriate baselines for calculating losses and gains, time-related issues and uncertainties in assessment [START_REF] Quétier | Assessing ecological equivalence in biodiversity offset schemes: Key issues and solutions[END_REF][START_REF] Dale | Measures of the effects of agricultural practices on ecosystem services[END_REF].

The concept of ES considerably developed at the end of the 1990s following the ecological economics work of [START_REF] Costanza R, D'arge | The value of the world's ecosystem services and natural capital[END_REF] and [START_REF] Daily | Nature's Services: Societal Dependence on Natural Ecosystems[END_REF] which led to a growing interest and debate around the valuation of ecosystem services. Subsequently, several studies have explored the integration of agricultural provisioning services from the viewpoint of yield, benefits and economic value; these services are then qualified as "direct ecosystem benefit-related ES" instead of ES (e.g. [START_REF] Costanza | Twenty years of ecosystem services: how far have we come and how far do we still need to go?[END_REF]. Here, we thus explored only non-production and non-direct benefit-related ES as this topic was already and still well documented.

Our aim in this article was to set up and execute for the first time a systematic evidence map on the ES identified in conventional farming systems. This included agriculture based on monocultures, mechanisation and the use of agrochemicals. An evidence map allows to show, at a glance, the points that have been most studied, and highlight key gaps in the evidence base [START_REF] Mckinnon | Sustainability: Map the evidence[END_REF].

Then, we present and discuss easily measurable and operational indicators of identified ES that should improve existing methods of ES from conventional farming measurement and allow better incorporation of ES into conservation and restoration programmes.

We specifically sought to identify:

(1) Growth trends in research on ES from the conventional agricultural landscape, (2) Agricultural land ecosystems focused on and their characteristics, (3) ES proxies that could be used as indicators.

Materials and methods

Selection of studies

The systematic literature search and screening was performed in April 2019 by generating keyword combinations in the ISI Web of Science database (Core Collection, 1985-2019). No language restrictions were applied to the search. Three keyword strings were applied concomitantly to the article "TITLE": "ecosystem service*" OR "agroecosystem service*" AND "agricultu*" OR "farm*" OR "agr*" OR "arable*" OR "orchard*" OR "cultiv*" OR "vineyard*" OR "plantation*" OR "crop*" OR "past*" OR "rangeland*" OR "grove*" NOT "wind farm*" OR "urban" Alternative search strings (e.g. including all agricultural system types and not only conventional ones) or a "TOPIC" search were performed but resulted in an unmanageable number of articles suggesting a topic too broad to be covered in a single article. Moreover, this search also often yielded not relevant articles to our topic.

We have thus decided to use a screening approach with a title search to obtain a purposive sample of the literature rather than a comprehensive analysis of all potential articles. Using a purposive sample of the literature allowed us to select relevant and information-rich articles appropriated to our article goal. The title search was also chosen to only include references where the authors had used with intent the search terms to entitle their work [START_REF] Johnson | Why all this fuss about codified and tacit knowledge?[END_REF][START_REF] Huber-Stearns | Social-ecological enabling conditions for payments for ecosystem services[END_REF][START_REF] Sattler | Methods in ecosystem services governance analysis: an introduction[END_REF]. To achieve representativeness of our literature sample and even if we acknowledge the importance of other form of knowledge such as so called grey literature and written in other languages than English, we chose to base our analysis only on an international scientifically validated corpus by peer review [START_REF] Johnson | Why all this fuss about codified and tacit knowledge?[END_REF][START_REF] Pirard | Classifying market-based instruments for ecosystem services: a guide to the literature jungle[END_REF][START_REF] Huber-Stearns | Social-ecological enabling conditions for payments for ecosystem services[END_REF][START_REF] Sattler | Methods in ecosystem services governance analysis: an introduction[END_REF]. We are aware that Web of Science database has its own biases related to an incomplete corpus of journal and the absence of "grey literature"; however, it is not presumed to yield any specific point of view concerning ecosystem services provided by conventional agroecosystems [START_REF] Pirard | Classifying market-based instruments for ecosystem services: a guide to the literature jungle[END_REF].

The initial search yielded a total of 616 publications, many of whose contents were not relevant to our research questions. To include only articles measuring ecosystem services identified in conventional agroecosystems, we therefore screened studies through three reading steps: (1) title and keywords, (2) abstracts and (3) full publication content. Articles that did not meet our inclusion criteria were removed from the final list of articles. We thus excluded articles only examining (1) non-conventional agriculture, (2) production and ecosystem benefits (already well documented), (3) disservices, ( 4) ES economic valuation and (5) trade-off between services and disservices, because identification of ES is a fundamental first step before studying these topics [START_REF] Cowling | An operational model for mainstreaming ecosystem services for implementation[END_REF].

After reading the title, keywords and abstract, the number of studies was narrowed down to a total of 191 articles. Ultimately, after reading the full articles, 189 articles were included in the study: 123 original articles and 66 reviews, editorials or opinion articles (see Supplementary material A, Table A.1).

The full dataset of the 189 articles was used for the bibliometric analysis in order to perform a first systematic evidence map showing the points that have been most studied and highlight key gaps in the characterisation and measurement of ES provided by conventional agroecosystems. The inclusion of reviews, editorials or opinion articles is crucial because it is informative of the focus of research efforts. Thereafter, quantitative analysis was only performed on data extracted from the subset of original articles (123) in order to avoid replication bias in the statistical analyses.

Bibliometric analysis of the selected articles

To quantitatively assess the productivity and quality of the conventional agroecosystem service research field, we applied bibliometric methods from R package "Bibliometrix" to the 189 articles. We measured annual scientific publication growth from 1985 to April 2019, the most productive countries, the collaboration index, the journals in which scientists most frequently published their articles and the conceptual themes within the field.

ES identification and characterisation from the selected articles

To determine the ES and their characteristics that are studied in conventional agroecosystems, we checked each article for the presence or absence of supporting, provisioning (excluding production for economic benefit), regulating and cultural services, based on the Millennium Ecosystem Assessment (2005) conceptual framework. These categories were recorded following [START_REF] Costanza R, D'arge | The value of the world's ecosystem services and natural capital[END_REF] and the Millennium Ecosystem Assessment ( 2005) classification (Table 1 part A). Lastly, we recorded (also coded presence/absence) the type of agricultural land use studied, the study scales, the ES measurement methods and the ES beneficiaries specified in the consulted articles (Table 1 part B). When articles compared conventional farming systems with other types of agricultural management, we only extracted data concerning conventional farming systems whether or not the supply of ES was significantly different between agricultural management types. Indeed, no quantitative comparisons were planned in this study in which identification of ES provided by conventional agroecosystems was the major aim.

ES proxies from original articles: identification and selection of ES indicators

We recorded each ES proxy (i.e. indirect measurement of an ES) detected in the 123 original articles. After homogenising terms (i.e. classifying under the same term identical proxies for which authors used different terms), a total of 349 proxies were identified. The measurements most commonly applied to ES, and the appropriacy of these proxies as indicators (i.e. the identification of proxies showing strong relationships with ES), were assessed through analysis on extracted data.

Quantitative analysis of data extracted from original articles

ES identified from the subset of original articles (123) were subjected (Table 1 part A) to ordination analysis based on non-metric multidimensional scaling (NMDS). NMDS is used over a large number of scientific fields because it is a potent statistical tool which allows to visualise complex multivariate datasets in a lowered number of dimensions. Moreover, it is an adaptable technique for analysing many different data types which produces a handily interpreted output and is robust to out of distribution data (Borg and Groenen 2005). NMDS analyses (metaMDS function) based on Jaccard distances were run using 40 random starting configurations in 1 to 15 dimensions. The run with the lowest stress value was applied. All recorded parameters (Table 1 part B) were further fitted onto this ordination (envfit function). The significance of the correlation between ordination variation and fitted parameters was assessed using 999 permutations. All statistical analyses were run in R (version 3.6.3) using the vegan package.

Indicator criteria

A first subset of "indicators" was selected from significantly correlated ES proxies on NMDS axes. The [START_REF] Dale | Measures of the effects of agricultural practices on ecosystem services[END_REF] criteria for the definition of ecological indicators of ES were then used to select a second subset of "indicators". The proxies selected thus met the following criteria: (1) easily measured; (2) sensitive to changes in the system; (3) anticipatory, i.e. signify an impending change in key characteristics of the ecological system; (4) predict changes that can be averted by management actions; (5) integrative: the full suite of indicators provide a measure of coverage of the key gradients across the ecological systems (e.g. gradients across soils, vegetation types, temperature, space, time, etc.) and ( 6) have known response to ecological system changes.

Results

Growth trends in research on ES from the conventional agricultural landscape

The literature search resulted in a total of 189 articles of which 65.1% were original articles (N = 123); 31.7% were reviews, meta-analysis, or book chapters (N = 60); and only 3.1% were editorial or opinion articles (N = 6). The articles were mainly published in environmental sciences, ecology and environmental studies We b of Scien ce categ ories (see Supplementary material B, Table B.1). It is only since 1999 that interest has been shown in ES from the agricultural landscape. There was a slight increase from 2004 to 2011, followed by a sharp increase that reached a peak in 2018 with 40 publications (Fig. 2). The annual growth rate was 15.5%, with a collaboration index which is a co-authors per article index calculated only using the multi-authored article set of 4.8.

