Resolving a 50-year debate: fragmentation decreases biodiversity on multiple scales

The debate over fragmentation has taken several forms over the past years, most notably questioning whether conserving a single large or several small habitat areas is best for biodiversity (the SLOSS debate) or whether habitat amount drives species richness rather than fragmentation. Because conservation efforts often must prioritize what habitat to conserve, understanding the extent to which fragmentation reduces biodiversity can better guide decision-making processes to maximize conservation benefits.

The key to resolving this debate is quantifying how patterns of biodiversity are related at different scales. At local scales, species diversity decreases in smaller, more isolated patches (⍺ diversity). But because fragmentation creates environmental heterogeneity and can limit dispersal, species turnover may increase between patches (β diversity). This increase in species turnover between patches might lead to higher species diversity across the landscape ( diversity). However, it remains unclear whether these relationships truly exist in fragmented landscapes, or whether they seem to exist simply because it’s difficult to measure fragmentation at multiple scales and appropriately control for confounding variables.

Recent research aimed to definitively determine fragmentation effects on ⍺, β, and diversity as a way to resolve these issues. Researchers used 37 datasets made up of over 4,000 taxa and distributed over 6 continents that recorded the number of individuals and species in both fragmented forest patches and nearby continuous (unfragmented) forest. Using these datasets, they calculated ⍺, β, and diversity for fragmented and continuous forests, accounting for the spatial distance between samples and sampling effort.

The study found that both ⍺ diversity and β diversity decreased in fragmented habitat compared to continuous habitat. Importantly, this trend held true even when accounting for habitat amount and other confounding variables, indicating that the effects of fragmentation, like edge effects or loss of connectivity, play a role in biodiversity loss in addition to habitat loss.

Reduced diversity in fragmented landscapes meant that changes in β diversity were unable to compensate for biodiversity loss in individual habitat patches. Although β diversity did increase in the fragmented landscapes, this was entirely due to the increased distance between samples in habitat fragments, rather than fragmentation-related processes like dispersal limitation. When accounting for these underlying sampling effects on diversity, fragmentation decreased diversity.

The main takeaway is that fragmentation reduces biodiversity at multiple spatial scales in ways that are quantifiable and unrelated to sampling techniques. Habitat fragmentation is widespread across the globe and increasing under human-modified landscapes, and understanding its effects is essential for prioritizing conservation strategies. While small patches of habitat can play important roles in conservation, conserving and connecting intact ecosystems is essential for maintaining biodiversity and supporting vital ecological processes.

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