Loss of seed-hauling animals spells trouble for plants in warming world | Science

A slow death is creeping through Earth’s forests and other green landscapes. As animals are killed by hunters or forced away by logging, for example, the plants that depend on them to carry their seeds begin to disappear. Over time, trees and other plants may vanish. Climate change is accelerating this process, a new study suggests—and it may ultimately harm not just biodiversity, but the ability of ecosystems to store carbon and provide food and clean water.

“This is a very impressive, sobering finding,” says ecologist Rodolfo Dirzo of Stanford University, who was not involved with the work.

Rooted in place, many plants need help to move about the world. Although the seeds of dandelions and other plants take to the air with feathery wings, about half of all species rely on birds and mammals to eat or carry their fruits and nuts to new places. When these partners disappear, forests and plant communities can struggle to regenerate.

Climate change is making things worse. As temperatures warm and rainfall withers, for example, plant populations will have to extend their ranges by as much as tens of kilometers per year to stay in their optimal climate conditions.

Evan Fricke, an ecologist at Rice University, wanted to know how well plants all across the planet might be able to move to new habitat. He and his colleagues gathered existing data from thousands of studies that looked at how birds and mammals spread seeds, including how far they travel and what percentage germinate. They focused on seeds that are dispersed more than 1 kilometer, a distance that is great enough to significantly expand populations and help them recover from fire or other disturbances.

Then, the team developed a computer model that learned which physical traits, such as body size and diet, accurately predicted how many seeds might be spread by various birds and mammals and survive being digested. Survival is more likely if a bird doesn’t have a gizzard, for example, which grinds its food. And a small seed is more likely to survive being eaten by a pig, whereas a large seed might be crunched to bits.

The average ability of plants to keep pace with climate change has already declined by 60%, the model found. “That should certainly be ringing alarm bells,” says Fricke, whose team reports its findings today in Science. “At the same time that we’re ‘forcing’ plants to move these great distances, we’ve also substantially slowed their ability to do so.”

The loss of resilience is greater in temperate regions such as eastern North America and Europe, the researchers found. That’s because these areas have lost many populations and species of large fruit-eating mammals, such as bears, which can move seeds longer distances. Also, the challenge for plants is greater there, because when climate conditions change in relatively flat landscapes, species must move farther north to keep cool. In contrast, an alpine plant only has to move uphill to get to a cooler climate.

The situation could get worse. In another analysis, the researchers predicted what might happen if currently threatened and endangered seed-dispersing birds and mammals go extinct. This would reduce, on average, a typical plant’s ability to keep up with climate change by another 15%. The effect would be greatest in the tropics, where many of these plants live with relatively big seed dispersers, such as lemurs in Madagascar and large fruit-eating birds in Southeast Asia.

Some of the damage can be repaired by connecting isolated patches of habitat, such as forest remnants, with corridors of restored habitat. That will better allow species to move in response to climate change. Reintroducing large animals to their former ranges could also help, the team says, because they can move seeds long distances.

“This is a very loud wake up call for urgent conservation efforts” of fruit-eating and seed-dispersing animals, Dirzo says. What’s important, he says, is not just preventing extinction, but keeping populations big enough to help many plants reproduce.

Although it’s not possible for the model to make any predictions about how specific ecosystems will change in the coming decades, the overall concern is that if climate change shakes up forests, grasslands, and other plant communities, they may not absorb as much carbon or prevent floods or provide mushrooms or berries for local people, for example. Fricke says: “It’s hard to encapsulate the enormity of how much we rely on plants.”