Plants move too. They use their seeds to do it. One of the means of expansion is the wind. But not all of them are as light as dandelion. There are thousands of plant species that need animals to move. They attract them with sweet or nutritious fruits, sneaking their seeds into their stomachs until they excrete them. This is how they secure their future. But in Europe, extensive work has proven that interactions between dispersers and dispersers are being threatened because many of the former are on the path to extinction.
The new research, published today Thursday in the scientific magazine Sciencenumbers 2,248 European plant species that have evolved to have specific adaptations for the biotic dispersal (abiotic is wind, sometimes fire or water) of their seeds. “Its fleshy fruits, the pulp attracts dispersers. Others, like acorns, attract them because of their nutritional content,” recalls researcher at the University of Coimbra (Portugal) and first author of the study, Sara Beatriz Mendes. “Other species do not have fruits as we understand them, but they have hooks and other elements [para engancharse]”says Ruben Heleno, from the same university and senior author of the work. The authors clarify that there are many other species with non-specialized seeds that benefit from animals.
Mendes, Heleno and other researchers have gathered information on the conservation status of European frugivorous animals or animals that have fruit in their diet, from bears that gorge on wild cherries when emerging from hibernation, to ants. capable of carrying a seed 10 times heavier than them. Once a series of filters were applied (as if digestion makes the seed unviable), they counted 11,414 interactions between 1,902 plant species and 455 animal species. It could be thought that the main dispersers are birds and mammals. But almost a hundred arthropods also fulfill this ecological function, in addition to reptiles, mollusks and even some fish. Each animal species disperses the germ of 13 different plants on average. And, conversely, each plant has an average of nine dispersers. Some mammals and birds are superdispersers, such as the common deer, wild boar or blackbird, which spread more than a hundred different seeds. And the plants with the greatest number of allies are elderberry, blueberry and wild cherry.
Beyond this data, which had not been gathered until now, the accumulated information allows researchers to speak of a seed dispersal crisis in Europe. “The crisis is based on the fact that 30% of the dispersers are threatened,” says Mendes. “30% at least, because for many species, for example, invertebrates, we do not know their conservation status,” he adds. Although only 1% of dispersed plants appear on the Red List of the International Union for Conservation of Nature, for two-thirds of the total there is no data on their threat status. According to this work, 60% of European plants have at least one of their dispersers on the verge of extinction and a third of plant species have more than one. Overall, at least 30% of plant-animal interactions are threatened. The authors of the study have left the analysis of the situation of these interactions for a second part, but they have found at least 78 in a critical situation. And they give a preview: the possible local extinction of corvids such as the rook is complicating the situation for at least two species of oaks.
“A third of the animals are leaving, that is going to be a problem for the plants to move, right now when they need it most,” says Heleno. “Spain, Portugal, Italy, Greece… are on the path to desertification, in addition to habitat fragmentation. The plants have to escape to the north,” he adds. Both Heleno and Mendes emphasize that the seed dispersal crisis must be much deeper than what they have unearthed. There is no data on the conservation status of most European plants and even less on the situation of arthropods, the second group of dispersers after birds. “Ants, for example, are essential because there are plant species that are mostly dispersed by them, but since they are not as charismatic as other species, we know very little about their status,” says Mendes. Overall, the status of 85% of invertebrate species is unknown.
Andrew J. Green, research professor at the Doñana Biological Station (CSIC), remembers that “plants have to move to adapt to global warming and without animals they will not be able to.” To this growing danger we must add that Europe is the continent with the greatest fragmentation of its natural habitats. With so many cities, fields, infrastructure… trees and shrubs need the help of dispersers more than ever. His colleague in Doñana, Pedro Jordano, professor in the Department of Integrative Ecology, highlights to SMC Spain that “animal-mediated seed dispersal is essential for forest regeneration.” Unrelated to the study, the expert adds that “if we lose frugivorous animals and the countless interactions they have with plants, the forests would collapse.” And he gives two pieces of information: “In tropical forests, for example, more than 85% of woody species depend on frugivorous animals for seed dispersal, and in the Mediterranean forest we find up to 60%.”
Heleno and Mendes compare this crisis to that of pollinators. Much has been written and researched about the consequences of the decline of pollinating insects and birds. But only now is it beginning to be studied how the extinction or decline of some animal populations is compromising the future of many plant species.
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