Surprise, surprise: Habitat degradation and exploitation are the biggest animal killers.
Since 1970, more than half of the world's population of wild vertebrate animals have died off, according to a biennial report from the World Wildlife Fund.
The organization's "Living Planet Report" studies the populations of more than 10,000 populations of mammals, birds, reptiles, amphibians, and fish. According to the study, 52 percent of the overall total population of wild animals died between 1970 and 2010.
"We're gradually destroying our planet's ability to support our way of life," Carter Roberts, WWF's CEO, said in a statement.
According to the 180-page report, the decline is most notable in tropical regions and in poorer countries. In Latin America, for instance, the decline is roughly 83 percent.
"Habitat loss and degradation, and exploitation through hunting and fishing, are the primary causes of decline," the report suggested. "Climate change is the next most common primary threat, and is likely to put more pressure on populations in the future."
Among all vertebrate animals, terrestrial species are down 39 percent, as are marine species. The major decline, the group says, is in freshwater-dwelling fish and amphibians, where scientists have seen a 76 percent drop in overall animal numbers.
The group uses what it calls the "Living Planet Index" to calculate these numbers, which it says are calculated from trends in 10,380 populations of 3,038 vertebrate species.
"These species groups have been comprehensively researched and monitored by scientists and the general public for many years, meaning that a lot of data is available to assess the state of specific populations and their trends over time," the report says.
Of course, the WWF is an organization that is deeply committed to preserving the planet's biodiversity, so this report should probably be taken with a grain of salt or two. But, as far as wildlife organizations go, it's been one of the most trustworthy, and it's not as if its findings are all that out-of-line with what you'd expect. It's no secret that the Earth is undergoing its sixth mass extinction event, and a lot of the evidence points to the idea that we're the driving force behind a lot of that.
Add to that the well-known phenomena affecting amphibians and coral reefs, the dwindling numbers of giant fauna in Africa, massive oil spills and environment destruction in the Gulf of Mexico, the proliferation of micro plastics in our waterways, and the clear cutting and oil extracting that's going on in the Earth's rain forests, well, can you say you're surprised?