Endangered fruit-eating animals play an outsized role in a tropical forest—losing them could have dire consequences

A new study by researchers at the University of Washington shows that losing a particular group of endangered animals—those that eat fruit and help disperse the seeds of trees and other plants—could severely disrupt seed-dispersal networks in the Atlantic Forest, a shrinking stretch of tropical forest and critical biodiversity hotspot on the coast of Brazil.

The findings, published Oct. 12 in the Proceedings of the Royal Society B, indicate that a high number of plant species in today's Atlantic Forest rely on endangered frugivores—the scientific term for animals that eat primarily fruit—to help disperse their seeds throughout the forest. As a result, losing those endangered frugivores would leave a high proportion of plants without an effective means to disperse and regenerate—endangering these plants, reducing diversity in the Atlantic Forest and crippling critical portions of this ecosystem.

"Tropical forests contain this incredible diversity of trees," said lead author Therese Lamperty, a UW postdoctoral researcher in biology. "One of the main processes forests use to maintain this diversity is dispersal. If you're not dispersed, you're in a crowd of trees that are just like you—all competing for resources. And there are a lot of plant enemies already in the area or that can be easily recruited, like harmful animals or plant diseases. Your chance of survival is higher when you get transported away from your mother tree to an area without trees like you."

The Atlantic Forest, which lies east of the rainforests of the Amazon Basin, once encompassed an area twice the size of Texas. Some 85% of it has been lost over the centuries due to deforestation, industrial development and urbanization in eastern Brazil, according to The Nature Conservancy. The forest is home to a variety of frugivores, from primates to birds, which disperse seeds by regurgitating or excreting them. The seeds of some plant species can't even germinate until they pass through the gastrointestinal tract of a frugivore.

Lamperty and senior author Berry Brosi, a UW associate professor of biology, analyzed a dataset published in 2017 that incorporated data on the diet and distribution of fruit-eating vertebrates in the Atlantic Forest. The data, compiled from 166 studies spanning more than half a century, allowed Lamperty and Brosi to paint a comprehensive picture of the interactions between hundreds of frugivore species—331 total—and 788 tree species.

"For reference, the entire state of Washington only has 25 native tree species," said Lamperty.

Lamperty and Brosi deduced how important those frugivore species are for the forest by modeling how many tree species would be left without seed-dispersal partners if certain frugivores died out. According to the International Union for the Conservation of Nature, only 14% of the frugivore species they analyzed are endangered, but losing them left about 28% of the plant species they analyzed without a means of dispersing seeds. Losing endangered frugivores led to a worse outcome than losing even "generalist" frugivores, which eat fruits and nuts from a variety of species and were previously believed to be the most important group of frugivores for seed dispersal networks.

"A lot of frugivores are generalists. But in the Atlantic Forest, it turns out that a lot of plants are specialists," said Brosi. "The size and the toughness of their fruit and their distribution in the forest can really limit which animals can perform this important role for them."

Nearly 55% of the specialist plant species in the dataset relied solely on endangered frugivores to disperse their seeds.

Losing a species—like an endangered frugivore—is bad enough. But this study serves as a reminder that what appears to be one loss has numerous "secondary effects," said Lamperty. Researchers don't always know these effects until in-depth studies that span years and incorporate many species linked by different interactions, like this one, are conducted. That can also keep the public unaware about the long-term consequences of losing endangered species.

"It's a reminder that we should try to understand better what ecological roles and interactions we lose when endangered animals disappear—not just these seed dispersal networks, but other roles, too," said Lamperty. "Endangered animals have co-evolved with many species in these ecosystems, and I'm not sure we know enough about the roles they play in the health and well-being of places like the Atlantic Forest."

"It's an alarming finding, and a sign that we should pay more attention to these interactions between species when considering conservation and land protections," said Brosi.