Nina Melendez

Virginie M Stevens

Michel Baguette
email: michel.baguette@mnhn.fr

Freshwater Reservoir, Ecological Traps and Source-Sink Dynamics

Keywords: freshwater ecosystem, biodiversity, desiccation, Odonata, dragonfly, damselfly, dispersal, metapopulation, lotic community, lentic community Conceptualization, M

Odonata are biodiversity indicators that adequately represent many aquatic and semiaquatic animal species. We recorded over two field seasons a rich lentic community of Odonata (18 species) in a large artificial freshwater reservoir (ca. 55 ha) built 40 years ago. The release of water from the dam in summer for crop irrigation leads to the desiccation of large parts of the reservoir, which prevents the reproduction of half of the species of this Odonata community. We identify two adaptations that allow eight species to cope with desiccation, i.e., a precocious breeding period allowing the emergence of adults before the retreat of water, or a delayed adult emergence due to egg diapause from oviposition to the end of winter. The reservoir acts thus as an ecological trap for individuals of 10 species that developed elsewhere and were attracted to the site without successfully breeding there. As consequence of the local population extinction at each generation, the presence of individuals of these 10 species at the reservoir depends on source-sink population dynamics in the landscape. In the context of global warming that encourages the creation of artificial freshwater reservoirs, the multiplication of such sinks could threaten the persistence in the landscape of species maladapted to desiccation.

Introduction

Freshwater ecosystems cover only ~0.8% of the Earth's surface but support almost 6% of the diversity of all described existing species [START_REF] Dudgeon | Freshwater biodiversity: Importance, threats, status and conservation challenges[END_REF]. Freshwater is unquestionably considered the most vital natural resource for both biodiversity and humanity, yet directly threatened by various human activities from land cover changes to urbanization, through water provision aiming at maximizing human access to water [START_REF] Vörösmarty | Global threats to human water security and river biodiversity[END_REF]. Indeed, the management of freshwater resources often focuses on human water security rather than on natural ecosystem integrity and biodiversity conservation [START_REF] Harrison | The freshwater biodiversity crisis[END_REF]. Changing climate is among the most challenging threats to water biodiversity [START_REF] Reid | Emerging threats and persistent conservation challenges for freshwater biodiversity[END_REF]. Among various impairments to freshwater ecosystem functioning linked to climate change, humans try to mitigate shifting temperature and precipitation by constructing reservoirs to store water and enhance irrigation capacity for crop cultivation [START_REF] Reid | Emerging threats and persistent conservation challenges for freshwater biodiversity[END_REF]. The raising of dams on watercourses not only alters flow regimes and prevents or disrupts the free movement of aquatic organisms [START_REF] Reid | Emerging threats and persistent conservation challenges for freshwater biodiversity[END_REF], but also creates temporary lentic ecosystems whose impact on biodiversity is still poorly known. The creation of freshwater reservoirs in a landscape could be seen as a mitigation measure for the drainage and destruction of many ponds, marshes or wet meadows resulting from the intensification of agriculture-wetland loss is three times as high as forest loss [START_REF] Harrison | The freshwater biodiversity crisis[END_REF]. However, the question remains to assess the real effectiveness of these artificial freshwater ecosystems in terms of hosting biodiversity. Here, we address this question by focusing on a water reservoir in the French Pyrénées. This reservoir captures two watercourses and retains water that is released from the dam during the three driest months in summer for irrigation of crops in the watershed below the reservoir. During summer, the flow of the two watercourses feeding the reservoir is very low. When the water is used for irrigation, the reservoir does not dry completely. Due to the gentle slope of the reservoir, at the end of the water release period, the shore of the reservoir recedes locally by 150 m, thus exposing vast surfaces previously covered with water. These surfaces are then gradually submerged again during the abundant autumn rains, which swell the flow of the two watercourses. To investigate the consequences of this large-scale desiccation on biodiversity, here we focus on the local community of Odonata, i.e., those adult damselflies and dragonflies that fly and try to reproduce around the reservoir. Species of Odonata are frequently used as biodiversity indicators for the following reasons: they are reasonably diverse and benefit from a rather well-established and stable taxonomic framework; they have a complex life cycle with larvae living underwater, whereas adults are aerial but depend on aquatic habitat for breeding; both stages are predaceous; they constitute all at once a keystone, umbrella and flagship group; they are easily accessible and amenable to quantitative sampling by standardized techniques (e.g., [START_REF] Oertli | The use of dragonflies in the assessment and monitoring of aquatic habitats[END_REF]).

We monitored around the reservoir adult presence, age-teneral (freshly metamorphosed) or adult individual-and behavior of damselflies and dragonflies during two flight seasons. We compiled from existing databases the species-specific traits of Odonata habitat selection to identify which species were attracted to this artificial lentic ecosystem in an area previously strictly visited by lotic species. Then, for each of the lentic species we extracted from the databases phenological and behavioral traits to predict which species of this lentic community should be able to cope with the reservoir desiccation. Specifically, we compare the requirements of each species in terms of egg-laying site, resistance to desiccation and speed of development with the presence or the absence of water during the different periods of their life cycle. Finally, we compared these predictions to the indices of successful reproduction that we recorded in the field, i.e., the presence of newly emerged adults that reveals that the species has managed to complete its lifecycle in this artificial lentic ecosystem. We acknowledge that our study is local and concerns only one reservoir. However, at this state of knowledge, we are not aware of no other studies that address this crucial research question.

Materials and Methods

Study Site

The reservoir of Mondély (43 • 03 08" N, 1 • 26 16" E, Figure 1) is located in France, in the Occitanie region, more precisely at the foothill of the Pyrénées in the Ariège deparment. This water impoundment was created in 1980 by building a dam on the Lèze river that has an upstream watershed of 13.6 km 2 . Its surface ranges between 51.3 ha and 56.6 ha according to seasonal water release. Its maximum depth is 23.7 m for a maximal water capacity of four million m 3 . The reservoir is used to irrigate 1700 ha of crops in the downstream watershed of the Lèze valley. Water release from the dam is decided by the managers of the reservoir (SMHVL: Syndicat Mixte d'Aménagement Hydraulique de la Vallée de la Lèze) according to the water requirements of crops.

Figure 2 shows the debit of water released each month from the dam during the years 2019, 2020 and 2021; water is released between May and October. Figure 2 illustrates that the peak of water release does not occur at the same time each year: there is a shift of approximately 4 weeks between 2019 and 2020 on the one hand, and 2021 on the other hand.

