Joy Lian

A new study led by a team at Yale has found that “natural enemies,” like insects and fungi that maintain tree diversity in forests, tend to weaken near forest edges, leading to the loss of diversity at borders of fragmented forests.

Forest fragmentation is increasing around the world due to rising deforestation and the splitting of forests for human use. But although it is known that biodiversity declines near the edges of forest fragments, the mechanism behind this phenomenon is poorly understood.

In a field study performed in Karnataka, India, researchers led by Meghna Krishnadas GRD ’18, a Yale plant ecologist and researcher, found that weakened “natural enemies” near forest edges partly explain the loss of biodiversity at these edges. The study was published on Oct. 30 in the journal Nature.

“We know a lot about the patterns of how biodiversity seems to change in these forests, but we don’t have as good of a handle on why these patterns are occurring. In other words, we don’t know what the processes that are leading to patterns of biodiversity change are,” Krishnadas said.

Previous studies in the field have focused largely on changes to abiotic, or nonliving, factors near forest edges, showing that forest edges tend to be drier and warmer, according to Liza Comita, a professor of tropical forest ecology at Yale and co-author of the study. The new research, however, demonstrates that biotic factors, such as the activities of fungi and insects, also play a role in reducing biodiversity at forest edges.

“When a species that is doing really well becomes abundant [in the forest], its natural enemies knock back its numbers, which allows rare species to persist and hang on in the system,” Krishnadas said.

The weakened activities of natural enemies near forest edges therefore remove a key mechanism that maintains biodiversity, she said.

To test this hypothesis, Krishnadas and her colleagues sprayed fungicide and pesticides to suppress natural enemies at both forest edges and the interior of forests. They found that the diversity of seedlings, or newly sprouted plants, relative to the diversity of seeds was lower in forest interiors.

However, seedling diversity relative to seed diversity did not change at the edges. The researchers concluded from this difference that natural enemies play a role in altering plant diversity during the seed to seedling stage — when seeds sprout into young plants.

Several potential explanations exist for why natural enemies may be weakened at forest edges. One hypothesis is that drier and warmer environments near edges alter activities of these fungi and insects, thereby removing a key mechanism that maintains biodiversity. Another suggests that natural enemies are less specific in attacking individual plants near edges, waging more general attacks on all plant species.

“My study sort of opens up the avenue to test which of these possibilities might actually be happening,” Krishnadas said.

The study also raises more questions about biodiversity near forest edges, including how their findings can be generalized to other tropical forests, according to Comita.

Since Krishnadas’ research also focuses on the role of natural enemies in a plant’s seed to seedling stage, which is the earliest stage of plant development, there is room to explore whether the finding also applies to later stages of plant lives, Comita said.

70 percent of remaining forest is within one kilometer of the forest’s edge and is thus subject to the degrading effects of fragmentation, according to a study by Nick Haddad, a researcher at the North Carolina State University.

Jasmine Su | i-shin.su@yale.edu

Correction, Nov. 28:  A previous version of the article stated that the diversity of seeds relative to the diversity of seedlings was lower in forest interiors. In fact, seedling diversity was lower than seed diversity.