How does fragmentation affect biodiversity? A controversial question at the core of conservation biology

By Richard B. Primack, Abraham J. Miller-Rushing, Vincent Devictor

“A [tree] which it has taken two centuries to perfect, rising by slow stages into the heavens, has this afternoon ceased to exist… Why does not the village bell sound a knell? … The squirrel has leaped to another tree; the hawk has circled further off, and has now settled upon a new eyrie, but the woodman is preparing [to] lay his axe at the root of that also.”  Thoreau in his Journal. 

Does fragmentation harm biodiversity? For many years, most conservation biologists have understood the answer to be “yes.” It seems obvious—fragmentation divides landscapes into smaller patches that support fewer species. Edge effects further erode the ability of small patches to support many species. The negative effects of fragmentation are taught to students in introductory biology, ecology, and conservation courses, and affect conservation strategies and management. 

Forest Fragment in Costa Rica.

However, as we describe in a recent editorial, Fahrig and co-authors recently argued that the evidence base for these ideas, recommendations, and actions is not as strong as many think, largely due to the confounding effects of scale, habitat amount, and fragmentation. Their analysis even suggests that in many cases fragmentation might enhance biodiversity. Other conservation biologists have strongly disagreed with these findings, arguing that habitat fragmentation does harm biodiversity.

Getting the answer right in this debate is critical because it allows us to (1) understand the consequences of roads, development, and other fragmentation-inducing human actions on biodiversity; and (2) prioritize the protection and management of lands in cases when deciding between protecting large intact landscapes or fragmented landscapes with the same total amount of habitat. 

This debate highlights the need to make sure we continue to investigate questions central to conservation and check the evidence supporting our understanding and decisions. We think this process is healthy for the field, especially if we can keep dialogues productive and respectful. 

Key papers in the debate are linked below:

Fahrig, L. (2017). Ecological responses to habitat fragmentation.

Fahrig, L. et al. (2019). Is habitat fragmentation bad for biodiversity?

Fletcher Jr, R. J. et al. (2018). Is habitat fragmentation good for biodiversity?