Abstract
Because Allee effects 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 20 populations of Trichogramma wasps raised under similar conditions, of which 3 were subject to an Allee effect. We showed that these 3 populations were also characterized by lower maximum growth rate and lower carrying capacity, 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 population, especially during the establishment phase.
Highlights
IntroductionAllee effects (or “demographic” Allee effects, Stephens et al 1999) characterize small populations in which per capita performance increases with population density, a phenomenon known as positive density-dependence (Dennis, 1989; Courchamp et al, 2008)
Allee effects characterize small populations in which per capita performance increases with population density, a phenomenon known as positive density-dependence (Dennis, 1989; Courchamp et al, 2008)
Our analysis demonstrated the presence of an Allee effect in some, but not all, Trichogramma populations raised in laboratory conditions
Summary
Allee effects (or “demographic” Allee effects, Stephens et al 1999) characterize small populations in which per capita performance increases with population density, a phenomenon known as positive density-dependence (Dennis, 1989; Courchamp et al, 2008). In presence of “weak” Allee effects, population growth is reduced at low density but always remains positive (Courchamp et al, 2008). As the presence of an Allee effect is often considered a major risk factor for population extinction (Lande, 1988; Boukal & Berec, 2002), Allee effects are commonly included in population dynamics models for the management of small populations, either for conservation or management purposes (Tobin et al, 2011; Molnár et al, 2014; Hutchings, 2015; Wittmann et al, 2018; Bajeux et al, 2019). One of the most common heuristic population dynamics model accounting for an Allee effect is a modified version of the logistic model (Lewis & Kareiva 1993; Amarasekare 1998; Boukal & Berec 2002, with analogous models existing in discrete time, Liebhold & Bascompte 2003; Tobin et al 2007) that reads:
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