Animals that eat fruit and spread seeds in their droppings offer a complete transportation service. half the world’s flora. New research suggests that as more seeds-dispersing mammals and birds die globally, some plant species will lose the ability to move to keep up with climate change.
“When you hear the headlines about the biodiversity crisis, some call it the sixth mass extinction, that decline of birds and mammals also means the decline of seed dispersers,” Evan Fricke, lead author of the new Study, recently published in Science, said.
Fricke and colleagues found that 60% of animals-dispersed plants are less able to track climate change because of the loss of birds, mammals, and other species.
This number “is somewhere in the alarm bell territory,” he said. “I hope [this finding] focuses people’s attention on the importance of seed-disperser biodiversity for plant adaptation to climate change.”
“If there are no animals available to eat their fruits or carry away their nuts,” Fricke said in a Press release “animal-dispersed plants aren’t moving very far.”
Many species will have to relocate as the climate warms. This could mean that they move upslope by a few to tens per year on a mountain. For climate change to be effective, organisms must move to the poles. This could take them hundreds of kilometers. Flat land is harder for plants to track because it is easier to determine the speed at the which suitable climate zones move across a landscape (also known by the climate change velocity).
Plants are unable to pick up and move like animals. So the question, Fricke said, becomes: “How many seeds disperse at least that distance that the climate has shifted during the year? How many seeds are dispersed far enough to keep pace with that climate change?”
In the past, scientists have studied what the loss of seed-dispersing animals means for plants in ecosystems, and they’ve also studied how plant populations respond to climate change. But combining those two catastrophes — climate change and mass extinction — on a global scale has been a tougher nut to crack.
The researchers used data from hundreds past studies to train a machine learning model to estimate and draw conclusions about the loss in seed-dispersal services. These data sets included IUCN data from all over the world, which showed which animals disperse seeds; where and how far they travel; and how long it takes for seeds to pass through their dispersers’ guts.
They found that seed-dispersal losses are greatest in temperate regions such as North America, Europe and South America. Extinction of the world’s current endangered species would most impact dispersal in tropical regions in South America, Africa and Southeast Asia.
“This paper is an elegant analysis of how the loss of animals will affect plants under climate change scenarios,” said Mauro Galetti, a seed-dispersal researcher from the University of Miami who was not involved in the study. “The results are worrisome because most natural ecosystems’ large fruit-eating animals are vanishing.”
The scientists also found that even just a small decline in the number of animal species leads to a massive decline in plants’ ability to track climate change. “One might expect that if a location loses 10% of its seed-dispersing animals, we would see a 10% decline in dispersal,” Fricke said, “but this is not the case.” When animals die off in an ecosystem, we’re often first losing the large ones — those that are the best at long-distance dispersal.
“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 said.
“From elephants and gorillas in Africa, to toucans and tapirs in South America, large seed dispersers are vanishing rapidly and their dismissal will have strong consequences on seed dispersal,” Galetti said. “Many plants will be trapped in space without seed dispersers.”
This first global analysis of the loss of seed-dispersers, according to Fricke, demonstrates the critical interconnectedness of the climate change and biodiversity crises — two of the Neun planetary boundaries identified by scientists. Human interference could lead to the destabilization or overshoot one or more of these boundaries, which could lead to the failure of critical Earth operating system.
“Biodiversity of seed-dispersing animals is key for the climate resilience of plants, which includes their ability to continue storing carbon and feeding people,” Fricke . “Extinction and habitat loss damage complex ecological networks. This study shows animal declines can disrupt ecological networks in ways that threaten the climate resilience of entire ecosystems that people rely upon.”