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With fewer animals to spread their seeds, plants could have trouble adapting to climate change

The impact of past seed disperser declines has been greatest in areas including North America, Europe and the southern part of South America.

Written by Evan Fricke, Alejandro Ordonez, Jens-Christian Svenning and Haldre Rogers

Picture a mature, broad-branched tree like an oak, maple or fig. How does it reproduce so that its offspring don’t grow up in its shadow, fighting for light?

The answer is seed dispersal. Plants have evolved many strategies for spreading their seeds away from the parent plant. Some produce seedlings that float on the wind. Others have fruits that actually explode, ejecting their seeds.

And more than half of all plants rely on wildlife to disperse their seeds. This typically happens when animals eat fruits from plants or carry away their nuts, then excrete or drop the seeds somewhere else. In tropical rainforests, animals disperse the seeds of up to 90 per cent of tree species.

A key way plants adapt to climate change is through migration. But about half of plant species rely on animals for dispersal.

Our (w @anordonez1 @haldre1 @JCSvenning) new paper asks how seed disperser losses impact plants’ ability to track climate change. https://t.co/LhjALFjCcf pic.twitter.com/iV42uluq5t

— Evan Fricke (@ecfricke) January 13, 2022

Today the Earth is losing species at a rapid rate, potentially representing the sixth mass extinction in its history. In a newly published study, we examine what this loss means for seed dispersal, focusing on birds and mammals that disperse fleshy-fruited plants.

We assessed how seed dispersers help plants shift their geographic ranges to reach habitats newly suitable for growth — a crucial mechanism for surviving climate change. If not enough seeds disperse to track the environmental conditions like temperature and precipitation that plants require, the plants could be stuck in settings where they will struggle to survive. This could lead to losses of plant species, along with the valuable products and services they provide, ranging from food to carbon storage.

A new era for plant movement

Animals have been dispersing seeds for millions of years, but the relationships between plants and their seed dispersers have changed dramatically in our modern era.

Berries in California are no longer eaten by grizzly bears, which disappeared from the state a century ago. On the island of Madagascar, seeds no longer travel in the bellies of gorilla-sized lemurs, which went extinct there about 2,300 years ago. In France, seeds don’t catch a ride on the fur of lions or between the toes of rhinos that once lived there, as shown in prehistoric cave paintings. When animals disperse seeds today, their movement is often hampered by roads, farms or built-up areas.

For most animal-dispersed plants — especially those with large seeds, which require large animals like tapirs, elephants and hornbills to spread them — these changes mean a big reduction in seed dispersal, and a great slowdown of plant movement.

Research by our team and work by many colleagues have uncovered the negative ecological consequences that occur when seed dispersers disappear. Now researchers are assessing how seed dispersal decline is affecting plants’ responses to climate change.

Quantifying what’s been lost

Only a small fraction of the thousands of seed disperser species and tens of thousands of animal-dispersed plant species have been studied directly. Many seed disperser species are extinct or so rare that they can’t be studied at all.

To overcome this challenge, we pulled together data from published studies showing which bird and mammal seed dispersers eat which fruits, how far they spread the seeds, and how their digestive systems’ effects on the seeds help or hinder germination. These three steps together describe what’s required for successful seed dispersal: A seed must be removed from the mother plant, travel some distance away from it and survive to become a seedling.

Next, we used machine learning to generate predictions for seed dispersal, based on the traits of each species. For example, data on a medium-sized thrush in North America could help us model how a medium-sized thrush species from Asia dispersed seeds, even if the Asian species wasn’t studied directly.

Using our trained model, we could estimate seed dispersal by every bird and mammal species ? even rare or extinct species for which there isn’t any species-specific data on the seed dispersal process.

The last step was to compare current seed dispersal to what would be happening if extinctions and species range contractions hadn’t happened. For fleshy-fruited plants, we estimate that because of bird and mammal losses, 60 per cent fewer seeds are being dispersed far enough worldwide to keep pace with climate change by shifting locations. Further, we estimate that if currently endangered seed disperser species such as bonobos, savanna elephants and helmeted hornbills became extinct, global seed dispersal would decline by an additional 15 per cent.

The impact of past seed disperser declines has been greatest in areas including North America, Europe and the southern part of South America. Future losses of endangered species would have their most severe impacts in areas including Southeast Asia and Madagascar.

With fewer seed dispersers present, fewer seeds will be moved far enough to enable plants to adapt to climate change by shifting their ranges.

