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Pygmy sloth, for example, may be one of the most endangered mammal species, but it is also one of the youngest, having diverged from its closest relative 9,000 years ago. The aardvark, on the other hand, is the last survivor of a formerly very important group of mammals that separated from the others. million years ago. Losing pygmy laziness would be like tearing oneself away from a small branch of the mammalian family tree; losing the aardvark would be like sawing a whole branch.
To determine the extent of these reductions, Davis and his colleagues first built a family tree for all mammals, past and present, going back 130,000 years, up to the late Pleistocene. By summing up the length of all missing twigs and branches, they calculated that prehistoric humans were flying at mammals over 2 billion years of unique evolutionary history. Since the 16th century, we have erased 500 million years of evolutionary history and we risk losing another 1.8 billion years over the next five decades. "It's stunning," Davis says.
Indeed, our actions were much more destructive than if we had just killed random species. Indeed, as another group has shown earlier this year, we have disproportionately targeted the larger species. There were lazy floor giants and armadillos at the size of a car; they are all gone. There were six elephant-like mammal species in North America alone; there are only three left in the world.
And "those great things were also the most evolutionary distinct things," says Davis. "They were often on their own branches of the tree. We do not see this trend in previous mass extinctions. According to him, the humans managed to obtain a result close to the scenario of extinctions in the mammals. We would hardly have been able to destroy more phylogenetic diversity if we had planned to do it.
When the past is so dark, the future is too. Imagine that we are sparking a massive and well-funded global campaign for conservation, which effectively saves all existing mammals from extinction. Imagine also that all the survivors produce new species twice as fast as their historical rates, on par with African cichlids, which are classic examples of extremely rapid evolution. Even in this unlikely optimistic scenario, it would take half a million years for mammal diversity to return to the zenith of its ice age.
More realistic, given the speed with which mammals evolve and certain living species will inevitably disappear, the complete return will probably take between 3 and 7 million years. "This puts us at the same level as the previous mass extinctions," said Davis. "What we are currently experiencing could impact as big as the asteroid" that killed most dinosaurs.
But phylogenetic diversity is only one way to weigh the number of victims. We could also look functional diversity, which focuses on which animals make in their environment. Some play a crucial role as seed bearers, pollinators and nutrient providers. Pygmy laziness may be a young species, but if it "performs a unique function in its ecosystem, its extinction can have dramatic consequences," said Advait Jukar of the Smithsonian National Museum of Natural History.
These contributions are difficult to study and measure, and Davis feels that it will take longer to replace them. That is, even after the appearance of new species of mammals, they will not necessarily participate in the ecological gaps created by the disappearance of old ones. And these vacancies themselves will change as the world warms and the environment changes.
Regardless of these uncertainties, "it is difficult to imagine that complete recovery or phylogenetic or functional diversity can be achieved in a timely manner," said Shan Huang of the Center for Research on Biodiversity and Climate Change. Senckenberg. "But by giving conservation priority to unique and distinctive lines, we can at least slow down the losses."
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