Damselflies and dragonflies were monitored in a creek of 2 ha located at the western part of the reservoir (Figure 1) and enclosed between two watercourses that were former tributaries of the Lèze river. This sampling site was chosen because due to its gentle slope the water level recedes 150 m during the summer months of June, July and August, which increases the emerged part of the study site by 1.25 ha (Figure 3). During periods of strong water release, the water recedes extremely quickly, in less than a week. When the water level is at its minimum, the confluence between the two Lèze tributaries emerges. Over the whole reservoir, this site is the part where the emerged area is the most important during summer. Moreover, the creek has a natural character that the few other gently sloping banks of the site do not have because they have been fitted out for swimming and nautical activities, which generates there a very high tourist load from May to October. Our study area is thus the only suitable place for Odonata reproduction around the reservoir, by its shallow depth and absence of disturbance by humans. Figure 2 shows the debit of water released each month from the dam during the years 2019, 2020 and 2021; water is released between May and October. Figure 2 illustrates that the peak of water release does not occur at the same time each year: there is a shift of approximately 4 weeks between 2019 and 2020 on the one hand, and 2021 on the other hand. Damselflies and dragonflies were monitored in a creek of 2 ha located at the western part of the reservoir (Figure 1) and enclosed between two watercourses that were former tributaries of the Lèze river. This sampling site was chosen because due to its gentle slope the water level recedes 150 m during the summer months of June, July and August, which The vegetation of the majority of the site can be attached to the group of eutrophic wet meadows. In particular, the area that emerges each summer is dominated by the water mint (Mentha aquatica). The vegetation of three depressions located to the west and northwest of the site can be linked to Typha low marshes and aquatic vegetation. The edges of the Lèze tributaries are occupied by narrow, non-continuous strips of trees dominated by willows and alders.

level is at its minimum, the confluence between the two Lèze tributaries emerges. Over the whole reservoir, this site is the part where the emerged area is the most important during summer. Moreover, the creek has a natural character that the few other gently sloping banks of the site do not have because they have been fitted out for swimming and nautical activities, which generates there a very high tourist load from May to October. Our study area is thus the only suitable place for Odonata reproduction around the reservoir, by its shallow depth and absence of disturbance by humans. The vegetation of the majority of the site can be attached to the group of eutrophic wet meadows. In particular, the area that emerges each summer is dominated by the water mint (Mentha aquatica). The vegetation of three depressions located to the west and northwest of the site can be linked to Typha low marshes and aquatic vegetation. The edges of the Lèze tributaries are occupied by narrow, non-continuous strips of trees dominated by willows and alders.

Sampling Methods

Adult damselflies and dragonflies were monitored by performing modified Pollard's walk [START_REF] Pollard | A Method for Assessing Changes in the Abundance of Butterflies[END_REF] on the accessible parts of the study site (Figure 4). This method is widely used to estimate the relative abundance and diversity of butterflies and has been adapted to Odonata. Observation surveys are carried out along fixed transects. Adult damselflies and dragonflies were observed in a 5 m strip on both sides of the transect, while walking a slow and steady pace and their identity and numbers were recorded. The route taken between transects during each sampling session is shown in Figure 4. Routes are modified to follow the variation of the water level (Figure 4).

Sampling Methods

Adult damselflies and dragonflies were monitored by performing modified Pollard's walk [START_REF] Pollard | A Method for Assessing Changes in the Abundance of Butterflies[END_REF] on the accessible parts of the study site (Figure 4). This method is widely used to estimate the relative abundance and diversity of butterflies and has been adapted to Odonata. Observation surveys are carried out along fixed transects. Adult damselflies and dragonflies were observed in a 5 m strip on both sides of the transect, while walking a slow and steady pace and their identity and numbers were recorded. The route taken between transects during each sampling session is shown in Figure 4. Routes are modified to follow the variation of the water level (Figure 4). Each time a species new to the community was observed, it was photographed so that its identification could be confirmed with certainty. For each individual encountered, the observer noted its behavior, looking for signs of reproduction (mating, egg-laying) and its age (teneral or adult). Tenerals are adult individuals freshly hatched from their larvae that can be recognized by their pale colors and their soft integuments (see Figure 5c, Aeshna affinis teneral male). Adult damselflies and dragonflies were sampled during two calendar years beginning in July 2020. Sampling visits were performed two or three times a month, weather permitting, between July and November in 2020, between February and November in 2021 and between April and June in 2022, which provided a total of 53 visits. pair, Sympecma fusca (Vander Linden, 1820) ovipositing pair, Calopteryx virgo (L., 1758) displaying male, Platycnemis pennipes (Pallas, 1771) male, Ischnura elegans (Vander Linden, 1820) mating pair, Enallagma cyathigerum (Charpentier, 1840) mating pair, Coenagrion puella (L., 1758) ovipositing pair. (b) Odonata species recorded in the Mondély reservoir [START_REF] Vörösmarty | Global threats to human water security and river biodiversity[END_REF]. From left to right and top to bottom. Erythromma lindenii (Selys, 1840) ovipositing pair, Pyrrhosoma nymphula (Sulzer, 1776) ovipositing female, Ceriagrion tenellum (de Villers, 1789) male, Aeshna mixta (Latreille, 1805) ovipositing female, Aeshna affinis (Vander Linden, 1820) ovipositing female, Anax imperator (Leach, 1815) ovipositing female, Boyeria irene (Fonscolombe, 1838) male, Cordulegaster boltonii (Donovan, 1807) female. (c) Odonata species recorded in the Mondély reservoir [START_REF] Harrison | The freshwater biodiversity crisis[END_REF]. From left to right and top to bottom. Libellula quadrimaculata (L., 1758) male, Libellula depressa (L., 1758) male, Orthetrum cancellatum (L., 1758) male, Sympetrum sanguineum (Müller, 1764) ovipositing pair, Sympetrum fonscolombii (Selys, 1840) male, Sympetrum striolatum (Charpentier, 1840) male, Sympetrum meridionale (Selys, 1841) male, Aeshna affinis (Vander Linden, 1820 teneral. All photographs were taken in Mondély, except C. virgo meridionalis and P. pennipes, from the nearby Arize valley, and Ceriagrion tenellum, from Mazère (Ariège). Photographs by Michel Baguette, all rights reserved.

Odonata Life History Traits

Species-specific habitat preferences and life history traits were extracted from existing databases [START_REF] Waller | The odonate phenotypic database, a new open data resource for comparative studies of an old insect order[END_REF][START_REF] Harabiš | European database of the life-history, morphological and habitat characteristics of dragonflies (Odonata)[END_REF]. First, we investigated whether each species recorded on the study site prefers lotic, lentic or mixed water bodies. Next, for each lentic species we recorded a suite of life-history traits to investigate the possible completion of its life cycle by coping with the summer desiccation of the creek. We compiled the following traits: female egg-laying strategy, larval location, female egg-laying time, egg hatching time and larval development time. Female egg-laying strategy is subsumed by the type of substrate in which females lay their eggs: either underwater (endophytic or exophytic), or aerial (endophytic or in muddy soil). Larval location provides information on the habitat selection of larvae. The timing of the species' life cycle is described by the period during which females lay their eggs, the eggs hatching time, including the possibility of diapause, and the time required for the development of the larvae, from egg-hatching to the metamorphosis of the adult. This analysis provides us with a subset of candidate lentic species potentially having a stable reproductive population in the reservoir.

Results

Table 1 presents the composition of the Odonata community recorded on the study site over the period 2020-2022. Adults of the 23 species recorded in Mondély are illustrated in Figure 5. Counting data performed during the field surveys were aggregated over each flight seasons. Numbers in Table 1 reflect the mean abundance of the species over the 3 flight seasons. We also mention in Table 1 the records of breeding attempts, either the observation of mating or of egg-laying females, and the records of tenerals, i.e., these newly emerged adults that unambiguously demonstrate that the species has completed its life cycle in the study area.

