Correlations and variance among species traits explain contrasting impacts of fragmentation and habitat loss on functional diversity

Context Understanding how landscape fragmentation affects functional diversity, defined as the distribution of functional traits in an assemblage, is critical for managing landscapes for biodiversity and ecosystem functions. Despite some scattered evidence, we lack a clear understanding of how patterns of fragmentation drive changes in functional diversity, and its relationship with species richness as habitat is lost from a landscape. Objectives To develop testable predictions about how landscape fragmentation, relative to the effects of habitat loss, impacts functional diversity and its relationship with species richness. Methods We used a spatially explicit metacommunity model that evaluates communities that vary in the distribution of response traits (traits involved in species responses to environmental change) and the correlation between response and effect traits (traits associated with species’ effects on ecosystem functioning). Results Compared to effects of habitat loss, relative effects of fragmentation on functional diversity increased as the variance in the distribution of response traits was high and the correlation among traits was high. Functional richness decreased faster than species richness in highly fragmented landscapes as habitat was lost. However, functional diversity remained unchanged or even increased in fragmented landscapes when either response and effect traits were not correlated (or weakly correlated), or when the proportion of generalist species with high dispersal capacities was high. Conclusions Compared to effects of habitat loss, the relative effects of fragmentation on functional diversity and species richness are more dependent on the type of community evaluated. A careful evaluation of the variance in the distribution of response traits within a community, as well as the correlation among response and effect traits, can help to determine when it is important to manage landscape fragmentation to protect functional diversity.