Among the victims of the success of human expansion across the planet are the other members of the animal kingdom. But the case of birds is one of the most dramatic: some 600 species have become extinct in the last 130,000 years, according to a new study published today Thursday in Science. But this work goes beyond numbers, highlighting that with each bird that leaves, it takes with it the function it fulfilled in nature. Key roles such as pollination, insect control or carrion removal are compromised. The situation will get worse. The authors of the research fear that more than 1,300 avian species will disappear in the next 200 years.
Beyond the cataclysms (meteorites, supernovas or megavolcanoes) that have caused the various mass extinctions, the disappearance of a species in the past was exceptional. Among birds, the natural rate of loss was estimated to be no more than 0.1 per million species per year. But, shortly after humans began their great expansion across the planet, the number tripled. The new study, supported by a review of the archaeological record and the main taxonomic collections of large museums, estimates that since the end of the Pleistocene, about 130,000 years ago, at least 610 avian species have disappeared. Almost all (562) did so due to anthropogenic causes such as hunting, habitat destruction or the introduction of invasive species, especially domestic or assimilated species, such as cats and rats. With the remaining 48, researchers are not clear what happened, so it cannot be ruled out that the human factor was also key.
One aspect that this new work confirms is the acceleration of the rate of disappearances. Since 1500, the time of great Western exploration, the extinction rate has multiplied by 28. Other studies maintain that humans have increased the natural rate by 100 times. These data are the basis on which many scientists maintain that we are facing the sixth mass extinction, the first caused by a single species and in a very short time. Even the meteorite impact took many thousands of years to wipe out the last of the dinosaurs. According to this research, if the 1,300 that are already extinct are added to those that could do so in the next 200 years, almost 20% of the 10,000 species of birds that were on the planet before human expansion.
And currently, the situation has been complicated due to the emergence of other factors. “Climate change, invasive species that arrive more easily due to increased mobility of people or loss of habitat, are some of the problems that species face, a scenario that becomes complicated if several of these impacts are combined,” explains the researcher at the Center for Ecological Research and Forestry Applications (CREAF) and co-author of the study, Ferrán Sayol Sanyol.
“It is not just the number of species that have been lost or could be lost,” adds the researcher. “Each one could have an important role, the novelty here is that we try to quantify what consequences that number of species has for the ecosystem,” he adds. And he gives an example: “We have observed that there is a tendency for species that play a unique role in the ecosystem to become extinct. Among them, the iconic Dodo dispersed large fruit seeds on the island of Mauritius and few birds replace this function.” Something similar must have happened with the extinction of the moa. “They were giant birds that lived in New Zealand and grazed. They would be like the large herbivores there, because there were no land mammals,” says Sayol.
The extinction of moas, dodos and the so-called elephant birds allows us to detect some of the traits that most endangered the birds: all three were large, none retained the ability to fly and they lived on islands. In fact, insularity is behind up to 80% of past extinctions. Bred and developed in the absence of humans and everything that came with that colonization, they disappeared shortly after they arrived, mainly due to hunting and the introduction of invasive species. It could be thought that these are circumstances that occurred in prehistory. But no, most of the disappearances have occurred in relatively recent times, since the 15th and 16th centuries. It is the time of great explorations. As Jorge Orueta, researcher and species expert at SEO BirdLife, says, “these have not been so much extinctions caused by humans in general, but by Western man in particular.”
Altogether, the authors of the study have estimated the loss of functional diversity, the decrease in the ecological functions that the disappeared birds fulfilled, at 20%. A figure that would rise to 27% if the rate of species disappearance estimated by scientists for the next 200 years is maintained. In some ecosystems and for a certain role, extinction has compromised the entire ecosystem. On some of the Hawaiian Islands, for example, the extirpation of many frugivorous species is facilitating deforestation: without fruit-eating birds, there are no ones to disperse their seeds. Human pressure and fires fueled by climate change complete a desert future for the archipelago.
The first author of the study, Thomas J. Matthews, from the University of Birmingham (United Kingdom) gives the examples of the Mauritius and Hawaii islands, where all or almost all native frugivores. “Frugivory is an important function, since by eating the fruits and then moving, the birds disperse the seeds of the plants to which the fruits belong,” he recalls. One of the consequences is that of secondary extinctions, which is illustrated by the first of the islands. “For example, Mauritius has a large number of threatened tree species.”
The British scientist adds another example, this one more recent, so much so that the problem is starting now: “The loss of scavengers (for example, vultures), which eat and recycle dead animals, has led to an increase in the number of animal corpses.” that remain in the environment and then an increase in the prevalence of certain diseases in the human populations that live there. Orueta, the BirdLife SEO expert, who was not involved in the study, points out that scavengers do not need to become completely extinct. “In South Asia, in India or Bangladesh, there are still vultures, but the decline in their populations has caused their functions to become extinct without them becoming extinct.” In fact, cases of rabies among humans are increasing in the region because there are no longer birds to remove carrion from circulation. Scientists fear that, with bird extinctions doubling in the next two centuries, cases of ecological functions that do not have anyone to perform them will multiply.
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