The USA, France and Spain were the three most productive countries in the field (Fig. 3). Multiple country publication was frequent. Australia, the UK and Denmark are three remarkable cases where most of the papers were from multiple countries.

Agricultural land ecosystems focused on and their characteristics

In most cases, multiple types of ES were addressed in the same article (73%) (Fig. 4A). Regulating services were the most frequently studied (85.7%), followed by supporting (58.7%) and provisioning services (excluding production for economic benefit), (55.6%), while cultural services received the least attention (24.9%).

The aspects of ES that were focused on include nutrient cycling (49.7%), pest control (48.1%), carbon sequestration (46.6%), biodiversity (40.7%), water regulation (37.03%), pollination (36.5%), soil structure (35.4%) and climate regulation (30.2%) (Fig. 4B).

Studies focussed most on ES in arable crops (68.8%), followed by permanent crops (51.1%) and grasslandspastures (43.9%). Remarkably, most studies addressed several agricultural land uses (54%) (Fig. 5A).

The most common study scales were landscape (63%), multi-plot (43%), multi-farm (42%) and farm (29.1%) scales, while plot, country, global and European scales accounted for under 10% of occurrences. Almost half the articles (48%) used several of these scales (Fig. 5B).

The method most frequently used to measure ES was in situ measurement (74.6%), followed at some distance by modelling (34.9%), and well behind them spatial or geographic measurement (20.6%), interview-questionnaire (16.4%), database literature (12.7%), synthetic indicator (6.3%) and lab experiment (4.2%). Half the studies involved multiple measurement methods (51.9%) (Fig. 5C).

The main beneficiaries of ES identified in our dataset were farmers (95.2%), then local society (88.4%) and global society (61.9%). Multiple beneficiaries were found in 87.3% of the articles (Fig. 5D).

Concerning the NMDS, the greatest reduction in stress was achieved with a six-dimensional solution. ES distribution along the NMDS axes was related to a certain degree to ES type (Fig. 6). Cultural ES, and to a lesser extent supporting and provisioning ES (excluding production for economic benefit), were mainly explored in studies devoted to them alone. On the other hand, regulating services were examined together with provisioning and supporting ES. Within regulating services, weed regulation, pest control and pollination were often studied together. Disease regulation, human health and pesticide retention were another group of ES explored jointly (Fig. 6). Moreover, carbon sequestration, water regulation, water purification, toxic waste retention, climate regulation, air purification and protection from disturbance were studied together.

Agricultural land use fitted on NMDS revealed that grasslands and pastures were significantly correlated with cultural ES, while arable crops were correlated with weed and pest regulation and pollination. Studies on biodiversity, genetic resources and habitat provision mainly referred to permanent crops (Fig. 7A).

Concerning study scale, landscape scale was significantly correlated with cultural ES while multi-plot scale was significantly correlated with supporting services such as soil formation, primary production and nutrient cycling. Carbon sequestration, water purification and regulation, toxic waste retention, climate regulation and air purification were studied at plot scale but also at country or more global scales. Disease regulation, human health, weed and pest regulation and pollination were more often examined at farm or multi-farm scales (Fig. 7B). In terms of measurement methods, NMDS also revealed that weed and pest regulation, pollination, pesticide retention, human health and disease regulation were significantly correlated with in situ measurement and, to a far lesser extent, with lab experiments. Interviews and questionnaires were significantly used to measure cultural ES. Modelling, spatialized information and database literature were mainly used to measure carbon sequestration, water regulation and purification, soil structure, water provision, toxic retention, climate regulation, air purification and sediment trapping. Protection from disturbance was often assessed via synthetic indicators (Fig. 7C).

All three ES beneficiaries were significantly correlated with NMDS axes. Farmers benefited the most from soil formation, primary production and nutrient cycling. Water regulation and purification, soil structure, water provisioning, toxic retention, climate regulation, air purification and sediment trapping were of benefit to global and local society. Carbon sequestration was of the greatest benefit to global society and protection from disturbance to local society (Fig. 7D).

ES proxies that could be used as indicators

A total of 74 out of 349 proxies were significantly correlated with NMDS axes such as land cover, pest damage, pollinator visit rate, bulk density, erosion rate… (see Supplementary material B, Table B.3). After applying the Dale and Polasky (2007) criteria, we selected a total of 50 indicators from these proxies such as land cover, pest damage, fruit set (pollinator visit rate for example did not meet the criteria), bulk density, erosion rate, pH… (see Supplementary material B, Table B.3). Seventeen involved biodiversity provision by ES associated with a species and/or a taxon (e.g. honeybee). As the validity of the proxies identified could not be tested statistically, their use as indicators is only discussed below.

Discussion

Growth trends in research on ES from the conventional agricultural landscape

The growing number of references and the growth rate found in our study attest to increased interest in this field, despite a history of neglect of agricultural land ES compared to ES from "natural" ecosystems [START_REF] Van Zanten | European agricultural landscapes, common agricultural policy and ecosystem services: a review[END_REF]. The first reason is scientists' growing interest in and knowledge of functional approaches and ES (e.g. Lavorel 2013; Nelson and Colours represent ES types: purple for regulating services, blue for supporting services, green for provisioning services and orange for cultural services (NMDS stress: 0.05, six dimensions). For ES codes, see Table 1 part A. [START_REF] Nelson | Modelling ecosystem services in terrestrial systems[END_REF][START_REF] Seppelt | A quantitative review of ecosystem service studies: approaches, shortcomings and the road ahead[END_REF][START_REF] Vihervaara | Trends in Ecosystem Service Res: Early Steps and Current Drivers[END_REF]). Added to this, the synthesis by the Millennium Ecosystem Assessment (2005) stressed the decline and unsustainable use of ES, leading to awareness of the urgent need to conserve and restore ES. This likely explains the increase in such studies in the early 2000s. Later, the Convention on Biological Diversity, which set the maintenance of ES as one of its main objectives in the Strategic Plan for Biodiversity 2011-2020, was probably the driver of the sharp increase observed after 2011.

Scientists' heightened interest in this field is also due to heightened recognition of ES from agricultural land. The amount of land covered by farming and its human management provide opportunities to deepen our knowledge of ES and to improve the provision of services and their sustainability [START_REF] Tscharntke | Landscape perspectives on agricultural intensification and biodiversityecosystem service management[END_REF][START_REF] Wratten | Ecosystem services in agricultural and urban landscapes[END_REF]. Moreover, the collaboration index and the number of multiple country articles found here show that research in this field is collaborative. All the scientific output descriptors revealed solid growth within this research field, with increasing scientific production and research collaborations. The research is mainly performed by rich countries, a large percentage of whose land is occupied by conventional agricultural systems; Sweden is the only exception. Seven of the ten most productive countries are European (Table 2). This result is consistent with previous reports that research production concerning ES is largely driven by the USA, China and European countries (FAO 2016;[START_REF] Seppelt | A quantitative review of ecosystem service studies: approaches, shortcomings and the road ahead[END_REF]).

Agricultural land ecosystems focused on and their characteristics

Studies focused mainly on regulating services from agricultural land, as found in previous reviews of multiple ecosystems studies [START_REF] Egoh | Integrating ecosystem services into conservation assessments: a review[END_REF][START_REF] Martínez-Harms | Methods for mapping ecosystem service supply: a review[END_REF]Malinga et al. 2015). In contrast to their findings, however, here, supporting services were the second most frequently studied ES type, followed by provisioning services (excluding production for economic benefit). This result is probably linked to the fact that we excluded production and direct ecosystem benefit-related ES from this article. The interest in regulating and supporting services is likely due to their association with production, human wellbeing and commonly valuable services (Millennium Ecosystem Assessment 2005). It follows logically that the studies focused mainly on services that could support and/or provide a benefit, such as, for example, pollination services, pest control and nutrient cycling. Moreover, it must be specified that our study is focused on conventional farms, where agrochemicals are used to eradicate pests and weeds. The identification of ES related to pest and weed control might therefore be the result of the use of external inputs, which can have a negative impact on other ES and human health, rather than a natural pest and weed control [START_REF] Stavi | Soil functions and ecosystem services in conventional, conservation, and integrated[END_REF][START_REF] Palomo-Campesino | Exploring the connections between agroecological practices and ecosystem services: a systematic literature review[END_REF].

Carbon sequestration (46.6%) was often measured, testifying to the recent high concern over rising atmospheric CO2 levels. Biodiversity provision (40.7%) was often assessed, indicating that species approaches remain popular even within the ES framework and in species-poor conventional farming systems. This may be linked to evidence from several studies that high species diversity is then needed to maintain ES [START_REF] Isbell | High plant diversity is needed to maintain ecosystem services[END_REF][START_REF] Harrison | Linkages between biodiversity attributes and ecosystem services: a systematic review[END_REF].

