Humans have killed off 600 bird species, annihilating their role in nature

Human expansion increases at the expense of other members of the animal kingdom. 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 Thursday in the journal Science.

With each bird that disappears, the function it fulfilled in nature also vanishes. Key roles such as pollination, insect control and 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.

Aside from cataclysms such as meteorites, supernovae or megavolcanoes that have caused the various mass extinctions down the ages, the disappearance of a species in the past was exceptional. Among birds, it is estimated that the natural rate of loss was no more than 0.1 per million species per year. But shortly after humans began their relentless expansion across the planet, the figure tripled. Based on a review of archaeological records and vast taxonomic collections in major museums, the new study estimates that since the Late Pleistocene era, some 130,000 years ago, at least 610 avian species have disappeared, almost all of them (562) due to anthropogenic causes such as hunting, habitat destruction and the introduction of invasive species, especially domestic or assimilated species such as cats and rats. The researchers are not clear what happened in the case of the remaining 48 species, so the possibility of humanity playing a key role in their extinction cannot be ruled out.

The study confirms that the rate of extinction has accelerated. Since 1500 — the Age of Discovery — extinctions multiplied by 28. Other studies claim that humans have multiplied the natural rate by 100. These data have led many scientists to argue that we are facing the sixth mass extinction — the first caused by a single species and over a very short period of time. Even the deadly meteorite impact took many thousands of years to wipe out the last of the dinosaurs. According to this research, if we add the 1,300 birds that could become extinct in the next 200 years to those already extinct, that would account for almost 20% of the 10,000 bird species that existed on the planet before human expansion.

Nowadays, the situation has become more complicated due to other factors. “Climate change and invasive species that arrive more easily due to the increased mobility of people or habitat loss, are some of the problems faced by birds, a scenario that is complicated if several of these factors are combined,” explains Ferrán Sayol Sanyol, a researcher at the Center for Ecological Research and Forestry Applications (CREAF) and co-author of the study.

“It’s not just the number of species that have been lost or may be lost,” adds the researcher. “Each one could have an important role, and we are trying to quantify what consequences that number of species has for the ecosystem,” he adds. “We have observed that there is a tendency for species that play a unique role in the ecosystem to become extinct. Among them is the iconic dodo which dispersed large fruit seeds on the island of Mauritius — few birds replace this function.” Something similar 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 used to be no land mammals,” Sayol adds.

The extinction of moas, dodos and the so-called elephant birds allows us to pinpoint some of the traits that put these birds in danger: all three were large, none retained the ability to fly and all lived on islands. In fact, insularity causes up to 80% of past extinctions. Having bred and evolved in the absence of humans, they disappeared soon after their arrival, mainly due to hunting and the introduction of invasive species. One might think that these were circumstances that occurred in prehistoric times. But most of the disappearances have occurred in relatively recent times, since the 15th and 16th centuries — the Age of Discovery. As Jorge Orueta, a researcher and species expert at SEO BirdLife, says, “These have not been extinctions caused by humans in general, but by Western man in particular.”

Overall, the authors of the study have estimated the loss of functional diversity and the decrease in the ecological functions these birds used to perform at 20%. This figure would rise to 27% if the rate of extinction estimated by scientists over the next 200 years is realized. In some ecosystems and for a certain role, extinction has compromised the entire ecosystem. On some Hawaiian islands, for example, the extinction of many of the fruit-eating bird species is facilitating deforestation: without the fruit-eating birds, there are no seed dispersers. Human pressure and fires driven by climate change add to the probability of a desert future for the archipelago.

The study’s lead author, Thomas J. Matthews, from Birmingham University in the U.K., flags up the islands of Mauritius and Hawaii, where all or almost all native frugivores are located. “Frugivory is an important function, because by eating the fruits and then moving around, the birds disperse the seeds of the plants,” he says. “For example, Mauritius now has a large number of threatened tree species.”

Matthews also mentions an emerging problem. “The loss of scavengers (e.g. vultures), which eat and recycle dead animals, has meant an increase in the amount of animal carcasses left in the environment and, subsequently, an increase in the prevalence of certain diseases in the human populations living there.”

According to Orueta, “In South Asia, in India or Bangladesh, there are still vultures, but the decline of their populations has led to the extinction of their functions without their actual extinction.” In fact, cases of human-to-human rabies are on the rise in the region because there are no longer enough birds to remove carrion from circulation. Scientists fear that as bird extinctions double in the next two centuries, the number of unperformed ecological functions will multiply.

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