Posts Tagged 'population'

False Hope of the Boom-and-Bust Model

It has been a long time since my last post. The problem has not been “writer’s block” but the lack of an interesting topic. It has been very quiet in the southern U.S. regarding exotic Pomacea. Though the USFWS in Alabama continues to struggle against Pomacea insularum at two locations, officials in the rest of the South seem complacent regarding exotic Pomacea. In Florida, the official dogma is that “they tend to boom-and-bust without causing much harm. Besides, they have already spread everywhere.” I am less sanguine.

Regarding the boom-and-bust rationale for official inaction, Dr. Daniel Simberloff and Leah Gibbons (2004) said it best: “Substantial populations of invasive non-indigenous species occasionally collapse dramatically. Although disease is often invoked, the causes are rarely studied experimentally and/or quantitatively, and some collapses remain quite mysterious. Except for the few species in which spontaneous collapse has been repeatedly observed, the possibility of such an event is unwarranted as a potential rationale for a do-nothing approach to management.”

In an exhaustive study of the impact of alien species in the Mediterranean Sea, Dr. Charles Boudouresque et al. (2005) concluded, “The boom-and-bust model predicts the eventual decline of the invasive species and the recovery of the native ecosystem. In fact, species introductions are irreversible, even at a geological scale, and the natural decline of introduced species is quite uncommon. Data have been misinterpreted, leading to the generalization of the probably rare boom-and-bust model.” The graph below depicts the typical population volatility of an introduced species which is not to be confused with a permanent “bust.”

Often, the interaction between a species’ population size and its habitat is subtle. Populations fluctuate due to density-dependent factors, such as disease,
parasitism, predation, and competition, and due to density independent factors, like the weather. As I was driving my tractor through the smoky haze blanketing my farm while disking fire lanes, I pondered, “What is the primary factor causing the perceived bust in the exotic snail populations? What if it is not subtle in this case but so obvious that it is hidden in plain sight?” Then, it hit me – – the exceptional drought plaguing the Southern U.S. has temporarily stemmed the proliferation of exotic Pomacea and spawned official complacency. It makes sense because current “exceptional drought” map of the U.S. generously overlaps the range of P. insularum.

While flooding is documented to assist the range expansion of exotic Pomacea, if not the eggs, one can logically conclude that extreme drought should be detrimental. To test my hypothesis, I took a walk around Wellman Pond, the test site for our apple snail traps and my favorite place to observe Pomacea insularum. I had not visited the site for months, and while I expected some impact of the recent drought, I was amazed to see the lowest water level ever. As I walked on the dry lake bottom within the periphery of emersed vegetation, I saw numerous dead snails, stranded egg clusters, and an army of foraging fire ants (See “Rematch: Pomacea versus the Red Fire Ant”). Clearly, the exotic Pomacea have had a difficult spring season in 2011 at Wellman Pond . . . and likely elsewhere in the South.

The image above is typical of the entire shoreline of Wellman Pond. Fire Ants are devouring most of the snail eggs, while the adult snails have lost access to the refuge from predation and egg laying substrate provided by the emersed vegetation. These hard times for exotic apple snails will surely end on Wellman Pond and elsewhere in the southern United States. The current rainfall deficit will inevitably swing the other way. In some future wet period, the expansion of the exotic Pomacea will likely resume with a vengeance. The snails are more vulnerable to control efforts now than ever. This is no time for official complacency based on the false hope of the boom-and-bust model. Posted by Jess Van Dyke