Cultural services from the agricultural landscape received the least attention, contrary to findings from other ES reviews [START_REF] Egoh | Indicators for mapping ecosystem services: a review[END_REF][START_REF] Martínez-Harms | Methods for mapping ecosystem service supply: a review[END_REF]. Even though agroecosystems are recognised as important cultural ES providers [START_REF] Assandri | Beautiful agricultural landscapes promote cultural ecosystem services and biodiversity conservation[END_REF], the more conventional agricultural systems do not preserve cultural ES. This could explain the lower cultural benefits derived from such agricultural land, which is mainly composed of ordinary nature (species-poor open-field grasslands and intensive arable crops, intensive vineyards or orchards). Moreover, cultural ES do not sustain agricultural production and are thus underexplored [START_REF] Swinton | Ecosystem services and agriculture: Cultivating agricultural ecosystems for diverse benefits[END_REF]).

ES from different agricultural land uses

ES were primarily studied in arable crops, which use 11.6% of global land area, and in permanent crops, which only use 1.3%. Although grasslands and pastures occupy the largest land area (25.8%), ES studies there were less common. This could be explained by the fact that grasslands have received substantially less attention in the ES framework compared to other production systems [START_REF] Bengtsson | Grasslands-more important for ecosystem services than you might think[END_REF]. Seminatural grasslands and pastures do not have a central place in the debate on delivery of ES, despite their generally high value in terms of biodiversity, production and cultural heritage in comparison with intensive arable crops. Moreover, while arable crop and permanent crop areas have increased in response to higher demand for food production, grassland and pasture areas have decreased through conversion to arable land for annual cropping and/or abandonment and scrub encroachments (FAO 2016).

Our results indicate that it is the agricultural management system that drives the relationship between agricultural land use and the ES types studied. For instance, ES supporting and/ or of benefit to agriculture, i.e. weed and pest control and pollination, was more frequently studied in arable crops, which are the most intensively managed ecosystems. Moreover these services are also particularly threatened by agricultural intensification [START_REF] Dale | Measures of the effects of agricultural practices on ecosystem services[END_REF][START_REF] Wratten | Ecosystem services in agricultural and urban landscapes[END_REF]). In the articles we reviewed, biodiversity provision was studied principally in permanent crops composed of agroforestry systems, olive groves and vineyards. This is likely due to the fact that these systems support higher levels of biodiversity compared to arable crops and artificial grasslands alone [START_REF] Torralba | Do European Agrofor Syst enhance biodiversity and ecosystem services? A meta-analysis[END_REF]. For example, the heterogeneity and diversity of the agroforestry system vegetation also create varied habitats useful for conservation [START_REF] Jose | Agroforestry for ecosystem services and environmental benefits: an overview[END_REF]. Cultural ES, on the other hand, tend to be linked to grasslands and Several authors have already argued that in agricultural land, it is often the ecosystem landscape pattern and the spatial structure of patches that is important for service provision [START_REF] Termorshuizen | Landscape services as a bridge between Landsc Ecol and sustainable development[END_REF][START_REF] Willemen | A multi-scale modelling approach for analysing landscape service dynamics[END_REF][START_REF] Van Zanten | European agricultural landscapes, common agricultural policy and ecosystem services: a review[END_REF]). Nevertheless, landscape-based metrics of structure and composition are often the most costeffective indicators of ecological systems [START_REF] Dale | Measures of the effects of agricultural practices on ecosystem services[END_REF]. Furthermore, [START_REF] Wratten | Ecosystem services in agricultural and urban landscapes[END_REF] showed that scarcely any ES are provided at one scale alone. ES can be measured at different scales, as our study shows: for instance, carbon sequestration can be measured at multi-plot scale from direct soil carbon content or proxies derived from vegetation cover [START_REF] Dale | Measures of the effects of agricultural practices on ecosystem services[END_REF]. The choice of an ES study scale is related to the ES scale of occurrence and the feasibility of its measurement [START_REF] Hein | Spatial scales, stakeholders and the valuation of ecosystem services[END_REF]).

ES and measurement methods

In situ measurement was by far the most common method used in the assessed articles. However, several other methods were often used, likely chosen after weighing up their limitations and reliability [START_REF] Seppelt | A quantitative review of ecosystem service studies: approaches, shortcomings and the road ahead[END_REF]. Cultural ES measurements mainly involved interview and scoring methods, as demonstrated in other studies (e.g. [START_REF] Boerema | Are ecosystem services adequately quantified?[END_REF]. There was greater variety in the methods used to measure other ES, but overall, biological control and pollination were studied in situ, while other ES like carbon sequestration or air quality were mapped and/or modelled. Furthermore, several different measurement types were used to quantify the same ES. This is consistent with other findings and demonstrates the complex reality of ES and the need for universally accepted indicators and methods [START_REF] Boerema | Are ecosystem services adequately quantified?[END_REF]).

ES and beneficiaries

Agroecosystems are managed to improve the supply of services, so it is natural that farmers were the main beneficiaries of ES in our article. However, we found that most of the studies addressed multi-beneficiaries. As ES is scale-dependent, ES beneficiaries are also scale-dependent [START_REF] Wratten | Ecosystem services in agricultural and urban landscapes[END_REF]. Farmers stand to benefit from small-scale ES and local and global society from larger-scale ES. Moreover, while several ES are produced, consumed and managed at the same scale, e.g. soil organic matter, other ES are produced at one scale but of benefit to people at several different scales, e.g. carbon sequestration (Raudsepp-Hearne and Peterson 2016).

ES proxies that could be used as indicators

Proper but also easy quantification of ES is fundamental. According to [START_REF] Cowling | An operational model for mainstreaming ecosystem services for implementation[END_REF], this should be the first step before either monetary valuation or evaluation of trends, trade-offs, synergies and disservices. First, ES need to be properly defined [START_REF] Boyd | What are ecosystem services? The need for standardized environmental accounting units[END_REF]; here, we found that several different terms were often used for the same ES. B.3).

However, one challenge that could be addressed now in further studies is to determine for each indicator the range of measurement values that should be considered positive for ES provision, which is likely ecosystem-dependent. Moreover, the scale of production and the beneficiaries need to be determined before selecting the measurement scale. As indicators are often proxies for non-measurable ES, it is important to know which part of the ES "cascade" is being measured (i.e. ecosystem property, function, service, benefit or value), to ensure reliability [START_REF] Boerema | Are ecosystem services adequately quantified?[END_REF]).

Conclusion: perspectives and constraints for researches on ES from conventional farming systems

result of interactions between agricultural management and ecological structures. It is thus important to know how land management impacts ES provision [START_REF] Van Zanten | European agricultural landscapes, common agricultural policy and ecosystem services: a review[END_REF][START_REF] Duru | How to implement biodiversity-based agriculture to enhance ecosystem services: a review[END_REF][START_REF] Rey | Integrating Ecol Eng and ecological intensification from management practices to ecosystem services into a generic framework: a review[END_REF]. Overall, it has been already proven that agroecological practices provide more nonprovisioning ES than conventional farming practices [START_REF] Stavi | Soil functions and ecosystem services in conventional, conservation, and integrated[END_REF][START_REF] Palomo-Campesino | Exploring the connections between agroecological practices and ecosystem services: a systematic literature review[END_REF]), and we thus must be aware that non-conventional types of managements are important in conservation and restoration plans. However, in this study, we wanted as a first crucial step to identify ES supplied even under conventional agriculture. This acknowledgement would allow taking into account ordinary nature from species-poor ecosystems. Then, identified indicators of ES provided by ordinary nature from conventional agriculture systems should be included into conservation and restoration plans. A following step would be to evaluate trade-offs, synergies between ES and disservices (Therond et al. 2017). Today, in a context of global change, it is also important to understand and consider the evolution of ES driven by changes such as agricultural intensification and increased drought duration.

Moreover, ES have multiple dimensions, i.e. biophysical, sociocultural and monetary. To date, studies on ES have been often based on monetary approaches and have taken a provisioning point of view [START_REF] Van Zanten | European agricultural landscapes, common agricultural policy and ecosystem services: a review[END_REF]. Now, there is a need for more integrated approaches considering all three ES dimensions of sustainability (de Groot et al. 2010;Millennium Ecosystem Assessment 2005).

To conclude, we are aware that there are limitations to our approach related to the literature screening method we used. Indeed, the conducted search by title yield a limited number of articles, and the exclusion of "grey literature" reduced the access to important other form of knowledge on ES. Nevertheless, we think that an evidence map was needed as a first stating point to understand the state of knowledge on ES, to suggest easily measurable and operational indicators of significant ES for conventional farming system and then to allow better incorporation of ES into conservation and restoration programmes.

Fig. 1

 1 Fig.1Comparison of a speciespoor mowed artificial meadow (conventional farming system, on the left) with a species-rich grazed semi-natural grassland (traditional farming system, on the right), France. Photograph by Christel Vidaller, 2020.

