Understanding the processes of coexistence of species within local assemblage is one of the main questions addressed by the community ecologist (e.g. Pianka, 1973, Schluter and Grant, 1984; Kikkawa and Anderson, 1986, Neet and Hausser, 1990). Niche theory predicts that the stable coexistence of species is associated with differentiated utilization of the resource in their environment (see Schoener, Hutchinson 1959; Pianka, 1974; Schoener 1974, 1986, 1989; Jenkins and Wright, 1988). Historically, the interspecific competition has been seen as the main process determining the structure of the communities (see reviews by Connell 1983; Schoener, 1983). However, recent studies have provided less importance of this mechanism of organization (e.g. Tilman, 1987; Pratchett, 2005). They show the possibility of predominant effect played simultaneously by non deterministic processes as predation, parasitism and environmental stochasticity (Rosenzweig and Abramsky, 1997; Tokeshi, 1999; Tilman, 2004). Generally, community ecologists focus on a taxonomic subset, such as mammals, plants, or fishes, of the local community to investigate the niche segregation among species. Frequently, ecologists work on a guild (Root, 1967). A guild is a subset of taxonomically related species using the same resource, such as cervids, grasses, or butterflyfishes (Jenkins and Wright, 1988; Marks et al., 1991; Pratchett, 2005). Regarding local bat faunas through the world, we see that bats are packed assemblages of numerous morphologically similar species, with few outlying forms. Moreover, these assemblages seem to be stable and predictable through species morphology. Consequently, competition must play a minor role in bats’ communities, because strong morphological resemblance suggests utilization of similar niches (e.g. Aldridge and Rautenbach, 1987; Findley, 1993; Arita, 1997). Numerous studies have shown that the majority of bats species are opportunistic foragers which can exploit patchily distributed, ephemeral resources, such as temporally and locally abundant food, which permit to avoid competitive niche partitioning among the species. Therefore, bats seem to constitute assemblage under the control of no-deterministic phenomenon (Findley, 1993; Arita, 1997). Because of this particular vision of the structure of the bats communities, the comprehension of the mechanisms underlying the coexistence among similar bats species is a major issue of bat ecology. In this sens, morphological and echolocation studies can provide some evidence of the structure of bats communities (e.g. Norbert and Rayner, 1987; Aldridge and Rautenbach, 1987; Kalko, 1995, Arita, 1997). But these studies make indirect observation on the ecological requirements of the species. They consider characters which permit to exploit resource rather than resource exploitation. They don’t emphasize potential spatial partitioning within morphologically similar species (Arlettaz, 1999). These methods have numerous intrinsic limitations; however Saunder and Barclay (1992) could emphasize relationships between ecological traits and morphology or echolocation. Few studies have been done on closly related insectivorous bats species. The complex of the sibling mouse-eared bats species Myotis myotis and Myotis blythii is a rare example where simultaneous data on the dietary niche, habitat use and the foraging strategy are available (Arlettaz, 1995). Without these data, it is difficult to understand how bat communities are assembled (Kalko, 1995). Moreover the food resources exploited by the gleaning bat species are less abundant and more constant 6 Introduction than those of aerial foragers. Therefore, competition may again play a role between these species. Consequently, the mechanisms, which permit coexistence among morphologically similar species (interspecific competition versus species-specific adaptations), are not clear and require further investigations. The demonstration of niche segregation does not explain the processes which were at its origin (competition vs. non-deterministic evolutive processes). However if niche segregation could be related to morphological and/or sensory adaptations, evolutive process are more likely to occur than competition. Role played by interspecific competitions are difficult to emphasize (Arlettaz, 1999), but they are more likely to occur among sibling species or strong morphologically convergent species, such as Plecotus auritus and Myotis bechsteini. Plecotus auritus and Myotis bechsteini are morphologically and ecologically similar, syntopic and potential competitors, but they have never been studied sympatrically until now. In previous studies, these species were compared separately with the Natterer’bat Myotis nattereri, regarding to echolocation or morphology (Swift and Racey 2002; Siemers and Swift, 2006). No comprehensive data about trophic niches, habitat selection and roost selection are simultaneously available from these two convergent species. Our study emphasizes and tests the hypothesis of niche segregation between Plecotus auritus and Myotis bechsteini.
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