Kerbiriou Christian
email: kerbiriou@mnhn.fr

Isabelle Leviol

Jiguet Frédéric

The impact of human frequentation on coastal vegetation in a Biosphere Reserve

Keywords: trampling, littoral, species richness, rare species, Biosphere Reserve

We aimed to assess the impacts of recreational trampling on rare species, plant communities and landscape structure in the Iroise Biosphere Reserve (western France). Focusing on coastal grasslands, we first identified indicators discriminating humaninduced short grasslands (i.e. maintained short by intensive trampling) from natural short grasslands (i.e. maintained by environmental constraints): the presence of lichens and succulent or woody species, which are known to be highly sensitive to trampling, as well as a shallow soil were good indicators of natural short grasslands. Recreational activities affected the majority of plots containing rare species, but one third of rare species (according to their habitat preference) appeared currently not threatened by recreational activities. The other rare species were found in grasslands with low trampling intensity and were not found in grasslands with greater trampling intensity. One lichen species (Teloshistes flavicans) was not affected by trampling intensity, while two plants species (Scilla verna and Ophioglossum lusitanicum) showed higher abundances when trampling was low to medium. When it occurs in natural short grasslands, tourist trampling reduced drastically plant species richness. However, when considering maritime high grasslands, we observed that species richness was higher under low trampling vs. no trampling, but decreased at higher trampling intensity. At a landscape scale, the mean annual rate of path creation was about 1,6% and trampling has already completely destroyed 3.5 ha of natural coastal vegetation, created 31 ha of short grasslands which represent 50.8% of the whole short grassland habitat of the island, and affected respectively 41% and 15% of natural short grasslands and maritime-high grasslands. One of the main suggestions for managers to minimise trampling impacts should be to protect totally areas of rocky soil covered by short grassland that are still non-trampled and not impacted. Fortunately, this appears compatible with a relatively free access of visitors to coastal areas, as tourists can be redirected towards high maritime grasslands, a habitat which is less impacted by tourism in terms of affected surface, soil cover, loss of species, or presence of rare species host.

Introduction

Protection status, arising from the presence of rare and endangered species, high biological diversity and/or preserved ecosystems sometimes leads to an increase in eco-tourism: increased use of natural and wild areas for eco-tourism and recreation has already been identified as a worldwide trend (Buckley, 2000). Tourism in protected areas can provide resources for sustainable development to regions with often little other economical activities [START_REF] Buckley | Ecological indicators of tourist impacts in parks[END_REF][START_REF] Ceballos-Lascurain | Tourism, Ecotourism, and protected area[END_REF][START_REF] Epler Wood | The Evolution of Ecotourism as a Sustainable Development Tool[END_REF][START_REF] Guerrero | Ecotourism to the galapagos island, a world heritage site[END_REF]U.S. Congress, 1992). However, many of these protected areas have been designed for species and habitat protection, with limited or no consideration for tourism access or accommodation [START_REF] Ashton | Fundamentals of Ecotourism. A workbook for on profit and travel programs in Ecotourism Management Workshop[END_REF][START_REF] Boo | Ecotourism: The Potentials and Pitfalls[END_REF]. Some authors recently suggested that eco-tourism may generate important perturbation of wildlife and alteration of habitats (Buckley, 2000;[START_REF] Buckley | Ecological indicators of tourist impacts in parks[END_REF][START_REF] Kelly | Impacts of tourism on threatened plant taxa and communities in Australia[END_REF]Taylor and Knight, 2002). As recreation is a legitimate use of wilderness areas, the challenge for managers is to evaluate the acceptable level of negative impacts on resources based on wilderness management goals and mandates. Furthermore, studies underlined that tourist perception of the impact (soil erosion, path…) can degrade the quality of visitor experiences [START_REF] Roggenbuck | Defining acceptable conditions in wilderness[END_REF][START_REF] Vaske | Social and environmental influences on perceived crowding[END_REF]. These perceptions are based on how visitors believe impacts affect the overall attributes of the setting like scenic appeal or solitude [START_REF] Whittaker | Types of norms for recreation impacts: Extending the social norms concept[END_REF]. However, international and national reports have underlined the small amount of data about tourism impact on biodiversity (Bhandari 1999;[START_REF] Harriot | Marine tourism on the Great Barrier Reef[END_REF]IUCN, DFID and European Commission, 2002; Minister of the Environment and Governments Services Canada, 1998; Office of Technology Assesment, 1992; U.K. CEED, 2000; U.S. Congress, 1992).

One of the first degradation resulting from the presence of tourists is vegetation trampling, which often affects ecosystems of high conservation value (Andres-Abellan et al.,. 2005;[START_REF] Liddle | A survey of tracks and paths in a sand dune ecosystem[END_REF][START_REF] Liddle | recreation ecology: The ecological impact of outdoor recreation and ecotourism[END_REF][START_REF] Yorks | Toleration of traffic by vegetation: Life form conclusions and summary extracts from a comprehensive data base[END_REF]. Many ecological studies of tourist impact on vegetation have been conducted on sand dunes (Blom, 1976;Lemauviel, 2000;[START_REF] Lemauviel | Response of three plant communities to trampling in a sand dune system in Brittany (France)[END_REF][START_REF] Liddle | A survey of tracks and paths in a sand dune ecosystem[END_REF][START_REF] Van Der Maarel | Plant species diversity in relation to management[END_REF], forest [START_REF] Roovers | Experimental trampling and vegetation recovery in some forest and heathland communities[END_REF]Sun and Walsh, 1998), riparian areas [START_REF] Marion | Spatial and temporal variation in soil and vegetation impacts on campsites[END_REF] alpine environments [START_REF] Whinam | Impacts after four years of experimental trampling on alpine/subalpine environments in western Tasmania[END_REF], but there have been few studies on coastal vegetation on rocky soil (Gallet and Rozé, 2001). Such vegetation is likely to be highly sensitive to trampling, due to poor productivity. Plants are generally resilient to perturbation, but their ability to recover after intensive trampling disturbance vary substantially across habitats, with slower recovery in less productive environments such as alpine grasslands [START_REF] Hartley | Visitors impacts at Logan Pass, Glacier National Park: A thirty-year vegetation study[END_REF][START_REF] Stohlgren | Vegetation and soil recovery in wilderness campsites closed to visitor use[END_REF]Parson 1986, Whinam and[START_REF] Whinam | Impacts after four years of experimental trampling on alpine/subalpine environments in western Tasmania[END_REF]. It has been shown in alpine grasslands that recovery can run on many year [START_REF] Godefroid | Impact of fencing on the recovery of the ground flora on heavily eroded slopes of a deciduous forest[END_REF], so that the effects of the disturbance can sometimes be detected 20 years after it occurred [START_REF] Charman | Long term vegetation recovery after vehicle track abandonment on Dartmoor, South-West England. UK[END_REF]. Coastal cliff vegetation is very similar to alpine grasslands (close species such Armeria sp, Festuca sp., Scilla sp, Jasiona sp, Sedum sp…), is characterized by a growth on thin soil and poor productivity, so that trampling could potentially generate major, long-lasting damage to this vegetation as well.

Coastal habitats (maritime-high grassland and dry heathland) are classified as Habitat of Community Value by the European 'Habitats' Directive on the conservation of natural habitats (Directive 94/43/CEE; Romao, 1997) and may shelter species of great conservation concern [START_REF] Rodwell | British Plant Communities[END_REF][START_REF] Rodwell | British Plant Communities[END_REF]Thompson et al., 1995). Nevertheless, nearly total destruction of such vegetation had already occurred on large coastal areas, due to tourism activities at several sites along the French Atlantic coast (for example, "Pointe du Raz" in Brittany). Landscape rehabilitation of such areas has led to large public spendings (10 millions euros) and ecological restoration runs on many years (official source from Bretagne Regional Council and Syndicat mixte pour l'aménagement de la pointe du Raz et du Cap Sizun).

Here, we explore the impact of tourism on the coastal vegetation of Ouessant, a small French island hosting rare plant and animal species, as well as noticeable habitats, but receiving increasing numbers of tourists, which represent the main source of income for the inhabitants. By first checking that short coastal grasslands were natural plant communities maintained by environmental constraints, not by human trampling, we studied the impact of trampling at different scales and from different perspectives:

From a Park manager point of view, we assessed the potential and actual impacts of trampling by visitors on rare species. This was performed first at a landscape scale by mapping tourist flow and rare species. Several studies have shown that recreation impacts a relatively small proportion of wilderness areas (see review in [START_REF] Leung | Recreation impact and management in wilderness: A state-of-knowledge review[END_REF] and that effects are usually distributed unevenly due in part to visitor use patterns [START_REF] Lucas | How wilderness visitors choose entry points and campsites[END_REF]. However, even localized impact can harm rare or endangered species. Thus, in addition to studying trampling at a landscape scale, we also identified the habitats of rare species, some of which (e.g. maritime-high grassland and short grassland) are more submitted to tourist frequentation than others, depending notably on accessibility via vegetation height and absence of thorny species like blackberry bush (Rubus sp.) or gorse (Ulex sp.). Finally, at a species scale, we assessed how rare species responded to human perturbation.

From a conservation biologist point of view, we considered the potential and actual impacts of trampling on plant communities by assessing the proportion of trampled habitat and by analysing species richness and species assemblage in trampled vs. preserved areas.