Using our trained model, we could estimate seed dispersal by every bird and mammal species — even rare or extinct species for which there isn’t any species-specific data on the seed dispersal process.

The last step was to compare current seed dispersal to what would be happening if extinctions and species range contractions hadn’t happened. For fleshy-fruited plants, we estimate that because of bird and mammal losses, 60 per cent fewer seeds are being dispersed far enough worldwide to keep pace with climate change by shifting locations. Further, we estimate that if currently endangered seed disperser species such as bonobos, savanna elephants and helmeted hornbills became extinct, global seed dispersal would decline by an additional 15 per cent.

The impact of past seed disperser declines has been greatest in areas including North America, Europe and the southern part of South America. Future losses of endangered species would have their most severe impacts in areas including Southeast Asia and Madagascar.

With fewer seed dispersers present, fewer seeds will be moved far enough to enable plants to adapt to climate change by shifting their ranges.

Seed dispersers also promote biodiversity by helping to ensure that a large number of plant species can survive and thrive. Ecosystems that contain many plant species with diverse genetic makeups are better equipped to handle uncertain futures, and to sustain the ecosystem functions that humans rely on, such as storing carbon, producing food and timber, filtering water and controlling floods and erosion.

There are ways to increase seed dispersal. Making sure patches of similar habitats are connected helps species move among them. Restoring populations of important seed dispersers, ranging from toucans to bears to elephants, will also help. And global models of seed dispersal like ours can help scientists and land managers think about seed dispersers as a nature-based solution for addressing climate change.

— Evan Fricke is from Rice University, Alejandro Ordonez & Jens-Christian Svenning from Aarhus University and Haldre Rogers from Iowa State University.

With Fewer Animals to Move Their Seeds, Plants Are Stuck in Threatened Habitats

A study published this month in the journal Science found that 60 percent of all plants globally are already having trouble keeping up with climate change as seed-spreading species face major drops in population numbers. The study highlights the role larger animals have in carrying seeds over long distances and the impact wildlife declines have on the symbiotic relationship, New Scientist’s Adam Vaughan reports.

“That should certainly be ringing alarm bells,” says study author Evan Fricke, an ecologist at Rice University, to Science’s Erik Stokstad. “At the same time that we’re ‘forcing’ plants to move these great distances, we’ve also substantially slowed their ability to do so.”

Previous seed-dispersal studies focused on threats to specific ecosystems, such as tracking how bird habitat loss in Brazil has impacted trees’ abilities to spread their seeds. However, similar data has never been analyzed on a global scale, per Scientific American.

To see the impact globally, the team gathered data on 302 animal species and the seeds each animal is known to disperse. They also gathered information on how far the seeds travel and how long they survive after being digested and expelled in animal feces, New Scientist reports. Researchers used machine learning and modeling to fill in missing data for all animal and plant species. With the model, the team could predict mutualistic interactions between the plants and animals for rare or even extinct species.

Together, researchers created an index that detailed how many seeds could spread more than a kilometer by a given number of birds and mammals. After analyzing the data, the team found seed dispersal declined at an alarming rate. Mammal and bird losses cut a plant’s ability to adapt to climate change globally by 60 percent, per the study.

“We found regions where climate-tracking seed dispersal declined by 95%, even though they’d lost only a few percent of their mammal and bird species,” Fricke says in a statement.

The loss of plant resilience was more severe in temperate regions like eastern North America and Europe because these areas have lost many fruit-eating mammals, per Science. Whereas mountain environments that vary in elevation feature different ecosystems within tens or hundreds of kilometers apart, animals living on flat terrain in temperate climes have to travel further to find new habitats, per Scientific American.

When using the data model to see what would happen if the birds and mammals listed as vulnerable or endangered by the ICUN Red List went extinct, Southeast Asia and Madagascar faced the most extreme losses. In these regions, the responsibility of seed dispersal is being carried out almost exclusively by threatened species, Scientific American reports. Based on this prediction, a plant’s ability to adapt to climate change would be reduced by another 15 percent on average, Science reports.

For possible solutions, the researchers suggest strengthening biodiversity by reintroducing large animals to their original ranges or connecting patches of habitat with restored areas using wildlife passages.

“Animal biodiversity supports climate adaptation for the world’s plants,” says Fricke to New Scientist. “This is a really clear intersection of the biodiversity crisis heavily impacting the climate crisis.”

Elizabeth Gamillo is a daily correspondent for Smithsonian and a science journalist based in Milwaukee, Wisconsin. She has written for Science magazine as their 2018 AAAS Diverse Voices in Science Journalism Intern.