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

For many types of vegetation, the only way to migrate to a more favorable range is through the guts of mammals and birds

A close up image of a cedar waxwing eating a red berry. The bird is sitting among tree branches.
Mammal and bird losses cut a plant’s ability to adapt to global climate change by 60 percent. Pictured: Cedar waxwing Andrew C via Wikimedia Commons under CC BY 2.0
Half of all plant species rely on animals to scatter their seeds through hitchhiking in scat, fur, or beaks. When animal populations decline, so does the ability plants have to disperse their seeds and adapt to climate change. Against the backdrop of a heating planet, species are shifting away from their historically-adapted climate conditions. Threats like deforestation, poaching and urbanization are causing declines in mammals and bird species that can scatter seeds and help plants enter more livable ranges, reports Scientific American’s Andrea Thompson.

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.”