You may not have noticed, but we are in the early to middle stages of the Earth’s sixth large extinction event. Compared to the fossil record’s extinction rate of between about 10 and 100 species each year, scientists believe we may now be losing more than 27,000 species annually. This mass extinction is different from those that preceded it in that Homo sapiens are now the primary driver, and it is marked by the loss of large animals, particularly predators. Top predators have failed to cooperate with lab experiments testing their impact on global ecosystems, so evidence about what this means has been weak. But a new study shows what a predator-less ecosystem might look like.

A recent study in the journal Science, “Trophic Downgrading of Planet Earth,” examines how the near disappearance of upper levels of the food chain–big cats, wolves, bison and great whales–has altered ecosystems around the world.

Twenty-four scientists searched through decades of experimental and observational data for evidence of “trophic downgrading,” the cascading effects in ecosystems from the loss of large “apex consumers.” What they found indicates “the loss of these animals may be humankind’s most pervasive influence on nature,” according to the authors. “Trophic cascades have now been documented in all of the world’s major biomes.”

Examples of such cascades are all around us. The elimination of top predators, once thought to be merely passengers atop larger immutable ecosystems, are actually the determining factor in many biomes. Removing predatory fish from tropical reefs or cold-water streams can quickly convert them into degraded algae-covered systems. The presence of wolves and big cats keeps forests from Alaska to Panama from vanishing as fodder for herbivores. The treeless areas in once-forested parts of the Scottish isles such as Rum, where wolves disappeared about 250 to 500 years ago, is a cautionary tale about the processes underway in many American forests being overgrazed by deer. The same story, scientists say, is playing itself out all around us, from Africa to the oceans.

This shifts one of the central assumptions in ecology. Traditionally, scientists and resource managers have studied and managed ecosystems from the bottom up. But that is “only half of a very complex equation,” writes James Estes, one of the study’s lead authors. Top consumers in the food web are “enormous influencers of the structure, function, and biodiversity of most natural ecosystems.” Predators must be part of the conceptual equation if there is “to be any real hope of understanding and managing the workings of nature.”

[Image: Flickr user Dennis from Atlanta]