Sitting on an airplane from Boston to Miami, en route to Marsh Harbour, I realized that this trip marks the 20th anniversary of my Bahamian fieldwork. It was as a callow new postdoc at the University of California, Davis, that I first embarked to the Bahamas in May, 1991, setting forward a research program that has brought me back every year in the past two decades, some times more than once per year. I’ve lost track of how much time in total I’ve spent there, but it’s been more than a year of my life (of course, I should point out that my colleagues in crime have been going there even longer, Dave Spiller since the 80’s and Tom Schoener, since the 70’s).
What keeps bringing us back? Despite what you might think, it’s not the beaches, or even the casinos! The primary reason is that in many areas of the Bahamas, there are a large number of very small islands. They are officially termed “rocks,” and aptly so: they are craggy dots of limestone sharpness, ranging in size from a few square meters on up. There are several things that are great about these islets. First, they have complex ecosystems, but not too complex: a few species of bushes and trees, a variety of insects and other arthropods, and often only one species of lizard, the brown anole, Anolis sagrei. As the islands get bigger, they become lusher and more species rich in everything, including lizards. Second, many of the islands are just the right size: big enough to have lizards (the smallest islands generally don’t), small enough that we can easily census the populations of lizards, spiders, plants, and other creatures. Third, there are lots of islands, so we can use them as test tubes to look for generality in our studies of ecological and evolutionary mechanisms. Finally, fourth, the Bahamian government is very enlightened in their approach toward scientific research. In particular, they have allowed us to translocate lizards to islands on which they don’t occur. Now, don’t get your knickers in a bunch, it’s not really a problem. What we have done is taken a species—such as A. sagrei—that occurs on the nearby larger island and put them on some of these rocks. Realize that the larger island is usually a stone’s throw away, usually within 100 meters. And, the lizards colonize these islands naturally—we’ve monitored islands for several decades now, and have recorded islands that were, sadly, lizardless suddenly sprouting a lizard population (a joyous find, indeed), the result of a waif individual making the crossing (anoles float, and are adept at clinging to vegetation thrown into the water, which may be quite frequent during the stormy season); our ongoing genetic studies are beginning to show that such colonization may occur at a higher rate than even we anticipated. If you’re still bothered, consider this: the reason that most of these islands are lizardless is not because the populations can’t survive there—we’ve shown clearly that they can. Rather, it’s because hurricanes periodically sweep through the area and wash away the lizards on all low-lying islands (our islands are usually less than 5 m in elevation above sea level), as we have now documented—to our dismay, as several experiments have ended prematurely—several times.
So, by taking advantage of these islands, we can conduct the sort of replicated ecological and evolutionary experiment that is the hallmark of laboratory science. What have we learned? Well, quite a lot. I can’t summarize it all here, but look at Lizards in an Evolutionary Tree or Google “Thomas Schoener” or “David Spiller” and check out our papers (like this one). But, in short, here’s some of what we’ve found, focusing on our two main questions. First, we asked: what role do lizards—specifically A. sagrei—play in the ecosystem? In particular, A. sagrei eats insects, and insects eat plants, so one might suspect that adding lizards to an island will be good for the plants. But hold on a second—the lizards also eat spiders, and spiders eat insects. So, which effect is stronger: the direct negative effect of lizards eating insects, or the indirect positive effect of lizards eating the spiders that eat the insects? The answer is the former. Although the lizards hammer the spiders, they more than compensate for the removal of these aranean predators, and as a result, insects decline, and with them, so does damage to plants. Anoles are good horticulturists!
Our second line of research looks at natural selection and adaptation. From many decades of work by many people, we have a good understanding of how anoles, at least in the Greater Antilles, adapt to different circumstances. One particular feature involves their hind limbs: when they use broad surfaces, they evolve long hindlimbs, and when they use narrow surfaces, the legs become shorter. Our first work on this topic, on Staniel Cay in the middle of the Bahamas, found that experimentally translocated populations of A. sagrei had differentiated, and a relationship existed, among populations on different islands, such that the broader the vegetation the lizards used, the longer their legs. More on Staniel Cay and that study next week.
Our current ongoing work is based out of Great Abaco in the northern end of the Bahamas. For the last decade and a half, we have been combining the two lines of research by adding another level to the food web: we’ve introduced a ground-dwelling, predatory lizard, the charming curly-tailed lizard, Leiocephalus carinatus (as with our previous studies, curlies occur on Great Abaco, naturally colonize these rocks, and are wiped out by hurricanes). This experiment had two purposes. First, we’ve added another layer to the food chain: curlies eat A. sagrei (a lot, much more than we ever anticipated), and they also eat spiders, so what will the net effect be on the insects and the plants? And, the brown anoles, no dummies, take one look at the curlies and head for the hills…or at least up the bushes (they do come down to the ground some times, though, and that’s when they get eaten). There are few big trees on these islands, so becoming arboreal means using narrow vegetation, and we know what that should lead to: natural selection for short legs!
Our results so far are interesting: the food web effects are kind of complicated. Strong effects of curlies on anoles (negative) and spiders (positive), but less consistent effects on insects and plants. With regard to evolution, we recorded strong selection on anoles over the course of the first generation. Unfortunately, nature has been misbehaving. The experiment has been wiped out twice by hurricanes, not letting it proceed long enough to look at the evolutionary consequences. Our specific prediction is that, forced into the bushes, A. sagrei will start eating different prey, having a stronger impact on arboreal species and a lesser one on terrestrial ones (which will, however, suffer from the curly onslaught). However, over time, we hypothesize that the browns will adapt to the arboreal life, and as a result, their impact on arboreal prey will increase through time. The experiment has now been running for three years (after a four year hiatus to let the islands recover from the last hurricane). Early results are promising, but, as always, we never know what we’ll see when we get to the islands. Last year, brown anole populations on islands with curly tails were way down. Will that continue? Will any populations be extirpated? We’ll know soon enough.
And now, I’ll continue my fruitless effort to garner sympathy. It’s hard work! The sun beats down on us. And, sad to say, it’s not really that pretty there. Sure, we’re on the water, and every now and then dolphins escort us from one island to another, and we see sea turtles (man, they’re fast) and rays and now and again a very big shark. And, yes, there often is a nice breeze and the lizards are always charming. But, you know, it’s a pretty dry area, really, and the vegetation is scraggly dry forest, and the lizards don’t occur on the beaches (well, the curlies do) and we don’t even really like beaches (at least I don’t), and the biodiversity is kind of limited: the lizards are great, but there are no poisonous snakes or jaguars or monkeys or lots of other cool stuff, like in Costa Rica. Really, it’s a dirty job, but someone has to do it, and I’m ready to accept your expressions of gratitude.