Chopping up a forest into smaller bits seems like a bad idea for biodiversity. It risks isolating animals, making it difficult for them to sustain large enough numbers to find a mate, preserve genetic diversity, or withstand a catastrophe such as a drought.
But for years, some scientists have argued that breaking a landscape into smaller pieces can be a boon for biodiversity. After all, real islands in the ocean are rich with unique species because isolated populations can follow different evolutionary paths. The Galapagos is a classic example.
The outcome of the debate is more than academic. If conservationists want to counter the biodiversity crisis, driven in large part by habitat destruction what’s the best way to do it: Protect large contiguous sections of land or a patchwork?
A new study in Nature has delivered a potentially decisive answer to the debate: “Fragmentation is bad,” said Nate Sanders, a University of Michigan ecologist and co-author. “This paper clearly shows that fragmentation as negative effects on biodiversity across scales.”
The long-running debate stems partly from different measurements of biodiversity. Think of them somewhat like different levels of magnification. There’s how many different species you have within a single patch of forest (alpha diversity), the number of species when comparing one patch to another (beta diversity), and the number of species across an entire landscape, whether it’s a big continuous patch of forest, for example, or a lot of forest “islands” that are all in the same area (gamma diversity).
While there is a broad consensus that fragmentation can reduce the number of species in a single patch (alpha), it’s possible that enough differences in the species found in individual patches might add up to greater biodiversity across a fragmented landscape then in a contiguous one.
“The heart of the debate is that people who argue that fragmentation isn’t so bad say that because you have isolated habitats, you have different species composition, which means at a large scale, it’s good. If they are different, we can assume that the gamma diversity is going to be higher,” said Thiago Gonçalves-Souza, the lead author and a postdoctoral researcher at the University of Michigan.
To test whether that scenario holds up in practice, a group of 28 scientists, people in Europe, South and Central America and Australia, compiled data from 37 studies of biodiversity spanning six continents and more than 4,000 species. That data enabled them to compare the number and kinds of species found in large sections of land (between 1,000 and 300,000 hectares) with the same measurements from a collection of smaller fragments from similar ecosystems nearby.
At the big, landscape scale, the scientists found that fragmented areas had, on average, 12% fewer species than contiguous areas, with a range of between a 10.7% and 18% deficit. The deficit remained significant even when accounting for differences in the amount of habitat.
That loss in biodiversity was similar to the amount of biodiversity lost to fragmentation at the alpha level (the individual patch level). Even at the intermediate scale, the scientists found that the supposed biodiversity bonus from islands vanished when taking into account the distance separating the pieces of land. When scientists counted species, the sampling locations were, on average, 22% further apart in fragments than in a continuous forest, increasing the likelihood they would find different organisms.
“This paper resolves a half-century old debate about how to conserve biodiversity in natural areas, one started by scientific luminaries including E.O. Wilson and Jared Diamond,” said co-author Nick Haddad, a researcher at Michigan State University.
Does this mean smaller fragments aren’t worth bothering with? Not necessarily, said Gonçalves-Souza. “In many, many countries there aren’t many large, intact forests remaining. Therefore, our focus should be on planting new forests and restoring increasingly degraded habitats. Restoration is crucial for the future, more so than debating whether it’s better to have one large forest or many smaller fragments.”
Gonçalves-Souza, et. al. “Species turnover does not rescue biodiversity in fragmented landscapes.” Nature. March 12, 2025.
Image: Wikimedia
