The loss of large animals is wreaking havoc on Earth's ecosystems, according to a scientific review published in Science on Friday, causing food chains to fall into disarray, clearing the way for invasive species, and even triggering the transmission of infectious diseases. The decline and disappearance of these large animals, due in large part to human factors such as hunting and habitat loss, has such strong and wide-ranging effects that the review's authors say it may well be "humankind’s most pervasive influence on nature." How the Heck:
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The researchers reviewed data from recent studies investigating the loss of so called "apex consumers," large predators and megaherbivores, from terrestrial, freshwater, and marine ecosystems around the world.
Whether on land or at sea, the researchers found, the result was the same: Remove the apex consumer and the whole ecosystem suffers, as the initial loss sets off a cascade of changes all the way down the food chain. “Predators have a huge structuring influence," ecologist Stuart Sandin, one of the researchers, told LiveScience. "When you remove them you change the biology, which is typically profound and complex. And in many cases it's not necessarily predictable." While removing an ecosystem's top dog---or shark, or wolf, or elephant---is bound to have a big impact, just what that impact will be varies widely.
Wolves. When wolves were extirpated from Yellowstone National Park, elk and deer populations soared---and the ever-more-abundant herbivores ate a huge number of willow and aspen saplings, destroying the habitats of smaller animals and even altering the course of some streams. Likewise, the dearth of wolves across North America means many more deer---which not only gobble up gardens, but carry Lyme-disease-laden ticks.
Wildebeest. Outbreaks of rinderpest, a disease that struck livestock and grazing animals, decimated wildebeest populations in East Africa until the disease was eradicated there in the 1960s (it has since been eradicated worldwide). Without large and hungry herbivores, shrubs and small trees proliferated, making it easy for dangerous wildfires to spread. Free from rinderpest, the wildebeest populations went back up; their grazing kept the shrubbery---and the fires---in check.
Sea Otters. In coastal ecosystems in the Pacific Ocean, sea otters dined on sea urchins. When the otter populations shrank, the sea urchins multiplied and gobbled up large swaths of the kelp forests growing nearby.
One way of addressing the problem is to bring back native species wherever possible. Wolves, for instance, were recently reintroduced in Yellowstone.
The disappearance of apex consumers isn't a new phenomenon. Much of the megafauna that once populated the planet---mastodons, giant kangaroos, saber-toothed tigers---has been extinct for millennia. For buttressing these ancient ecosystems, since the apex consumers can't simply be brought back, some conservationists advocate rewilding: subbing in modern approximations for the extinct species of yesteryear.
Reference: James A. Estes et al. "Trophic Downgrading of Planet Earth." Science, July 15, 2011. DOI: 10.1126/science.1205106
Image: US Fish & Wildlife Service