Finally, we attempted to assess the current dynamic induced by recreation activities, via indicators of damage widely used in previous studies, such as variation of path length (Cole, 1995;[START_REF] Roovers | Experimental trampling and vegetation recovery in some forest and heathland communities[END_REF][START_REF] Whinam | Impacts after four years of experimental trampling on alpine/subalpine environments in western Tasmania[END_REF]. Because there is a strong social and economical pressure for developing tourism in protected, and notably coastal, areas [START_REF] Laurans | Les pour la recréation de sites littoraux : Quelles dimensions économiques ?[END_REF], zero perturbation is generally not an acceptable goal. Conservation biologists should better assess the rates of habitat alteration and diversity loss, and evaluate a sustainable level of perturbation, or propose compensatory landscape management that will ensure a suitable and a sustainable habitat conservation state. Tourist pressure on natural costal resources is becoming really obvious in Ouessant, a small island at the western tip of Brittany, France. This island is highly protected because it hosts rare plant and animal species and noticeable habitats, but it receives nowadays an increasing number of tourists, with currently about 140,000 visitors per year. Tourism is nowadays the main source of income for local inhabitants since the abandonment of traditional agriculture and the decline of resident human population. Given that tourism development is vital for the island economy and given the importance of Ouessant for the conservation of many threatened plants and habitats [START_REF] Bioret | Essai de synthèse de l'intérêt du patrimoine phytocoenotique des îles Bretonnes[END_REF]Bioret and Magnanon, 1993), we carried out a study for assessing the impact of tourism frequentation on coastal vegetation, the latter being considered at various spatial scales, from local species assemblage to the landscape. Most of the developed analyses could be hindered by the fact that the trampling pressure in coastal habitats may itself creates short grasslands. One of the first goal of this study is thus to discriminate grasslands kept short by human trampling from those kept short by environmental constraint such as wind, sea spray and soil depth and composition.

Materials and methods

Study area

Ouessant is a small island (1,541 ha) located 20 km off the western end of Brittany, France (48°28'N, 5°5' W). Ouessant has long been recognised as an exceptional ecosystem, representative of the biodiversity of the Atlantic domain, and was therefore listed in national and European inventories such as Important Bird Areas or Special Protection Areas. The island is thus highly protected, as it has been declared as a Regional Nature Park since 1969 and as a Man and Biosphere Reserve since 1988; in addition, the 40 km of the island coastline have received the status of "Classified Site" in 1979, which led to total protection against any human construction (fig. 1) From the sea border to inland, a limited number of vegetation series occur on the island. On the coastal line, vegetation of Atlantic sea cliff consists of three classes: (1) maritime rock crevice vegetation; (2) maritime-high grassland; (3) short grassland on rocky soil. The second main vegetation series of the coast stripe is European dry heathland, consisting of two main classes (tab.1). More inland, pastures with low grazing pressure and crop abandonment had led to the development of fallow-lands. Currently fallow lands, dominated by braken (Pteridium aquilinum) and brambles (Rubus fructicosus) occupy more than half of the island area [START_REF] Gourmelon | Land-use changes and implications for management of a small protected island off the coast of Bretagne[END_REF]. Species and habitats of high conservation values are present in Ouessant; for some of them (e.g. Isoetes histrix, Ophioglossum lusitanicum), the island is one of very few stations in Western France [START_REF] Annezo | Bilan régional de la flore bretonne. Les carnet de la nature en Bretagne Conservatoire[END_REF][START_REF] Bioret | Contribution à l'étude de la flore et de la végétation de quelques îles et archipels Ouest et Sud armoricains[END_REF]. Man exploited the entire island until the early 20 th century. During the 20 th century, inhabitant number declined drastically from 2661 in 1911 to about 1000 in 1999 (Péron, 1997). Consequently, cropping, which was exclusively directed towards self-production of food resources, almost disappeared from local farming practices. Only residual sheep breeding maintained a remnant traditional agriculture [START_REF] Gourmelon | Land-use changes and implications for management of a small protected island off the coast of Bretagne[END_REF]. During the last fifty years, tourism activities have greatly increased on the island: annual number of ferry passengers increased from 10,000 in 1950 to about 140,000 in 2004. The annual growth rate of the ferry traffic was about +2,500 passengers per year during the last twenty years (fig. 1). Tourism is currently the main source of economical incomes of the island. Most visitors undertake a one-day excursion to the island, and are mainly interested by discovering coastline landscapes by walking or cycling along paths all around the island.

Data collection and analysis

Generally, the impacts of trampling on vegetation are studied via a comparison of protected vs. experimentally trampled plots (Cole, 1995;[START_REF] Gallet | Long-term effects of trampling on Atlantic Heathland in Brittany (France): resilience and tolerance in relation to season and meteorological conditions[END_REF][START_REF] Whinam | Impacts after four years of experimental trampling on alpine/subalpine environments in western Tasmania[END_REF]. This was not achievable on Ouessant, where most land is privately owned which renders plots enclosure very problematic. Hence, we chose to consider the whole costal area of Ouessant Island as a life-size experience and to take advantage of the existing variation in trampling intensity.

Spatial pattern in tourist visits

During the 1993 and 1994 summers and year round between 1995 and 2001, numbers of tourists walking along the main tourist flow (tips of the island, main beaches, main littoral paths...) were routinely counted by the same observer in 12 coastal areas, where botanical patrimonial index was also estimated (see below). The average coastline length of these areas is 2.6 Km  0.13 (Fig. 1). Counts were performed at least once a month (sometimes many more times depending on the daily activity of the observer), on average 7 times a day, between 8 am and 8 pm. Overall, there were a total of 7,915 counts, split among 1113 days, which represents an average of 8 records per area and per month. To transform tourist number into a flux of tourists crossing the area, a specific survey of the time tourists spend at a given location was carried out in two specific sites (Ouessant tips: Porz Doun in 1998 and Kadoran in 1999) for a total of 56 hours of survey. Most tourists walked along the coastline and their presence in such area was ca. 21  4 minutes. Estimates of the annual number of tourists per area were obtained after accounting for monthly and daily variations.

Average of tourist within in a typical day of a month (m) in a tourist area x (ADMnTx) : ADMnTx = ∑i (ni/Ni).Jm.(60/sx) Were :

i is the hour of the day concern, n is the count of tourist at the hour i, N is the number of count at the hour i, Jm is the number of days in the month, sx is the average of time (in minutes) that tourist stay in a tourist area (Tx) Then estimates of the annual number of tourists in an area are asses through the addition of the 12 tourist month estimation.

Botanical sample used to discriminate short grassland origin

Some impacts of tourist trampling on vegetation, such as path creation, appear quite easy to identify and quantify. But when tourist impact occurs for a long time, the increase number of paths, which eventually connect to one another, can create large areas of short grassland and even soil erosion. To distinguish short grassland arising from natural, environmental constraints from those created by human trampling, we performed 157 botanical samples. 137 samples were performed on short grassland (European habitat classification 1230-6, Sedum anglicum-Dactlis glomerata or Scilla verna-Jasione crispa maritima or Plantago coronopus-Armeria maritime) This samplings were split among the 12 tourist areas following, as much as possible, the surface of grassland present in each area (stratified sampling, yielding between from 4 and 34 sampling sites per area , linear regression between grassland areas and number of sampling sites: R²=0,72). An intensit y of human trampling was estimated for each sampling, to assess correlation between species, soil characteristics and tourist trampling impact. Whenever possible, sampling was performed on plots of standardized size (10 m 2 ); however, because of field constraint (size of patch vegetation), some samples were done on slightly smaller areas (minimum 6m², average 9.59 ± 0.59). We listed all species present in each plot and attributed a cover index for each species (1: cover < 5% ; 2: 5% < cover < 25% ; 3: 25% < cover < 50% ; 4: 50% < cover < 75% ; 5: cover > 75%; [START_REF] Braun-Blanquet | Plants Society. The study of plants Communities[END_REF]. All these samples were performed in areas where there was no doubt on the intensity of human trampling. The rate of human visits on the plot was estimated with four classes: none, low, medium and high. Plots free from human trampling were located in non-accessible areas (n = 48 samples, 54 species have been founded and Plantago Coronopus, Cladonia furcata, Thrincia hirta, Agrostis stolonifera maritima and Jasiona montana occurred the most). Plots with low trampling were located in areas known to be faintly visited (n<15 000 visitor per year) or on secondary paths (n = 19 samples, 47 species have been founded and Plantago Coronopus, Agrostis stolonifera maritima, Festuca rubra pruinosa, Jasiona montana and Thrincia hirta, occurred the most). Plots located on secondary paths of the main tourist areas (n>15 000 visitor per year) were attributed a medium intensity of trampling (n = 38 samples, 49 species have been founded and Plantago Coronopus, Agrostis stolonifera maritima, Thrincia hirta, Festuca rubra pruinosa and Scilla verna occurred the most), whereas those located on the main pathways of the main tourist areas were scored as high human trampling (n = 32 samples, 34 species have been founded and Agrostis stolonifera maritima, Plantago Coronopus, Festuca rubra pruinosa, Thrincia hirta, and Lotus corniculata occurred the most). Twenty additional samples were performed near the frontier between maritime-high grassland (5 to 15 cm high) and natural short grassland (under 5 cm high), to provide potential indicators of the factors limiting the establishment of short grassland. Maritime grassland matches to the European classification 1230-3, Festuca rubra-Armeria maritima or Festuca rubra-Holcus lanatus or Festuca rubra-Daucus carota gummifer. 27 species have been founded and Festuca rubra pruinosa, Daucus carota gummifer, Armeria maritime, Lotus corniculata and Agrostis stolonifera maritima occurred the most). In addition to human trampling intensity, distance from the sea, slope, orientation and soil depth were recorded for each plot. Soil depth was calculated as the mean of 30 measurements performed every 5cm along the two diagonals of each plot. Principal Components Analysis and Discriminant Analysis were used to identify potential indicators of human trampling.

