The Synergistic Effects of Stochasticity and Dispersal on Population Densities

Abstract

Using laboratory experiments, simulation models, and analytical techniques, we examined the impact of dispersal on the mean densities of patchily distributed populations. Even when dispersal leads to no net additions or removals of individuals from a population, it may nonetheless increase mean population densities if the net immigration rate is positive when populations are growing and negative when they are declining. As a model system for exploring this phenomenon, we used the yeastlike fungus Aureobasidium pullulans. In a laboratory experiment, we showed that dispersal can both ensure persistence and increase mean population densities even when dispersal among populations causes no direct addition or loss of fungal cells. From the laboratory data, we constructed a plausible model of A. pullulans dynamics among apple leaves within an orchard. This simulation model demonstrated that the effect of dispersal on mean densities is enhanced by three factors: weak density dependence of the dynamics within populations, high environmental variability affecting population growth rates, and lack of synchrony among the fluctuations of populations. Using an analytical model, we showed that the underlying mechanisms for this phenomenon are general, suggesting that a large effect of dispersal on mean population densities is possible in many natural systems.

Stability via Asynchrony in Drosophila Metapopulations with Low Migration Rates

“…Contrary to a previous report that migration leads to increase in population size (19), the mean subpopulation and metapopulation sizes of CMs in our study were significantly higher than those of LMMs or HMMs (Fig. 1C).…”

“…Thus, rigorous tests of theoretical predictions regarding the effects of migration rate on metapopulation dynamics are rare (13). Similarly, despite a large corpus of theoretical studies (16-18), the effects of migration rates on mean population size have rarely been investigated experimentally (19).Here, we report the effects of low (10%) and high (30%) migration rates on the dynamics of replicated laboratory metapopulations of the fruit fly Drosophila melanogaster in a 21-generation experiment (20). We quantified constancy stability (21) of the metapopulations and subpopulations with the use of a dimensionless measure of amplitude of fluctuation in population size over time (22).…”

“…Thus, rigorous tests of theoretical predictions regarding the effects of migration rate on metapopulation dynamics are rare (13). Similarly, despite a large corpus of theoretical studies (16-18), the effects of migration rates on mean population size have rarely been investigated experimentally (19).…”

“…1C). However, the controls used in that study (19) underwent no subpopulation extinctions, whereas subpopulations in our CMs frequently went extinct and were restarted (25). This influx of flies is the probable reason for increased mean subpopulation size in our CMs.…”

A review of extinction in experimental populations

“…Ives et al (2004) have shown that when negative density dependence is weak, migration can increase population size if immigration rate is positive when populations are growing and negative when they are declining, even when migration leads to no net additions or removals from a population. This is because migration acts to average population densities between patches.…”