Policymakers may not intend to keep us trim when they're pondering how to manage fisheries and other wild food resources. But a new study indicates that our current food-harvesting practices are making the stuff we eat smaller—very quickly.

The study, published this week in the Proceedings of the National Academy of Sciences, found that plants and animals being harvested aggressively around the world from the wild (rather than from farms) are changing more than two and a half times faster than would be expected under natural conditions.

"Two and a half times is pretty big," says Stephan Munch, an assistant professor of fisheries ecology at Stony Brook University in Long Island, N.Y.

Scientists have long assumed that humans can—and do—affect the plants and animals that live around us (with pollution and by introducing invasive species). But this new work, which analyzed data from dozens of other studies, found that our intense food-gathering practices have substantially changed the size and breeding schedule of at least 29 species in as few as 20 years.

Unlike, say, wolves or lions, which target the smallest, weakest members of a group, humans tend to select the biggest and juiciest ones—be it caribou or cod—for our meals or trophies. As a result, the study says, plants and animals harvested from the wild are now about 20 percent smaller than they were just generations ago—and, in the case of the bighorn sheep than they were just three decades ago.

These animals are also reproducing at younger and younger ages, because mature individuals are more likely to be caught. When animals reproduce earlier, they often have fewer offspring, which means there will be an even smaller population in the future, according to the study.

Researchers aren't sure whether a change in the genetic pool or plasticity (individual adaptation to the environment) is responsible for these changes. But lead study author, Chris Darimont, an evolutionary ecologist at the University of California at Santa Cruz (who started the research at Canada's University of Victoria), is more concerned about the outcome.

"We’re changing the face of biodiversity at very rapid rates," he says. And any shift in animal size can alter predatory-prey relationships, impacting entire ecosystems.

The next big question, says Munch, is how long it will take for these altered populations to change back.

It's "kind of like this great experiment that hasn't been tried yet," Darimont says, because these species are still being harvested intensively. He hopes that will change soon, though, because as Munch points out, "No one wants to have very, very tiny cod that mature in two years."