Localization of hot spots of rare species in Ouessant

As the preservation of rare species is one of the main goals of the various protection statuses, we built a patrimonial flora index to localize hot spots of rare species. We considered only plants protected at the European (Annexe II of European Community Habitat Directive 21 may 1992), National (law of the10 July 1976 and 20 January 1995) or Regional (law of the 23 July 1987) scale, listed on the national Red List [START_REF] Olivier | Le livre rouge de la flore menacée de France, tome 1 : espèces prioritaires[END_REF] or listed in the appendix I of the regional Red List (published by the National Botanical Conservatoire of Brest, [START_REF] Annezo | Bilan régional de la flore bretonne. Les carnet de la nature en Bretagne Conservatoire[END_REF]. The retained species were: Centaurium marinum, Cytisus scoparius ssp. maritimus, Heterodermia leucomelos, Isoetes histrix, Ophioglossum lusitanicum, Rumex rupestris, Scilla verna, Solanum dulcamara ssp. marinum, Solidago virgaurea ssp. rupicola, Teloschistes flavicans, and Trichomanes speciosum. Two species of lichens considered as very rare ([Nom des espèces] Pr Masse / Rennes University comm. pers.) were retained to build up the index. Two species (Daucus carota ssp. gadecaei and Silene dioïca ssp. zetlandica) were removed due to obvious problems during field identification. All detected stations of each species were mapped on aerial ortho-photographs (IGN 2002), implemented in a Geographical Information System (G.I.S. Arc-Info, Environmental Research System Institute Inc). The data were then pooled using a geo-referenced grid of 250m squares (n = 292 squares) to calculate the patrimonial index, using the formula: Index value of a square = ∑i (ni/Ni) Where ni is the number of spots of rare species a in the considered square.

Ni is the total number of spots of rare species on Ouessant island.

As the delimitation of local stations was not easy for species covering large unfragmented areas, like Scilla verna or Cytisus scoparius maritimus, we considered only presence/absence of these two species in each square.

Distribution of rare species along coastal habitat and their response to trampling intensity.

Using the result from the analysis of trampling effect on vegetation, a coastal vegetation map was drawn using field plotting and aerial ortho-photographs interpretation, and implemented in the G.I.S. Habitats were categorized according to vegetation height and floral composition, while for short grasslands two other features were considered: trampling intensity (trampled or non-trampled) and their potentiality (corresponding to the vegetation expected on a plot without human trampling, derived from results of Discriminant Analyses on natural vs. trampled short grassland). As paths could not be accurately digitalised directly from aerial photographs, they were first defined as lines. In a second step, path width was measured in the field and their vegetation cover was recorded. A polygon cover of paths was then created as a buffer along the path length cover, using data on path width as the buffer coefficient. The final coastal vegetation cover was obtained by updating the vegetation cover with the polygon path cover. We super-imposed this vegetation coverage with the rare species coverage to describe the distribution of rare species within coastal habitat. We further used Logistic models to explore variations of rare species occurrence (Presence/absence in samples) along the trampling gradient established previously (see Discriminant analysis).

Plants community diversity.

We studied variations in species richness as a function of levels of human trampling. Assumptions of equal detectability of all species are generally not met, which can invalidate comparisons of plots [START_REF] Boulinier | Higher temporal variability of forest breeding bird communities in fragmented landscapes[END_REF][START_REF] Nichols | Inference methods for spatial variation in species richness and community composition when not all species are detected[END_REF]. We therefore used statistical methods, derived from the capture-recapture approaches used in population or community dynamics, and incorporating differences in species detectability, to provide an estimator of species richness and of changes in community composition. The basic estimator of species richness is the jack-knife estimator proposed by [START_REF] Burnham | Robust estimation of population size when capture probabilities vary among animals[END_REF] and implemented in COMDYN (Hines et al., 1999). We studied variations in species richness in two vegetation types (maritime-high grassland and natural short grassland) across the four trampling levels (none, low, medium and high). Within each vegetation type, sample sizes varied across trampling levels. To correct for sample size bias, we set all sample sizes to the minimum sample size observed within each group (n = 14 for short grassland and n = 5 for maritime-high grassland) via sampling methods: we sampled randomly 14 (respectively 5) plots in each of the three trampling levels containing supernumerary plots. This sampling was performed 20 times, and the resulting samples were analyzed with COMDYN, to obtain estimators of (1) species richness, (2) extinction rate Φ in pair wise comparisons (i.e. proportion of group 1 species found in group 2) and rate of species turnover γ (i.e. proportion of group 2 species present in group 1).

Relationship between trampled areas and tourist visits

We used Linear models to explore correlation between trampled areas assess by GIS (m²) and tourist visits (number of tourists per year).

Rate of creation of artificial short grassland

As creation of new areas of short grassland is mostly linked to path creation, we used the rate of additional path length from 1998, to 2000 and 2004 as indicators of short grassland creation dynamic. Aerial photographs and field measurements were used for this estimation.

Results

Spatial distribution of tourists

The distribution of tourists (Fig. 2) showed great spatial variations, ranging from 100,000 visits per year at the North-Western tip to only 3000 in the South-Eastern Island. In the followings analyses, we used this annual average estimation of tourist number obtained in each coastal area.

Distinguishing natural from human-created short grassland

We observed a significant correlation between soil depth, vegetation type and trampling intensity (Fig. 3 ; F4,152 = 22.9, P< 0.0001). Soil depth appears to constrain the establishment of natural, non-trampled short grassland (Fig. 3), which was encountered only on soil shallower than 10cm. This type of vegetation had an average soil depth of 4.8  0.2 cm. This pattern was not attributable to a spurious correlation with distance to seashore (soil generally gets shallower as distance to seashore decreases, and vegetation type is also constrained by proximity to the sea), as no correlation between soil depth and distance to seashore was observed in our data (not shown). Maritime-high grassland appeared not to be able to settle in areas with a soil shallower than 10 cm. We can therefore assume that trampled vegetations with a soil depth under 10 cm were previously natural short grassland and those with a soil depth over 10 cm are kept short by human trampling only. Soil depth thus appeared as a first powerful indicator to classify natural vs. human-maintained vegetation, and was used to build a map of the whole coastline (see below).

To identify additional indicators of human trampling, a PCA was performed on the floristic composition of the 137 samples of short grassland using their cover indices. This analysis revealed an obvious gradient in species composition across trampling levels (Tab. 2, Fig. 4), located roughly along the second principal component axis. Natural (nontrampled) short grassland appeared to be characterized by the presence of Cladonia furcata, Cladonia firma, Sedum anglicum, and Jasiona crispa maritima.

We performed parametric a Discriminant Analyses and then assess error rate of estimation in classification (Mathsoft 1999). We test how species composition of botanical sample (n = 137; species cover as variable, only the first 45 most frequent species were taken into account) from distinct trampling intensity (none, low, and a third group gathering medium and high trampling levels) could distinct trampling intensity. This technique provides an upper limit of error count estimates, and cross-validation is necessary. The crossvalidation technique uses functions computed from all data except the group being classified. Conceptually, each observation is systematically dropped, the discriminant function is reestimated, and the excluded observation is classified. All error rate estimates presented in this study are based on posterior probabilities (see Mathsoft 1999 and McLachlan 1992 for more details). Multivariate Analysis of Variance was highly significant (Wilk's Lambda = 0.051, F90,180 = 6.873, P< 0.0001). Overall, this classification generated only 4% of posterior error in jack-knife classification, most of these errors coming from the low intensity group (Tab.3). Furthermore, none of the samples of medium to high trampling group were classified in the "non-trampled" group, and only one sample in the converse way.

According to Sidak's test (Tab 4), lichens such as Cladonia firma and C. furcata contributed highly to discriminate the natural short grassland group from trampled short grassland. In addition, Aira caryophylea, Armeria maritime, Jasiona crispa maritima, Sedum anglicum, Plantago coronopus, Bellis perennis discriminated the natural short grassland group from short grassland with medium and high trampling intensity, with the first five species being mostly specific of non-trampled sites, whereas Plantago coronopus and Bellis perennis are specific of highly trampled sites.

Location of rare species hotspots

As excepted, rare taxa on Ouessant are species or subspecies linked to marine environmental constraints and are hence naturally found essentially along the coastline (Fig. 5). Littoral 250m x 250m cells (i.e., cells including a fragment of coastline, n = 117) have an average of patrimonial index (0.095 ± 0.013) twenty times higher than inside cell (0.005 ± 0.002). The North coast, more exposed to dominant winds, appeared to host the largest part of rare species.

Distribution of rare species along coastal habitats and their response to trampling intensity.

Some habitats were obviously devoid of human trampling (Table 1), such as submerged or partially submerged sea caves, maritime rock crevice or other little human trampled habitats such as European dry heaths; they host species that are not found elsewhere (Rumex rupestris, Trichomanes speciosum, Cytisus scoparius maritimus, Solanum dulcamara and Solidago virgaurea) and that were then not considered in the following, as we chose to focus on human-accessible habitats only. In such habitats, one species was noticed only in non-trampled habitat (Teloshistes flavicans). Conversely, some species such as Isoetes histrix, Ophioglossum lusitanicum, Centaurium maritimum, Scilla verna, and Heterodermia leucomelos occurred both in natural and trampled short grassland. We thus focused our subsequent analyses on these species, studying their occurrence throughout samples to discriminate short grassland origin. Except S. verna and O. lusitanicum, rare species have their maximum occurrence frequency in non-trampled short grassland (Table 1). But no significant difference could be detected between non-trampled and low trampled group for I. Histrix (², ddl 1 = 2.31, P=0.13 ), C. maritimum (², ddl 1 = 0.05, P=0.82 ) and H. leucomelos (², ddl 1 = 0.05, P=0.82 ). S. verna and O. lusitanicum appeared to have their optimum in short grassland with medium trampling intensity; the difference with the non-trampled group were significant for O. lusitanicum (², ddl 1 = 4.21, P=0.04 ) and non-significant for S. verna (², ddl 1 = 0.53, P=0.47).

In addition, as human trampling created new areas of short grassland, we focused on two widespread species, I. histrix and O. lusitanicum, and identified plots located in this newly-created areas. Twenty-five % of the 159 I. Histrix plots and 38% of the 195 O. lusitanicum plots were found in the human-maintained short grasslands (the other plots were all found in natural short grasslands). These species were never found in tall grasslands).

Plants community diversity.

Species richness (estimated via capture-recapture methods) significantly decreased with increasing intensities of trampling in maritime high grasslands (F1,78 = 134.87; P < 0.0001; Fig. 6). The same pattern was found in maritime-high grassland (F1,78 = 3.83; P = 0.05; Fig. 6), although the decrease was less significant due to an initially relatively poor species richness in non-trampled maritime-high grasslands. In addition to species richness, we also consider estimators of species changes across the gradient of trampling, to test whether the observed decreased species richness with increased trampling intensity could be linked with losses and/or replacement of species.

