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With the disappearance of animals, the death of plants

Rise in temperature, variable rainfall, intensification of heat waves… Global warming puts plants to the test. To survive, most must migrate in search of more suitable weather conditions. Without legs, wings or fins to move about, more than half of them use animals to move to new lands. But according to a study published on January 13 in the journal Science, the gradual disappearance of bird and mammal populations would have reduced the ability of plants to adapt to climate change. And this, from 60 %. Eventually, this could lead to the extinction of certain plant species.

To achieve this result, the research team collected and analyzed data from several thousand studies of how mammals and birds move plant seeds around the world. This dissemination can take several forms, resulting from several million years of co-evolution. Burdock seeds, for example, have tiny hooks that cling to the fur of passing quadrupeds. Other species move through the digestive tract of animals. Trapped in fruits with an enticing color and taste, the seeds are eaten and then excreted a good distance from where they were produced. African savannah elephant (Loxodonta africana)for example, can carry seeds 65 kilometers from where it ingested them. Birds can also transport seeds over long distances, but to a lesser extent than larger animals. »says the lead author of this study, Evan Fricke.

Normally, dissemination allows plants to flourish in new territories and avoid competing with their descendants for access to light. A mechanism now crucial for their survival. As the climate changes, suitable habitats for plants evolveexplains Evan Fricke. They must move from a few hundred meters to several tens of kilometers per year to manage to maintain themselves in suitable conditions. »

The human activities involved

This process is endangered by human activities. Over the past five decades, the biomass of wild animals has declined by 82 %, according to the latest report from the Intergovernmental Platform on Biodiversity (IPBES). The rate of extinction of species is now 10 to 100 times higher than the average of the last ten million years, a speed that leads scientists to believe that the planet today would live its sixth mass extinction ». In question, in particular, the destruction of natural habitats, poaching or the introduction of invasive species. Many plant species are lagging behindnotes Jonathan Lenoir, researcher in ecology at the CNRS (who did not contribute to this study). If humans had not impacted this network of interactions, the dispersal capacity of plants would be much greater. »

Evan Fricke evokes the example of elephants. When they are killed for their ivory, their stomachs are full of seeds. Gorillas, brown bears, and other large animals are often valued for their charisma, but they’re not just charismatic, they’re also some of the best seed dispersers. »

Pixnio/CC0/Ruggiero Richard/USFWS

The consequences of the disappearance of these species for plants are particularly important in North America, Europe and southern Latin America. Megafauna capable of dispersing seeds over long distances (such as grizzly bears in California) have indeed disappeared from these regions for a longer time. In the future, the erosion of mammal and bird populations could threaten tropical ecosystems, especially in Madagascar and Southeast Asia. Animals can scatter up to 90 % of tropical forest tree seeds. Many of them, like the Brazilian tapir (Tapirus terrestris) or the Helmeted Hornbill (Rhinoplax vigil), are now classified as endangered or vulnerable by the International Union for Conservation of Nature (IUCN). The team of scientists estimates that the disappearance of these species could reduce by 15 % additional plants’ ability to adapt to climate change.

Habitat destruction

Other factors explain the delay in the migration of certain plant species, specifies Jonathan Lenoir. One of the most important is the fragmentation of habitats by humans. As cities expand, as a large part of the spaces are paved over, natural habitats become more and more fragmented and isolated. Some species therefore find it difficult to migrate. The dispersal agents themselves may not reach the next habitat. »

The lifespan of some plant species can also compromise their ability to adapt to climate change. The oak, for example, which can live for hundreds of years, may start producing seeds only after several decades. What characterizes climate change is its unprecedented speedrecalls Jonathan Lenoir. We often talk about curves in the shape of a hockey stick. Average temperatures are changing too quickly compared to the ability of some species to respond to them. » The researcher also mentions the heterogeneity of microclimatic conditions » in certain natural habitats (in particular forests), which sometimes slows down the migration of plants, but cannot for the moment be transcribed precisely in the models.

This study, hopes Evan Fricke, should however highlight the very deep links between the collapse of biodiversity and climate change. If the disseminating species disappear, many plants could decline or even become extinct. » With a cascade of dramatic consequences: For forests, this would lead not only to a loss of plant biodiversity, and therefore of habitats for animals, but also to a loss of carbon stored by plants, which would accelerate climate change. » Hence the importance, according to the researcher, of protecting disseminating species and fighting against the fragmentation of their habitats. Everything is interconnected ; plants, animals and humans are deeply dependent on each other. »

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