There are fewer animals to sow the seeds

Imagine a mature, broad-branched tree like an oak, maple, or fig tree. How do you reproduce so that your offspring don’t grow up in your shadow, fighting for the light? The answer is seed dispersal.

Plants have developed many strategies to spread their seeds away from the mother plant. Some produce seedlings that float in the wind. Others have fruits that explode, expelling their seeds.

And more than half of all plants depend on wildlife to disperse their seeds. This usually happens when animals eat fruit from plants or pick up their nuts and then excrete them or throw them elsewhere. In tropical forests, animals disperse the seeds of up to 90% of tree species.

Today, the Earth is losing species at a rapid pace, which could represent the sixth mass extinction in its history. On a recently published study, we examine what this loss means for seed dispersal, focusing on birds and mammals that disperse fleshy-fruited plants.

We evaluated how seed dispersers help plants change their geographic areas to reach new habitats suitable for growth, a crucial mechanism for surviving climate change. If they don’t disperse enough to track the environmental conditions like temperature and precipitation that plants require, plants can get stuck in environments where they’ll have a hard time surviving. This can lead to the loss of plant species, along with the valuable products and services they provide, from food to carbon storage.

A new era for the plant movement

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

Grizzly bears, which disappeared from the state a century ago, no longer eat berries in California. On the island of Madagascar, seeds no longer travel in the bellies of gorilla-sized lemurs, which became extinct around 2,300 years ago. In France, the seeds are not mounted on the skin of lions or between the fingers of the rhinos that lived there, as prehistoric cave paintings show. When animals disperse them 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 such as tapirs, elephants and hornbills to spread them, these changes mean a large reduction in dispersal and a large slowdown in plant movement.

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

Quantifying what was 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 dispersers are extinct or so rare that they cannot be studied.

To overcome this challenge, we gathered data from published studies showing which seed-dispersing birds and mammals eat which fruits, to what extent they scatter seeds, and how the effects of their digestive systems on seeds help or hinder germination. These three steps together describe what is needed for successful seed dispersal: a seed must be removed from the parent plant, travel some distance from it, and survive to become a seedling.

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We then use machine learning to generate seed dispersal predictions, based on the characteristics of each species. For example, data on a medium-sized thrush in North America can help us model how a medium-sized Asian thrush species disperses seeds, even though the Asian species has not been studied directly.

Using our trained model, we were able to estimate seed dispersal by each bird and mammal species, including rare or extinct species for which there is no specific data on the seed dispersal process.

Analyzing the changes

The final step was to compare the current seed dispersal with what would have happened had there not been extinctions and species distribution contractions. For plants with fleshy fruits, we estimate that, due to losses to birds and mammals, 60% fewer seeds are being dispersed enough around the world to keep up with climate change as they move. Furthermore, we estimate that if species currently threatened by seed dispersal, such as bonobos, savanna elephants, and helmeted hornbills, were to become extinct, global seed dispersal would decline by a further 15%.

The impact of the decline of seed dispersers in the past was greatest in areas such as North America, Europe and the southern part of South America. Future losses of endangered species would have their most severe impacts in areas such as Southeast Asia and Madagascar.

With fewer seed dispersers present, fewer seeds will move enough to allow plants to adapt to climate change by altering their ranges.

Seed dispersers help maintain forests

Seed dispersal also helps forests and other natural ecosystems recover from disturbances such as wildfires and deforestation. This means that mammals and birds play an important role in maintaining natural vegetation.

Most of the recovery of forests around the world takes place through seed dispersal and regeneration of natural forests, rather than people planting trees. Animal dispersal of seeds is especially important for tropical forests, which can grow relatively quickly after being cut down or burned.

Seed dispersers also promote biodiversity, helping to ensure that large numbers of plant species can survive and thrive. Ecosystems that contain many plant species with diverse genetic makeup are better equipped to deal with uncertain futures and to maintain the ecosystem functions on which humans depend, such as storing carbon, producing food and wood, filtering water, and controlling floods and inundations.

There are ways to increase seed dispersal

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

This article was written by Evan Fricke, faculty member in ecology and evolutionary biology at Rice University; Alejandro Ordonez, assistant professor of biology of global change at the University of Aarhus; Haldre Rogers, associate professor of ecology, evolution, and organismal biology at Iowa State University; and Jens-Christian Svenning, professor of ecology at Aarhus University. It is republished from The Conversation under a Creative Commons license. Article in English

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