Fig. 2

 2 Fig. 2 Growth of absolute publication output concerning ecosystem services from agricultural land from 1999 to December 2018.

Fig. 3

 3 Fig. 3 Number of articles concerning ecosystem services from agricultural land produced by the ten most productive countries, and number of articles from multiple countries.

Fig. 4

 4 Fig. 4 Percentages for ecosystem service (ES) types (A) and ES (B) in all articles on ES from agricultural land.

Fig. 5

 5 Fig. 5 Percentages for (A) agricultural land use, (B) study scale, (C) measurement method, and (D) beneficiary of ecosystem services (ES), in all articles studying ES from agricultural land.

Fig. 6

 6 Fig. 6 Non-metric multidimensional scaling (MDS) based on Jaccard distance of ecosystem services (ES) identified in the original article subset. Only the first two axes (MDS1 and MDS2) are shown here. Colours represent ES types: purple for regulating services, blue for supporting services, green for provisioning services and orange for cultural services (NMDS stress: 0.05, six dimensions). For ES codes, see Table 1 part A.

Fig. 7

 7 Fig.7 (A) Agricultural land use, (B) study scale, (C) measurement method, and (D) beneficiary of ecosystem services (ES), fitted on nonmetric multidimensional scaling (MDS) of ecosystem services identified in the original article subset. Only the first two axes (MDS1 and MDS2) are shown here. Colours represent ES types: purple for regulating

  Only one set of terms, such as those provided by the Millennium Ecosystem Assessment (2005) framework or by the Common International Classification of Ecosystem Services (CICES 2012) classification, should be used, to avoid misunderstandings when ES are studied and/or incorporated in conservation and restoration plans. Second, ES quantification implies the use of several relevant indicators (Dale and Polasky 2007). Almost three quarters of the studies in this article analysed several ES types in concert. Future perspectives include using these identified bundles of ES, i.e. sets of positively correlated ES, as indicators (Raudsepp-Hearne et al. 2010). Our evidence map yielded 74 frequently used indicators, 50 of which are easily measurable and fulfil the Dale and Polasky (2007) criteria (see Supplementary material B, Table

Table 1 (

 1

	A) Recorded ecosystem
	service types and ecosystem	A			B
	services (ES) and (B) ES	ES type	ES	ES code	ES characteristics	Modality
	characteristics, following
	Costanza et al. (1997) and the	Regulating	Pest control	Pest con	Agricultural land use	Arable crop
	Millennium Ecosystem		Carbon sequestration	Carb seq		Permanent crop
	Assessment (2005) classification.		Water regulation	Wat reg		Grassland and pasture
			Pollination	Pol	Study scale	Landscape
			Climate regulation	Clim reg		Multi_plots
			Water purification	Wat pur		Multi_farms
			Weed regulation	Wat reg		Farm
			Disease regulation	Dis reg		Plot
			Protection from disturbance	Prot dist		Country
			Air purification	Air pur		Global
			Toxic waste retention	Tox ret		Europe
			Human health	Hum hea	Measurement method	In situ measurement
			Pesticide retention	Pesti ret		Modelling
			Seed dispersal	Seed dis		Spatial or geographic
		Supporting	Nutrient cycling	Nut cyc		Interview questionnaire
			Soil structure	Soil str		Database literature
			Primary production	Prim pro		Synthetic indicator
			Soil formation	Soil for		Lab experiment
			Sediment trapping	Sed tra	ES beneficiary	Farmer
		Provisioning	Biodiversity	Biodi		Local society
			Habitat	Habi		Global society
			Fresh water	Wat
			Genetic resources	Gen res
			Medicines	Med
			Wild food	Wfood
		Cultural	Aesthetics	Aes
			Recreation, ecotourism	Rec eco
			Cultural heritage	Cult her
			Sense of place	Sense pla
			Spiritual, religious	Spiri
			Educational	Edu
			Inspirational	Insp

  Table 1 part A. For correlation significance of variables see Supplementary material B, Table B.2.

Table 2

 2

	Ten countries producing
	most research on ecosystem	Productivity ranking	Country	Agricultural land (% of land area) in 2016
	services (ES) from agricultural
	land, and percentage of	1	USA	44.37
	agricultural land in each country	2	France	52.45
	from FAO (2016).	3	Spain	52.58
		4	Australia	48.24
		5	Germany	47.68
		6	Italy	43.23
		7	Sweden	7.44
		8	United Kingdom	71.71
		9	China	56.21
		10	Denmark	62.01

Agron. Sustain. Dev.(2022) 42:22

Acknowledgements We are grateful to Dr. Marthe Lucas, lecturer at Avignon University, for answering questions on policies. We thank Marjorie Sweetko for her careful reading of the manuscript and especially for her helpful comments. We also thank two anonymous reviewers and associate editors of Agronomy for Sustainable Development for their valuable comments on the first version of this paper.

Funding This work was supported by ANR CompAg for financial support (CompAg ANR-17-CE32-0014-02).

Data availability All articles analysed during this study are available online.

Code availability Not applicable.

Declarations

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1007/s13593-021-00740-w.

Authors' contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Christel Vidaller. The first draft of the manuscript was written by Christel Vidaller, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Ethics approval Not applicable.

Consent to participate Not applicable.

Consent for publication Not applicable.

Conflict of interests

The author declare no competing interests.


Aguir

Professor Moez Bejar
email: bejar_moez@yahoo.fr

Sabrine Smiy

Essebti Dhahri

Tomas Fiorido

Marc Bendahan

Khalifa Aguir

Ozone detection based on nanostructured thin films

Keywords: perovskite, drop-coating, thin films, semiconductor, gas sensor, ozone perovskite, drop-coating, thin films, semiconductor, gas sensor, ozone

    Ozone detection based on nanostructured L a 0.8 P b 0.1 C a 0.1 F e 0.8 C o 0.2 O 3 thin films

Cover letter

for consideration for publication in the Journal of Alloys and compounds.

It has not been published elsewhere and that it has not been submitted simultaneously for publication elsewhere. In this manuscript, we investigated the response of compound toward the Ozone oxidizing gas.

Thank you for your consideration, Yours Sincerely;

Pr. Moez BEJAR

Prime Novelty Statement

The sample has been prepared using the drop coating method. The structural and Ozone gas-sensing properties were reported. The gas sensing properties of shows that ours sensor can detect low concentrations of Ozone-gas and we deduced that our sensor can be used as promising candidates to detect Ozone-gas added to that our captor is able to detect this dangerous gas at different concentration.

I-INTRODUCTION

Recently, the synthesis of advanced materials for many applications has been widely investigated in literature. In particular, the interest for the perovskite materials (Ln: lanthanide and M: trivalent cations), destinated for innovative technological applications, such as catalysis, magnetocaloric and gas sensing field, have grown steadily

[1-4].
The is a potential functional material which is used in many fields such as gassensing in the detection of many gases, including , and

[5-9].
Also, derivates of are very used in the detection of dangerous gases. In line with our interest in the study of gas sensing, we report in the present research the synthesis of a thin film gas sensor based perovskite-type over a / substrate with interdigitated Pt-electrodes using a drop coating as a new method.

II-EXPERIMENTAL DETAILS

II-1) Preparation of perovskite nano-powder

Nano-powder of has been prepared by mixing different nitrates: and as precursors according to the proportion indicated in the equation below:

Then, the collected powder has been sintered at 300, 600 and 900 °C for 24h for each temperature.

II-2) Thin film preparation

The perovskite thin-film was deposited over the substrate with interdigitated Pt-electrodes by the drop coating method. Firstly, a slurry of of perovskite mixed with of 2-propanol has been prepared and ultrasonicated for at to ensure a good homogenization for the mixture. Subsequently, the deposition of the thin film was assured by coating the substrate drop by drop using a micropipette. The process has been combined with heating at after each drop to remove the solvent. The thin film properties were optimized at 8 drops which makes a total of . Finally, after the deposit of 8 drops a heating for one hour at will take place .

II-3) Ozone sensing measurement:

To investigate the ozone sensing properties of films, the sensors have been introduced in a test chamber allowing the control of the sensor temperature under variable gas concentrations. Dry air has been used as a reference gas.

Ozone gas was generated by oxidizing oxygen using a pen-ray UV lamp (Stable Ozone

Generator UVP/ ). The intensity of the UV radiation was varied by shifting a shutter around the lamp. The different ozone concentrations are obtained in the range of with a flow rate of dry air maintained at .

The operating temperature of the sensor has been adjusted between 160 °C and 280 °C to determine the best sensor response. In this case, our sensor was exposed to ozone-concentration for in order to identify the best operating temperature.

The applied dc voltage for sensor measurement is 1V and the current is measured using a computerized sourcemeter type Keithley 4250. The sensor response is defined as

, where and are the resistance under air and the resistance when the sensor is exposed to ozone-gas, respectively.

