Large, undisturbed forests support greater biodiversity 

A recent investigation shows that large, unbroken forests support more biodiversity than fragmented landscapes.

While ecologists have long known that habitat loss contributes to declines in biodiversity within forest remnants, a debate persisted about whether conserving several smaller plots or one larger tract is more beneficial. 

This study – published in the journal Nature and directed by University of Michigan (U-M) ecologist Thiago Gonçalves-Souza – takes a definitive stance, indicating that fragmentation undercuts biodiversity on multiple levels.

“Fragmentation is bad,” said study co-author Professor Nate Sanders. “This paper clearly shows that fragmentation has negative effects on biodiversity across scales. 

“That doesn’t mean we shouldn’t try to conserve small fragments when we can with our limited conservation dollars, but we need to be wise about conservation decisions.”

Global perspective on fragmentation

The experts examined 4,006 species of vertebrates, invertebrates, and plants gathered from 37 sites worldwide, thereby generating a global overview of biodiversity differences between continuous and fragmented forests. 

The findings suggest a significant drop in species diversity within fragmented zones, illustrating a 13.6% decline at smaller (patch) scales, and 12.1% at broader (landscape) scales.

Additionally, the researchers noticed that generalized species – those flexible enough to thrive in a wide range of settings – tend to dominate in fragments. In contrast, more specialized organisms flourish in intact forests.

Alpha, beta, and gamma diversity

To illustrate the varied impacts of fragmentation, the team looked at alpha, beta, and gamma diversity. Alpha diversity refers to how many species occur in a single piece of forest, whereas beta diversity measures how different species compositions are from one patch to another. 

Gamma diversity represents total biodiversity across a larger region, whether that region is made of disjointed fragments or an uninterrupted forest expanse.

“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 Gonçalves-Souza.

“They say the opposite for large tracts of land: because this is a continuous and homogeneous patch, the species composition is too similar.”

Previously, much research only covered one aspect of diversity or compared a few continuous forests against numerous fragments, potentially skewing the findings. The new study tackled this imbalance with thorough analyses and corrected for variations in sampling.

“One reason that this has been such a long-standing and unresolved debate is that we simply have not had the appropriate data and statistical tools to systematically evaluate the question at both smaller and larger scales,” noted Jonathan Chase, a professor at the German Center for Integrative Biodiversity Research.

Fragmentation’s clear impact

Gonçalves-Souza and colleagues found that fragmentation lowers species numbers across all groups, and that heightened beta diversity in smaller forest fragments does not compensate for the overall loss of species variety at the wider landscape scale.

“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 Nick Haddad, a researcher at Michigan State University.

Alongside reduced biodiversity, scientists argue that fragmentation can also threaten carbon sequestration in forests.

“People are also comparing these two situations and finding that we are losing the ability for landscapes to store more carbon in fragmented landscapes,” Gonçalves-Souza said. “Fragmented landscapes are not only going to affect biodiversity by decreasing alpha and gamma diversity, but it also has implications for carbon stock as well.”

Rethinking future conservation efforts

Ultimately, Gonçalves-Souza hopes that the study’s results will move the conservation community beyond debates about whether multiple small fragments or a single extensive forest is preferable. Instead, the authors push for forest restoration as a top priority.

“I don’t know if it’s useful to think about continuous vs. fragmented landscapes. We need to protect biodiversity and I think this debate is not helping to actually support conservation,” he said.

“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.”

With many forest ecosystems already facing pressures- from logging and agriculture to climate change – restoration may be the pivotal step to ensure that habitats regain some of their lost ecological complexity.

By renewing expanses of continuous forest wherever possible, conservationists can maximize carbon storage while also supporting the wide variety of species needed for healthy, enduring ecosystems.

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