- The scientific community has been divided since the 1970s as to which sort of forest offers more protection for biodiversity: a set of many small patches of forest, or a single large tract?
- A newly published study has rekindled the debate, backing the thesis that large expanses of green space are more important for species conservation, particularly for larger animals that require a more extensive range.
- The debate could help policymakers better direct conservation efforts and funding, but researchers agree that all standing forest, regardless of size, must be protected.
Which provides more shelter for species biodiversity: a large, continuous tract of forest, or a number of small forest fragments that add up to the same land area? This question has divided the scientific community for more than 50 years, and a recently published study led by a Brazilian scientist has brought the topic back to the table.
“It’s a debate dating back to the 1970s when researchers were trying to define the best way to plan conservation units [protected areas] so they would protect biodiversity,” says Thiago Gonçalves-Souza, a biologist at the University of Michigan, U.S., who authored the study together with more than two dozen other researchers from eight different countries.
Some scientists say a region with many small forest fragments can be just as rich or richer in species diversity than a single large tract of forest. This is because each of these fragments has unique characteristics that favor the development of the different species living there. When added together, the sum is greater than that found in a large, continuous green space.
“Those who defend this thesis say that, even with the loss of species on a local scale, the increase in heterogeneity between the different fragments would increase biodiversity in that region overall,” Gonçalves-Souza says.
However, his study supports the hypothesis that conservation of large tracts of forests is actually more efficient for protecting biodiversity. He and his colleagues analyzed data from 37 other studies done in landscapes spread across six continents, comparing species diversity found in large forest areas with that in small forest fragments.
The team concluded that there’s 12% less species diversity in fragmented areas of forest than in large, uninterrupted forests — even when summing up the fragments as a set. “We showed that the loss on a local scale is so great that the increase in biodiversity variation doesn’t make up for the loss,” Gonçalves-Souza tells Mongabay. “The landscape’s biodiversity as a whole is reduced, and this is extremely relevant.”
Lenore Fahrig, a biology professor at Carleton University, Canada, and one of the main proponents of the idea that small forest fragments do just as much as large blocks of forest, disagrees. In an email to Mongabay, Fahrig questions the methodology used by Gonçalves-Souza and team, saying “his results could be used wrongly to suggest that small remnants hold little importance for biodiversity conservation. This could lead to the loss of protection for smaller remnants.”
Those championing the “bigger is better” thesis say that certain animals like large wildcats need vast territories to hunt and reproduce. As the green space shrinks, so does their population, until they’re replaced by more versatile species like rodents and possums.
“One jaguar alone needs thousands of hectares to raise a family, and a certain number of families are necessary to avoid inbreeding,” says Fernando Guedes Pinto, CEO of SOS Mata Atlântica, a conservation nonprofit that focuses on the Atlantic Rainforest, one of Brazil’s most deforested biomes. “There need to be thousands of hectares for this to happen — and that’s very hard to come by in the Atlantic Rainforest.”
Subject to predatory exploitation since Portuguese colonizers arrived in the 16th century, the Atlantic Rainforest is one of Earth’s most biodiverse biomes, which at one time covered 130 million hectares (321 million acres) across Brazil, Paraguay and Argentina. Today, only 24% of the original cover remains standing in Brazil, 97% of which is composed of small, isolated fragments no larger than 50 hectares (about 120 acres) each.
“The rainforest has been cut down and fragmented over time,” says Pinto, who wasn’t involved in the recent study. “The large tracts that remained are in areas that can’t be farmed, like the Coastal Range in between the state of Paraná up through São Paulo and Rio de Janeiro.”
These “islands” of standing forest are also more susceptible to degradation because they tend to border highways, cities or farms, making them vulnerable to invasions, wildfires and variations in temperature and humidity. “The periphery of the forest isn’t appropriate habitat for more ecologically demanding species,” Pinto says.
The importance of understanding which areas are home to more biodiversity helps inform policymakers on how and where to direct always-scarce conservation funding. But according to Lucas Ferrante, a researcher at the University of São Paulo (USP) and the Federal University of Amazonas (UFAM), the debate can’t be reduced just to the size of the area to be protected.
“I think it’s minimalist to defend either a single large area or many small ones as an ideal conservation model,” Ferrante, who wasn’t involved in the recent study, tells Mongabay. “Exactly why is it that we create a conservation unit? Is it to slow down deforestation in Amazonia? In that case, placing large conservation units in strategic locations would be the best option.”
But, he goes on, “If it’s to protect endangered species, I have to look at where these species live within the landscape. [There are] many species endemic to Brazil that today only live in small forest fragments.”
Gonçalves-Souza, who also works on raising funding for restoration projects inside biomes like the Atlantic Rainforest, says he agrees: “It’s not that fragments aren’t important.”
In 2024, the recuperated area within the Atlantic Forest was larger than the deforested area for the first time. “If we don’t recuperate what still exists, the loss of biodiversity will be much more drastic,” he says.
Banner image: Large felines like jaguars need large areas to hunt and reproduce. Image courtesy of Steve Winter/Panthera.
This story was first published here in Portuguese on June 16, 2025.
Citation:
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
Fahrig, L., Arroyo-Rodríguez, V., Bennett, J. R., Boucher-Lalonde, V., Cazetta, E., Currie, D. J., … Watling, J. I. (2019). Is habitat fragmentation bad for biodiversity? Biological Conservation, 230, 179-186. doi:10.1016/j.biocon.2018.12.026
Credits