III-RESULTS AND DISCUSSION

III-1) Scaling analysis

The Rietveld refinement of the X-ray diffractogram of nanopowder (Fig. 2) has revealed that our sample crystallizes in the orthorhombic structure with the space group. The lattice refined parameters and are nd . 4 Ǻ, r sp ctiv ly. Th Miller indices corresponding to the main reflexion planes of our sensor are . M r v r, th n lysis using "X0 rt High Sc r lus" pr gr m has shown the presence of as secondary phase.

The crystallite size of the powder has been evaluated using the Williamson-Hall method [15] (Fig. 3). The sensor has shown a nanometric crystallite size equal to .

(Eq. 2)

Where is the width at half height, is the coefficient due to structural defects, represents the grain form factor , is the radiation wavelength and is the angle corresponding to each peak.

III-2) Ozone sensing properties:

Fig. 4 shows the variation of the electrical resistance as a function of time under dry air with a temperature increasing from 25 to 180 °C. The results show that the resistance decreases with increasing the temperature, which proves the semiconductor behavior of sensor [START_REF] Van Der Ziel | Solid State Physical Electronics[END_REF].

In order to determine the operating temperature of our compound, the sensor responses in presence of of ozone have been carried out at different temperatures between 160°C and 280°C. (Eq. 3)

(Eq. 4)

(Eq. 5)

(Eq. 6)

The chemical reactions process between free adsorption site S and the triatomic gases, as Ozone (O3), are [26]:

(Eq. 7)

(Eq. 8)

IV-CONCLUSION

In this research paper, the sensor has been successfully

prepared by the drop coating method and tested as an ozone sensor. The prepared thinfilm showed sensitivity to -levels of ozone gas. The resistance study as a function of time at different working temperatures depicted the good sensitivity of our sensor especially at the operating temperature of . In addition, our sensor can be used in the distinction of the air quality for the ozone and sub-ppm levels detection becomes possible.

Our study shows the possibility of the use of our sensor in the field of the detection of ozone-gas. Another necessary part of our work is the study of the selectivity of sensor.

  Pr. Moez BEJAR Laboratoire de Physique Appliquée, Faculté des Sciences, B.P. 1171, 3000 Sfax, Université de Sfax, Tunisie. bejar_moez@yahoo.fr 22/12/2019 Dear Editors of the Journal of Alloys and compounds. I am pleased to submit our manuscript entitled: "Ozone detection based on nanostructured thin films"

Fig. 5

 5 shows the resistance variation versus time at 170, 190, 260 and 280 °C during ozone exposing time of . The results show that the sample displays reversibility and the curves present good stability of the baseline. Furthermore, the sample resistance has declined upon exposure to an oxidizing gas [17], indicating the p-type semiconductor behavior of sensor [18]. In fact, with an oxidizing gas, such as Ozone, the conductivity increases due to the enhancement of the holes of concentration. Table.1 summaries the ozone (O3)-responses values of sensor. The maximum of the sensitivity has been reached at when our sensor was exposed to for 1min, is considered as the operating temperature of the sensor. The sensor performances have also been evaluated with different ozone concentrations and the results are presented in Fig. 6(a). The results exhibit the sensing ability of the thin-film in all the studied ozone concentrations. In the same way, Fig. 6(b) shows that the response intensities increase together with the ozone concentration and reach a maximum of 3.6 for the concentration of . The recorded values are depicted in

[ 3 ].

 3 In contrast, sensor has low response time values compared to other perovskite materials, such as [3]. The response, signal and response time are influenced by the quantity and quality of oxygen adsorbed on the surface of our sensor [23]. On the surface, atmospheric oxygen can be adsorbed very easily and atoms or molecules become chemisorbed ions or . When the temperature increases the state of oxygen on the surface undergoes the following reactions (from Eq. 3 to Eq. 6) and this phenomenon is demonstrated theoretically by using the SnO2 model [24]. In fact, our thin film presents an increase in hole concentration because when oxygen species are adsorbed in the sensor surface will capture electrons from our sensor which is a p-type semiconductor and consequently a depletion region of electrons is thus formed [25]. Added to that, at the operating temperature we have a maximum of oxygen adsorbed on the surface of sensor [1].

Figure 1 :

 1 Figure 1: (a) Optical sensor view after drop coating of sensor and (b) the zoom of the figure.

FigureFigure 2 :

 2 Figure

Figure 3 :

 3 Figure 3: The Williamson-Hall analysis plots of

Figure 4 :

 4 Figure 4: Resistance vs. time of

Figures 5 :

 5 Figures 5: Ozone gas response of

Figures 6 :

 6 Figures 6:

Click here to view linked References

	Saoudi et al. [12] have reported that introducing Cobalt (Co) in B-site in
	perovskite materials will increase the response because the
	sample with x=0.05 shows the higher sensitivity of	-gas compared to
	sample.
	The ozone-gas has been emerged in many fields, e.g. textile, food, water treatment,
	pharmaceutical and chemical industries [3]. Consequently, ozone constitutes one part of
	the summer toxic smog. However, for a concentration above	in the atmosphere,
	Ozone could cause serious health problems (e.g. burning eyes, lung damage and
	respiratory irritation) [13, 14]. Moreover, the air quality is classified referring to the
	Ozone amount in the atmosphere: very good	-	, good	-	, poor
	and very poor	[3]. For all the above reasons, the
	monitoring of the Ozone concentration in our atmosphere is becoming a serious issue.
	In our previous published work related to this framework, we have demonstrated the
	high sensing properties of		towards the NH3-gas [1].

Song et al. have proven that introducing Pb in the A-site to replace Lanthanum (La) increase the response of material [10]. In addition, the introduction of Ca in has caused the increase of sensitivity of to alcohol [11]. In another study, H.

*Manuscript

Table 2 .

 2 TheseThe response time is another important parameter to evaluate the sensor, which is the time required to reach 90 % of the electrical resistance when the sensor is exposed toOzone gas [1]. The response time values, recorded and gathered in Table3, have revealed that decreases when increasing the concentration at the operating temperature of 170 °C. The same trend was observed previously by Masami Mori and al.

values are better compared with those reported earlier [19-22].

Table 1 :

 1 The relationship between response

	and temperature	of

Table

Table 2 :

 2 Response values of sensor (our study) compared to others materials considered for Ozone sensing application.

	Sensors	T (°C)	C (ppb)		References
	(EG : 50µm)	250	200	1.7	[19]
	(EG : 50µm)	250	400	2.3	[19]
	(EG : 50µm)	250	800	2.8	[19]
		300	50	1.25	[20]
		300	100	1.42	[20]
		300	150	1.78	[20]
		300	175	2.25	[20]
		250	75	3	[21]
		200	400	3	[22]
		170	50	1.6	Our study
		170	100	1.9	Our study
		170	200	2.5	Our study
		170	300	3.3	Our study
		170	400	3.9	Our study

Table 3 :

 3 Response times

			of			sensor at different
	concentrations	at the operating temperature of 170 °C compared to the
	sensor.
	Sensors	T (°C)	C (ppb)	)	References
			170	50	3.5	Our study
			170	100	3.0	Our study
			170	200	2.5	Our study
			170	300	2.2	Our study
			170	400	2.1	Our study
			200	50	8.4	[3]
			200	300	5.2	[3]
			200	500	4.0	[3]
			200	3000	1.2	[3]

Graphical abstract

Graphical Abstract (for review)

Research Highlights



sample crystallizes in the orthorhombic structure with Pnma space group.

 Our compound presents a nano-metric size.



shows a p-type semiconductor behavior.

 sensor can detect the Ozone-gas even with low concentration equal to 50 ppb.

 T=170°C is the operating temperature of sensor.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

*Declaration of Interest Statement


Sandra Revolon

Iridescence as Affordance: On Artifacts and Light Interference in the Renewal of Life among the Owa (Eastern Solomon Islands)

This article focuses on identifying the technical and cognitive processes that imbue artifacts and practices with forms of efficacy central to the relations the Owa people have with surrounding invisible entities. In particular, it aims to describe the correspondences between the schemas at work, on the one hand, in material logics, by analyzing the making of magical artifacts and, on the other hand, in ritual logics, through an analysis of the practices and representations involved in Owa rituals. These two domains-technical action and ritual action-are both locally understood as ways of acting on and in the world.

This efficacy rests on the relationships established between vital processes and technical processes, or more precisely on the co-construction, by specialized humans and by invisible entities, of artificial devices that reproduce visible phenomena considered by the Owa to be visible manifestations of powerful entities' effective capacity to intervene (mana-ity / mana-ization) in the world. These include a broad category of phenomena that can be seen in the atmosphere and on the bodies of living organisms and which all display, in different ways, chromatic distortions that are mobilized and imitated by Owa carvers, who apply them to the surface of magical artifacts.

