The Pygmy Sloth, for example, is perhaps one of the most endangered species of mammals, but it is also one of the youngest that deviated from its closest relative 9,000 years ago. The aardvark, on the other hand, is the last survivor of a once-large mammal group that seceded from the others 75 million years ago. Losing the miniature sloth would be like ripping off a tiny branch from the mammal tree; Losing the aardvark would be like sawing off a whole branch.
To calculate the extent of these incisions, Davis and his colleagues first constructed a pedigree for all past and present mammals, dating back to 130,000 years and extending to the late Pleistocene. By adding up the length of all the missing branches and branches, they calculated that prehistoric humans robbed mammals of 2 billion years of unique evolutionary history. Since the 1
In fact, our actions were far more destructive than if we had arbitrarily killed species. As another group showed at the beginning of this year, we have disproportionately targeted the largest species. There used to be huge ground sloths and car armadillos; They are all gone. Previously, there were six species of elephant-like mammals in North America alone; now there are only three in the world.
And "these great things were also the most evolutionary discernible things," says Davis. "They were often on their own branches of the tree. We do not see this pattern in earlier mass extinctions." According to him, humans have achieved something akin to the worst case of mammalian death. We could hardly have destroyed more phylogenetic diversity if we had planned it.
If the past is so grim, so is the future. Imagine that we are pursuing a massive, well-funded global conservation spurt that effectively threatens extinction for all existing mammals. Imagine that all survivors produce new species at twice their highest historical rates, comparable to African cichlid fish, which are textbook pieces of an extremely rapid evolution. Even in this implausibly optimistic scenario, it would take half a million years for mammal diversity to return to its peak in the ice age.
Realistically, how fast mammals typically develop and that some living species inevitably die out the full comeback will likely take 3 million to 7 million years. "That puts us on the same scale as previous mass extinctions," says Davis. "What we're going through now could have as much impact as the asteroid," which killed most dinosaurs.
But phylogenetic diversity is just one way to weigh the loss of life. One could also consider functional diversity focusing on what animals do in their environment. Some play a crucial role as seed carriers, pollinators and nutrient providers. The pygmy sloth might be a young species, but if it "performs a unique function in its ecosystem, its extinction may have cascading effects," says Advait Jukar of the Smithsonian National Museum of Natural History.
These contributions are hard to find, study and measure, and Davis estimates that it will take even longer to replace them. That is, even as new mammalian species develop, they will not necessarily enter the ecological gaps created when earlier species have become extinct. And these spaces themselves will change as the world warms and the environment changes.
Despite these uncertainties, "it is hard to imagine that full recovery or phylogenetic or functional diversity can be achieved within human time scales," says Shan Huang from the Senckenberg Biodiversity and Climate Research Center. "However, by prioritizing conservation for unique and distinctive lines, we can at least slow down losses."