Abstract
Species abundance distributions (SADs) describe community structure and are a key component of biodiversity theory and research. Although different distributions have been proposed to represent SADs at different scales, a systematic empirical assessment of how SAD shape varies across wide scale gradients is lacking. Here, we examined 11 empirical large-scale datasets for a wide range of taxa and used maximum likelihood methods to compare the fit of the logseries, lognormal, and multimodal (i.e., with multiple modes of abundance) models to SADs across a scale gradient spanning several orders of magnitude. Overall, there was a higher prevalence of multimodality for larger spatial extents, whereas the logseries was exclusively selected as best fit for smaller areas. For many communities the shape of the SAD at the largest spatial extent (either lognormal or multimodal) was conserved across the scale gradient, despite steep declines in area and taxonomic diversity sampled. Additionally, SAD shape was affected by species richness, but we did not detect a systematic effect of the total number of individuals. Our results reveal clear departures from the predictions of two major macroecological theories of biodiversity for SAD shape. Specifically, neither the Neutral Theory of Biodiversity (NTB) nor the Maximum Entropy Theory of Ecology (METE) are able to accommodate the variability in SAD shape we encountered. This is highlighted by the inadequacy of the logseries distribution at larger scales, contrary to predictions of the NTB, and by departures from METE expectation across scales. Importantly, neither theory accounts for multiple modes in SADs. We suggest our results are underpinned by both inter- and intraspecific spatial aggregation patterns, highlighting the importance of spatial distributions as determinants of biodiversity patterns. Critical developments for macroecological biodiversity theories remain in incorporating the effect of spatial scale, ecological heterogeneity and spatial aggregation patterns in determining SAD shape.
Highlights
Species Abundance Distributions (SADs) describe the relative abundance of the species within a community
Our results suggest that the combination of S and N is not sufficient to determine SAD shape across spatial scales (White et al, 2012; Locey and White, 2013; Xiao et al, 2015), as the total abundance across the scale gradient did not exhibit any systematic effect on SAD shape
Our results strongly support the suggestion that both Neutral Theory of Biodiversity (NTB) and Maximum Entropy Theory of Ecology (METE) might be more adequate for smaller scales (McGill, 2010), while the traditional SAD distributions are unable to accommodate SAD shape variability with scale, as we have clearly shown with consistent empirical patterns across taxa, and for both marine and terrestrial habitats
Summary
Species Abundance Distributions (SADs) describe the relative abundance of the species within a community. Species Abundance Distributions Across Scales and community structure, and a key component of biodiversity theory and research. Multimodal SADs, i.e., with more than one mode of abundance can occur (Gray et al, 2006; Dornelas and Connolly, 2008). While this pattern had been mostly disregarded, a recent empirical meta-analysis showed that is multimodality more common than previously recognized, it is more likely to occur for communities encompassing larger spatial extents or with higher taxonomic diversity (Antão et al, 2017). Given the pivotal role of SADs in biodiversity research, these insights will further facilitate assessments of ongoing biodiversity change
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