By illustrating one of the ways in which Owa craftsmen imitate vital processes through artifacts, this article examines what this imitation indicates about how the Owa conceive the renewal of life and act upon life in an attempt to control it. The idea of "vital processes" should be understood here in a broad sense to include all the processes that benefit life and result from non-human action. * Aorigi is a coral island six square kilometers large, situated in the eastern Solomon Islands and inhabited by some eight hundred Melanesian fishers, farmers, and carvers who speak the Austronesian language of Owa. The Owa reside in sixteen hamlets and villages and are divided into six clans, split into two matrilineal, exogamous moieties. Although the Owa converted to Christianity in the 1950s, they

continue to place great importance on maintaining equilibrium between humans and the invisible beings (ataro) who govern their lives.

These invisible beings fall into two categories. The first category includes beings who never had human existence, who are called ataro si fenna, "beings of the earth, of inhabited places," or ataro ni matawa, literally, "beings of the abysses, of the horizon," also known as "beings of the sea" (ataro iesi). The second category is comprised of beings who come from humans who have died either in ordinary circumstances (from old age or illness)-these are called ataro ni nuni, "beings of persons"-or as a result of violence: ataro si gapu, "beings of blood." All of these beings contribute more or less directly to the reproduction and growth of living beings-in particular, humans and pigs-to the fertility of the land, to the growth of domestic plants and edibles, and to the proliferation of marine resources, fish, and crustaceans. What does the existence of these beings or agents teach us about Owa ethnotheories of life? How do the Owa understand vital processes such as growth, reproduction, or the degeneration that brings death? Is there, for the Owa, a common principle, shared by dissimilar beings, that explains why they are all considered "living"? To answer these questions within the context of the Oceanian world, we must understand the relations between these vital processes and mana.

Whether or not they have human existence, and regardless of their fields of action, invisible beings-who all possess, in an immanent way, a power referred to as mena or mwaru, a local variant of the Melanesian mana defined by Codrington (1891, 118-20)-seem to have always dominated human affairs. To this day, throughout the year and in all kinds of circumstances, individual and collective rituals are performed to honor them (wosagi, to worship), to make up for offenses (tara, "payment of compensation"), or to solicit their support (nungifa, "to ask"). All means are used to attract these beings during worship: areca nuts are consumed in order to cause trances during which invisible entities enter human bodies; an ataro's true name is pronounced in ritual expressions, rather than a circumlocution; offerings and sacrifices of prepared foods are made, including yam and taro puddings, bonito fish,1 and steamed pig quarters, as well as the first fruit from gardens and orchards, bonito blood, areca nuts, tobacco, shell currency, paper money, coconuts, the yam premices, and Canarium nuts. Beyond their seeming diversity, all these rites share the same goal: to summon the help of the ataro in executing human actions, and thus to benefit from the generative power of their mana, necessary for the reproduction and growth of all living beings. The beautiful objects used during rituals-individual and collective bowls, large canoes with sewn and caulked boards-reproduce certain optical phenomena seen in the atmosphere or on certain living organisms to which the Owa attribute a non-human origin [START_REF] Revolon | Les esprits aiment ce qui est beau: formes, sens et efficacité rituelle des sculptures owa (est des îles Salomon)[END_REF]. These objects play a key role in the stratagems that living beings use to co-ordinate their actions with those of invisible beings.

Whether it is a matter of anthropology of art, approach to "religious materialities", extension of "material culture studies" to the most physical aspects of technical practices or the development of cognitive approaches to the production as well as the reception of artifacts, contemporary research on objects is characterized by the combining of disciplines, itself the outcome of research since the late 1960s, in France and England more particularly, often conducted in Oceania. The first phase of this approach was to replace objects in their local context of production, exchange and use. As R. Firth (1973) reminded us, such objects must be considered "as works of art in their own right, to be judged as expressions of artists' original conceptions in the light of their cultural endowment". Based on this postulate and influenced by the seminal ideas of A. Forge and then A. Gell, one of the central concerns in anthropology has been to show "how objects 'work' in their cultures" [START_REF] Gell | The Technology of Enchantment and the Enchantment of Technology[END_REF]) and more specifically, to quote J. [START_REF] Coote | Anthropology, Art, and Aesthetics[END_REF], to determine "why art objects should function as they are supposed to do? How in fact they do so, and why and how they are vehicles of meaning? … Here, the importance of art objects as objects is considered, the way that they are supposed to work is investigated, and the forms they take explained, as well as their functions and meanings explored." (1992,3).

Today, in an effort to deepen the relation established by A. Gell between the notions of esthetics and efficacy, one of the major fields investigated by anthropology of the object is that of the cognitive and relational processes that endow artifacts with the capacity to arouse emotion. For Gell, esthetics "has nothing to do with universal qualities or standard of beauty, but with people's conceptualizations of the effects of their art works" [START_REF] Gell | The Technology of Enchantment and the Enchantment of Technology[END_REF]. He therefore proposes to see art as a system of action whose purpose is to change the world.

These forms of action involve a mechanical chain of causes and effects that bring into play the visual perception system and human thinking. Among Gell's methodological options, certain seem particularly productive for the analysis of Owa ethnographic material, first of all because the notion of efficacy itself is at the heart of the practices and discourses associated with material and ritual logics; and then because using C.S. Pierce's semiology, and the notion of indexicality in particular, can be useful in explaining the different figurations of the dead in this Melanesian society.

We can nevertheless regret that Gell's analysis of the object as a system of actions did not take into account recent developments in an anthropology of modes of action that is attempting to go beyond the dichotomies (tangible/intangible, living/dead, nature/culture, human/non-human) that were an obstacle to the study of humans' relations with materiality. For instance, the mechanisms he defines to explain the notion of "agency" applied to artifacts -as material indications of social agents' capacity for intervention -unfold in a paradigm unquestioningly based on the nature/culture division, which we now know not to be universal (Descola 2005). Since Gell's work, T. Ingold, for example, has suggested developing a "new kind of ecological anthropology, one that would take as its starting point the active, perceptual engagement of human beings with the constituents of their world" (2000: 60). J.-M. Schaeffer in turn has defined the esthetic experience as a specific cognitive behavior, "a modality of activating" (modalité d'activation) objects, events or actions produced in risk-laden communication situations -seduction, struggle, submission -encountered in humans and animals (2008: 174-6; 2009).

In the same vein, anthropologists have devoted increasing attention to the crucial role played, alongside living humans, by invisible beings (deceased humans and non-human beings) in the processes of production and reception of living beings, events and artifacts in Oceania and elsewhere [START_REF] Revolon | Les esprits aiment ce qui est beau: formes, sens et efficacité rituelle des sculptures owa (est des îles Salomon)[END_REF], 2012[START_REF] Santos-Granero | The Occult Life of Things: Native Amazonian Theories of Materiality and Personhood[END_REF][START_REF] Morgan | Religion and Material Culture: The Matter of Belief[END_REF], Lemonnier 2012[START_REF] Pitrou | Figuration des processus vitaux et co-activité dans la Sierra Mixe de Oaxaca (Mexique)[END_REF][START_REF] Coupaye | Growing Art, Displaying Relationships: Yams, Art and Technology amongst the Nyamikum Abelam of Papua New Guinea[END_REF]. The present article is situated within this particular theoretical framework, and my principal aim is to analyze how humans capture sensuous qualities they perceive in nature and transform them into material devices allowing them to co-ordinate their actions with those of invisible agents so as to obtain effects they know themselves incapable of producing on their own. What are the specific characteristics of material actions in ritual actions? What material and conceptual mechanisms allow the Owa to ensure the presence of ataro during acts that, without them, would have no effect? By exploring the articulations between perception and action among humans, this article illustrates one of the ways in which perceptible qualities attributed to certain objects in the world, natural phenomena, and human artifacts are locally perceived as acting effectively on the world. Methodologically, highlighting the vital processes that the Owa seek to produce in certain circumstances requires identifying, within certain rites, the elements that make it possible to represent these vital processes and the actions expected of the non-human agents said to produce them.2

Light, a Sign of Non-Human Agency

Luminous phenomena are ubiquitous in the Owa's visual environment: first, in the marine world, where an exceptionally high concentration of species generate a bioluminescence well known to navigators in the region, who have long used itthe light from corals in particular-to locate coastlines and to navigate at night [START_REF] George | Polynesian Navigation and Te Lapa: 'The Flashing[END_REF]. Second, phenomena of light are seen in the celestial world, on this archipelago located in a subduction zone where permanent seismic and tectonic activity generates many different electromagnetic phenomena [START_REF] Devereux | Earth Lights Revelation: UFOs and Mystery Lightform Phenomena: The Earth's Secret Energy Force[END_REF].

Humans have ample opportunity to observe and integrate into their conceptual universe all these phenomena from the viewpoint of their tiny island surrounded by the immense ocean and an unobstructed horizon. Thus, they have various terms to describe that which is luminous, most of which refer to things locally considered to be primary, direct sources or forms of light: a fire, a lamp, the glowing end of a cigarette, the stars. In common language, the verb toga applies to that which shines, sparkles, twinkles, or dazzles. A second sense of the word expresses the action of secondary sources of light, which do not themselves produce radiance but rather reflect and diffuse it. When these notions are discussed by holders of local knowledge, the verb toga and its noun form togatoga(na) take on a more precise meaning, and designate that which emits a blinding, and necessarily reflected, light.

