The global economy hasn't just opened up new markets for humans, it's been a catalyst for bringing species to into habitats where they don't belong. Biologists have recorded the many places where these invasives have thrown foreign ecosystems into disorder. However, there hasn't been a lot of research that looks at how globalization has changed the way we calculate biodiversity.

A new study, published in today's Nature, argues that human economic activity should be included as a major factor for predicting the total number of species in a given ecosystem. Based on research that compares shipping activity to the proliferation of lizards called anoles (pronounced ah-NOL-ee) in the islands of the Caribbean, the authors believe that economic activity should be added as a parameter to the theory of island biogeography, which attempts to explain an ecosystem's biological richness based on its size and a few other factors, including the rate at which new species arrive and old ones go extinct.

The theory can be used to estimate how many species will live on almost any isolated landmass (it needn't be surrounded by water, an "island" can be an oasis, an alpine lake, or even a patch of forest in a sea of farmland), based mostly on two parameters: size and isolation. An island's size tells scientists how much habitat is available, as well as how big a target it is to potential colonizing species. Its isolation, or distance, from the "mainland" (the closest, large, habitable area) also indicates how likely the island is to be colonized, since few plants and animals dispersing from a mainland source are likely to successfully make landfall on more remote islands. "It turns out, if you know an island's area, isolation, and a few other factors, you can predict the number of species you’ll see," said Luke Mahler, a biologist at UC Davis and co-author of the study.

The diversity of anole species across the Caribbean had big jumps after World War II and the Cold War. Matthew R. Helmus et al./Nature

But Mahler and his co-authors reasoned that as the Caribbean islands became more connected to the global economy, the number of ships coming in and out of their harbors would increase. All this marine traffic would inevitably bring new species of anoles, stowed away in cargo holds, to far flung islands, lessening the effect of isolation.

Not only did their findings support this theory, but they also found that historical economic data could explain the rate at which new species were being found outside their established ranges within the Caribbean. As the graph to the left shows, different species of anoles started appearing outside their established ranges with greater frequency after the respective endings of World War II and the Cold War. Global commerce increased in both periods.

Their findings led the authors to recommend that other researchers consider trade when they are calculating biodiversity. Mahler points out that as the hubs of global trade continue to shift, research like this could help places protect their ecological integrity. "We need to look more at how human trade and human travel affect the transport of species," said Mahler. "Anything that is introduced has enormous potential to cause extinction."