Changes occurring in natural short grassland affected by trampling

In trampled plots, the proportion of species observed in non-trampled natural short grassland decreased significantly with trampling intensity, by about 20% (Figure 6 and Table 5; Φ; F1,58 = 22.51; P < 0.0001). Similarly, the number of species observed at a given trampling intensity was always smaller than that of immediately lighter trampling intensity (Table 5; γ; F1,58 = 5.56; P =0.022). The proportion of new species appearing when trampling intensity increases remained relatively minor with respect to loss of species.

Changes occurring in maritime-high grassland affected by trampling

In trampled plots of maritime high grasslands, the proportion of species observed in nontrampled plots also decreased significantly with trampling intensity (Table 6; Φ; F1,58 = 10.98; P < 0.002) and the number of species observed at a given trampling intensity was smaller than that of immediately lighter trampling intensity (Table 6; γ; F1,58 = 50.02; P < 0.0001). However, the estimated species richness increased from 23 to 46 species between non-trampled plots and plots of low trampling intensity (Fig. 6), which results in a low γ (0.50, Table 6). As Φ, the proportion of species observed in non-trampled plots and still found at low trampling intensity, was relatively high (0.84 Table 6), we can infer that most changes between the two states are essentially due to a contribution of new species. Further increases in trampling intensity resulted in patterns similar to those observed in short grasslands, i.e. decreasing number of species and loss of species (Table 6).

Impact of human trampling at the landscape scale

Using the indicators of natural vs. human-induced grassland identified above (i.e. soil depth < 10 cm and presence of species non-tolerant to trampling, such as lichens, inferred from the Discriminant Analysis), a map of coastal grasslands was built in the GIS as described below. Four grassland types were distinguished: natural, non-trampled short grassland, maritime-high grassland, short grassland induced by human trampling (i.e. predicted to be maritime-high grassland without tourist impact) and short grassland affected by human trampling (i.e. predicted to remain short grassland without tourist impact).

As expected, path surface in a given area was strongly correlated with the number of tourists crossing the area (regression of path surface on number of tourists: y = 0.358x + 8004, F1,10 = 46.79 ; P < 0.0001). Total surface of trampled short grassland was also strongly correlated with the number of tourists crossing the area (regression of surface of trampled short grassland on number of tourists: y = 1.437x + 9292, F1,10 = 45.20 ; P < 0.0001). Indicators of trampling, such as soil depth or plant species and assemblages, and obvious field observations, such as tourist paths, were then used to infer the short grassland cover (of the whole coastline Fig. 7). Human trampling on the coast appears to have destroyed 3.5 ha of natural coast vegetation by creating numerous paths without vegetation cover. This trampling also created 31 ha of short grassland, which represents 50.8% of the whole short grassland of Ouessant, and appears to affect 41.4% of the original natural short grassland.

3.9. Creation dynamics of short grassland link with human trampling.

From 1998 to 2004, 7.5 km of additional paths were created by human trampling, so that Ouessant coastline is traversed today by a total of 85 km of littoral paths. The mean annual rate of path creation was 1.1% from 1998 to 2000 and 1.8 from 2000 to 2004.

Creation of new paths appeared more intensive in areas with large tourist flow (F1,10 = 4.89 ; P = 0.051) . Given the previously observed relationship between path surface and tourist flow (Regression of path surface on number of tourists: y = 0.358x + 8004, R² =0. 82), and the constant increase in the number of boat passengers (+2500 per year), we expected a 13.3 km increase in path length between 1998 and 2004. This prediction is somewhat higher than the estimation obtained by field surveys (7.5 km), but remains in the same order of magnitude.

Discussion

Trampling indicators

Our study underline that indicator species to discriminate natural short grassland from grassland maintained short by human trampling are mainly species that do not resist trampling, such as thallophyte (lichens: C. firma, C. furcata, T. flavicans, H. leucomelos), succulent (Sedum anglicum) or perennial woody-stems species (Armeria maritima). As shown by [START_REF] Yorks | Toleration of traffic by vegetation: Life form conclusions and summary extracts from a comprehensive data base[END_REF], this sensitivity is probably linked with their breakable nature, which increases in summer when plants, especially shrub lichens, dry. The species mentioned above thus appear as good indicators of non-trampled areas. A couple of species were associated with human-maintained short grassland: (1) P. coronopus and B. perennis, which is probably due to the presence of a rosette stage in their life cycle, yielding greater resistance to trampling [START_REF] Liddle | A survey of tracks and paths in a sand dune ecosystem[END_REF][START_REF] Cole | Effects of three seasons of experimental trampling on five montane forest communities and a grassland in western Montana, USA[END_REF][START_REF] Enoul | Restaurer nos espaces naturels. Bilan d'un savoir-faire en Bretagne[END_REF] and ( 2) graminoids (grass, here Festuca sp.), which are already known as trampling-resistant species (Cole, 1995;[START_REF] Yorks | Toleration of traffic by vegetation: Life form conclusions and summary extracts from a comprehensive data base[END_REF].

Rare species

Focusing on patrimonial species, we showed that at least on third of rare species occurring in potentially-trampled habitats were never found in trampled sites. One species, T. flavicans, was never seen in trampled vegetation, even under a low intensity of trampling, which suggests zero tolerance to trampling. However, most rare species on Ouessant seem to be able to tolerate a low intensity of trampling (xxx visitors per year), and disappear only at greater trampling intensities. Two species, S. verna and O. lusitanicum, even appear to be favoured by low and medium trampling. This may be caused the fact that trampled soil are generally less permeable, so that raining water can stagnate in winter, creating suitable conditions for these species. For I. histrix and O. lusitanicum, trampling has even created a rather large number of new stations, respectively 40 (+25%) and 74 (+38%). Recent extinctions of spots of O. lusitanicum in Ouessant could be detected through differences between a map drawn during the period 1984[START_REF] Stohlgren | Vegetation and soil recovery in wilderness campsites closed to visitor use[END_REF][START_REF] Bioret | Contribution à l'étude de la flore et de la végétation de quelques îles et archipels Ouest et Sud armoricains[END_REF]) and this study. The few detected extinctions (at least 5) are mainly located inland and are probably associated with fallow-land process rather than tourism impacts; a single extinction might have been due to human trampling.

Community scale

When it occurs in natural short grassland, coastal tourist trampling appears to reduce drastically species diversity and results mainly in loss of species not tolerant to trampling. Such results are globally consistent with earlier studies [START_REF] Lemauviel | Response of three plant communities to trampling in a sand dune system in Brittany (France)[END_REF][START_REF] Roovers | Experimental trampling and vegetation recovery in some forest and heathland communities[END_REF][START_REF] Whinam | Impacts after four years of experimental trampling on alpine/subalpine environments in western Tasmania[END_REF]; however, in contrast to other studies, species were lost gradually with increasing trampling intensity, i.e. we did not observed a more severe loss of species in the first stage (no trampling to low-intensity trampling) than in the later stages.

When trampling occurs on maritime-high grassland, low intensity of trampling results in larger species richness than in non-trampled plots. At higher trampling intensities, species richness decreases with increased trampling, as in natural short grasslands. Such transient increase in number of species at low intensity of trampling has been also observed by [START_REF] Van Der Maarel | Plant species diversity in relation to management[END_REF] on sand dune. This result can be explained by the fact that dominant species of maritime-high grasslands, which are mainly perennial (Agrostis stolonifera maritima, Armeria maritima, Dactylis glomerata oceanica, Festuca rubra pruinosa) are not resistant to trampling, so that small clearings produced by trampling enable establishment of numerous annual species such as Aira sp., Catapodium loliaceum, or Radiola linoides.

Landscape scale

Considering the landscape scale is crucial for managers who want an accurate picture of how natural patrimony is affected by tourism pressure. In contrast to many studies (see review by [START_REF] Leung | Recreation impact and management in wilderness: A state-of-knowledge review[END_REF], recreational impacts on Ouessant are not limited to a relatively small and negligible proportion of the habitat of rare species: the major parts of noticeable habitats are affected. According to vegetation coverage, trampling currently affects 12 ha of natural short grassland (i.e. 41% of this vegetation type) and 31 ha of maritime-high grassland (15%) and tourist visits have already led to the destruction of 3.5 ha.

In addition, yearly rate of increase in tourist number (+2500), dynamics of path creation (+1.6% per year) and decrease in vegetation cover (-6.6% per year on shallow soil (< 10 cm) and -0.6% on thicker soil (> 10 cm), unpublished survey of 12 permanent quadrats between 1998 and 2003) are a source of major concern for the conservation of the natural patrimony of the Biosphere Reserve.

Management implications

The results of this study have direct implications for managers attempting to minimise trampling impacts in wilderness recreation areas with nature preservation goals [START_REF] Cole | Wilderness ecosystems, threats, and management[END_REF][START_REF] Gómez-Limón | Recreational activities and loss of diversity in grasslands in Alta Manzanares Natural Park, Spain[END_REF]Lande et al., 2000;[START_REF] Yorks | Toleration of traffic by vegetation: Life form conclusions and summary extracts from a comprehensive data base[END_REF].

Our results are relevant to issues such as how natural patrimony is affected by this process on this Biosphere Reserve; which plants communities are more affect by trampling impacts and whether concentration or dispersal of tourists is more appropriate in a given environments.

Managers should influence spatial distribution of visitors at a large scale to (1) fully protect non-trampled areas of natural short grassland, where rare and non-tolerant to trampling species like T. flavicans are present, and (2) protect areas favourable to the establishment of natural short grasslands that have been turned into bare soil by trampling, to limit the current decrease in vegetation cover. Fortunately, a compromise between tourist access to the coast and species protection seems possible. Maritime-high grasslands, dominated by perennial grass species like Festuca rubra pruinosa and Agrostis stolonifera maritima, appears currently less impacted by tourism in term of affected surfaces, soil cover or species loss. Tourist flow should thus be redirected towards such habitat. With a moderate and controlled flow of visitors flow in this habitat, new areas of short grassland would be created, which could enable the establishment of rare species.

