Abundance data of anuran species in forest fragments

Understanding the effects of random versus niche-based processes on
biodiversity patterns is a central theme in ecology, and an important tool
for predicting effects of habitat loss and fragmentation on biodiversity.
We investigated the predictive power of random processes to explain
species richness and species dissimilarity of amphibian assemblages in a
fragmented tropical landscape of the Atlantic Forest of South America. We
analyzed a large database of amphibian abundance and occupancy, sampled in
21 forest fragments ranging in size from 1.9 to 619 ha. We compared
observed species richness and species dissimilarity with the outcomes of
two null (random placement) models: 1- the traditional Coleman’s
area-based model and 2 – an abundance-based model (based on the number of
individuals observed in each fragment). We applied these models for all
species combined, and separately for forest‐dependent and
habitat-generalist species. The abundance-based model fitted the observed
species richness data better than the area-based model for all species,
forest-dependent species, and generalist species. The area-based and the
abundance-based models were also able to significantly explain species
dissimilarity for all species and for generalists, but not for
forest-dependent species. The traditional area-based model assigned too
many individuals to large fragments, thus failing to accurately explain
species richness within patches across the landscape. Although niche-based
processes may be important to structuring the regional pool of species in
fragmented landscapes, our results suggest that part of the variation in
species richness and species dissimilarity can be successfully explained
by random placement models, especially for generalist species. Evaluating
which factors cause variation in the number of individuals among patches
should be a focus in future studies aiming to understand biodiversity
patterns in fragmented landscapes.