Among this community of 23 species of adult damselflies and dragonflies, 2 (Calopteryx virgo and Cordulegaster boltonii) are strictly lotic species living in streams and small rivers. Their presence is certainly to be related to the tributaries of the Lèze river surrounding the study site, where all adults of both species have been recorded. Three species are considered as having mixed preferences for lotic and lentic water bodies. Adult of two of these three species (Platycnemis pennipes and Boyeria irene) were represented by only one individual in our data set, with no cue of reproduction detected. Adults of the third one (Pyrrhosoma nymphula) were scarce, but we observed an ovipositing female (Figure 5b).

The 18 remaining species are thus strictly lentic species that are attracted to the reservoir. Among these 18 species, our life history trait compilation (Table 2) indicates that one of them (Sympecma fusca) has the possibility of completing its life cycle before the beginning of the desiccation. The two species of the genus Sympecma are the only Odonata that overwinter as adults in Western Palearctic. Adults of S. fusca emerge in May-July in our study area and leave the water surroundings. They come back to the study site from February onwards and begins to reproduce at this time. During the desiccation period of the reservoir, this species is at the adult stage but in pre-reproductive period outside the study area. Chalcolestes viridis is another species of damselfly with a particular life cycle. Adults breed from August onwards, i.e., at the peak of the desiccation period. Females lay their eggs at 100-150 cm high into aerial, thin shoots of soft trees such as willows (Salix sp.) surrounding water. The egg stage is long (5-6 months), then the larval hatches and drops into the water where its development is rapid (6-8 weeks). After emergence, the adults undergo a protracted, pre-reproductive, summer diapause (up to 3 months) before mating and ovipositing. During the desiccation period of the reservoir, this species is at the egg stage sheltered in shoots of trees overhanging the study area. Lestes barbarus has a life cycle closely related to the previous species. Adults mate between June and August, and females lay their eggs in aerial vegetation, in thin shoots of large herbaceous species (Juncus sp., Carex sp., . . . ). Eggs hatch after several months, the larval development is fast, whereas there is no summer diapause after adult emergence unlike the previous species but a classical pre-reproductive period. During the desiccation period of the reservoir, this species is at the egg stage sheltered in shoots of large herbaceous species. Females of Aeshna affinis oviposit in July-August in the study area. Tenths of ovipositing females were observed alone or in tandem, and all of them lay their eggs in mud. Eggs hatch after 6-7 months, and the subsequent development of the larva last about 3 months. During the desiccation period of the reservoir, this species is at the egg stage sheltered in mud. Four species of Sympetrum, i.e., S. fonscolombii, S. meridionale, S. sanguineum and S. striolatum have the potential to cope with the summer desiccation. Adults of these four species are on the wings and breed during or after of the peak of water retreat (from mid-May to mid-September for S. fonscolombii, from June to September for S. meridionale, from June to October for S. sanguineum, and from June to November for S. striolatum). Females of these four species lay their eggs in shallow water or in mud. The eggs of S. fonscolombii laid in May either will undergo a direct development and produce a new adult generation that will emerge in September or enter in diapause during winter and will hatch at the end of winter, to produce the spring adult generation. Similarly, the eggs of S. striolatum will hatch directly or can enter diapause to produce the new adult generation the next June. All eggs of S. meridionale and S. sanguineum enter in diapause, hatch at the end of winter and produce a single adult generation the next June.

According to this analysis of the species' life cycles, the 8 species listed above should be able to cope with the desiccation of the study site during summer. Conversely, the remaining 10 species seem not be able to develop permanent population on the site, as for all of them the water retreat occurs their larval development, while their larvae do not have the possibility of escaping. During our surveys, teneral individuals were found for six species (Table 1), all of them being among the eight candidate species to perform their life cycles despite the desiccation. For the two remaining candidates for which no teneral individuals were found (Lestes barbarus and Sympetrum meridionale), it is worth noticing that reproduction cues were observed (egg laying for L. barbarus, mating for S. meridionale). The lack of teneral detection could be explained by their low to very low adult abundance on the study site, but we cannot exclude that they do not reproduce locally. There is also a small probability that we missed certain species as tenerals, especially if individuals of those species have strongly synchronized hatching occurring in the time interval between our sampling sessions. Searching for exuviae would have been helpful to solve this issue, but it revealed to be difficult to find exuviae in a site with moving shore.

Table 1. Abundance and reproduction index of the 23 species of damselflies and dragonflies recorded in Mondély. Reproduction index indicates whether reproduction clues of the species were observed in the study site. For each species, we mentioned the most indicative clue of reproduction observed, using the rule teneral > egg laying > mating > no reproduction clue observed.

Species Abundance Class Reproduction Index

Calopteryx virgo meridionalis 2 -

Lestes barbarus 1 E

Lestes viridis 4 T

Sympecma fusca 4 T

Ischnura elegans 2 M

Enallagma cyathigerum 3 M

Coenagrion puella 3 E

Erythromma lindenii 2 E

Pyrrhosoma nymphula 1 E

Ceriagrion tenellum 1 -

Platycnemis pennipes 1 -

Aeshna mixta 2 E

Aeshna affinis 2 T

Anax imperator 1 E

Boyeria irene 1 -

Cordulegaster boltonii 1 E

Libellula quadrimaculata 2 -

Libellula depressa 1 E

Orthetrum cancellatum 1 -

Sympetrum striolatum 4 T

Sympetrum fonscolombii 3 T

Sympetrum sanguineum 3 T

Sympetrum meridionale 1 M

Abundance class cumulated over the field visits-1: 1-5 adults.

Discussion

The current species pool of damselflies and dragonflies currently censused in the Occitanie region (South-Western France) amounts to 73 species [START_REF] Charlot | Liste Rouge des Odonates d'Occitanie[END_REF]. The richness of the Odonata community of our 2 ha study site bordering the Mondély reservoir (23 species) could thus be considered as rather low. However, the ecosystem diversity of Occitanie is very high, from glacial relict peat bogs in altitude to brackish fens along the Mediterranean coast, through large rivers such as the Aude, the Hérault or the Garonne. The different communities of Odonata specialists associated with each of these ecosystems contribute to increase the regional species pool. In fact, the community of Mondély is a respectable species assemblage representative of the low altitude water bodies of Occitanie [START_REF] Charlot | Liste Rouge des Odonates d'Occitanie[END_REF]. Among these 23 species, 18 are lentic specialists that were attracted in the area by the construction of the dam and the resulting flooding of the reservoir. Damselflies and dragonflies have a complex life cycle, during which newly emerged adults engage in a pre-reproductive period and leave the water bodies where the larvae developed. At the end of this time, which is variable among species mature adults wander through the landscape in search of suitable breeding sites that they locate using a suite of species-specific, hierarchical cues [START_REF] Corbet | Behavior and Ecology of Odonata[END_REF]. Given this high vagrancy of adults, it is therefore not surprising that lentic damselflies and dragonflies have colonized the artificial reservoir of Mondély especially because the construction of the reservoir dates back more than 40 years ago.