According to these exegetes, these terms refer to phenomena produced by a very diverse set of surfaces and materials: the skin of bonito fish and the mark it leaves on the torsos of young men when they are initiated; the ocean surface at dawn, sunset, or when the moon is reflected on it; certain clouds; the shell of the pearly nautilus when it is polished; or coral lime powder.

At the perceptual level, all these phenomena display light interferences and, in particular, a chromatic aberration found throughout nature and known to biologists since Buffon: iridescence-also called irisation or goniochromism-: a property of certain bodies that, because of their structures, reflect solar light while at the same time decompose it in such a way that only certain colors appear.

Photometeors, visible in the atmosphere, are created by the reflection, refraction, or diffraction of solar or lunar light; hydrometeors are created by water particles suspended in the atmosphere or settled on objects on the ground. The skin of bonitos is covered with iridophore pigment cells and diffracts light, generating blue and green iridescent shades, which mark the chests of initiates on whom the skin is placed. When cut, the flesh of these small tuna fish also displays iridescent reflections. Irisations can be seen at the edge of clouds, appearing as an arch whose colors, from violet to red, are created by sunlight filtered through drops of water in the atmosphere. The iridescent hues of the sunset are also due to the refraction, dispersion, and selective absorption of sunrays, as is the anti-twilight arch known as the Belt of Venus, where the bluish shadow of the Earth can be seen just above the horizon; above this, there is a glow of warm colors, from pink to purple, that comes from the scattered light of the rising or setting sun. As for nacre, its iridescence is due to its composition: superposed thin layers of aragonite and conchiolin are bonded to ionized water molecules and reflect light rays; depending on the refraction angle of the light, this causes certain colors to appear or disappear. Coral lime, which is made of calcium carbonate, presents iridescent properties similar to those of nacre. The vibrant, moving, column of light formed by the moon's reflection on the ocean surface and connected to variations in atmospheric refraction produces an optical effect known as scintillation. Bioluminescence, another phenomena of light that can be seen in nature, is a biochemical reaction that allows certain organisms, such as glow-worms or fireflies, to produce fluorescent or phosphorescent light-this light is one of the forms entities take for the Owa and many other Pacific populations. In the depths of the ocean, cephalopods-nautiluses in particular-equipped with light organs produce a cold light that allows them to hide their silhouettes in the dimly lit deep waters. Finally, certain corals produce light.

These phenomena of reverberation, saturation, and contrasts of dark (anthracite gray, jet black, dark blue, purple) and light (blinding white, gray, light blue or pink), dusky hues, iridescence, scintillation, and luminescence are characteristics of the beings, artifacts, places, and temporalities that have relations of proximity to invisible entities-the dead in particular. Thus, certain clouds (roto na rarapuru) are considered the manifestation of the malevolent deceased, as are the sudden, heavy rains that fall from them and cause fevers, which, without the intervention of a healer, can be fatal. Sunset-faufau tai nuni, literally "the hour when one cannot recognize people"-because of the nature of its light, the source of which can no longer be perceived, is for the Owa the moment when things blur together and beings, living or dead, encountered on a path or the seashore, are indistinguishable. Representations associated with bonitos accord their blood specific qualities: like nautiluses and coral lime, their blood it is supposed to immanently contain mana . Other shells-such as cowries (Ovula ovum) or motherof-pearl (Pinctada maxima), which are worn as ornaments or used as lure for bonitos-are considered vehicles for this force. In other words, whether natural or created by humans, these plays of light make tangible both the presence of invisible powerfull beings and their capacity to act on the world; their mana.

Materializing the Performative Dimension of Action

Within the Owa conceptions, mana is understood as a potentially unlimited capacity for action, recognizable through the events it is thought to cause, and cognizable within the relationship between living humans and supernatural entities (Codrington 1981;Drew and Fox 1915;Fox 1925;Hogbin 1936;Firth 1940;Ivens 1972;[START_REF] Keesing | Kwaio Religion: The Living and the Dead in a Solomon Island Society[END_REF][START_REF]Rethinking Mana[END_REF]2000;[START_REF] Kolshus | Codrington, Keesing, and Central Melanesian Mana: Two Historic Trajectories of Polynesian Cultural Dissemination[END_REF]). It is a transverse phenomenon, as light, which, in its physical, biological, and artifactual manifestations is the visible sign of mana. According to Owa understandings, mana results from the transformation of a vital principle called afigona that occurs at a person's death.

Afigona is defined as "the living part of a being," the site of life and health, located in the head, and it is seen as the origin of every form of life, whether plant, animal, or land; the island itself has a site considered to be the seat of its afinona. While alive, a human person does not inherently, spontaneously possess mana, and does not produce it. On the other hand, he or she has this "living part," afigona, which, at the moment when an embryo (apena, the corporeal wrapping) is conceived, animates it by giving it a material entity (anunu, which is distinct from the body itself, apena) and breath (manawana). Neither the living part of the being (afigona), nor the material part (anunu), nor the breath (manawana) contains mana. It is at the moment of death that the living part, afigona, transforms into a being (ataro), which leaves the bodily wrapping and possesses mana immanently. As for the material part, anunu, it detaches from the body after five days and leaves the world of the living for good. Thus, afigona and mana are like two states of the same principle: in its living form, afigona animates the flesh at the moment of conception and keeps the body it inhabits alive; its metamorphosis into a being (ataro) animated by mana leads to the body's metamorphosis into a cadaver. Five days after this change of state-as many days, it is said, as it takes yams to germinate 3 -the alteration of the flesh allows the material part of the body, anunu, to detach in turn. The body is then definitively emptied of all the principles that kept it alive, 4 and can be buried.

One of the first visible signs of a body's change of state from life to death may be iridescence-for example, a golden-green reflection, or a rainbow-seen on the raw flesh. This phenomenon is especially common on tuna muscles, and undoubtedly can also be observed on human flesh. Iridescence, an unstable and moving effect, is for the Owa an indication of this metamorphosis, and it visually marks the change in state of the afigona-mana principle that underlies the existence of all animated beings. Thus, iridescence is the perceptible quality of phenomena that can mana-ize or that are in a state of mana-ity. More broadly, it is my hypothesis that for the Owa, light interferences are tangible manifestations of the performative dimension of action, which by definition produces effects: here, iridescence marks the transformation of the imperceptible form of the afigona principle into mana, a form that can be assimilated by living humans. The change of state from afigona to mana, which takes place independently of all human and nonhuman action, has an immanent agency, just like the afigona principle itself, which is necessary to all forms of life but whose origin and modes of action remain mysterious and out of human control. This is not the case with mana, which can and must be captured by living beings either temporarily or definitively, through complex, costly (Schaeffer 2009), multisensory (Lemonnier 2012), and potentially dangerous material procedures. Thus, the livings attribute to themselves a role and 3 This indicates that the process of decomposition is combined with a process of germination/regeneration. 4 Fox also discussed the relationship between afigona and mena in his works on the island of Makira: "There is a great source of mena, whose source is the serpent afigona, and in the last instance in a personal centre of mena, the spirit Agunua; from this central source mena is imparted to adoro (ghosts) and to material objects" (Fox 1925, 236). We find mention of this principle on many of the eastern Solomon Islands: vi'gona at the south of Guadalcanal (Hogbin 1936, 244); hi'ona on Ulawa; and li'oa on Sa'a (Ivens 1972). In general, the term refers to a supernatural principle or entity that may be manifested in the form of a giant serpent. On Aorigi, we may say, the term afigona refers to a disembodied vital principle. a responsibility in the reproduction of the conditions necessary to the existence and preservation of life on earth. One of the procedures used for this is the spreading of iridescent bonito cells on the chests of boys during their initiation. As adults, other procedures allow men to literally incorporate the mana of a powerful dead man; this mana will remain attached to their bones well after their own deaths. A transmission of mana may occur among the livings, between a father and son, between two brothers, or from an uncle to a nephew, and may take several forms. In one, the two protagonists share an areca nut mixed with Pipper leaves and coral lime powder, which the bearer of mana chews first, after pronouncing the appropriate ritual formula. He may also place the same mixture on a cut on his successor's arm so that it will mix with his blood. The transmission of mana from one man to another is called tafagaroa or mafata iana taro, which means "to share with the spirit," and conveys the idea that the same principle is present in nonhuman beings, humans, places, and objects.

Transmitted mana is thought to have been literally and definitively placed in the bodies of living beings. A man who has incorporated the mana of a dead man is said to have "the son of a dead man" in his body, as this is how the being atarowhich is literally born from the human body at the moment of death, and which contains mana immanently-is designated. The seat of a dead man's mana is his skull, which explains why the lower jawbone of mana-bearing men is particularly used in their descendants' worship.

