|Land snail richness and abundance along a sharp ecological gradient at two sampling scales: Disentangling relationships
|Oxford University Press
|Journal of Molluscan Studies
It is generally recognized that the number of land snail species increases with the number of individuals in samples. This correlation may be an artefact of sample size, but it may also reflect the similarity in microhabitat preference of many species. Biotic interactions (competition) might also influence this relationship: an upper limit to coexistence at a particular scale. Here, we model the relationships between number of species, abundance of live individuals and environmental factors in the snail faunas of treeless fens covering a broad range of calcium content. The models considered two scales of sampling: (1) a 12-l sample collected at a 4 × 4 m plot (187 samples) capturing at least 80% of the site's known species pool, and (2) a sampling of 25 × 25 cm plots (115 samples) usually capturing <40% of the site's species pool. At the larger scale, the number of species responded to the calcium gradient more tightly than did total densities, but this was reversed at the smaller scale. Variation in abundance explained about the half of the species richness variation at both scales, but the number of individuals collected was not limiting for the number of species recorded at the larger scale. Using simulated communities, we discovered that an asymptotic relationship between number of species and individuals can arise from a nested pattern of species distributions towards calcareous sites and thus that it may not be simply a rarefaction effect. The shape of a species accumulation curve may be only a weak indication of sampling efficiency when there is environmental variation among sample areas. Further, the observed hump-shaped relationship between the numbers of live individuals and species at the smaller scale raises a classical question about interspecific competition in land snail assemblages.
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