What our study shows, which was unexpected, is that more than half of these lentic species attracted to the reservoir seem not able to develop a local population on the site. The key point is the rapid water retreat resulting in the rapid emergence of a large area of land previously occupied by shallow water. Our analysis of species' life cycle shows that 8 species have the potential to survive to this fast desiccation process. These species have either a precocious breeding period allowing the emergence of adults of the new generation before the retreat of water, or conversely a delayed adult emergence due to egg diapause from oviposition to the end of winter, and then a rapid larval development. We indeed found tenerals for six out these eight species that indicate the full completion of their life cycle on site. Adults of the remaining 10 species are thus attracted to the site and attempt to reproduce there but they seem to fail-we did not record any teneral of these species, whereas egg-laying females were observed for some of them. It is worth investigating to what extent the traits we identified as potential driver of resistance to desiccation could be plastic and/or rapidly evolve to allow the completion of the life cycle for each of these 10 species. Moreover, further investigation of this phenomenon could be to sample benthic community to search for larvae on the study site during the period there is water. In this way, we could rule out indisputably less parsimonious hypotheses as to the absence of these species than the summer desiccation of the site, such as predation or competitive exclusion.

Ecological traps occur when "good animals love bad habitats" [START_REF] Battin | When good animals love bad habitats: Ecological traps and the conservation of animal populations[END_REF], which is probably the case here for adults of these 10 species. Ecological traps result from errors in habitat assessment by individuals due to some mismatch between the environmental cues they use to select habitats and the actual quality of the habitat [START_REF] Battin | When good animals love bad habitats: Ecological traps and the conservation of animal populations[END_REF][START_REF] Hale | Ecological traps: Current evidence and future directions[END_REF]. Ecological traps result thus from maladapted individual behavioral processes resulting in low adult survival and/or reproductive success [START_REF] Robertson | A framework for understanding ecological traps and an evaluation of existing evidence[END_REF]. As the fitness of individuals caught in ecological traps is, by definition, lower than that of those settled in normal habitats [START_REF] Robertson | Is selectively harvested forest an ecological trap for Olive-sided Flycatchers?[END_REF], we expect that consequences of these individual processes on population and metapopulation dynamics will occur [START_REF] Hale | Ecological traps: Current evidence and future directions[END_REF][START_REF] Hale | Evaluating the metapopulation consequences of ecological traps[END_REF]. Given their lifestyle where adult females must anticipate during their aerial life the aquatic conditions favorable to the development of young stages, Odonata are poorly equipped to escape ecological traps. The analysis of behavioral sequences of habitat selection used by female Odonata indeed indicates the use of a hierarchical process, first to locate a water body in which to lay eggs, then to select a suitable part of this water body to find the required substrate for egg-laying, and finally to choose the best egg-laying substrate (e.g., [START_REF] Corbet | Behavior and Ecology of Odonata[END_REF]) This hierarchical process of habitat selection entails the use by the female of a succession of different stimuli (visual, tactile, thermal and perhaps olfactory) according to a binary decision-making process (e.g., [START_REF] Corbet | Behavior and Ecology of Odonata[END_REF]). The evolution of these behavioral sequences to cope with the existence of ecological traps is probably possible, but the key question is whether it could occur in a time frame compatible with the rate of current environmental changes.

The most probable consequences of the summer desiccation of the Mondély reservoir on the 10 species that are trapped there is a source-sink metapopulation dynamics [START_REF] Dias | Sources and sinks in population biology[END_REF] at the landscape scale. Those damselfly and dragonfly adults that are attracted into the Mondély reservoir acting as a demographic sink had to have developed elsewhere in sources, i.e., in water bodies allowing their full development from eggs to adults. These water bodies had to be located into the "vagrancy range" of immature adults wandering through the landscape during their maturation phase.

Besides small pools up-and down-stream of the two watercourses feeding the reservoir, there is only a single large (66 ha) water body located at ca. 2.5 km as the crow flies from Mondély. This water body of Filleit also serves as a water reservoir for irrigation, but its desiccation in summer is much lower than in Mondély. We thus suggest that those adults of the 10 species that cannot complete their life cycle in Mondély could at least from some of them come from Filleit. Unfortunately, the current state of knowledge of Odonata life histories is sorely lacking data on adult dispersal: the current databases on European Odonata traits either does not address movements and dispersal [START_REF] Waller | The odonate phenotypic database, a new open data resource for comparative studies of an old insect order[END_REF] or mention movements and dispersal as a binary trait according to adult migration [START_REF] Harabiš | European database of the life-history, morphological and habitat characteristics of dragonflies (Odonata)[END_REF]. This black box about dispersal strongly limits the understanding of Odonata spatial dynamics in comparison with butterflies that are alternative iconic insects with a much more detailed understanding of their dispersal data [START_REF] Stevens | A meta-analysis of dispersal in butterflies[END_REF][START_REF] Stevens | How is dispersal integrated in life-histories: A quantitative analysis with butterflies[END_REF]. Anyway, large scale landscape genetic studies [START_REF] Manel | Landscape genetics: Combining landscape ecology and population genetics[END_REF][START_REF] Manel | Ten Years of Landscape Genetics[END_REF] would be very helpful to solve the issue of the geographic origin of the species colonizing Mondély at each generation, and more generally to provide insights on the spatial scale of Odonata metapopulations, which has a great potential value for the design of conservation measures.

Whatever their origin damselfly and dragonfly adults of 10 species seem trapped in a demographic sink in Mondély. At the landscape scale, metapopulation dynamics of species facing such source-sink systems can be affected in two ways, with rather different consequences for species conservation. Firstly, if individuals spend all their adult lifetime and waste all their reproductive effort in the sink, the dynamics of the metapopulation can be negatively affected, which can threaten its persistence in the landscape [START_REF] Hale | Evaluating the metapopulation consequences of ecological traps[END_REF][START_REF] Dias | Sources and sinks in population biology[END_REF]. The key parameter is thus the ratio at each generation between the productivity of adults in source habitat vs. the loss of adults in sinks. Given the high spatial and temporal variability of insect demography, only long-term studies would inform on the stability of such source-sink systems. Landscape genetic studies mentioned above could also be very helpful here, in identifying which local populations in the landscape act consistently as sources of first-generation migrants, making them priorities for conservation action. Secondly, damselfly and dragonfly adults confronted to the degradation of their habitat due to its fast desiccation can take the decision to leave the site to find out better conditions elsewhere. In such a situation, the reservoir can act as a stepping stone for those individuals that disperse further from the source [START_REF] Hale | Evaluating the metapopulation consequences of ecological traps[END_REF]. If these individuals manage to locate a suitable habitat and breed there, such movements can be beneficial for the persistence of the species in the landscape by leading to the (re)colonization of empty habitats or the reinforcement of small populations (e.g., [START_REF] Hanski | Metapopulation Ecology[END_REF]). However, the probability of these appealing theoretical events has apparently never been measured in the field.

