“If you can imagine walking into a huge, 1,000-kilometer square [386-square-mile] tropical forest … it’s moist and damp [with] rich soil and an overstory. You imagine walking into a 10-meter [33-foot] patch of forest and it’s just a totally different thing. It’s drier, it’s more open, it’s more harsh, and there’ll be far fewer species,” says Thomas Crowther, ecology professor at the Swiss Federal Institute of Technology Zurich (ETH Zurich).

Fragmentation, the process by which large areas of intact forest become broken up into smaller pieces, is increasing in most of the world’s forests, according to a new Science study authored by Crowther and other researchers at institutions in Switzerland, Australia, China, the U.S. and the UK.  

The study finds that more than 50% of the world’s forests became more fragmented between 2000 and 2020. Tropical forests fared the worst, with the findings indicating that up to 80% were fragmented over 20 years.

This has profound implications for global biodiversity and ecosystem health, Crowther says.

“[T]he scary thing is, even if we kept the same amount of forest area on the planet, if we’re turning those big intact ones into all the tiny fragments, we’re losing a lot of the ecological functionality.”

Fragmentation can happen in different ways and for different reasons. Shifting cultivation can pockmark intact forests with clearings. New roads into previously intact areas can bring miners or loggers, creating a dendritic pattern of forest loss. Development can eat away at a forest’s perimeter. Stands of trees might be chopped down or large swaths can completely disappear as wildfires sweep through.

The spatial layout and distribution of forest patches — as well as the overall amount of intact forest — is important for wildlife and ecological processes. Many species rely on connected forests for population movement and gene flow. Numerous studies have shown that fragmentation reduces biodiversity at various scales.

The new Science study finds that 51-67% of forests globally became more fragmented in just the first two decades of the 21^st century. That’s a more dire picture than some previous studies found.

“We were very sad to learn that fragmentation is much more widespread than previously thought,” Crowther says.

However, the study also finds that for tropical forests, protection was remarkably effective in keeping forests more intact. 

The growing threat of fragmentation

To get a nuanced picture of what was happening in forests, researchers grouped different ways of measuring fragmentation (metrics) to create three composite indices: a connectivity-based fragmentation index that considered how connected areas of forest were to each other, an aggregation-based fragmentation index that looked at how clustered they were and a structure-based fragmentation index that focused on the physical characteristics — size, number and length — of forest patches. Then they ran computational models using global forest cover data to figure out how these captured different fragmentation patterns.

The researchers found that the three indices didn’t always interpret landscape changes in the same way. Some were more sensitive to certain patterns of fragmentation than others.

The team also used a statistical technique called principal component analysis to look at how different metrics aligned with metapopulation capacity, a measurement that describes a landscape’s ability to support healthy wildlife populations. They found that the connectivity-focused index aligned most closely with metapopulation capacity, indicating that it better describes fragmentation in terms of how it impacts biodiversity, says Yibiao Zou, doctoral candidate at ETH Zurich and the Science study’s lead author.

The team used the three composite indices to calculate the percentage of forests that had become more fragmented between 2000 and 2020 in different biomes around the world.

Analysis using the connectivity-focused index indicated that 51-67% of forests globally and 58-80% of tropical forests showed increased fragmentation over the 20-year period.

In contrast, when the team ran its analysis using a structure-focused index, it found that 30-35% of forests showed increased fragmentation. That result is more closely aligned with previous estimates, the authors note, including a 2023 Nature Communications study, which, using structure-based metrics, found that more than 75% of forests had experienced declining fragmentation over the same period.

The results of the new Science study are “insightful,” Jun Ma, associate professor at Fudan University and first author of the 2023 Nature Communications study told Mongabay by email. However, he says that “the authors do not provide direct evidence linking connectivity indicators to species habitat conservation.”

Moreover, while the two studies used different methods and came up with different numerical values, both paint a broadly similar picture of fragmentation, Ma says. 

“Our work highlights the need to prioritize the protection of regions where fragmentation is intensifying, particularly in tropical forests,” he says.

Running the analysis with different metrics shows the benefits of using more than one way to capture fragmentation, Zou says. “Then we have a more comprehensive picture of the landscape change.”

Zou said the team will next be looking more closely at how measurements of fragmentation relate to changes in biodiversity and ecological functions.

 Drivers and solutions

The researchers also examined the causes of fragmentation in different regions. They found shifting cultivation accounted for more than 60% of fragmentation in the tropics, while forestry was the major driver in the temperate forests, and wildfires, followed by forestry, were major drivers in boreal regions.

In a hopeful note, the researchers were “unbelievably surprised” by the degree to which protected status kept tropical forests intact, Crowther says.

In the tropics, strictly protected areas showed 82% less fragmentation than similar non-protected areas; less strictly protected areas were less effective at preventing fragmentation but still showed a drop of 45%.

“There are an increasing number of good news stories that indicate that if we make decisions to protect nature, it actually can work,” Crowther says.

Banner image: In this Sep. 26, 2017 photo released by Ibama, the Brazilian Environmental and Renewable Natural Resources Institute, members a firefighter fighta a forest fire affecting Porquinhos Indigenous lands in Maranhao state in Brazil’s Amazon basin. Image by Felipe Werneck/Ibama via AP.

Citations: 

Zou, Y., Crowther, T., Smith, G., Ma, H., Mo, L., Bialic-Murphy, L., … Zohner, C. (2025). Forest fragmentation increased in over half of global forests during years 2000-202. Science. doi:10.5194/egusphere-egu25-9004

Ma, J., Li, J., Wu, W., & Liu, J. (2023). Global forest fragmentation change from 2000 to 2020. Nature Communications, 14(1). doi:10.1038/s41467-023-39221-x 

Gonçalves-Souza, T., Chase, J. M., Haddad, N. M., Vancine, M. H., Didham, R. K., Melo, F. L., … Sanders, N. J. (2025). Species turnover does not rescue biodiversity in fragmented landscapes. Nature, 640(8059), 702-706. doi:10.1038/s41586-025-08688-7

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