Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Romuald N. Lipcius


Allee effects occur when fitness suffers at low population size or density. This can lead to reduced population growth rate and a critical density below which extinction occurs. Allee effects can occur in reproduction or survival. Critical densities are higher in reproductive Allee effects than in juvenile survival Allee effects, making reproductive Allee effects more severe (higher extinction probability). In terms of the critical Allee effect threshold, reproductive Allee effects are more severe in r-selected populations, while juvenile survival Allee effects are more severe in K-selected populations. For populations with negative density dependence, there is a maximum critical Allee threshold, above which a population of any density goes extinct. Critical thresholds are altered by changes in mortality, such that Allee effects can interact with exploitation to cause non-linear population collapse. Adding two Allee effects together has unpredictable consequences depending on the life history. Adding negative density dependence makes Allee effects more severe. I tested Allee effects in the field in the queen conch Strombus gigas, a marine gastropod. Mating and egg production are reduced in low-density areas. This could be due to low encounter rates, habitat quality or ontogenetic differences. to distinguish between these hypotheses, mature conch were translocated between high and low density sites. Conch were at equal high density in all enclosures, but high source-site density produced higher reproductive activity. Conch from high-density source sites also had thicker shell lips, indicating that they were older. These data reject the hypothesis of an Allee effect; possibly reproduction is delayed after morphological maturity. In model simulations, a reproductive Allee effect and delayed reproduction could both produce non-linear population collapse. I show that predators can generate an Allee effect in prey if they have a Type II functional response and the aggregative or numerical response is not Type III (or vice versa). A literature review reveals several unrecognised examples of predation-driven Allee effects from the literature. This mechanism for Allee effects does not arise from the specifics of prey life history, which makes it difficult to predict and prepare for.



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