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

2 Responses to “False Hope of the Boom-and-Bust Model”


  1. 1 BOB BON GIORNO June 20, 2011 at 12:54 pm

    BOB, FROM SUBURBAN WATER GARDENS, INC.LOCATED IN NEW YORK HERE. I HAVE AN ALMOST NEVER ENDING BATTLE WITH SMALL POND SNAILS, AND RAMS HORN SNAILS EVER SINCE SEVERAL CLIENTS ROUGHT ME A GIFT OF THEIR EXCESS WATER HYACINTHS SEVERAL YEARS AGO. I HAVE TRIED FREEZING THE GREENHOUSE IN THE WINTER, THEN TOTALLY DRYING IT OUT IN THE LATE WINTER BEFORE GROWING ON NEW STOCK. A COMPLETE FAILURE. THIS PAST WINTER, I ADDED CHLORINE TO THE SYSTEM, WHERE THE CHLORINE LEVEL WAS OFF THE CHART ABOVE 10 PPM FOR A WEEK, A SUCCESS FOR ABOUT 2 MONTHS, THEN FAILURE AGAIN. I HAVE INTRODUCED ABOUT 25 LARGE GOLDFISH, INTO A 4000 GALLON WATER LILY GROWING TANK. THEY ARE GETTING BIGGER AND FATTER, WITH OUT BEING FED ANY OUTSIDE FOOD, BUT THE POPULATION OF POND AND RAMSHORN SNAILS ARE STILL OUT OF CONTROL. IS THERE ANYTHING I CAN ADD TO THE WATER IN OR OUT OF SEASON THAT WILL DO THE JOB?

    THANK YOU……….BOB email koi@suburbanponds.com
    site http://www.suburbanponds.com

  2. 2 snailbusters1 June 20, 2011 at 2:35 pm

    Hi Bob! Most of my experience is with trapping the larger apple snails without using chemicals. I have never worked with ramshorns. However, I did conduct some successful bench tests using chelated copper (see link below). Copper sulfate might be easier for you to find, than chelated copper. That might be your best bet. To be on the safe side, I recommend removing the fish first, then using an initial rate that is VERY low. Slowly, increase the rate until you kill the snails. Err on the side of safety for the sake of your plants. Good luck! Jess

    http://snailbusters.wordpress.com/2009/07/06/captain-copper/


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About Snail Busters

The Snail Busters Blog was created to facilitate communication between aquatic resource managers who are fighting the spread of invasive, South American apple snails, specifically Pomacea maculata (formerly P. insularum) and P. canaliculata, in the U.S.

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