In addition to landscape management, a threshold of sustainable number of visits on the coast should be defined to preserve species and habitat of great value, as the continuous increase in visitor number currently observed will eventually creatie serious and irreversible damage to the coastal ecosystem, even if tourist paths are strongly constrained. Species status: Species of the European directive habitat (1), species with national protected status (2); species listed in the National Red List (3); species listed in the Regional Red List (4), Lichens consider as rare at National Scale, Pr Masse comm.. pers., (5). X presence of this specie in this habitat.

Figure 1 :

 1 Figure 1: Location of protected areas on Ouessant, survey areas and variations of the number of tourists through time.

Figure 2 :

 2 Figure 2: Spatial distribution of tourists on Ouessant Island. The estimated annual number of visitors is given for each survey area.

Figure 3 :Figure 4 :Figure 5 :Figure 6 :

 3456 Figure 3: Soil depth in plots of maritime high grasslands and short grasslands under various trampling intensity (Maritime grassland n = 20 ; Short grassland with no trampling n = 48, with low trampling n = 19, with medium trampling n = 38 and with important trampling n = 32).

Table 1 .

 1 Protection status of rare species and number of spots where they were observed on Ouessant; habitats of Annexe I directive habitat hosting these species. For short grassland habitat, occurrence (%) of species under various intensities of trampling is given

		Number	Species			Annexe I European Habitat
		of plots	status
				Caves (8300)		Vegetated sea cliff of the Atlantic coasts	European dry heaths
						(1230)			(4030)
				8300-1 Submerged or	1230-1 Crithmum maritimum-	1230-3 Festuca rubra-Armeria	1230-6 Sedum anglicum-Dactlis glomerata or, Scilla verna-	4030-2 Calluna	4030-3 Calluna
				partially	Spergularia rupicola	maritima or, Festuca rubra-	Jasione crispa maritima or Plantago coronopus-	vulgaris-	vulgaris-
				submerged sea	: maritime rock	Holcus lanatus or, Festuca	Armeria maritima : short grassland	Scilla verna :	Ulex gallii :
				caves	crevice	Gummifer: maritime grassland rubra-Daucus carota ssp.			heath	heath
								Trampling intensity
							none	low	medium	high
	Rumex rupestris	10	1,2,3,4		X
	Trichomones	8	1,2,4	X
	speciosum
	Isoetes histrix	159	2,4				0.19	0.05	0.05	0
	Daucus carota	1 +	3,4			X
	gadecaei
	Solidago	23	3,4						X
	virgaurea
	rupicola
	Silene dioica	3 +	4			X
	zetlandica
	Ophioglossum	195	4				0.02	0.05	0.13	0.06
	lusitanicum
	Centaurium	49	4				0.13	0.11	0	0
	maritimum
	Cytisus	28*	4						X	(X)
	maritimus
	Scilla verna	75*	4				0.50	0.53	0.58	0.28	X
	Solanum dulcamara	8	4		X
	marinum
	Heterodermia	53	5				0.13	0.11	0	0
	leucomelos
	Teloshistes	40					0.06	0	0	0
	flavicans

+ : minimum number, due to problem with field determination ; * Species with spread distribution : plots correspond to 250x250m squares and not to populations ;

Table 2 : First five axes of the principal component analysis performed for floristic composition

 2

		PC1	PC2	PC3
	% Variance	18,1	13,7	11,5
	Cumulative %	18,1	31,7	43,2

Table 3 : Result of a Discriminant Analysis performed on 137 short grassland samples using six variables.

 3

		In classification matrix	In jack-knife classification matrix
	Trampling intensity None Low Medium	None	Low	Medium to
				to high			high
	None	47	0	1	41	6	1
	Low	1	15	3	5	8	6
	medium to high	0	0	70	0	4	66

Table 4 : Species contributing significantly to group discrimination under 95% simultaneous confidence intervals, using the Sidak's test of SPLUS package.

 4

	Species

Table 5 : Changes in species composition of natural short grassland along a gradient of trampling intensity. Φ is the proportion of group 1 species still present in group 2 and γ is the proportion of group 2 species present in group 1, with group 1 (lines) being less trampled than group 2 (columns).

 5

	Φ

Table 6 : Changes in species composition of maritime grasslands along a gradient of trampling intensity. Φ is the proportion of group 1 species still present in group 2, γ is the proportion of group 2 species present in group 1, with group 1 (lines) being less trampled than group 2 (columns).

 6

	Φ

Acknowledgements

We are especially grateful to Sylvie Magnanon and Marion Hoden from the Conservatoire Botanique National de Brest, Frédéric Bioret (Professor at Brest University) and Louis Masse (Professor at Botanical laboratory, Rennes I University) for their help with species identification. We are also grateful to Francoise Gourmelon, Mathias Rouan and Manuel Giraudet for their assistance with GIS. Vincent DeVictor provided helpful comments on the use of COMDYN software.


Christian Kerbiriou
email: kerbiriou@mnhn.fr

Bruno Bargain

Isabelle Le Viol

Sandrine Pavoine

DIET AND FUELLING OF THE GLOBALLY THREATENED AQUATIC WARBLER ACROCEPHALUS 1 PALUDICOLA AT AUTUMN MIGRATION STOPOVER AS COMPARED WITH TWO CONGENERS

Keywords: diet, Acrocephalus paludicola, Acrocephalus scirpaceus, Acrocephalus schoenobaenus

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

A decline in long-distance migratory songbirds has been repeatedly observed. The causes of this decline are numerous: climate change [START_REF] Both | Climate change and population declines in a long-distance migratory bird[END_REF], degradation of wintering, breeding habitats [START_REF] Robbins | Population declines in North American birds that migrate to the Neotropics[END_REF], or loss and fragmentation of stopover site (Huotto, 1998). Recently, the vital importance of the presence and quality of migration stopover sites to en route songbirds has come to the forefront of avian conservation [START_REF] Petit | Habitat use by landbirds along nearctic-neotropical migration routes: implication for conservation of stopover habitats[END_REF]. Longdistance migration requires exceptional reserves. Migratory songbirds must rest and deposit fat reserves in restricted stopover. There, the often high density of birds together with heavily depleted food supplies lead to a severe competition both within and among species [START_REF] Newton | Population limitation in migrants[END_REF]. Accordingly, it is known that high-quality habitats at stopover sites and a preserved network of stopovers should be considered an essential component of strategies for the conservation of migratory bird populations [START_REF] Ktitorov | The importance of landscape context for songbirds on migration: body mass gain is related to habitat cover[END_REF][START_REF] Newton | Population limitation in migrants[END_REF]. However, the quality value of a site may differ among species and the reserve managers need explicit recommendations.

The aquatic warbler (Acrocephalus paludicola) is a rare long-distance migratory bird species and is considered one of the most threatened western Palaearctic migratory passerines [START_REF] Collar | Birds to Watch 2. The World List of Threatened Birds[END_REF]. Its populations suffered an important decline mainly due to the loss in their breeding habitat [START_REF] Dyrcz | Breeding statistics of the Aquatic Warbler Acrocephalus paludicola on the Biebrza marshes, northeast Poland[END_REF][START_REF] Kozulin | Factors affecting fluctuations of the Aquatic Warbler Acrocephalus paludicola population of Byelarussian mires[END_REF]. In addition, like many insectivorous birds that breed in northern Europe and winter in sub-Saharan Africa, aquatic warbler crosses wide ecological barriers, which requires long uninterrupted flights fuelled by large fat deposits. The migration strategy includes departure date, flight duration, habitat and diet selection, and is known to be under considerable selection pressure [START_REF] Bairlein | New Aspects on Migratory Physiology of Trans-Saharan Passerine Migrants[END_REF]. Northern aquatic warbler populations migrate through western Europe in autumn, chiefly visiting marshes in the Netherlands, Belgium and western coastal regions of France.

France hosts the largest number of individuals in migration [START_REF] Julliard | Identifying autumn migration routes for the globally threatened Aquatic Warbler Acrocephalus paludicola[END_REF]. However, important, rapid losses in marsh areas have occurred on its migratory route: 50% of marsh areas in France were lost in the 1970-1990 period [START_REF] Bernard | Rapport d'évaluation sur les politiques publiques en matière de zones humides[END_REF]); 40% of freshwater wetlands were destroyed or degraded in the Netherlands in only a 10-year period [START_REF] Holland | Wetland degradation and loss in the rapidly urbanizing area of Portland[END_REF].

As highlighted in the European Action Plan [START_REF] Heredia | Action Plan for the Aquatic Warbler (Acrocephalus paludicola) in Europe[END_REF], the effective conservation of these threatened migratory passerines requires a thorough description of its ecological needs at stopover sites. Yet, to the extent that we are aware, few studies have analysed aquatic warblers' diet, and these studies have focused on the breeding period only. Unfortunately, the ecological needs and the network of stopover sites of aquatic warbler cannot be derived from information on congeners, as species within the Acrocephalus genus can exhibit very different migration strategies [START_REF] Bibby | Autumn migration strategies of Reed and Sedge Warblers[END_REF]. The direct observation of aquatic warbler feeding on stopover is hardly possible due to the rarity of this bird and to poor visibility in marsh habitats. In addition, indirect studies of diet through faeces analysis are hindered by differential prey digestibility between preys. To circumvent these difficulties, we chose to compare faeces of aquatic warbler and two more common congeners known to exhibit differential strategy (reed warbler, Acrocephalus scirpaceus, and sedge warbler, Acrocephalus schoenobaenus) within the same stopover area. This comparison revealed diet specificities of aquatic warbler with the underlying assumption that digestibility bias is equal among the three closely related species. We then identified the taxa that made a major contribution to the diet of each species and the taxa that distinguished the diet of aquatic warbler from the two other warblers. In addition, we studied the correlations between aquatic warbler's main prey and habitat.

