The fewer animals there are, the worse it is for plants

The Osage orange is a large fruit that mammoths and other large mammals fed on twelve millennia ago.

Its trees (

) were abundant throughout North America. After the extinction of the megafauna, its habitat dwindled until it was limited to a small area of ​​what is now the state of Texas (United States) at the time of Columbus. For biologists, it is a typical case of an orphan species that lives on borrowed time: its fruit is so large that the animals that replaced those giants could not ingest it whole and take it to colonize new territories. Like this false orange tree, throughout the planet many plant species became extinct or their habitat diminished following the disappearance of large herbivores, marking the end of the Pleistocene. Today and also throughout the Earth, half of the trees and shrubs are limiting their dispersal by not having anyone to take their seeds. And that leaves them without their last resort against climate change, to emigrate.

There are many plants that entrust the fate of their offspring to the elements (air, water and even fire), but more than half of the trees and shrubs require an animal to eat their fruit, dropping its seeds (local dispersal) or swallowing them and then regurgitating or defecating them (distance dispersal). The second is the best colonization tool that species that are tied to the ground have and is one of the best examples of the mutualism described by Darwin: I feed you and you plant me further. But the ongoing sixth great extinction raises the question: How is the reduction in numbers, if not the outright disappearance of many vertebrates, affecting plants? It might be thought that the absence of many herbivores is good news for the plant kingdom, but the opposite is true.

Rice University ecologist Evan Fricke leads a group of researchers that has just published research on seed dispersal in the context of climate change. They analyzed data from some 18,000 mutualistic relationships of 302 animal species with plant species from all over the planet. The results, published in

, are not good: "Areas with the greatest decline in birds and mammals are experiencing the greatest declines in seed dispersers," he says in an email. The crisis begins with the largest. "Large dispersers that move seeds over great distances are very often the species that first disappear from ecosystems," adds Fricke. And that exposes the plants they fed on to extinction.

"Large dispersers that move seeds over great distances are being the species that first disappear from ecosystems"

Evan Fricke, an ecologist at Rice University (United States)

The boko (

) is a tree that dominates the heights of the jungles of central Africa.

Its seed reaches nine centimeters and another five in diameter, one of the largest known.

The accelerated extinction of elephants, their great disperser, is causing the replacement rate of old trees for new ones to be negative.

But the problem this time is not limited to megafauna, as it has been in the past. Now small mammals are also extinct and, in particular, birds, which shared the work of carrying the seeds to new territories. Alejandro Ordóñez, co-author of the study when he taught at the University of Aarhus (Denmark), puts figures on the changes that the flora-fauna connection is undergoing. "Our analyzes indicate that the loss of vertebrates experienced to date has severely reduced long-distance seed dispersal, more than halving the number of seeds dispersed far enough to keep pace with climate change."

The connection between loss of animal biodiversity and the impact of climate change on flora is one of the main contributions of this work.

Science has been looking at how plants are coping with global warming for years.

The main strategy is simple: they are moving to higher and higher latitudes, further and further north (in the northern hemisphere) or to higher altitudes, higher and higher, to recover the climatic conditions they had.

But everything indicates that the warming runs faster than the plants.

The orange tree of the Osages developed large fruits that were eaten by mammoths in North America.

With the extinction of the megafauna, it almost became extinct.

It was saved first by the Osage Indians and later by European settlers.Gale French/Wikimedia Commons

As the authors of the study project into the future, trees and shrubs with fleshy fruits (the scope of the study), in the coming decades there will be an extra reduction in long-distance dispersal capacity of 15%. “The problem is not only the loss of species, but also [there is] climate change. When we put the two problems together, only a quarter of the evaluated plant species will be able to disperse their seeds enough to keep pace with climate change," Ordóñez details.

This summer, the scientific journal

published research on the same problem, and its conclusion was very grim. On that occasion, the work analyzed about a thousand cases of mutualism between migratory birds and trees in European forests. They observed that 86% of plant species are dispersed by birds on their migration to the south (from the north of the continent to the Mediterranean and North Africa and from here to sub-Saharan Africa). Only 35% of the trees see their seeds carried north. The percentages add up to more than 100 because there are species that take advantage of the double meaning.

The biologist from the University of Cádiz Juan P. González Varo is the main author of that study of birds. "The fruits of the forest are not like the ones we see in the greengrocer, they are mostly small, so that the birds can swallow them," he recalls. For him, investigations like his prove the drama that life faces: “There is an overlap between plants and migration. The plants fruit in late summer and fall, when the postbreeding migration south takes place. But with climate change, in the south there will be increasingly higher temperatures”, he warns. González Varo believes that we are facing an unprecedented scenario, "there have been climate changes in the past, but none as fast as this one," he concludes.

The most recent, the end of the ice age, occurred about 12,000 years ago, coinciding with the extinction of the megafauna. In fact, many specialists link both facts. Pedro Jordano is a CSIC researcher at the Doñana Biological Station and has been investigating ecological interactions such as mutualism for years. In this area, he has studied the impact of the extinction of large mammals on the flora of the past. “When the megafauna was lost, some plant species collapsed. But extinction did not always occur. Some found alternative dispersers, such as smaller mammals or reptiles, and local dispersal remained,” says Jordano. But, in the long term, there was a reduction in the populated area and other processes, such as a decrease in the genetic pool that have compromised its future. In some cases,like that of avocado or cocoa, humans replaced gomphotheres, related to elephants, or the giant sloths. "In nature, long-delayed responses are produced that we call extinction debt: with the disappearance of the megafauna, extinctions that we have not yet seen will follow one another," he says.

“In nature, long-delayed responses are produced that we call extinction debt: with the disappearance of megafauna, extinctions that we have not yet seen will follow one another”

Pedro Jordano, CSIC biologist at the Doñana Biological Station

The past illustrates how the future of the planet's flora can be. The decrease of the dispersers that, together with the pollinators (also in decline), are the gardeners of the forest, will cause a remodeling on a global scale. For Jordano, "key pieces of the web of life are being lost." As Ordóñez says, "the impact of the loss of a species is an event that transcends other components of an ecosystem, and starts a chain of events that can accelerate the current diversity crisis." Regarding the forests of the future, González-Varo speaks of a

of the European continent, but "it is most likely that totally new communities will emerge, with species from the past and present."

In all this process, humans can have a final impact that allows us to return to the story of the oranges of the Osages.

The Osage Nation is a Native American people of the Great Plains of the United States.

They saved the tree from extinction, they valued the elasticity and hardness of its wood for their bows.

It was valued by settlers for its resistance to rot and was the main material for fences and boundaries.

Today, thanks to a 1930s initiative to combat erosion, orange trees in the Osages parallel many highways in the central and midwestern United States.

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