These two scenarios are not mutually exclusive, i.e., they can affect the same species depending on the year: as shown in Figure 2, the peak of the desiccation period of the reservoir is variable according to the year, which can differentially influence the reproduction of the species. Anyway, we expect dragonflies to have a better chance of being able to locate a new habitat according to scenario 2 because their abilities of movement and therefore of exploration are higher than those of damselflies. Finally, we want to stress that the existence of a source-sink dynamics could preclude the apparition of evolutionary responses to desiccation. Indeed, the incessant flow of genes from source populations to sinks should limit the establishment of a life cycle adapted to desiccation, or even the perception by adults that a body of water is in fact an ecological trap.

Conclusions

To our best knowledge, our study is the first to address the consequences of the summer desiccation of an artificial freshwater reservoir on Odonata biodiversity (but see [START_REF] Carmignani | Ecological impacts of water drawdowns on lake littoral zones: A review[END_REF] for a thorough review of the consequences of winter desiccation on multiple taxa). Here, we report the presence of a rich lentic community of Odonata (18 species) in a large reservoir (ca. 55 ha) over 40 years old. Odonata are biodiversity indicators that adequately represent the many aquatic and semi-aquatic species. The release of water from the dam in summer for crop irrigation leads to the desiccation of large parts of the reservoir, which prevents the reproduction of more than half (10/18) of the species in the Odonata community. We identify two different adaptations that allow eight species to cope with desiccation, i.e., either a precocious breeding period allowing the emergence of adults of the new generation before the retreat of water, or a delayed adult emergence due to egg diapause from oviposition to the end of winter. The reservoir acts thus as ecological trap for individuals of these 10 species that developed elsewhere and were attracted to the site. As consequence of the local population extinction of these 10 species at each generation, the presence of individuals at the reservoir is suggestive of source-sink population dynamics in the landscape. In the current context of global warming that leads to severe changes in the precipitation regime and encourages the creation of artificial freshwater reservoirs, the multiplication of such sinks could threaten the persistence in the landscape of species maladapted to desiccation. The question remains about (1) the evolution of traits allowing those species to adapt to such environmental changes, and (2) the apparition of species adapted to such habitats that move northwards taking advantage of global warming. The current northward shift in Europe of several North African dragonfly species is a strong signal in this direction.

Figure 1 . 15 Figure 1 .

 1151 Figure 1. Map of the Mondély reservoir showing the location of the study site (red rectangle). The dam is visible at the upper side of the reservoir.

Figure 2 .

 2 Figure 2. Dynamics of water release at the Mondély reservoir dam during the period 2019-2021 (data courtesy of the SHMVL).

Figure 2 .

 2 Figure 2. Dynamics of water release at the Mondély reservoir dam during the period 2019-2021 (data courtesy of the SHMVL).

Figure 3 .

 3 Figure 3. Emerged (left panel) and submerged part (right panel) of the study site showing the extent of the desiccated area between June and August.

Figure 3 .

 3 Figure 3. Emerged (left panel) and submerged part (right panel) of the study site showing the extent of the desiccated area between June and August.

Figure 4 .

 4 Figure 4. Map of the location of the Pollard walks in the study site. Transects are following the course of the water retreat during summer. White and orange traits indicate transect walked during the flood or the desiccation of the creek, respectively.

Figure 5 .Figure 5 .Figure 5 .

 555 Figure 5. Cont.

2 : 6 -

 26 25 adults. 3: 26-125 adults. 4: 126-625 adults. Reproduction index--: nothing. M: Mating. E: Egg-laying. T: Teneral adults.

Table 2 .

 2 Egg-laying strategy, larval location, egg-laying time, egg hatching time and larval development time of the 18 lentic species observed in Mondély. Mixed larval location means that larvae occur both on sediments and on vegetation. Data from references[START_REF] Waller | The odonate phenotypic database, a new open data resource for comparative studies of an old insect order[END_REF][START_REF] Harabiš | European database of the life-history, morphological and habitat characteristics of dragonflies (Odonata)[END_REF].

	Species	Egg-Laying Strategy	Larval Location	Egg Laying/Hatching Time	Larval Development
	Lestes barbarus	Out of water, endophytic	On sediments	June-August/4-21 weeks	4-8 weeks
	Lestes viridis	Out of water, endophytic	On sediments	August-September/28-32 weeks	8-12 weeks
	Sympecma fusca	In water, endophytic	On sediments	April-May/3-6 weeks	8-12 weeks
	Ischnura elegans	In water, endophytic	On vegetation	June-September/1-3 weeks	2-3 weeks/40-50 weeks
	Enallagma cyathigerum	In water, endophytic	On vegetation	June-August/2-3 weeks	40-50 weeks
	Coenagrion puella	In water, endophytic	On sediments	June-July/3-5 weeks	24-40 weeks
	Erythromma lindenii	In water, endophytic	On vegetation	June-July/	24-48 weeks
	Ceriagrion tenellum	In water, endophytic	On sediments	July-August/3-4 weeks	36-72 weeks
	Aeshna mixta	In water, endophytic	Mixed	August-September/winter diapause (28-36 weeks)	16-20 weeks
	Aeshna affinis	Out of water, in mud or endophytic	No data	August/ winter diapause (28-36 weeks)

Table 2 .

 2 Cont.

	Species	Egg-Laying Strategy	Larval Location	Egg Laying/Hatching Time	Larval Development
	Anax imperator	In water, endophytic	On vegetation	June-September/3-6 weeks	12-100 weeks
	Libellula quadrimaculata	In water, exophytic	Mixed	June-July/2-7 weeks	52-156 weeks
	Libellula depressa	In water, exophytic	On sediments	June-July/1-5 weeks	52-104 weeks
	Orthetrum cancellatum	In water, exophytic	On sediments	July-August/5-6 weeks	52-156 weeks
	Sympetrum striolatum	Out or in water, exophytic	On sediments	August-September/4 weeks or after winter diapause	20 weeks
	Sympetrum fonscolombii	In water, exophytic	On sediments	June-September/1-3 weeks	Direct development or winter diapause
	Sympetrum sanguineum	Out or in water, exophytic	Mixed	July-August/winter diapause	6-10 weeks
	Sympetrum meridionale	Out or in water, exophytic	Mixed	August-October/winter diapause	8-16 weeks

Acknowledgments:

We acknowledge support from the CHANGE-EVOL team of the Theoretical and Experimental Ecology Station of the CNRS in Moulis (CNRS UAR 2029), and especially from Delphine Legrand and Hervé Philippe. We received constructive comments from a Diversity Editorial Board member and from four reviewers that considerably improved a first draft of the manuscript.

M.B. and V.M.S. are members of the LabEx (Laboratory of Excellence) TULIP. This research was funded by grant TULIP 10-LABX-0041 from the ANR (Agence National de la Recherche, French National Research Agency).

Institutional Review Board Statement: Not applicable.

Data Availability Statement: Data are available on request from the authors.

Diversity 2022, [START_REF] Robertson | Is selectively harvested forest an ecological trap for Olive-sided Flycatchers?[END_REF] 1021

Conflicts of Interest:

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.


