Spatial variation in Allee effects influences patterns of range expansion

In simulations comparing a population spread in a spatially homogenous (left) versus patchy (right) landscapes, variation in the Allee effect induces variation in spread patterns and increases the overall rate of spread.

By Jonathan A. Walter, Derek M. Johnson, and Kyle J. Haynes

Because of the significance to biological invasions and range shifts accompanying climate change, there is considerable interest in understanding what factors govern patterns of spread. One of these is the Allee effect, which can depress rates of population growth rates and even cause extinctions in small populations. As range edge populations are often small, Allee effects can affect movement of the range boundary; the presence of Allee effects has been shown empirically and theoretically to slow invasion rates.

Our study investigated how spread rates are affected when the Allee effect varies; recent empirical studies indicate that environmental heterogeneity can cause the strength of Allee effects to vary spatially, but how such variations influence spread patterns was unknown. We used a general model to simulate spread through landscapes where the strength of the Allee effect varied with different spatial configurations. The spatial configurations we investigated were: random variation, fine patches, coarse patches, and an increasing gradient, and these were compared to spatially homogeneous landscapes. These hypothetical configurations were based on different ways environmental variables vary in real landscapes. We also changed the average strength of the Allee effect and its range of variability.

Our main findings are that spatial variability in Allee effects shapes variation in spread patterns—spread was faster and populations reached higher densities in areas where the Allee effect was relatively weak—and, importantly, that spatial variability in Allee effects also influences mean rates of spread, not merely variability. An important implication is that isolated patches having relatively weak Allee effects could act as stepping stones for range expansion; this could pose a challenge to efforts to restrict the spread of invasive species, but be a boon to poleward or upslope range shifts in light of climate change.

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