The hidden side of the Allee effect: correlated demographic traits and extinction risk in experimental populations

Abstract

Because Allee effects (i.e., the presence of positive density-dependence at low population size or density) have major impacts on the dynamics of small populations, they are routinely included in demographic models for the evaluation of extinction risks. However, the structure of most common models implies that other demographic parameters (like the maximum growth rate) are modified by the inclusion of an Allee effect, which also affects in return the extinction risk of the population. Whether such correlations between demographic traits occur in natural populations or merely reflect a practical constraint related to model formalism is of primary importance to understand better the dynamics of small populations. We investigated this question using 30 populations of Trichogramma wasps raised under similar conditions, of which 8 were subject to an Allee effect. We showed that these 8 populations were also characterized by lower maximum growth rate and lower population size, and that their extinction probability was higher than for non-Allee populations. These results provide the first empirical demonstration of a correlation between the presence of positive density-dependence and impaired demographic performance, which increases the extinction risk of a population, especially during the establishment phase.