The sixth mass extinction will be an event triggered by people and will hit the biggest animals the hardest. “There is no past event that looks biologically like what’s happening today,” says lead study author Jonathan Payne of Stanford University. “Processes like warming and ocean acidification are not the dominant cause of threat in the modern ocean.” Gizmodo reports: A paleontologist by training, Payne and his research group started compiling data on modern marine organisms several years back, in order to study how body size and ecological traits have changed over evolutionary time. Payne, who has studied the End Permian extinction event that wiped out more than 95 percent of all marine species 250 million years ago, soon realized that his dataset — which included living and extinct members of nearly 2,500 marine genera — could serve another purpose. By comparing the extinction threat faced by modern marine genera (as indicated by their official conservation status) with their ancestral counterparts, Payne and his colleagues discovered that modern extinction threat is more strongly associated with body size. Larger animals face a greater risk of disappearing than smaller animals. Today, the dominant driver of marine extinction is people, and people aren’t terribly selective about which environments they pluck animals from. We go for the biggest game, fishing down the food web and removing top predators. Within species, too, we tend to hunt the largest individuals, which is why North Atlantic cod and Chesapeake oysters were historically much larger. “In a sense, we’re driving evolution [toward smaller individuals],” Payne said. What’s worth noting is that the Stanford researchers only looked at organisms whose extinction risk has been assessed by the International Union for the Conservation of Nature (IUCN), which creates a bias towards big, charismatic groups like fish, sea turtles, marine mammals, etc. The marine genera that were analyzed only had fossil counterparts, too. Gizmodo also notes that the study “excluded corals, which are currently in the midst of a catastrophic, global die-off.”
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