Prevalence of multimodal species abundance distributions is linked to spatial and taxonomic breadth

Abstract

Aim Species abundance distributions (SADs) are a synthetic measure of biodiversity and community structure. Although typically described by unimodal logseries or lognormal distributions, empirical SADs can also exhibit multiple modes. However, we do not know how prevalent multimodality is, nor do we have an understanding of the factors leading to this pattern. Here we quantify the prevalence of multimodality in SADs across a wide range of taxa, habitats and spatial extents. Location Global. Methods We used the second‐order Akaike information criterion for small sample sizes (AICc) and likelihood ratio tests (LRTs) to test whether models with more than one mode accurately describe the empirical abundance frequency distributions of the underlying communities. We analysed 117 empirical datasets from intensely sampled communities, including taxa ranging from birds, plants, fish and invertebrates, from terrestrial, marine and freshwater habitats. Results We find evidence for multimodality in 14.5% of the SADs when using AICc and LRT. This is a conservative estimate, as AICc alone estimates a prevalence of multimodality of 22%. We additionally show that the pattern is more common in data encompassing broader spatial scales and greater taxonomic breadth, suggesting that multimodality increases with ecological heterogeneity. Main conclusions We suggest that higher levels of ecological heterogeneity, underpinned by larger spatial extent and higher taxonomic breadth, can yield multimodal SADs. Our analysis shows that multimodality occurs with a prevalence that warrants its systematic consideration when assessing SAD shape and emphasizes the need for macroecological theories to include multimodality in the range of SADs they predict.

Systematic variation in North American tree species abundance distributions along macroecological climatic gradients

“…However, recent work has indicated that a small proportion of empirical SADs are, in fact, multimodal (e.g. Antão, Connolly, Magurran, Soares, & Dornelas, ; Dornelas & Connolly, ; Matthews & Whittaker, ; Vergnon, van Nes, & Scheffer, ). For example, a recent synthesis of 117 datasets found significant evidence of multimodality in c .…”

“…For example, a recent synthesis of 117 datasets found significant evidence of multimodality in c . 20% of cases (Antão et al, ). A number of potential causes of multimodality in SADs have been put forward, such as the amalgamation of different types of species within a single sample (e.g.…”

“…core and satellite species; Magurran & Henderson, ; Matthews & Whittaker, ) and the increasing taxonomic breadth, sampling variation and spatial extent (i.e. increasing ecological heterogeneity; Antão et al, ) of a study. However, variation in the prevalence of SAD multimodality at large scales and across ecological gradients is largely unknown.…”

“…However, recent work has indicated that a small proportion of empirical SADs are, in fact, multimodal (e.g. Antão, Connolly, Magurran, Soares, & Dornelas, 2017;Dornelas & Connolly, 2008;Matthews & Whittaker, 2015;Vergnon, van Nes, & Scheffer, 2012).…”

The stretched exponential as one of the alternatives for the power law to fit ranked species abundance data

“…One is inclined to think so if there is a 'universal law'. Review [2] is steeped with the idea of a relatively simple equation for SADs, but it presents the opposite too, that different communities have rather different SADs and that groups of species within a community have different SADs, making the community's SAD a mixed one [5]. The SAD is mostly treated as a histogram, based on the binning of data into frequency classes (for a probability mass or density function).…”