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Have Humans Created a New Geological Era?

Welcome to the Anthropocene.
Cuban brown anoles. (Photo: andrea_pauline/Flickr)

Cuban brown anoles. (Photo: andrea_pauline/Flickr)

The Nobel-winning chemist Paul Crutzen got people riled up when he proposed a new name for our current geological epoch: the "Anthropocene." The word—a riff on "Holocene," geologists' official name for our present post-Ice Age epoch—is meant to emphasize the massive impact that we've had on our planet. While some geologists doubt the term's scientific merit, the International Union of Geological Sciences is exploring the possibility of making the Anthropocene official. It would be an acknowledgement that humans have created their own geological epoch.

This may seem like just words, an academic debate or political argument that is more about environmental activism than science. But the science is genuine. Even if the geologists decide that the Anthropocene doesn't qualify as a distinct geological period, there is no doubt that, as Crutzen and his colleagues have argued, "humankind has become a global geological force in its own right." In other words, we've reached the point where scientists can't build accurate theories and models of the Earth's "natural" processes without taking humans into account.

The notion of the Anthropocene is not simply a political statement; the scientific consequences are real.

You can see how this works in a study published last week on the biogeography of Caribbean anole lizards. Islands have long been a proving ground for scientific ideas about evolution and ecology—most famously, the Galapagos islands and their finches were the inspiration for Darwin's discovery of natural selection. With their limited landmass, and with the ocean acting as an extreme physical barrier, islands are a great place to study how species evolve and what happens when they colonize new territory. Theories about ecosystems that are developed on islands can often be applied to other environments.

The authors of last week's study, a trio of scientists from Vrije University in Amsterdam, the University of California-Davis, and Harvard, tested an important idea about how an island's size and physical isolation influences the number of different species that live on it. A key idea in ecology is that the number of different species that exist on an island is determined by two major factors: island size (new species evolve more frequently on large islands because they offer more habitats), and isolation (more physically isolated islands have fewer non-native species because they're difficult for new species to colonize). Researchers use these two factors to model how colonization and evolution shape biogeographical patterns and answer the question: Why do species live where they do?

The researchers tested this theory of island biogeography by applying it to the Caribbean islands, each of which has a different mix of native anole lizard species. Compiling data from a variety of sources, they identified which species existed on which islands over the past 100 years. Not surprisingly, the older data showed that closely related anole species tended to cluster together geographically, because the lizards don't swim across long stretches of open ocean to colonize distant islands. In general, the older data fit the theory—larger islands had more species, while more isolated islands had fewer species.

But when the scientists looked at more recent data, they found "a reversal of the natural pattern"—the physical isolation of an island no longer had such a negative effect. In fact, in some cases the effect was flipped; more isolated islands had more species. In recent decades, over a dozen anole species have colonized new islands that are far from their native ones. These foreign lizard have thrived on isolated islands where there are fewer native species to compete with. It's no surprise that a species will thrive when it has no competitors, but there is a reason why isolated islands lack competitors: It's hard to get there. So how did the lizards get across the open ocean?

As stowaways, of course. It turns out that shipping in the Caribbean is, for ecological purposes, redefining what it means for an island to be isolated. The researchers found that if they replaced geographic isolation with economic isolation in their model, then the standard theory of island biogeography works quite well. For example, the U.S. economic embargo of Cuba increases that island's economic isolation, and therefore it has been colonized by fewer species than expected. The scientists predict that Cuba would gain about two additional lizard species if the embargo ended. "Unlike the island biogeography of the past that was determined by geographic area and isolation," they write, "in the Anthropocene ... island biogeography is dominated by the economic isolation of human populations."

RESEARCHERS ARE INCREASINGLY RECOGNIZING that humans have to be factored into models of the Earth's natural systems. Earlier this year, a different team of researchers at Stanford and the University of California-Berkeley looked at the biogeography of bats in Costa Rica and Panama, and found that, to explain the observed patterns, they needed a new model of "countryside biogeography" that takes into account the unique features of human-populated landscapes. They conclude that new models "that explicitly account for human-made habitats" are essential for predicting how species respond to environmental changes, and for designing conservation policies that actually work.

The notion of the Anthropocene is not simply a political statement; the scientific consequences are real. Regardless of how we feel about it, we have to face the fact that, as one group of scientists write, the human species "has become so large and active that it now rivals some of the great forces of Nature in its impact on the functioning of the Earth system." We have to include ourselves in our scientific models of the Earth. With these models, we can not only better understand the impact we're having, but we're more likely to recognize opportunities to achieve our ecological goals to manage invasive species, prevent extinctions, and protect the planet's biodiversity.