Much has been said about the loss of bird, mammal, reptile, and amphibian species around the world. By current estimates, at least 322 species have gone extinct in the last 500 years. And researchers estimate that 16 to 33 percent of the world’s vertebrate species — animals with developed spinal cords — are currently threatened or endangered. But a new article, published today in Science, paints an even more alarming picture, as scientists have found that the number of individual insects, crustaceans, worms, and spiders decreased by 45 percent on average over the past 40 years — a period in which the global human population doubled.
dramatic declines in abundance
"We had strong suspicions that the problem was largely with the vertebrates," said Rodolfo Dirzo, an ecologist at Stanford University, in an email to The Verge. "But it was surprising to see this now, also, among the invertebrates," or animals without developed spines. Dirzo calls this loss of animal life "defaunation," and he blames it on humans. "The richness of the animal world of our planet is being seriously threatened by human activities," he said. Many species have gone extinct and the ones that remain — mammals, birds, and insects alike — are showing dramatic declines in their abundance.
In the article, Dirzo and his colleagues reviewed past studies, and compiled a global index of all invertebrate species over the past 40 years. Overall, they found that 67 percent of the world’s invertebrates have declined in numbers by an average of 45 percent. In the UK, for instance, there has been a 30 to 60 percent decline in the number of butterflies, bees, beetles, and wasps. This, the researchers write, is important because too often we measure animal diversity in terms of number of species, or in terms of extinctions. But an animal’s contribution is about more than the mere presence of its species on the planet — it’s also about local shifts in populations that could impact everything from agriculture to human health.
Not just silent springs, but empty forestsDirzo calls this "a dire situation," because continuing on the same trajectory will lead to a "very impoverished planet." To illustrate the depth of the problem, Dirzo appears to borrow a page from Rachel Carson, except that instead of using the metaphor of a "silent spring," he thinks of the world’s future in terms of "empty forests" — places that might "look green, lush and seemingly healthy," he said, but that contain no animal life whatsoever.
Yet "empty forests" are really only a symbol of the extent to which this problem needs solving. If animal populations continue to decline, the risks to human health will be significant, because 75 percent of the world’s food crops necessitate insect pollination. Moreover, invertebrates contribute significantly to agricultural pest control — not to mention decomposition, which increases plant productivity.
That’s why Dirzo thinks that humans need to take action, by reducing or eliminating our current over-exploitation of animals, and by protecting their natural habitats. These, he said, are the main factors driving defaunation. If we can achieve change, he said, "We might able to ‘buy time’ to address the other causes of defaunation that will begin to impact life soon — particularly climate disruption."