Kelin Wang

Marco A Lopez-Sanchez

Sylvie Demouchy

Catherine Thoraval

Comment on "If Not Brittle: Ductile, Plastic, or Viscous?" by

In continuum mechanics, viscous materials are those that lack rigidity and elastic response under shear stress. We argue that using the term viscous to refer to the aseismic lithosphere is thus a misnomer because it denies the propagation of S waves through the lithosphere in total contradiction to decades of seismic surveys. Similarly, viscous materials lack yield stress, which is another feature expected in most situations within the aseismic lithosphere but is more difficult to assess. Aiming to reconcile the definitions of rheological terms between material and Earth and mineral sciences, we propose a decision tree chart for the use of the terms viscous, viscoelastic, plastic, and viscoplastic, all widely used terms in the materials and Earth sciences communities for describing fundamental macroscopic behavior of rocks under shear stresses.

The terms viscous and frictional-viscous transition are gaining traction lately to refer to the aseismic part of the lithosphere or the transition toward it as opposed to the term plastic or brittle-plastic transition used in seminal studies dealing with the rheology of crustal or lithospheric-scale faults [START_REF] Rutter | On the nomenclature of mode of failure transitions in rocks[END_REF][START_REF] Sibson | Earthquakes and rock deformation in crustal fault zones[END_REF][START_REF] Scholz | The brittle-plastic transition and the depth of seismic faulting[END_REF][START_REF] Kohlstedt | Strength of the lithosphere: Constraints imposed by laboratory experiments[END_REF]. Most rock rheology and mechanics textbooks avoid the term viscous to refer to the permanent deformation of rocks without fracturing, using instead terms such as plasticity, viscoelasticity, or viscoplasticity (e.g., [START_REF] Jaeger | Fundamentals of Rock Mechanics[END_REF][START_REF] Karato | Deformation of Earth Materials[END_REF]. The same applies to textbooks that deal with the rheology of all types of solids (e.g., [START_REF] Courtney | Mechanical Behavior of Materials[END_REF][START_REF] Malkin | Rheology: Concepts, Methods, and Applications[END_REF]. In a recent contribution, [START_REF] Wang | If not brittle: Ductile, plastic, or viscous? Seismol[END_REF] discusses, among other things, which term to use to refer to the macroscopic behavior of the aseismic lithosphere, raising important points on the problem of establishing a consistent terminology in Earth sciences. Unfortunately, [START_REF] Wang | If not brittle: Ductile, plastic, or viscous? Seismol[END_REF] favors the term viscous over others to refer to the overall rheology of the middle and lower crust through lithospheric-scale shear zones. This choice, yet again, leaves the rheology terminology inconsistent between continuum mechanics (followed in material science) and geophysics, in particular the meaning of the term viscous.

Problem Statement and Constraints

In continuum mechanics, viscous materials are those that lack elasticity (i.e., a null shear and Young's modulus) as opposed to terms such as viscoelastic, viscoplastic, or plastic, which all imply an initial elastic response (Fig. 1). Consequently, using the term viscous to refer to the lower crust and upper mantle would mean assuming a lack of elastic response in such parts of the lithosphere, in total contradiction to decades of seismic studies indicating that S waves propagate through the aseismic lithosphere. We think much of the confusion stems from the use of the parameter viscosity, widely used in rheological models of the lithosphere as a synonym for macroscopic viscous behavior. Additional problems in establishing a clear terminology in this respect, somewhat already pointed out in [START_REF] Wang | If not brittle: Ductile, plastic, or viscous? Seismol[END_REF] are:

(1) the definition of a plastic behavior does not present a consistent definition across or even within the same (e.g., Earth sciences) scientific branch and ( 2) the introduction of the term yield stress in brittle deformation (Coulomb plasticity model).

The aim of this contribution is twofold:

1. Propose guidelines to refer to the fundamental macroscopic behavior of rocks under shear stresses within the aseismic part of the lithosphere. 2. Keep the meaning of the terms consistent across different branches of science.

Because the topic of material behavior can be approached from different points of view, it is necessary to specify what we mean here with macroscopic material behavior. In the past, most terminology to refer to the lithosphere was based on two different concepts (e.g., [START_REF] Rutter | On the nomenclature of mode of failure transitions in rocks[END_REF][START_REF] Blenkinsop | Deformation Microstructures and Mechanisms in Minerals and Rocks[END_REF]: (1) how strain distributes, that is, the degree of homogeneity of strain on a macroscopic scale or (2) deformation mechanisms at the microscopic scale. The former uses terms such as ductile or brittle deformation, whereas the latter uses terms such as intracrystalline plasticity, cataclasis, or mass transfer. The problem with terminology based on the degree of homogeneity was already addressed by [START_REF] Rutter | On the nomenclature of mode of failure transitions in rocks[END_REF] and covered by [START_REF] Wang | If not brittle: Ductile, plastic, or viscous? Seismol[END_REF], and thus will not be considered again here. Terminology based on deformation mechanisms refers primarily to phenomena occurring at the nano-and microscale, and scaling these phenomena to the macroscale is complex, first, because different deformation mechanisms may coexist during deformation, and second, because different deformation mechanisms or combinations between them can result in a rather similar (macroscopic) rheological behavior.

For the sake of simplicity, we will limit to basic terms such as elastic, plastic, viscous, and their combinations, which are all well entrenched in the material and Earth sciences communities and serve to describe the fundamental macroscopic rock behavior under shear stresses.

Viscous versus Plastic Definition and Their Combinations

The basis for understanding viscous behavior is:

1. Viscous materials resist no shear stress and thus flow under any shear stress. Accordingly, the degree of permanent deformation depends on how much time has passed, and a material undergoing viscous flow is said to be time dependent. 2. For fixed stress (or applied force), the strain rate primarily depends on temperature. Therefore, material undergoing viscous flow is said to be strongly temperature dependent.

When the strain rate is linearly proportional to the applied stress, the behavior is said to be Newtonian or linear; otherwise it is said to be non-Newtonian or nonlinear. In the basic mathematical expression representing a linear viscous behavior, E Q -T A R G E T ; t e m p : i n t r a l i n k -; d f 1 ; 5 3 ; 3 8 1 τ μγ ̣ ; 1 in which the τ is the shear stress, γ ̣ is the shear-strain rate, and μ is the viscosity that accounts for the resistance of the material to flow (μ = shear-stress ÷ shear-strain rate). In particular, the shear-strain rate accounts for the time dependency of the flow (dγ=dt), whereas the viscosity for the temperature dependency commonly through an Arrhenius-type equation of the type μ μ 0 expQ=RT, with Q being the activation energy, R the gas constant, and T the absolute temperature. The concept of stiffness (or strength) has no meaning in viscous materials because their rigidity (shear) modulus is by definition zero, and any shear stress will cause permanent deformation, that is, they behave like fluids, and indeed, this is the definition of fluid in continuum mechanics. Because of the time dependence of the deformation, it is not possible to derive the magnitude of the stress from the attained strain because any stress can result in any strain given sufficient time. Stress is thus related to strain rate via viscosity.