The strategies that underpin long migratory distances differ among species. Some birds -such as reed warblers -are known to move in many short steps, others -like sedge warbler -negotiate the same distance in a few jumps with very long flights [START_REF] Bibby | Autumn migration strategies of Reed and Sedge Warblers[END_REF][START_REF] Bensch | Autumn Migration Speed of Juvenile Reed and Sedge Warblers in Relation to Date and Fat Loads[END_REF]. Consequently, physiological requirements and ecological and time constraints are different. Indeed, moving in a series of short flights requires smaller fat reserves on board. The comparison of mass gain during stopovers between aquatic, reed and sedge warblers is thus expected to inform us on strategies underlying long migratory distances. This information is of conservation concern because moving in a series of short flights requires many different suitable stopover sites en route. In this case, the removal of one site is less tragic, as these 'hoppers' can easily move to the next site. However, for species exhibiting long-haul flights, the disappearance or degradation of a critical stopover site would seriously impair migration.

METHODS

Focal species

Aquatic warbler is a globally threatened species [START_REF] Collar | Birds to Watch 2. The World List of Threatened Birds[END_REF] whose breeding range shrank dramatically during the last decades. The species disappeared from its former breeding grounds in Austria, Belgium, France and the Netherlands [START_REF] Bargain | Phragmite aquatique Acrocephalus paludicola[END_REF]. The European population comprises 13,000 to 21,000 singing males, which were mostly found in Belarus, Ukraine and Poland (Aquatic Conservation Team, 1999). Despite yearly fluctuations, there is strong evidence that the aquatic warbler population keeps declining in Europe (Birdlife International, 2004).

Study area

The study was carried out in the Audierne marsh (western France,W4°19'14,0229 N47°55'15,0881). Three main vegetation types dominated the landscape from the coast to the inland: reed bed, fen mire and hygrophilous meadow. Reed beds surrounded the coastal lake and were dominated by common reed Phragmites australis; the water table was above ground level for most of the year. Fen mire comprised medium herbaceous vegetation (up to 1 m) and, in summer, the water table was only a few centimetres above ground level and sometimes dried up. Fen mires were dominated by numerous plant species including Scirpus spp, Juncus ssp, Eleocharis spp, Iris pseudacorus, Oenanthe spp. Hygrophilous meadows were grazed extensively and were dominated by Agrostis spp and Dactylis glomerata.

We performed standardised mist-netting between 1988 and 2006 (same mist-net type, localization and functioning period), which resulted in the capture of up to 60,000 sedge warblers, 26,000 reed warblers, and 1,200 aquatic warblers (for more details on the method used see [START_REF] Bargain | Importance des marais de la baie d'Audierne (Bretagne) pour la migration du phragmite des joncs Acrocephalus schoenobaenus[END_REF]. Due to technical constraints, i.e. mist netting could not be set up in fen mires or meadows, we were only able to capture aquatic warbler on reed beds, however mist net were localized close to fen mire: less than 100 meters (for more detail on localization and habitat see [START_REF] Bargain | Importance des marais de la baie d'Audierne (Bretagne) pour la migration du phragmite des joncs Acrocephalus schoenobaenus[END_REF]. The Audierne marsh is known as an important national breeding ground for reed warbler, whereas sedge warblers hardly ever breed in these marshes. However, sedge warblers that transit at the site during migration period represent 2% of the European breeding population [START_REF] Bargain | Importance des marais de la baie d'Audierne (Bretagne) pour la migration du phragmite des joncs Acrocephalus schoenobaenus[END_REF]. Moreover, this area is likely to constitute a major world stopover for aquatic warbler [START_REF] Julliard | Identifying autumn migration routes for the globally threatened Aquatic Warbler Acrocephalus paludicola[END_REF].

Faecal analysis

The diet of the three warblers was assessed by faecal analysis. Between 2001 and 2004, we collected 128, 78 and 28 samples of aquatic, sedge and reed warbler faeces respectively (with just one faecal sample by bird), during ringing operations in August and September. In order to collect faeces, we placed birds in special bags with a plastic-coated bottom, fifteen minutes before their release. Identifiable chitinous fragments were counted in each sample with the aim to estimate the minimum number of individuals of each taxonomic group (e.g. four Odonata wings were counted as one individual). This method likely led to some bias in diet evaluation, since soft-bodied or small preys are less readily detected. However, Davies (1977) demonstrated that there is a strong correlation between prey remains in the faeces and the composition of the true diet in other insectivorous passerines.

Identifying the specificity of the aquatic warbler's diet

We first conducted a Canonical Correspondence Analysis (CCA; [START_REF] Palmer | Putting things in even better order: the advantage of canonical correspondence analysis[END_REF] in order to evaluate the contribution of each prey species to the diet composition of aquatic, reed and sedge warblers. Furthermore, we used the apportionment of quadratic entropy (APQE), an analysis which allows diversity decomposition according to a given hierarchy [START_REF] Pavoine | The apportionment of quadratic entropy: a useful alternative for partitioning diversity in ecological data[END_REF]. Here, the hierarchy comes from Acrocephalus faeces and prey species in each faeces. This analysis evaluates (1) whether the diversity in diet composition was higher among faeces within warbler species than expected randomly (within-species diversity in diet composition) and (2) whether it was higher between faeces among warbler species than expected randomly (among-species diversity in diet composition). The significance of this hierarchy was tested using the permuting approach (n= 1000). Given that diet data mostly came from one month in one year (Table 1) we restricted these analyses (CCA and APQE) to August 2003 diet data, although similar results were obtained with the full data set.

Relationship between aquatic warbler's prey and habitat

To increase our knowledge on aquatic warbler's foraging habitat selection, we combined three semi-quantitative invertebrate sampling methods among the three major habitats of the Audierne marsh: (1) we made a pitfall trap, with unattractive conservative liquid, in order to assess invertebrate density-activity in the ground. However, as pitfall traps collected few of aquatic warblers' preys, they were not detailed in this study. (2) We used a yellow bowl trap for invertebrates collected in a medium level of vegetation (2 stations per habitat, 1 bowl trap per station, collection after 4 days of operation, total of 15 samplings per habitat). ( 3) We performed a standardised sweep-net in order to collect invertebrates in the upper part of the vegetation (2 samplings per habitat, walking a 25-m distance, done the same day for the 3 habitats). Variations in prey abundance among habitats were assessed using a Student's t-Test with p-values adjusted for multiple comparisons using [START_REF] Hochberg | A sharper Bonferroni procedure for multiple tests of significance[END_REF] correction.

Comparing diet diversity of aquatic, sedge and reed warblers

We assessed prey richness within each warbler's diet, using faeces. Taking into account closeness in terms of phylogeny or mass, the fairly similar prey digestibility could be considered a robust assumption for the three warblers studied. However, equal detectability of all prey species is probably not met. For example, beetles are probably more detectable than Diptera. Hence, estimating diet richness using the classic cumulative curve approach is inappropriate. We therefore used statistical methods derived from capture-recapture approaches. However, instead of capturing individuals, we capture species; and instead of assessing population size, this approach provides an estimator of community size, here prey species richness. This method relies on a table with faeces samples as columns, species as rows and presence-absence as entries that constitutes the "capture histories matrix". This approach models richness with heterogeneous species detection probabilities. Prey species richness was estimated with the jackknife estimator [START_REF] Burnham | Robust estimation of population size when capture probabilities vary among animals[END_REF]. For more detail on methods see recent studies [START_REF] Selmi | Does time of season influence bird species number determined from point-count data? A capture-recapture approach[END_REF][START_REF] Lekve | Spatio-temporal dynamics of species richness in coastal fish communities[END_REF][START_REF] Kerbiriou | The impact of human frequentation on coastal vegetation in a Biosphere Reserve[END_REF] addressing richness estimation and detection probabilities from species count data and using COMDYN software (Hines et al., 1999). As reed warblers had the smallest faeces sample sizes, we performed 50 random re-samplings of faeces samples for each warbler to obtain identical sample sizes of faeces (n=10) (i.e. 50 "captures-histories" matrix constituted by 10 "captures" events) before the assessment of detectability and richness. Species richness between warblers was compared using Student's t-Test with p-values adjusted for multiple comparisons using Hochberg correction.

Comparing mass gain strategies during stopover

To compare mass gain strategies across the three warbler species, we analysed changes in body mass between capture/recapture events within a same year and stopover site. Between 1988 and 2006, ringing operations were conducted, during the post-breeding migration period: from early July to late September [START_REF] Bargain | Importance des marais de la baie d'Audierne (Bretagne) pour la migration du phragmite des joncs Acrocephalus schoenobaenus[END_REF]. Whenever weather permitted, the ringing station was opened for a total of 77 effective days per year (SE±4 days; extreme: 44; 115). Each captured bird was ringed and when safety time between capture and release was not overtaking, birds were weighed and aged (two classes: adult and young, i.e. born within the year). When birds were captured several times within a day, we retained the first measure only. For each bird captured more than once, we recorded the change in body mass between two capture events (the vast majority of individuals were recaptured only once, which generated one data point per individual). At the Audierne marshes, we collected a total of 6,724 body mass changes for sedge warbler, 6,470 for reed warbler and 47 for aquatic warbler. We used Generalised Linear Models (GLM, with F test in order to account for overdispersion), to analyse whether body mass change was explained by the number of days between two capture events. Important factors are known to affect body mass of bird in migration such as age. Moreover we expect changes in mass during a day or over the seasons [START_REF] Schaub | Stopover durations of three warbler species along their autumn migration route[END_REF]. In addition, the mass gain of insectivorous bird could also vary across years due to great variations in prey availability. In order to limit biases due to variations in bird mass in the daytime, we only considered data from 7 to 11 am. Indeed, during this period we did not detect any significant difference between the time of capture and the time of recapture (respectively for the sedge, aquatic and reed warbler, F1, 2480=0.39, P=0.53 ; F1,58=0.84 P=0.36 ; F1,3150=2.59, P=0.11). In addition, there was no significant interaction between the day and the time of the day (respectively for the sedge, aquatic and reed warbler F1, 629=1.53, P=0.18 ; F1,279=0.01, P=0.91 ; F1,694=174.38, P=0.15). The other factors, age, season (i.e. day of the year), and year were included in GLM modelling with each variable tested adjusted to all the other variables.

