FACTORS INFLUENCING THE CHANCE OF POPULATION ESTABLISHMENT: IMPLICATIONS FOR RELEASE STRATEGIES IN BIOCONTROL

Abstract

Successful establishment of biological control agents is hampered by factors that put small populations at risk of extinction; among these are demographic stochasticity, environmental variability, and Allee effects. I used a stochastic simulation model to tease apart the influences of these three factors on the relationship between release size and the probability of population establishment. The fates of simulated colonies indicated that demographic stochasticity alone is not likely to be important in the establishment of biological control agents, but both environmental variability and Allee effects are crucial. These two factors are distinct in the way they interact with the initial colony size. The presence of an Allee effect results in a threshold response; below a critical colony size a population is doomed, and above that size establishment is likely. In contrast, environmental variability reduces the likelihood of establishment over the entire range of colony sizes. In combination, these factors have an intermediate effect, that is, a gradual increase in establishment rate with initial colony size. These results can be used to design release strategies that will maximize the probability of successful establishment given the inherent trade-off between the size of each release and the number of releases. In a variable environment, a large number of very small-sized releases will maximize the chance of overall establishment. When an Allee effect is present (in a constant environment), a single large release is optimal. For some combinations of both factors, there will be an intermediate optimum release size (and corresponding number of releases). This study provides an avenue for improving the rate of successful biological control through wisely designed releases. The results of this study have important implications for the conservation and reintroduction of threatened populations as well.