With fewer animals to spread their seeds, plants could struggle to adapt to climate change

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

Imagine a mature tree with wide branches like an oak, maple or fig tree. How does it reproduce so that its offspring do not grow up in its shadow, fighting for the light?

The answer is seed dispersal. Plants have evolved many strategies to spread their seeds away from the parent plant. Some produce seedlings that flutter in the wind. Others have fruits that explode by ejecting their seeds.

And more than half of all plants depend on wildlife to disperse their seeds. This usually occurs when animals eat the fruits of plants or take away their nuts, then excrete or deposit the seeds elsewhere. In tropical rainforests, animals disperse the seeds of up to 90% of tree species.

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

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

A new era for moving plants

Animals have been dispersing seeds for millions of years, but the relationship between plants and their seed dispersers has changed dramatically in modern times.

California berries 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 died out there about 2,300 years ago. In France, the seeds do not travel on the fur of lions or between the toes of the rhinos that once lived there, as shown in prehistoric cave paintings. When animals disperse seeds today, their movement is often impeded by roads, farms or settlements.

For most animal-dispersed plants – especially large-seeded ones, which require large animals like tapirs, elephants and hornbills to propagate them – these changes mean a big reduction in seed dispersal and a big slowdown plant movement.

Our team’s research and the work of many colleagues has highlighted the negative ecological consequences that occur when seed dispersers disappear. Now researchers are evaluating how declining seed dispersal affects plant responses to climate change.

Quantify what has been lost

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

To overcome this challenge, we collated data from published studies showing which bird and mammalian seed dispersers eat which fruits, how far they spread seeds, and how the effects of their digestive systems on seeds help or hinder sprouting. These three steps together describe what is needed for successful seed dispersal: a seed must be removed from the parent plant, travel some distance away from it, and survive to become a seedling.

Next, we used machine learning to generate seed dispersal predictions, based on the characteristics of each species. For example, data from a medium-sized thrush in North America could help us model how a medium-sized thrush species from Asia dispersed seeds, even though the Asian species has not been studied directly.

Using our trained model, we could estimate seed dispersal by each species of bird and mammal? even rare or extinct species for which there is no species-specific data on the seed dispersal process.

The final step was to compare current seed dispersal to what would happen if species extinctions and range contractions had not occurred. For plants with fleshy fruits, we estimate that due to bird and mammal losses, 60% fewer seeds are dispersed far enough around the world to keep pace with climate change by shifting locations. Additionally, we estimate that if currently threatened seed disperser species such as bonobos, savannah elephants and helmeted hornbills were to become extinct, global seed dispersal would decrease by a further 15%.

The impact of past declines of seed dispersers has been greatest in regions such as North America, Europe, and the southern part of South America. Future losses of threatened species would have their most severe impacts in regions such as Southeast Asia and Madagascar.

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

Using our trained model, we were able to estimate seed dispersal by every species of bird and mammal – even rare or extinct species for which there are no species-specific data on the seed dispersal process. .

The final step was to compare current seed dispersal to what would happen if species extinctions and range contractions had not occurred. For plants with fleshy fruits, we estimate that due to bird and mammal losses, 60% fewer seeds are dispersed far enough around the world to keep pace with climate change by shifting locations. Additionally, we estimate that if currently threatened seed disperser species such as bonobos, savannah elephants and helmeted hornbills were to become extinct, global seed dispersal would decrease by a further 15%.

The impact of past declines of seed dispersers has been greatest in regions such as North America, Europe, and the southern part of South America. Future losses of threatened species would have their most severe impacts in regions such as Southeast Asia and Madagascar.

With fewer seed dispersers present, fewer seeds will be moved far enough for 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 cope with uncertain futures and to maintain the ecosystem functions on which humans depend, such as carbon storage, food and timber production, filtration water and flood and erosion control.

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

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

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