As possible differences in mass gain are expected between birds with different mass, we used relative mass gain (G') instead of gross mass gain to illustrate the relationship between mass change and stopover duration.

Mc Mc Mr G ) ( '  
Mc is the mass measured during the first capture and Mr is the mass measured during the recapture. In order to cure heteroscedasticity in GLM analyses we log transformed Mr and

Mc. In order to distinguish reed warbler breeders from migrants, we then used the same GLM analysis on birds for which the foreign origin was known (birds ringed during the breeding season in another country, n=23). For aquatic warbler, we used the entire national data in order to test the existence of regional differences in mass gain. Yet, complementary data came from Sandouville (W0°19'15 N49°29'51), Chenac-Saint-Seurin-d'Uzet (W0°49'58 N45°29'59) and Frossay-Le Massereau (W1°55'54 N47°14'41) where the same standardised mist-netting protocols were carried out.

RESULTS

Taxa that make a major contribution to the diet of aquatic, reed and sedge warblers.

In the faeces samples, we recorded a total of 1,731 prey items. In terms of prey abundance, the diets of aquatic and reed warblers were dominated by Diptera (38 and 54%,respectively) and aphids (21 and 22%, Table 2) whereas that of sedge warbler was dominated by aphids (67%), followed by Diptera (17%). Using a predictive model of the relationship between body length and invertebrate group mass [START_REF] Ganihar | Biomass estimates of terrestrial arthropods based on body lenght[END_REF], the contribution of Odonata, Araneida, Orthoptera, Diptera and Lepidoptera to consumed biomass was 43%, 13%, 12%, 9% and 8% respectively for aquatic warbler. For reed warblers, Diptera represented 33% of consumed biomass, aphids 16% and Hymenoptera 15%. For sedge warbler, aphids represented 48% of consumed biomass, Odonata 12%, and Diptera 10% (Table 2).

Taxa that underlined the specificity of aquatic warbler's diet, when considering abundance The Canonical Correspondence Analysis (CCA) approach revealed that Lepidoptera, Araneida, Orthoptera, Odonata, Coleptera, Atlidae contributed to distinguishing the aquatic warbler's diet from that of the two other warblers (Fig. 1). Aphids mainly contributed to the sedge warbler's diet while wasps and, to a lesser extent, flies contributed to the diet of reed warbler (Fig. 1). These differences in diet composition among warbler species were significant, as shown by the APQE analysis (P=0.001), whereas no significant variation in composition was detected among faeces samples within warbler species (P=0.91).

Availability of aquatic warbler's prey among habitats.

The availability of the five principal preys in terms of biomass (Odonata, Orthoptera, Araneida, Lepidoptera and Diptera) varied across habitats (Fig. 2). The abundance of Araneida species was significantly higher in fen mires than in pasture (P<0.001 whatever the sampling method) or in reed beds (P=0.04 for bowl trap and P<0.001 for sweep net). The abundance of Odonata was higher in fen mires than in pasture (P=0.002 for bowl trap and P=0.04 for sweep net), but did not differ from reed beds (P=0.06 for bowl trap and P=0.21 for sweep net). Orthoptera abundance was high in both fen mires and pasture but no difference could be detected between the two habitats whatever the method used (P=0.11 and P=0.71 for bowl trap and sweep net, respectively). No significant difference was found between habitats for Diptera abundance (P>0.20 whatever the sampling method used). Lepidoptera (moth) were almost exclusively collected in fen mires.

Diet diversity of aquatic, sedge and reed warblers

Significantly fewer preys were found in aquatic warbler faeces (4.9 preys per faeces sample; se=0.4) than in sedge warbler faeces (13.2 preys; se=1.7) (P<0.0001). Yet, no significant difference was found between the number of preys of aquatic warbler and reed warbler (6.2 preys; se=0.8) (P=0.52).

According to the species richness estimate assessed with jackknife estimator, the aquatic warbler had a less diversified diet (16.9 species; se=1.3; on average in 10 faeces) than the other two warbler species (reed warbler: 22.2 species; se=2.5; sedge warbler: 28.8 species; se=4.6; P=0.02 and P=0.007 respectively). The average detection probability was generally high (0.77; se=0.07 for aquatic warbler, 0.72; se=0.02 for reed warbler and 0.72; se=0.04 for sedge warbler) and not significantly different across warbler species (GLM, F2,147=1.58, P=0.20).

Finally, aquatic warblers consumed larger preys (average 9.2 mm; se=0.4) than reed (5.1mm; se=0.3; t value = 7.31, P<0.0001) and sedge warblers (4.6mm; se=0.3; t value = 4.57,

P<0.0001).

Differences in mass gain strategies of aquatic, sedge and reed warblers during stopover.

Significant differences in mass between age classes were detected for the tree warbler studied: 10.99g se=0.02 for young and 12.02g se=0.06 for adult GLM, F1,6709=448.44; P<0.0001 for sedge warbler; 10.97g se=0.01 for young and 11.20g se=0.03 for adult F1,6195=112.83; P<0.0001 for reed warbler; 11.31g se=0.03 for young and 11.78g se=0.17 for adult F1,1093=7.70; P=0.006 for aquatic warbler. The number of days between two capture events, significantly was influenced by the age class for sedge wabler (F1,6709=15.92; P<0.0001) and reed warbler (F1,6469=303.52; P<0.0001) but not for aquatic warbler (F1,46=0.22; P=0.65).

Except for the estimate of mean daily mass gain and figure 3, all the analyses were carried out on relative mass gain (G') with log transformation. No correlation between the relative mass gain and the number of days spent was detected for reed warbler (Table 3, Fig. 3). As there was probably a small proportion of local reed warbler breeders captured and recaptured that could have induced bias since they were not in migration behaviour (birds involved in late reproduction or in partial moult), we performed the same analysis on a subset of data including reed warblers known to be migrating due to foreign ring identities. Again, no correlation could be detected (F1,20=2.51; P=0.13 and, moreover, the trend was slightly negative -0,05g/days).

In contrast to reed warbler, the mass in sedge and aquatic warblers increased according to the number of days spent on the Audierne marshes migration stopover (Table 3, Fig. 3).

According to the linear regression between gross mass gain and time spent between capture and recapture, the mean daily mass gain was 0.21g se=0.01 for sedge warbler and 0.38g se=0.06 for aquatic warbler.

When all French data of aquatic warblers' mass gain are considered, no impact of year, season or age is detected (F16,68=1.33; P=0.20; F1,68=1.83; P=0.18 and F1,68=0.31; P=0.57, respectively). In addition, no variation among the main sites where aquatic warblers were captured (Audierne marsh, Sandouville, Chenac-Saint-Seurin-d'Uzet and Frossay/Le Massereau) were detected (F14,68=1.12; P=0.35). However, the same pattern of mass gain in relation to stopover duration as observed in Audierne is noted (F1,68=6.59; P=0.01).

Mass gain varied significantly across the years for sedge and reed warblers (Table 3). Yet, there was no sign of unconditionally good or bad years, as yearly differences depended on the species: daily mass gain was significantly larger in 1993, 2000, 2003 and 2004 for sedge warbler, but significantly lower in 1991, 1994, 2000, 2002, 2003 and 2005 for reed warbler.

DISCUSSION

Diet specificity

The diet composition of aquatic warbler observed at the migration stopover sites of Audierne marshes is similar to that observed by [START_REF] Schulze-Hagen | Brutzeitliche Beutewahl beim Seggenrohränger Acrocephalus paludicola[END_REF] in the species' breeding areas: the diet predominantly consists of Araneida, Diptera and Coleoptera (30%, 22% and 15% respectively in Schulze-Hagen's study and 14%, 38% and 6% in this study). Small numbers of larger prey species such as Orthoptera, Lepidoptera, Odonata are also reported in both studies. Both studies also concur on the average large size of prey: 9.2 mm at Audierne marshes vs. 8.4 mm [START_REF] Schulze-Hagen | Brutzeitliche Beutewahl beim Seggenrohränger Acrocephalus paludicola[END_REF]. [START_REF] Leisler | Oko-ethologische voraussetzungen für die entwicklung von polygamie bei rohrsängern (Acrocephalus)[END_REF] found 12.1-mm prey sizes at breeding sites. The major difference between the Schulze-Hagen et al. study and ours is the presence of caterpillars in the former study, whereas none were detected here, which is probably due to the scarcity of such prey in late summer when aquatic warblers visit the stopover site. Although large prey species (Odonata, Araneida, Orthoptera) are found in small numbers (25% of total preys) in the aquatic warbler's diet, they significantly contribute to the total biomass consumed (68%). These three large prey groups only represented 23 and 20% of consumed biomass for sedge and reed warbler, respectively. Due to the potential differences in prey digestibility, the value of this result is mainly qualitative and the strength of the result lies in the comparison between warbler species. Accordingly, diet of aquatic warblers differs only slightly between the breeding and the migration period but its diet is definitely different from that of the two other warblers.

