Did the lizards on this island survive Hurricane Sandy?
Long time AA readers will recall that every year, a team of researchers go to the Marsh Harbour, Abaco, Bahamas to study how curly-tailed lizards affect brown anoles and, in turn, how the presence of brown anoles affect the rest of the ecosystem on tiny islands. This research, initiated by Tom Schoener and Dave Spiller in the 1980s, has been long-lasting and has produced many interesting discoveries about anoles, ecology and evolution.
But in the last few years, the research has been bedeviled by hurricanes. In 1998 and then again in 2004, studies on the effects of curly-tails on brown anoles were terminated prematurely by hurricanes. We did learn a lot about how hurricanes affect ecosystems, including lizards, but it wasn’t so helpful with regard to understanding how brown anoles evolve in response to predation pressure. After waiting several years for the islands and lizard populations to recover, we re-initiated the experiment again in 2008. At the same time, we started a second experiment by introducing a single pair of lizards to each of seven islands, creating a replicated experiment on the effects of founder events on evolutionary adaptation.
Both experiments were moving along nicely when—what else?—another hurricane hit, Irene in 2011. As everyone recalls, Irene was a big one, and we feared the worst, but by good fortune, its effects were less than catastrophic because it hit at low tide—minimizing the extent of the storm surge—and from a direction that tended to minimize the storm surge on the particular islands we were studying. The result was that some—though not all—island populations were hit hard, but very few were demolished. In fact, I speculated that studying the effect of a population bottleneck on populations created by founder events might be particularly interesting, because some theories suggest that it takes multiple population crashes to lead to great evolutionary changes.
Hurricane Irene ripping its way through the Bahamas. Abaco is the island to the right of the arrow labelled “Freeport.”
So along comes 2012 and… another direct hit on our islands, this time by the infamous Hurricane Sandy. Although initially quite large, by the time Sandy hit Abaco, its wind speeds were less than Irene, giving us hope. But, on the other hand, Sandy slowed to a crawl, meaning that it remained over Abaco for a long time, not just at low tide, and also from a more threatening direction than Irene’s. All dangerous portents.
Photo by Jason Kolbe
Thus, with great trepidation, we returned to Abaco earlier this month, anxious to learn how our island buddies had fared. A trip to Abaco last December—admittedly, not the best anole time, even in the Bahamas—had indicated that a number of the lowest lying islands seemed lizardless. And our visit confirmed that finding. The seven islands in our founder effect study are all small and low-lying, and the lizard populations on five of them were wiped out (one of them had a female last December, but she’s apparently gone). One of the remaining islands contains but three anoles—a male and two females—and it will be interesting to observe the effect of this population bottleneck. That population—like all in this experiment—was founded by two individuals and quickly grew in size. Will it rebound again, and just as quickly?
In addition, we have been following another 18 islands, 14 in our curly-tailed lizard predation experiment, plus another four. Last year, two of these island populations had perished, but the other 16 were mostly doing fine. This year, the story was not so rosy—populations on another six islands, all low-lying, were extirpated, and a seventh (on the island pictured above) was reduced to a single male plus, maybe, a female (possibly seen once, then never again). Several other populations were reduced to single digits.
So, where does that leave us? Amazingly, despite the loss of populations, the curly-tailed predator introduction experiment is still going strong. Relatively healthy populations still remain on four islands with curly-tails and two without (controls). Among these islands, the effect of the presence of the predators is still clear. Population sizes are smaller and individuals tend to perch high off the ground in the predator islands, whereas on the controls, the lizards are often found low down, even on the ground, and populations are much higher. We have measured individuals on all of these islands it will be interesting to see whether morphological differences have arisen already as a result of differences in habitat use.
However, the project now has an added direction: studying how populations recover from population bottlenecks and whether the presence of predators has an effect. Three of our islands—one experimental, two control—were reduced to a handful of lizards, as were several other islands mentioned above. It will be interesting to see how quickly, if at all, the populations recover. Moreover, we have morphological measurements and genetic data for the pre-bottleneck populations, so we’ll be able to see what effect, if any, the hurricanes have had.
Of course, this assumes that no more hurricanes hit the islands. Two years in a row is enough! Keep those fingers and toepads crossed.
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Pat Shipman
Very interesting experiments! What is the likelihood — and what would the mechanism be — for anoles or curly-tails getting to any of these islands on their own? Also, a mundane question. I realize anologists are much better than I at spotting anoles, so how long do you survey before you decide there are none that survived?
Jonathan Losos
Good questions, Pat! Both anoles and curly tails can swim, but most likely they disperse by floating on debris. Ironically, big storms may also be a positive force, knocking lots of lizard-clad vegetation into the water, as well as wiping out lizard populations on islands.
As for declaring an island lizard-free, we search each island carefully, the amount of time depending on the size of the island. Anywhere from 20 minutes to more than an hour. However, on any given day, a lizard may not be active because of temperature, humidity, or maybe it’s just taking the day off, so it’s important to go back multiple times. After about five visits, we can feel confident there’s nothing there, although it’s always possible that there’s one wary or hibernating individual that has been missed. In our experiment in which we put two lizards onto seven islands, all of the offspring the next year on each island were genetically traceable to the two founding individuals on their island, confirming that the islands had been lizard-free, or at least that any lizards on the island prior to our introduction did not contribute to the next generation’s gene pool.