TROPHIC RESOURCE PARTITIONING AND COMPETITION BETWEEN THE 2 SIBLING BAT SPECIES MYOTIS-MYOTIS AND MYOTIS-BLYTHII

1. Niche theory predicts that the stable coexistence of species within a guild should be associated, if resources are limited, with a mechan ism of resource partitioning. Using extensive data on diets, the prese nt study attempts: (i) to test the hypothesis that, in sympatry, the i nterspecific overlap between the trophic niches of the sibling bat spe cies Myotis myotis and M. blythii-which coexist intimately in their ro osts-is effectively lower than the two intraspecific overlaps; (ii) to assess the role played by interspecific competition in resource parti tioning through the study of trophic niche displacement between severa l sympatric and allopatric populations. 2. Diets were determined by th e analysis of faecal samples collected in the field from individual ba ts captured in various geographical areas. Trophic niche overlaps were calculated monthly for all possible intraspecific and interspecific p airs of individuals from sympatric populations. Niche breadth was esti mated from: (i) every faecal sample; (ii) all the faecal samples colle cted per month in a given population (geographical area). 3. In every population, the bulk of the diets of M. myotis and M. blythii consiste d of, respectively, terrestrial (e.g. carabid beetles) and grass-dwell ing (mostly bush crickets) prey. All intraspecific trophic niche overl aps were significantly greater than the interspecific one, except in S witzerland in May when both species exploited mass concentrations of c ockchafers, a non-limiting food source. This clearcut partitioning of resources may allow the stable, intimate coexistence observed under sy mpatric conditions. 4. Relative proportions of ground-and grass-dwelli ng prey, as well as niche breadths (either individual or population), did not differ significantly between sympatry and allopatry, showing t hat, under allopatric conditions, niche expansion does not take place. This suggests that active interspecific competition is not the underl ying mechanism responsible for the niche partitioning which is current ly observed between M. myotis and M. blythii.