Mean landscape‐scale incidence of species in discrete habitats is patch size dependent

Aim: A pervasive negative relationship between the species richness of an assemblage and the mean global range size of the species it contains has recently been identified. Here, we test for an effect of habitat patch size on the mean landscape-scale incidence (estimating local range size) of constituent species independent of variation in richness. Location: Global. Time Period: Contemporary. Major Taxa Studied: Various. Methods: We devised a new standardized patch-scale metric, Mean Species Landscape-scale Incidences per Patch (MSLIP). Positive values indicate a patch contains more widespread (high-incidence) species than expected relative to their frequency in the landscape; negative values indicate the presence of more narrow-range (low-incidence) species. For 202 metacommunity datasets (archipelagos, habitat islands and fragments) we regressed MSLIP on patch size, testing four hypotheses: (i) no effect (zero slope), (ii) more widespread species in smaller patches (negative slope), (iii) more narrow-range species in smaller patches (positive slope), (iv) a composite effect (curvilinear/unimodal). Results: There was a generally negative curvilinear relationship between MSLIP and patch size, consistent with smaller and larger patches respectively containing more high- and low-incidence species than expected. At intermediate patch sizes, mean species incidence approximates that of the overall landscape. Relative richness mediated this relationship; species-rich sites of any size increasingly favoured low-incidence species, species-poor sites high-incidence species. Results were consistent for taxonomic groups, and metacommunity types, except aquatic habitat islands. Main Conclusions: The dependence of mean species incidence on both patch size and relative richness has implications for biodiversity and community assembly theory, or when seeking to understand ecological processes sensitive to species composition (e.g., ecosystem functioning, trophic web structures). We propose the ‘patch size where the MSLIP regression intersects zero’ as a benchmark distinguishing smaller from larger patches, with the expectation small patches preferentially support more high-incidence and large patches more low-incidence species.