The core idea behind plastic deformation is that the material needs to reach a threshold shear stress value, called yield stress, to deform permanently. Below this threshold, the material undergoes recoverable (i.e., elastic) deformation under shear stresses; we ignore on purpose here the rigid-plastic model (e.g., [START_REF] Jaeger | Fundamentals of Rock Mechanics[END_REF] for which no elastic response exists before reaching permanent deformation because it is unrealistic in materials. As anticipated, plastic behavior is not as consistently defined as viscous or elastic across earth science textbooks. For example, some use a restrictive definition in which the plastic behavior requires constant stress for the material to deform at a constant strain rate above the yield stress (e.g., [START_REF] Stüwe | Geodynamics of the Lithosphere[END_REF]. Most material science and structural geology textbooks, however, refer to this behavior as perfectly plastic and present the term plastic (or general plastic) as less restrictive by allowing the material to harden or soften during the inelastic stage and thus varying the strain rate over time [START_REF] Poirier | Creep of Crystals: High-Temperature Deformation Processes in Metals, Ceramics and Minerals[END_REF][START_REF] Jaeger | Fundamentals of Rock Mechanics[END_REF][START_REF] Twiss | Structural Geology[END_REF][START_REF] Karato | Deformation of Earth Materials[END_REF].

The value of yield stress in the stress-strain space, which defines the elastic-to-plastic transition, depends on temperature (e.g., [START_REF] Paterson | Experimental deformation and faulting in wombeyan marble[END_REF][START_REF] Frost | Deformation Mechanism Maps: The Plasticity and Creep of Metals and Ceramics[END_REF], and thus plastic behavior is by definition strongly temperature dependent. Because there is a threshold shear stress for producing permanent deformation, plastic behavior is said to be time independent; which means that if the shear stress is below the yield stress, no permanent deformation occurs irrespective of the time that has passed. However, it should be noted that the amount of permanent deformation produced at shear stresses above the yield stress is only a matter of time, and deformation becomes time dependent [START_REF] Courtney | Mechanical Behavior of Materials[END_REF], blurring the difference between viscous and plastic behavior, especially for materials exhibiting very small yield stresses. However, it is possible to distinguish the time dependence in both cases by conducting an unloading exercise (e.g., [START_REF] Cooper | Load relaxation of olivine single crystals[END_REF].

Viscoelastic refers to materials that display both elastic and viscous properties depending on the time scale of the deformation. On short-time scales (e.g., seismic waves), viscoelastic materials behave elastically, whereas at large-time scales, they behave viscously (e.g., mantle convection) (Fig. 1). Viscoelasticity is therefore a time-and temperature-dependent material behavior.

Viscoplastic (or elastoviscoplastic or viscoelastoplastic) refers to materials that display elastic behavior below the yield stress and linear (steady state) viscous behavior above the yield stress. The difference with a viscoelastic material is that it requires exceeding the yield stress to undergo permanent deformation. Another way of looking at this is that the viscosity of the material below the yield stress tends to infinity but above it has a defined and constant value (at given P, T, and composition). Thus, under constant shear stress, viscoplasticity implies a temperature-dependent deformation of the material and a time-independent or time-dependent deformation below and above the yield stress, respectively.

A Flow Decision Chart for Aseismic Lithosphere Rheology

To ensure consistent terminology between geosciences and materials sciences, the term viscous should be reserved only for the behavior of melts in the lithosphere [START_REF] Dingwell | Transport properties of magmas: Diffusion and rheology[END_REF]. This should not be confused with the use of the parameter viscosity for modeling solid rock macroscopic response to shear stress, which is perfectly valid and indeed used in viscoplastic and viscoelastic numerical models. This point will be discussed further in the Subtleties and Final Remarks section. By the same logic, the use of the term frictional-viscous to refer to crustal or lithospheric-scale faults [START_REF] Schmid | Towards a genetic classification of fault rocks: geological usage and tectonophysical implications[END_REF][START_REF] Imber | Sibson-Scholz fault zone model: The nature of the frictional to viscous ("brittle-ductile") transition along a long-lived, crustal-scale fault[END_REF][START_REF] Handy | Continental fault structure and rheology from the frictional-to-viscous transition downward, in Tectonic Faults: Agents of Change on a Dynamic Earth[END_REF] should be avoided. Interestingly, in the synoptic fault model that likely causes the spreading of this term in the geophysics community (figure 6.7 in [START_REF] Handy | Continental fault structure and rheology from the frictional-to-viscous transition downward, in Tectonic Faults: Agents of Change on a Dynamic Earth[END_REF], it is stated that the rocks outside the viscous shear zone fall "below the rock yield stress," conflicting with the definition of the term viscous in continuum mechanics and materials sciences (Fig. 1).

To refer to the fundamental macroscopic behavior of rocks in the aseismic lithosphere, we propose a decision flowchart mainly built around two different concepts: the presence or lack of elasticity and yield strength (both related to the time dependency of deformation; Fig. 2). The ultimate goal is to standardize the meaning of these terms to send a clear message of what is meant when the terms are used. Similarly, we need to emphasize that it should always be specified when rocks under study (1) possess elasticity and a certain degree of rigidity (i.e., yield stress or a positive stiffness modulus), ( 2) possess elasticity and rigidity but can be ignored in the model, or (3) lack any strength at the time scale of observation (i.e., only have rigidity at short-seismic-time scales). The terminology proposed refers to the macroscopic behavior and is independent of deformation mechanisms and strain distribution.

Subtleties and Final Remarks

Most of these terminology issues in the realm of rheology primarily exist because fluids and solids are idealized. As such, the boundary between the two blurs under certain circumstances, with real materials exhibiting a mixture of liquid-and solidlike properties (e.g., partially molten rocks with low-melt fractions). Another subtler but very important rheological concept for the Earth sciences is the characteristic time scale under stress measured by the Deborah number [START_REF] Poirier | Creep of Crystals: High-Temperature Deformation Processes in Metals, Ceramics and Minerals[END_REF][START_REF] Twiss | Structural Geology[END_REF][START_REF] Malkin | Rheology: Concepts, Methods, and Applications[END_REF]. Thus, depending on the time scale of observation, a material can be treated as solid or fluid. For example, a rock that behaves like a solid in a deformation experiment might flow macroscopically like a fluid under very small stresses and over very long periods as shown in a few long-term (several years) creep tests in granite and gabbro bars [START_REF] Kumagai | Results of experiments of secular bending of big granite beams extending for 10 years and their analyses[END_REF][START_REF] Itô | Rheology of the crust based on long-term creep tests of rocks[END_REF]. In other words, the same rock may exhibit elasticity at high stresses (and then short-time scales) but also exhibit yield strength (solid-like behavior), for which value decreases with decreasing applied stress until it becomes negligible at very low-differential stresses and thus start to behave like an extremely flowresistant fluid (e.g., Deborah's number). As noted earlier, the general propagation of S waves through the lithosphere indicates that rocks do possess rigidity at short time scales. The only question then is whether the rocks in the aseismic lithosphere behave as viscoelastic or as plastic or viscoplastic materials or, to put it another way, whether they exhibit yield stress at deformation rates typical of lithospheric shear zones (Figs. 1 and2). The presence or lack of yield stress in the aseismic lithosphere is, however, a tougher issue and should be primarily confronted with evidence in nature because in the laboratory, it is impossible to reach strain rates typical of plate tectonics, within the range of 10 -13 to 10 -15 s -1 [START_REF] Pfiffner | Constraints on geological strain rates: Arguments from finite strain states of naturally deformed rocks[END_REF].

