Get ready to learn a new word: defaunation. Fauna is the total collection of animals—both in terms of species diversity and abundance—in a given area. So, defaunation, much like deforestation, means the loss of animals in all its myriad forms, including extinction, extirpation, or population declines. A special issue in Science today shines light on this little-discussed global trend and highlights how it’s impacting human society.
"Though for emotional or aesthetic reasons we may lament the loss of large charismatic species, such as tigers, rhinos, and pandas, we now know that loss of animals, from the largest elephant to the smallest beetle, will also fundamentally alter the form and function of the ecosystems upon which we all depend," writes Sacha Vignieri, an Associate Editor with Science in an introduction on the issue.
Starting with the bigger—more well-known—species, vertebrate populations on average have declined by over a quarter in the last forty years, according to a review paper in the issue. Such numbers are borne out by a lot of anecdotal reporting of the “empty forest” syndrome, where scientists are noticing more-and-more seemingly intact forests and other habitats that have been stripped of their medium to large vertebrates.
Meanwhile at least 322 vertebrates have gone extinct since 1500, a trend in human-caused extinctions that likely began during the Pleistocene. Many additional vertebrates remain unrecorded for decades and could be extinct.
But “fauna” also extends to invertebrates, which really comprise the vast bulk of the world’s animals. Most of these animals—which includes everything from insects to mollusks and jellyfish to spiders—have been far less studied than the world’s vertebrates and so much less is known about how imperiled they are and their population trends. Still, the data that we do have is not good.
A global review of 452 invertebrates find that these populations have fallen by 45 percent over the last 40 years. The best data is in the Lepidoptera family—moths and butterflies—which shows a drop in abundance of about 35 percent.