Shipping lanes of the Caribbean. Credit: Félix Pharand-Deschênes/Globaïa
Shipping lanes of the Caribbean. Credit: Félix Pharand-Deschênes/Globaïa

In the 1960s, the US ceased all trade with Cuba. The embargo choked the flow of goods between the nations, turning Cuba into an economic island as well as a physical one, with severe repercussions for its economy.

It also stopped the flow of lizards.

The Caribbean islands are full of small, colour-changing lizards called anoles. There are hundreds of species, and they’re everywhere: flashing bright throat flaps from tree trunks, scampering among grass, lying invisible against branches. For most of their history, anoles colonised these islands through natural means. They’re not strong swimmers, but floating driftwood and powerful storms occasionally carried them to new homes. Now, they have easier rides.

They stow away aboard our cargo vessels, or occasionally as deliberate passengers of the pet trade. Throughout the Caribbean, where humans go, anoles follow. We provided them with new ecological opportunities; they, in turn, provided opportunities to ecologists. They gave Matthew Helmus from VU University in Amsterdam a rare chance to check a longstanding theory about life on islands—one that has held powerful sway over decades of scientific thought but has never been tested at a large scale.

The science of island biogeography—what lives where—hinges on the simple idea that an island’s geography determines the number of species that live upon it. Two factors are especially important. The first is size. Bigger islands can hold more species because they have more habitats for newcomers to exploit. They also have more barriers like rivers or mountains that can separate different populations and allow them to chart their own evolutionary courses. And since they can hold larger populations, their residents are less likely to go extinct.

Isolation also matters. Islands that are far away from other islands or from the mainland tend to have fewer species. They are harder to get to, so they receive a meagre trickle of new colonists.

Together, these factors dictate an island’s roll call of species. Size sets the maximum length of the list, while isolation governs how full it is.

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Cuban Brown Anole (Anolis sagrei). Credit: Neil Losin.

These principles have a long history, but they were crystallised in 1963 by two ecologists, Robert MacArthur and E. O. Wilson. They have been incredibly influential in how we care for the wildlife of islands, and not just pieces of water-ringed land, but also national parks or nature reserves.

Still, the theory has been hard to test. Wilson and his student Daniel Simberloff did it by fumigating some small mangrove islands in the Florida Keys and watching their wildlife recover. But these land masses were tiny—no bigger than most studio flats. To really check the tenets of island biogeography, including the rise of new species and the effects of long distances, you need big experiments that run over long time-scales. And those aren’t feasible. As Helmus puts it: “You can’t really manipulate Cuba or Trinidad.”

Except, you can. We’ve been doing exactly that for decades by inadvertently shipping animals like anoles to new islands and allowing them to run free. We’ve already done the “natural experiment”. Helmus just needed to collect the results.

With help from Luke Mahler and Jonathan Losos, he catalogued the spread of anoles throughout the Caribbean, including 34 species that hav arrived on islands far from their usual homes. He found that human activity has changed the spread of anoles in the Caribbean, but in ways that MacArthur and Wilson’s principles predicted.

For example, foreign anoles have mostly colonised islands that had disproportionately few native species for their size. These unsaturated islands had the potential to support many more species that they actually did, and were prone to incursions. So, humans have greatly strengthened the link between island area and species richness, by transporting anoles around the Caribbean and filling even isolated islands with as many species as they can handle. We have pushed the roll calls to their limits.

By contrast, our boats and ships have weakened the opposing relationship between isolation and diversity. Oceans are no impediment to us, so they are no longer an impediment to anoles. Thanks to our vessels, geographic isolation doesn’t really exist in the Caribbean.

But economic isolation is still a reality. Different islands vary in how frequently they trade with each other, and with other nations. Puerto Rico and Trinidad are awash in the currents of commerce; Cuba stagnates in financial seclusion. And this, it turns out, affects the islands’ anoles.

Helmus used data on global maritime shipping traffic to measure the economic isolation of each Caribbean island, and found that this factor predicts the diversity of the resident anoles. Simply put: more trade, more lizards. The flow of human commerce is now the most important barrier between different islands, more important even than the water between them. Physical distance has been surpassed by a different kind of distance.

To Helmus, this is another sign that we’re living in the Anthropocene—an epoch where humans have radically changed the rhythms and characters of our planet’s ecosystems. The signs are everywhere: our dying wildlife; our changing climate; and now, our misplaced lizards.

But they’re being misplaced according to the old rules. Powerful though we are, humans haven’t changed the basic tenets of island biogeography; we’ve just pushed them to new extremes. “The study shows that the theory is valid, which is exciting because we haven’t been able to test it on a broad spatial scale,” says Helmus. “Ecologists and biologists have been using it for a long time and validly so. But now we’ve shown that geographic isolation isn’t that important and needs to be swapped out for economic isolation.”

Reference: Helmus, Mahler & Losos. 2014. Island biogeography of the Anthropocene. Nature

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