A test of the abundant-center hypothesis for stream fishes

The abundant-center hypothesis (ACH) provides a conceptual model for
predicting range-wide distributions of species abundance, suggesting that
abundance peaks in the center of the geographic range and declines towards
range edges. Empirical studies testing the ACH and its subsequent
derivations predominantly occurred in terrestrial systems and reported
mixed support. Moreover, none of these models consider the possibility of
multiple geographic areas of elevated abundance (which we refer to as
abundant cores). Naturally, dispersal limited species may exhibit multiple
abundant cores, requiring refinement of the ACH. We used fish species
abundances from 29,206 community monitoring surveys and weighted
geospatial kernel density estimation to identify the number of abundant
cores for 64 freshwater fish species. We regressed the number of abundant
cores against range size and body size to test if larger geographic
distributions and body sizes contain more abundant cores than smaller
distributions and body sizes. The two predictors are surrogates for
evolutionary age and dispersal ability, respectively, because older
species are generally associated with larger ranges, and large-bodied
fishes have greater dispersal ability than small-bodied fishes in
dendritic networks. For studied species, 43 exhibited multi-core
distributions, and 21 exhibited a single-core distribution. Species range
size, but not body size, was significantly and positively associated with
the number of abundant cores. The ACH was not a good descriptor of the
abundance patterns of most stream fishes we studied, suggesting that an
abundant center model may not be well-suited for freshwater fishes. Recent
geo-climatic events in evolutionary time have isolated populations of the
same species by a matrix of unsuitable habitat and/or hard dispersal
barriers, providing the basis for multi-core distributions. Biogeographic
and ecological mechanisms likely underpin observed multi-core patterns,
and our work indicates that the ACH and related concepts still present
opportunities for testing and refinement.