Similarly, the diet composition of sedge warbler estimated at the stopover site of Audierne marshes matched previous studies. The large contribution of aphids was already observed in the diet of sedge warblers in various breeding areas [START_REF] Koskimies | Autumn migration strategies of the Sedge Warbler (Acrocephalus schoenobaenus) in Finland: a preliminary report[END_REF][START_REF] Leivits | Ulevaade roolindude randeuurimis programmi "Acroproject" töötulemustest 1987. aasta sügelsel[END_REF][START_REF] Chernetsov | Foraging strategy of the Sedge Warbler (Acrocephalus schoenobaenus) on migration[END_REF] and on migration stopover [START_REF] Bibby | Autumn migration strategies of Reed and Sedge Warblers[END_REF]. Furthermore, observed aphid outbreaks around the study site [START_REF] Bargain | Importance des marais de la baie d'Audierne (Bretagne) pour la migration du phragmite des joncs Acrocephalus schoenobaenus[END_REF] are consistent with years of increased mass gain. However, a lot of alternative preys have been inventoried, [START_REF] Chernetsov | Foraging strategy of the Sedge Warbler (Acrocephalus schoenobaenus) on migration[END_REF] including Diptera, Coleoptera, Hymenoptera, and Araneida, which is consistent with our results: among the three warbler species, the diet of sedge warbler presented the highest prey species richness estimate.

Reed warbler also exhibited a diverse diet, which was yet centred on Diptera and, to a lesser extent, Hymenoptera and aphids. This type of diet composition was also observed by [START_REF] Bibby | Autumn migration strategies of Reed and Sedge Warblers[END_REF], [START_REF] Evans | Population changes, body mass dynamics and feeding ecology of Reed Warbler Acrocephalus scirpaceus at Llangorse Lake, South Powy[END_REF], [START_REF] Grim | Effect of habitat on the diet of Reed Warbler (Acrcephalus scirpaceus) nestling[END_REF], [START_REF] Grim | An exceptionally high diversity of hoverflies (Syrphidae) in food of the Reed Warbler (Acrcephalus scirpaceus)[END_REF], Rguibi Idrissi et al. (2004).

Once again, average prey size in the reed warbler's diet measured in this study (5.1 mm) was close to that observed by [START_REF] Leisler | Oko-ethologische voraussetzungen für die entwicklung von polygamie bei rohrsängern (Acrocephalus)[END_REF], 5.4 mm, or Rguibi-Idrissi et al. (2004), 4.5 to 5.4 mm.

The major part of the prey biomass in the aquatic warbler's diet that was distinct from the diet of the two other warblers was recorded in fen mires rather than reed beds. Spider families found in the aquatic warbler's diet, such as Clubionidae, Araneidae, and Tetragnatidae, and the absence of Lycosidae or Gnaphosidae, indicated that aquatic warbler did not forage on the ground level of vegetation (according to the functional group requirements of the families described in literature; [START_REF] Duffey | A Population Study of Spiders in Limestone Grassland[END_REF][START_REF] Roberts | The Spiders of Great Britain and Ireland[END_REF][START_REF] Marc | Maintaining spider biodiversity in agroecosystems as a tool in pest control[END_REF].

Mass gain

In Audierne's marshes and three other French marshes, Aquatic warblers' mass gain strategies were very close to those of sedge warblers: they both exhibited a significant increase in body mass during their stopover, suggesting the accumulation of fat reserves.

Sedge warblers, which migrate earlier and more rapidly than reed warblers, seem to accumulate fat in northern France or southern England and fly almost directly to West Africa over Iberia. In contrast, reed warblers migrate more slowly, thus over a longer period and break up the journey by refuelling [START_REF] Bibby | Autumn migration strategies of Reed and Sedge Warblers[END_REF][START_REF] Bensch | Autumn Migration Speed of Juvenile Reed and Sedge Warblers in Relation to Date and Fat Loads[END_REF].

Nevertheless, results from other stopover sites would be necessary to conclude that the aquatic warbler conducts a few-stop migration strategy as sedge warbler.

Conservation concerns

As regards the diet specificity of aquatic warbler, the choice and management of protected stopover areas for this species cannot only be based on existing knowledge on sedge and reed warblers. Moreover, according to the possible mass gain strategy and our initial knowledge on the stopover network of aquatic warbler (important refuelling and few migration stopovers), this species is thus expected to be more impacted by the degradation or loss of any important refuelling stopovers during migration. The current stopover known to be used by the aquatic warbler are thus of great importance for the conservation of this species. During the nesting period, the aquatic warbler is a habitat specialist species, preferring fen mires characterised by a mesotrophic level, a water table near the soil surface and intermediate vegetation height and density [START_REF] Kozulin | Breeding habitat, abundance and conservation status of the Aquatic Warbler Acrocephalus paludicola in Belarus[END_REF]Kloskowski & Krogulec, 1999;Kovacs & Végvari, 1999;[START_REF] Schaefer | Spatial behaviour in the Aquatic Warbler (Acrocephalus paludicola) during mating and breeding[END_REF][START_REF] Kozulin | Factors affecting fluctuations of the Aquatic Warbler Acrocephalus paludicola population of Byelarussian mires[END_REF]. As aquatic warbler are capture in reedbed certainly this vegetation plays a role for stopover, however our study underlined that higher abundance of several prey species occurs in fen mires. In addition, the first results found in France with radio-tagged birds in stopover migration also indicated that fen mires are very used by aquatic warblers (Provost et al. in prep.). This habitat plays an important role in allowing the complete life cycle of aquatic warbler's prey. Fen mire vegetation maximises the abundance of large Orthoptera prey Conocephalus discolor [START_REF] Baldi | Orthopteran assemblages as indicators of grassland naturalness in Hungary[END_REF][START_REF] Szövényi | Qualification of grassland habitat based on their Orthoptera assemblages in the Köszeg Mountains (W-Hungary)[END_REF] this study) and the densities of Clubionidae and Tetragnathidae [START_REF] Cattin | The impact of mowing as a management strategy for wet meadows on spider (Araneae) communities[END_REF] this study).

However fen mires in western European coast (i.e. the aquatic warbler migration route) are localized at the margin of reed beds due to hydrological constraints. The main threat for these small areas of fen mires is firstly direct human destruction such as drainage and agriculture (pasture or maize culture). A second threat is the encroachment of shrubs in marsh edge and reed vegetation of open wetlands (Kloskowski & Krogulec, 1999). In European Atlantic stopover sites, mostly comprising large areas of common reed, conservation measures should therefore aim at maintaining areas of medium vegetation height (50-100cm). Restoration management, such as clearing, should focus on marsh edges which are often colonised by shrub willow associated with common reed. However, reed cutting, especially cutting for commercial reasons, appears to affect the arthropod communities with, for instance, observed decreases in some passerine birds' prey, such as Coleoptera and Araneida, together with increases in other prey, such as aphids [START_REF] Schmidt | Reed cutting affect arthropod communities, potentially reducing food for passerines birds[END_REF]. To minimise negative effects, reed cutting should be restricted to small areas, connected with uncut areas, thereby allowing arthropod recolonisation [START_REF] Schmidt | Reed cutting affect arthropod communities, potentially reducing food for passerines birds[END_REF]. In addition, the creation of small ponds near reed beds is expected to provide habitat patches with exceptional densities of Diptera [START_REF] Brunel | Les Dolichopodidés et les Syrphidés des îles de la « réserve de Biosphère » d'Iroise (Finistère, France) : premier inventaire (Diptera)[END_REF] and Odonata.

Figure 2 :Figure 3 :

 23 Figure 2: Variation of abundance of the main Aquatic Warbler prey categories among the three main habitats (units: number of individuals collected, A: bowl trap, B: sweep net, errors bars represent standard errors, left axis represent Diptera abundance).

Table 1 :

 1 Number of faecal samples collected for each Warbler species across months and years in Audierne marshes.

		Aquatic Warbler Sedge Warbler Reed Warbler
	2001 August	9	1	-
	September	-	1	-
	2002 August	11	-	-
	September	12	-	-
	2003 August	50	64	21
	September	11	3	2
	2004 August	32	8	5
	September	3	1	-
	Total	128	78	28

Table 2 :

 2 Percentage of each arthropod group found in faeces samples of Aquatic, Sedge and Reed Warble. For each group of taxa, the percentages of biomass are given in brackets. Specificity of each Warbler's diet assess using a Canonical Correspondence Analysis, ordination of preys [Axis 1 (28%), Axis 2 (5%)]. AraInd, Araneida indeterminate; AraClu, Araneida Clubionidae; AraTet, Araneida Tetragnathidae; ColInd, Coleoptera indeterminate; ColAlt, Coleoptera Altisidae; ColCar, Coleoptera Carabidae; ColCur Coleoptera Curculionidae; DipInd, Diptera Indeterminate; DipDol, Diptera Dolichopodidae; DipSyr, Diptera Syrphidae; HetInd, Heteroptera indeterminate Heteroptera; HetHyd, Hydrometra stagnatorum; HomAph, Homoptera Aphid; HomCic, Homoptère Cicadelloidae; HymInd, Hymenoptera indeterminate; hymChr, Hymenoptera Chrysidae; HymIch, Hymenoptera Ichneumonidae; LepInd, Lepidoptera indeterminate; ZygIsc Odonata Coenagrionidae; OrtCon, Orthoptera Conocephalus discolor;

	Aquatic	Sedge	Reed
	Warbler	Warbler	Warbler

Acknowledgments

The mist-netting operation on the Audierne marsh was only made possible thanks to the help of many volunteers of the 'Bretagne Vivante' association, as well as volunteer bird ringers. Furthermore, we wish to thank P. Frebourg, G. Le Guillou, F. Baroteaux, Y. Beauvallet, O. Benoist, B. Caillaud, L. Demongin, B. Dumeige, P. Frebourg, P. Gautier, G. Goujon, C. Ingouf, G. Le Guillou, J. Pigeon, J. Pourreau, J. Pineau, P. Provost, ringers working for the Museum, who provided aquatic warbler data from other places than the Audierne marsh. Many thanks to Olivier Dehorter for the extraction of these data from the Museum ringing database. Finally, we thank Frédéric Jiguet, Romain Julliard, Jean-Pierre Moussus, Emmanuelle Porcher, Michael Schaub and two anonymous reviewers for providing us with sound advice on an earlier version of this manuscript. This study received financial support from the French Ministry of Ecology and the French National Centre for Scientific Research, and is part of the LIFE project 'Conservation of the Aquatic Warbler in Brittany', No. LIFE04NAT/FR/000086REV.