In order to enter into communication with powerful invisible beings, young men must go through an initiation during which they are visibly marked with an iridescent trace left by being anointed with iridophorous bonito cells. Here, a part of a living being becomes an artifact capable of promoting a vital process linked to mana. Afterwards, the transfer of substances (blood or saliva) allows some of the young men to incorporate the being generated from a deceased person. In doing so, they incorporate its capacity of action, which guarantees that their own actions will have greater efficacy than those of ordinary living beings. In exchange, they must regularly honor and satisfy the entity they carry within them by making it small, frequent offerings (of tobacco, betel, money) and by organizing a meal once a year for it, in which officiates place food (the lower jawbone of a pig, yam pudding) in ceremonial bowls and eat in order to feed "the child of the dead man" and reactivate its mana.

A similar mode of operation can be observed in the treatment of artifacts intended to become material medium that is, "receptacle[s] for temporarily manipulating a generative power whose destructive potential [must be] contained" (Chave-Dartoen 2013, 72). These artifacts are first marked with a brilliant sheen and then undergo ritual procedures to place them under the protection of the ancestor whose potential repository they become.

Imitating Nature to Reproduce the World

The last phase in the fabrication of magical objects consists in embedding finely cut and polished nautilus shell pieces on their surfaces, which have been varnished with black lacquer, thus giving them an iridescent mark similar to the one made on boys' chests during initiation. The black lacquer is first applied to the surface of carvings, on which coral lime powder is then spread or polished nautilus pieces inlayed, giving them gleaming lines that resemble the sinusoidal outlines of a serpent-as we have seen, snakes are associated with the vital principle of afigona throughout the eastern Solomons-or the "W"-shaped form of frigate birds with outstretched wings, whose feathers are also iridescent. The quality of sanding and the fineness of the lacquer must be such that there is no roughness, and the object is soft to the touch. The inlayed nacre pieces are explicit references to the skin of bonitos, and the care given to fashioning them corresponds to the need to make them as brilliant as "the skin of the bonito." Visually, the iridescence of the polished nautilus shell or the shimmering of the coral lime powder-which are always enhanced by a matte black background-condition the carved object's elevation to the status of a "beautiful object," and, de facto, its potential to enter into the sphere of ritual practices [START_REF] Revolon | Les esprits aiment ce qui est beau: formes, sens et efficacité rituelle des sculptures owa (est des îles Salomon)[END_REF].

Once these shining marks have been imprinted on their surfaces, these artifacts undergo ritual procedures that literally render them active (fagamamaru, "to activate"; mwaruai, "to transfer mana"; menaia, "to make powerful") and bind them to the entity they were created to worship, whose exclusive property they become. During these procedures, the sacrificer convokes the being by pronouncing its true name, orally signifying his desire to place the object under its protection.

Next, pirogues are anointed with bonito blood, and the sacrificer must consume yam pudding, canarium nuts, and pig meat from the ceremonial bowls; a portion of this food is then placed at the bottom of the bowl for the entity to whom the request is addressed. Once this has been done, the presence of these mana-ized objects outside of ritual contexts requires the occupants of a place to adopt an attitude of seriousness and to refrain from raising their voices or using harmful words, avoiding all questionable behavior, for these objects suggest the possible presence of the invisible entities to whom they now belong.

The Owa see these objects as capable of capturing the attention of invisible entities. The origin of the aesthetic canon governing their fabrication is attributed to the ancestors themselves, not to men. These numerous, complex formal conventions are the subject of decades-long teachings, at the end of which Aorigi carvers may becomes recognized experts, "men of skill" (mwane manira), who have mastered a certain body of technical, aesthetic, and mythological knowledge. These men are few (4 out of 800 inhabitants in 2011), and possess another remarkable skill: they are able to give material form to mental images transmitted to them by the deceased in their dreams. During their sleep, the dead come to them to mentally designate the prototype of the object that will be used for their worship.

The carved motifs are limited to a finite number of images, which are either figurative or abstract, and are linked to the sea (sharks, mackerels, bonitos, blue sardines, 5 frigate birds 6 ) or to the earth (dogs, land crabs, snakes, plant motifs), and they must display a certain realism: the animals represented on a carving must "resemble their living models," to the point that "they themselves appear to be alive" as owa carvers say. The forms within this aesthetic system are metaphorical references to the beings who govern the appearance of bonitos and facilitate their capture by men; the choice of forms is also conditioned by the fact that they all refer, 5 Spratelloides robustus. 6 Fregata minor.

13 in one way or another, to observable qualities in nature that men seek to appropriate. Anyone who has ever caught a bonito will not forget the power of this animal and the strength required to bring it on board. Sharks are great predators: effective, rapid, and precise, capable of detecting the blood of their prey from several dozen kilometers away, and their graphic representation on canoes corresponds to an explicit desire to attribute the qualities of these remarkable hunters to the watercraft and to the fishers who use them. 7 Frigate birds, also superior predators, are often represented by the Owa eating freshly-caught fish; recurrent graphical references to these birds, whose feathers are black with a bluish iridescence, undoubtedly testifies to men's desire to absorb their speed, agility, and the impressive piracy they display against other sea birds, capturing them in their talons and knocking them against the ocean surface to force them to drop their prey or regurgitate their food.

The co-operation between skilled carvers and invisible entities in different ways and at different moments in the conception and fabrication of artifacts guarantees that these objects meet the formal standards required for them to enter into the ritual sphere. Because these standards are based on conventions dictated by the ancestors themselves, once the artifacts are placed within the ritual context, they can and must function as lures (pa'a, the same word used to designate the blue sardine alevins eaten by bonitos) and thus they correspond to the belief that, just as the invisible entities have great appetites for the best food, they are also sensible to the aesthetic attributes that decorate the objects used to worship them. Subtle, sometimes almost imperceptible signs indicate the presence of such entity: a puff of wind, the rustling of leaves, the cry of a bird, an inflection in flames or in a sacrificer's voice-this alteration indicates that the being he is honoring is present in him and is speaking through his mouth.

Two types of representations are mobilized in the Owa aesthetic system: iconic representations though which men seek to appropriate qualities recognized 7 Similarly, in his study on the pirogues used during kula on the Trobriand Islands, [START_REF] Campbell | The Art of Kula[END_REF] showed that the animal characteristics depicted on boats' prows-"slipperiness, swift movement, and a quality glossed as 'wisdom'"-are those it is hoped will characterize the expedition itself. in certain living beings, and indexical representations of metamorphosis and the performativity of action, indicated by phenomena of light interference. The role of objects in representative procedures is plural and composite. On the one hand, because they are beautiful according to criteria defined by the ancestors, they are means humans can use to attract invisible beings to their places of worship (in addition to pronouncing the entities' true name and offering them a feast). On the other hand, they are ways for humans to appropriate qualities recognized in other living beings of nature. Eventually, the iridescent marks imprinted on the surface of magical objects indicate the presence of entities and their capacity for action. By producing these objects, men give themselves the ability to act on the invisible entities-which are by definition dangerous and partially uncontrollable-with whom they must cooperate in order to guarantee the vital processes necessary for the renewal of the world: the reproduction, multiplication, and growth of living beings.

*

In this socialization of nature-in particular of the sea and the seascapehumans attribute an imagined affordance [START_REF] Gibson | The Theory of Affordances[END_REF][START_REF]The Ecological Approach to Visual Perception[END_REF] to the phenomena of light interference that surround them, and have developed material and conceptual devices allowing them to capture these qualities and, with them, to build modalities of a new form of mediation with non-human beings considered to have powerful agency necessary for the reproduction of the world. Carvings occupy a central place in the instantiation of non-human agency, through technical devices based on using certain shells to create phenomena of light interference that are believed to be visible manifestations of entities' effective abilities to intervene (mana-ity/manaization) in the world. Among living beings, only the bonito, the nautilus, and coral spontaneously and visibly produce phenomena of light interference (iridescences and luminescence) both while alive-on their surfaces-and on their flesh, pearly interior, or lime when dead. This particularity places them in a liminal state [START_REF] Turner | Liminality and Community[END_REF]) as both living and dead, which undoubtedly helps explain the central place given to them in the ontologies of the eastern Solomon Islands. This examination of the means used by living beings to interact with invisible beings shows that, whether temporary or permanent, intrinsic or borrowed, possession of the polymorphic afigona-mana principle groups living humans, dead humans, and the non-human beings that have emerged from humans into the same category, separate from other beings, living or dead (plants or animals) that have a living part but do not generate mana at death. In Owa understandings, a line is drawn and materially emphasized in practices, between what is a (human or nonhuman) source of mana and what is not. At the same time, life and death appear as different states of single mode of existence in the world, in which mortal beings, in becoming intangible, acquire a capacity for action that extends and reinforces their social existence in the world of the living.

Grouped together under the generic term waiau, these include Auxis thazard (frigate tuna or frigate mackerel), Katsuwonus pelamis (skipjack tuna), and Euthynnus affinus (mackerel tuna), all related to tuna and part of the Scombridae family.

In a similar manner, Perig Pitrou has described and analyzed devices developed by the Mixe of Mexico to enlist supernatural entities in their enterprises[START_REF] Pitrou | Figuration des processus vitaux et co-activité dans la Sierra Mixe de Oaxaca (Mexique)[END_REF].