An American robin eats a winterberry

Paul Vitucci

Losses in the number of birds and mammals are limiting the capacity of plants worldwide to adapt to climate change by curbing seed dispersal.

About half of plants rely on animals to disperse their seeds, and research has shown the importance of large animals for transporting seeds over long distances. But the impact of wildlife declines on seed dispersal hasn’t been measured at a global level before, which Evan Fricke at Rice University, Texas, set out to address.

Fricke and his colleagues gathered data on 302 animal species and on the seeds of which plant species they help disperse, combining it with information from field studies including how far the seeds travel and how they survive after passing through animals’ guts.

Researchers “are nowhere close” to having evidence for all plant and animal species, says Fricke, so his team relied on machine learning and modelling to fill gaps. For example, in a case of missing data for a seed dispersed by a monkey, a similar-sized primate that had been studied stood in. The result was an index estimating how many seeds a given number of birds and mammals could spread by more than a kilometre.

The team found that seed dispersal globally had “steeply declined” compared with a model of a world that hadn’t experienced the bird and mammal losses recorded to date. The individual declines vary, but the biggest were in northern temperate regions rather than the tropics.

With some plant species having to shift their range by hundreds of metres or even several kilometres a year to be able to thrive in a rapidly warming world, the team also built an index tracking the ability of seed dispersal to outpace plants’ changing local climates. The loss of birds and mammals was found to have driven a 60 per cent cut in plant species’ ability to track climate change. A further 15 per cent fall is expected if today’s threatened animals go extinct.

Fricke says it is notable that the declines in seed dispersal are disproportionately bigger than the animal declines that triggered them. “Animal biodiversity supports climate adaptation for the world’s plants,” says Fricke. “I think this is a really clear intersection of the biodiversity crisis heavily impacting the climate crisis.”

Frank Schurr at the University of Hohenheim, Germany, says looking at the impact of animal losses on seed dispersals globally is new and important research. “It shows the power of combining large [biodiversity] data sets”, he says.

Journal reference: Science, DOI: 10.1126/science.abk3510

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