When does habitat fragmentation matter? A biome-wide analysis of small mammals in the Atlantic Forest

Aim While habitat loss is a primary driver of biodiversity declines worldwide, the role of habitat fragmentation per se is inconclusive, but likely depends on the amount of habitat left in a landscape. Here we aimed to tease apart the effects of habitat amount (percentage of native cover) and a fragmentation metric (number of fragments) on species richness and total abundance. Taxon Native small mammals. Location South American Atlantic Forest biome. Methods Small mammal species richness and abundance were obtained from a published database for 96 localities (groups of sampling sites). We then defined circular 100 km(2) landscapes centred on each locality. For each landscape, percentage of habitat cover and number of fragments were measured on time frames close to the sampling periods. Effects of habitat amount, fragmentation and their interaction were modelled considering all landscapes, and also within four classes of habitat cover: 0%-10%, 10%-30%, 30%-50%, and 50%-100%. Results Species richness was mainly affected by percentage of habitat cover, with a three-fold effect size compared to fragmentation. Yet, in landscapes with <10% or >= 50% of remaining cover, fragmentation positively affected species richness. Total species abundance also increased towards more fragmented landscapes. At the species level, three of the 20 species considered increased in abundance with fragmentation, while four species decreased. Main conclusion Percentage of habitat cover was the main driver of species richness when the entire cover range is considered, but the secondary effects of fragmentation were strong at the extreme ends of this range. Adding habitat patches in landscapes with low cover, or promoting habitat heterogeneity in landscapes with high cover, may boost species richness. However, further increases in species richness following fragmentation in high-cover landscapes are likely to correspond to disturbance-adapted species. In addition, such positive effects of fragmentation cannot be presumed to apply to all assemblages and species as some species are negatively affected.