In the aftermath of bushmeat hunting, pet trade harvesting, habitat fragmentation, selective logging, and other human intrusions, forests and national parks can still look surprisingly healthy. They may even feel like beautiful places for a walk in the woods. But these forests face what researchers in one recent study describe as a “silent threat” due to the rapid decimation of wildlife almost everywhere in the tropics. Without wildlife, forests rapidly deteriorate, losing their value for carbon storage and becoming unsustainable.

That’s because tropical forests, particularly in the Americas, Africa, and South Asia, are primarily composed of tree species that depend on animals to disperse their seeds. This is especially true for the tall, dense canopy trees that are best at carbon storage, a critical factor in climate change calculations. For instance, in a Smithsonian Institution research forest on Barro Colorado Island in the Panama Canal, roughly 80 percent of canopy trees produce large, fleshy fruits. A ravenous cacophony of animals zeroes in on these trees as the fruit ripens.

In a healthy forest like Barro Colorado, the customers may include monkeys, big pig-like tapirs, large rodents like the agouti, toucans with their long, colorful bills adapted for snatching fruit, and a long list of hungry birds, fruit bats, and other species. After these happy diners have eaten and wandered off, they excrete the seeds of the fruit they have eaten all around the forest. Most of the resulting seedlings inevitably die. But in patches of sunlight or otherwise suitable ecological niches, some find the conditions they need to flourish, ensuring that the tree species remains a part of the forest.

Without wildlife to disperse the fruit, on the other hand, the result is a rain of fruit and seeds in the immediate vicinity of a tree. Even more seedlings die, and the survivors grow more slowly because of crowding and competition for sunlight and water. That’s bad news for the tree species.