Another source of misunderstanding is that deformation models are often placed in the same category of describing the macroscopic material response to shear stress in terms of forces, stresses, strain rates, and/or strain. We think they are essentially different. The aim of establishing a rheological terminology is to provide a framework for understanding or conveying how a material deforms macroscopically under certain conditions in a qualitative manner. In contrast, models are mathematical idealizations of material behavior, and as such, their drive is to produce accurate enough predictions in a particular context at the cost of ignoring certain variables to simplify or just being mathematically tractable (compare to Ben-Zion, 2017), for example, ignoring the elastic response or the yield stress. An illustrative example of this would be the successful use of a viscous framework to predict the behavior of rocks under coseismic fault lubrication observed in laboratory experiments [START_REF] Pozzi | Coseismic fault lubrication by viscous deformation[END_REF]. In this case, the viscous model provides useful predictions of the process, but no one would argue that the suite of rocks tested (carbonates, sulphates, halides, and silicates) lacks elastic properties at upper crustal conditions. It is the lubrication process that shows viscous-like behavior, not the material itself. It is also important to recall that using the parameter viscosity to model the material response does not necessarily mean that the material has a viscous or perfect fluid-like behavior. The viscosity parameter is used indistinctly in viscous, viscoelastic, and viscoplastic models, for example, in the Bingham, Casson, and Hershel-Bulkley rheology models, all of which include a parameter defining the yield stress [START_REF] Malkin | Rheology: Concepts, Methods, and Applications[END_REF].

Finally, [START_REF] Wang | If not brittle: Ductile, plastic, or viscous? Seismol[END_REF] raised another potential source of misunderstanding in the geophysics community, which is the use of the concept yield stress or yield strength within the brittle domain, that is, the Coulomb plasticity model. The use of the term yield stress in such a way conflicts with material sciences and metallurgy, in which some materials fracture before yielding, and yielding can be favored by imposing a higher hydrostatic stress field limiting or suppressing the development of porosity and dilatancy caused by the fracturing process (e.g., [START_REF] Malkin | Rheology: Concepts, Methods, and Applications[END_REF][START_REF] Courtney | Mechanical Behavior of Materials[END_REF]. That is, they separate brittle failure (elastic to fracture transition) from yielding (elastic-to-plastic or -viscous transition), which strictly speaking would lead to a permanent deformation with a strong temperature dependency and no volume (or negligible) increase [START_REF] Paterson | Effect of pressure on stress-strain properties of materials[END_REF]. A similar distinction was made for rocks by [START_REF] Paterson | Experimental Rock Deformation-The Brittle Field[END_REF]. In Earth sciences, the coefficient of internal friction in Coulomb's law is usually presented as almost independent of temperature and resulting in pressure dependence due to dilatancy [START_REF] Paterson | Experimental Rock Deformation-The Brittle Field[END_REF]. Yet again, the aim here is to propose unambiguous terminology, not to question the utility of Coulomb's plastic model. For clarity, we suggest that if the terms yield stress or yield strength are used in this sense in Earth sciences, it should always be paired with the adjective brittle or, preferably, referred to as Coulomb yield stress.

In essence, the two provided figures capture the core message of the article. The ultimate goal is that when an Earth scientist uses one of these rheology terms, it sends an unambiguous message about its meaning that is in line with other branches of rheology.

Figure 1 .

 1 Figure 1. Key features of the terms viscous, viscoelastic, viscoplastic, and plastic as defined in continuum mechanics and material sciences. The color version of this figure is available only in the electronic edition.

Figure 2 .

 2 Figure 2. Decision flowchart on which rheological term to use to refer to the aseismic flow of rocks based on the macroscopic behavior under shear stress.

  Some fluids drastically change viscosity at a threshold shear stress value and are referred to as viscoplastic fluids (e.g.,[START_REF] Malkin | Rheology: Concepts, Methods, and Applications[END_REF]. We purposely ignore them here.

	How to refer to a material
	flowing under shear stress?	*
			* At a specific T, P, and mineral content
	Do seismic S waves propagate through material? (elastic response)		No	Viscous
				Linear (Newtonian) or nonlinear	e.g., magmas
	Yes
	Does the material require a minimum shear stress to flow? (yield strength/stiffness)	No	Viscoelastic
				Nonlinear viscosity, its elastic or viscous nature depends on the strain rate	e.g., thixotropic fluids, glasses above the glass transition temperature, some silicate melts, polycrystalline rocks.
	Yes
	Does the material behave above yield stress? as a linear viscous material		No	(or elastoplastic) Plastic
				Strain hardening and softening may occur in the anelastic path	e.g., single crystals deformed by dislocation creep, polycrystalline rocks
	Yes (or approximately)
	Viscoplastic (or elastoviscoplastic /		e.g., polycrystalline rocks*
	viscoelastoplastic)

*

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Volume XX • Number XX • -2022 • www.srl-online.org Seismological Research Letters

Acknowledgments

The authors thank A. Tommasi for informal discussions and an anonymous reviewer for constructive comments. S. D. is thankful for funding from the Centre National de la Recherche Scientifique (appel d'offre 'TELLUS'). M. A. L. S is thankful for funding from the Government of the Principality of Asturias and Fundación para el fomento en Asturias de la Investigación Científica Aplicada y la Tecnología (FICYT) (Spain) (Grant reference SV-PA-21-AYUD/ 2021/57163) under the Plan de Ciencia, Tecnología e Innovación 2018-2022 of Asturias (PCTI-Asturias).

Data and Resources

No data were used in this article.

Declaration of Competing Interests

The authors acknowledge that there are no conflicts of interest recorded.


domain: phys spi

text:
Integrated metrology devices dedicated to transient Harman measurement of silicon nano-meshes

Hafsa Ikzibane 1 , J. Canosa 1 , Emmanuel Dubois 1 and Jean-François Robillard In thermoelectricity, silicon nanostructures represent an interesting alternative to conventional thermoelectric materials due to Si abundance, non-toxic nature and its compatibility with CMOS technology. Researchers have investigated methods to enhance the silicon figure of merit zT by increasing the σ/κ ratio; decreasing the thermal conductivity κ by (i) using nanometric structure such as thin membranes [1,2,3] or nanowires [4,5], (ii) roughening [5,6], (iii) oxidization of the surface [7] achieved relatively low value of κ. Yet, few experimental measurements of zT in crystalline silicon nano-objects were presented with important data dispersion (Figure 1). Usually, the thermoelectric figure of merit is obtained through independent measurement of materials transport properties (κ, σ and S). The uncertainty of zT can easily reach 50% considering that each parameter has an uncertainty of 5% to 20% [8]. Harman's technique [9] is a simple and rapid method to measure zT directly in bulk materials. In this work, we propose an adaptation of the transient Harman technique to suspended crystalline nanomembranes. Additionally, following [10], a correction factor taking into account radiation, contact resistances and Joule effect is determined. Finally, we present a device implementation of the method that enable direct access to zT through purely electrical measurements without relying on separate measurements of transport parameters.

Figure 1 :

 1 Figure 1: Reported values of zT for crystalline silicon planar configurations.