The effect of fragment area on site-level biodiversity

Habitat fragmentation accompanies habitat loss, and drives additional biodiversity change; but few global biodiversity models explicitly analyse the effects of both fragmentation and loss. Here we propose and test the hypothesis that, as fragment area increases, species density (the number of species in a standardised plot) will scale with an exponent given by the difference between the exponents of the species-area relationships for islands (z similar to 0.25) and in contiguous habitat (z similar to 0.15), and test whether scaling varies between land uses. We also investigate the scaling of overall abundance and rarefaction-based richness, as some mechanisms make different predictions about how fragment area should affect them. The relevant data from the taxonomically and geographically broad PREDICTS database were used to model the three diversity measures, testing their scaling with fragment area and whether the scaling exponent varied among land uses (primary forest, secondary forest, plantation forest, cropland and pasture). In addition, the consistency of the response of species density to fragment area was tested across three well represented taxa (Magnoliopsida, Hymenoptera and herptiles'). Species density and total abundance showed area-scaling exponents of 0.07 and 0.16, respectively, and these exponents did not vary significantly among land uses; rarefaction-based richness by contrast did not increase consistently with area. These results suggest that the area-scaling of species density is driven by the area-scaling of total abundance, with additive edge effects (species moving into the small fragments from the surroundings) opposing - but not fully overcoming - the effect of fragment area on overall density of individuals. The interaction between fragment area and higher taxon (plants, vertebrates and invertebrates), which remained in the rarefied richness model, indicates that mechanisms may vary among groups.