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Understanding effects of landscape fragmentation on functional diversity

Predicting how landscape fragmentation - the breaking apart of habitat - affects species richness and ecosystem functioning is a central question in conservation biology. For many years, it was assumed that maximizing the protection of taxonomic diversity was enough to promote ecosystem functioning. However, multiple evidence has shown that the range of functions provided by a community depends much more on functional diversity, defined as the diversity of functional traits across species, than on species richness. Despite the elegance of trait-based approaches, spatially explicit evidence showing how landscape fragmentation influences functional diversity is still a big research gap. This information is crucial if we want to understand how to maximize the conservation of multiple components of biodiversity in human dominated landscapes.

In a new study, we used a spatially explicit metacommunity model to develop testable predictions about how landscape fragmentation, relative to the effects of habitat loss, impacts functional diversity and its relationship with species richness. We evaluated 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).

Potential paths by which fragmentation and habitat loss can influence functional diversity. Both habitat loss and fragmentation affect species richness and functional diversity by filtering species that show more vulnerability to landscape change depending on their response traits. The effects of fragmentation and habitat loss are moderated by both the correlation between response and effect traits, as well as the variance in the distribution of response traits across species. Fragmentation can also mediate effects of habitat loss through processes related to patch isolation, changes in matrix permeability and edge effects (pathway a.).

We show that compared to effects of habitat loss, relative effects of fragmentation on functional diversity increased when (1) the variance in the distribution of response traits increased, and (2) the correlation between response and effect traits increased. When these two conditions were met, habitat loss and fragmentation acted strongly on functional diversity, independently of their effects on species richness. We also show that unfragmented landscapes with greater than ~ 30% habitat remaining tend to promote the persistence of high levels of species richness with a diverse set of functional trait values. However, high fragmentation does not always necessarily mean low functional diversity, as functional diversity can remain unchanged or even increase in fragmented landscapes when either response and effect traits are weakly correlated, or when there is an increase in the proportion of high dispersal, generalist species with unique effect traits. In real communities, these patterns emerge when some species are replaced by stronger competitors with a new suite of traits. High functional diversity in fragmented landscapes can also occur when there is high complementarity across patches that favours species with different traits. Our results may explain why there is some empirical evidence that multiple small patches may have similar conservation value compared to large patches of equivalent area.

Our model shows that 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. Future work testing these patterns across different regions and landscape contexts will help increase understanding of the mechanistic pathways through which fragmentation affects functional diversity.

More info: Suárez-Castro, A.F., Mayfield, M.M., Mitchell, M.G.E., Cattarino L., Maron M., Rhodes, J.R. 2020. Correlations and variance among species traits explain contrasting impacts of fragmentation and habitat loss on functional diversity. Landscape Ecol .

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