Trophic niche size and overlap in temperate forest land snails are affected by their lifestyle and body size

Land snails represent a large diversity of species in the forest floor. Some species prefer to stay in the leaf litter, while others like to climb up vegetation. They are considered generalist herbivores/decomposers. Although the exact trophic position of most species is often difficult to determine, a low rate of trophic niche partitioning within an assemblage is assumed. We compared isotopic niche sizes and overlaps between and within four local land snail assemblages using stable isotope Bayesian ellipses. We found significant differences in trophic niche size (expressed by corrected standard ellipse area) as a function of body size and lifestyle. We hypothesised that the larger niche size of smaller and/or arboreal snail species is due to their limited mobility and the presence of spatially structured food resources. In contrast, a broad and similar mixture of leaf litter sources results in a homogeneous diet for individuals of larger, highly mobile, epigean species, i.e., smaller niche size. We documented multiple overlaps of isotopic niches between species within an assemblage, although arboreal species exhibited slightly different niche positions, indicating specific diets. When comparing all pairwise combinations, we found that isotopic niches of species using different lifestyle did not overlap, in contrast to species using similar lifestyle. In addition, the trophic niches of opportunists with respect to lifestyle were intermediate between those of species from the outermost categories. Our results fit the general concept of variable and overlapping trophic niches in co-occurring snail species. However, we were able to demonstrate differences in species that rarely occupy the same microhabitat.