Genetic Allee effects and their interaction with ecological Allee effects
Small populations, be they newly introduced or long-term residents on the verge of extinction, often face multiple challenges. First, it can be difficult for individuals to find mating partners. If this entails that populations below a certain critical population size tend to decline to extinction, we say that there is a strong ecological Allee effect. Second, even if individuals find mating partners, they are likely to be close relatives, potentially leading to inbreeding depression and loss of genetic variation. Additionally, small introduced populations may initially be maladapted to their new environment. It has been suggested that these genetic problems can also give rise to Allee effects, but such genetic Allee effects have so far received much less attention than ecological Allee effects. Our goal is therefore to 1) characterize the genetic Allee effects, if any, resulting from inbreeding depression, loss of genetic variation, or initial maladaptation and 2) understand and quantify the interaction of these genetic processes with a mate-finding ecological Allee effect. For this, we developed and analyzed stochastic models for the ecology and population genetics of small populations. To quantify and compare Allee effects, we used the success probability function, i.e. the probability that the population reaches a certain target population size given its initial population size, and the function's inflection point, which is the critical population size.
Our results suggest that in many cases inbreeding depression and loss of genetic variation reduce success probabilities, but do not generate the characteristic pattern of a strong Allee effect, i.e. there is no critical population size at which the success probability function has an inflection point. Nevertheless, adding these genetic problems to a strong ecological Allee effect can substantially increase the critical population size. Strong genetic Allee effects can arise, however, if the initial population's genetic composition is such that individuals have on average less than one offspring. For this scenario, we observed extreme interactions where a population of a certain initial size can establish in 100% of cases if only one type of Allee effect, ecological or genetic, is acting, and in 0% if an ecological and a genetic Allee effect are acting jointly. This can occur if genetic problems cause an initial phase of population decline which drives the population below the critical population size of the ecological Allee effect. Such examples highlight the importance of understanding the interaction of multiple Allee effects.