The interaction between dispersal,the Allee effect and scramble competition affects population dynamics

Many organisms experience an Allee effect: their populations do not grow optimally at low densities. In addition, individuals compete with one another at high densities. The Allee effect and competition thus create a lower and an upper bound to local population size. Local populations can, however, be connected through dispersal. By using a spatio-temporal integro-difference simulation model, parameterized for Drosophila melanogaster, we explore the consequences of the Allee effect, scramble competition and dispersal for different combinations of resource distributions, initial adult distributions and densities, modes of dispersal and boundary conditions. We found that the initial distribution and density of adults determines whether a population can establish, while resource availability, the ability to reach resources and heterogeneity are mainly responsible for subsequent population persistence. In our model heterogeneity was introduced by the distribution of resources, the initial adult distribution, and the boundary conditions. Although local population dynamics are inherently unstable, overall stability can be attained by (re)colonization processes. The averaged dynamics of the total population turned out to be reasonably smooth, so apparently upper and lower local population bounds, coupled with dispersal, created an effective stable mean population density for the system as a whole. This suggests that stable mean population densities for spatial populations can be emergent properties appearing at sufficiently large scales, as opposed to inherent properties occurring at all scales. We also found, in agreement with most literature but contrary to some recent literature, that population persistence can be facilitated by a leptokurtic dispersal mode, which has higher probabilities of traveling both short and long distances, but smaller probability of traveling intermediate distances than random dispersal