(This exercise is based on Schoener, T. W., D. A. Spiller, and J. B. Losos. 2001. Predators increase the risk of catastrophic extinction of prey populations. Nature 412: 183–186.)
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Thomas Schoener of the University of California at Davis and his colleagues have been conducting long-term studies of the abundances and distributions of various organisms in islands in the Caribbean.
Many islands in the Bahamas are inhabited by small anole lizards of the species, Anolis sagrei. In one set of their experiments, Schoener’s group introduced a larger predatory lizard, Leiocephalus carinatus, which preys on A. sagrei, among other organisms.
Tropical storms and hurricanes frequently travel over or near the Bahamas; some of these hurricanes are very strong. For instance, Hurricane Floyd made a direct hit on the Bahamas as a category IV hurricane on September 14, 1999, causing 3-meter storm surges at the study site. The effect of the hurricane on the anoles was severe; only juveniles (smaller than 35 mm) were found in the first survey of the islands two months after the event. The authors surmised that the anole lizards survived Hurricane Floyd in the egg stage.
Figure 1 shows the patterns of abundance of anole islands both before and after Hurricane Floyd. On the left are abundance patterns for islands without the predatory lizard L. carinatus, and on the right are abundance patterns for islands with the predatory lizard. (Note that the y axis shows abundance of A. sagrei on a log-scale).
Figure 1
Question 1
From these data, what conclusions can be drawn about the effects and interactions of biotic and abiotic features of the environment as they pertain to the distribution of anoles?
Question 2
In three of the islands with predatory lizards (A18, Z4, and Z1), the hurricane eliminated the predatory lizards. With respect to the anole populations, compare what happened on these three islands to what happened on the other three islands (N1, X10, and Z3), each of which had predatory lizards some or all of the time after the hurricane.
Question 3
With respect to islands Z4 and Z1, explain how the hurricane acted as a disturbance.
Question 4
On two of the islands with the predatory lizard, the anoles went extinct after the hurricane but were found again (in low numbers) a few months after they went extinct. What are some possible reasons for the reappearance of the anoles?
Question 5
A possible answer for Question 4 (regarding the reappearance of anoles after they went extinct) is that the anoles really weren’t extinct when the islands were sampled, but had been in the egg stage. Later on, these eggs hatched and anoles could be found on the islands again. How could one test this hypothesis? What type(s) of data would be useful?
