Understanding the processes that drive the spatial dynamics of biological invasions is a fundamental challenge in ecology. Longstanding theory has shown that invasion speed is primarily driven by dispersal and the density-dependent population growth rate, and until recently these drivers have been explored in a mostly ecological context. However, a growing body of evidence demonstrates that a suite of spatial evolutionary processes, acting on relatively short timescales, can accelerate invasions by selecting for increasingly dispersive and/or reproductive individuals in vanguard populations. Most of these evolutionary studies do not account for the fact that dispersal and the density-dependent growth rate may be genetically correlated, even though this correlation is likely to have important interactions with spatial evolutionary processes: negative correlations (‘tradeoffs’) should be expected to attenuate the accelerating response to evolution, while positive correlations should amplify it.
We used computer simulations to test how a range of correlations between dispersal propensity and density-dependent growth rate alter predictions for spatial selection. Additionally, we used a quantitative genetics framework to measure the genetic correlation between dispersal and the density-dependent growth rate in a population of the bean beetle Callosobruchus maculatus, which has previously been shown to exhibit evolutionarily accelerated invasion speeds.
Preliminary results from our simulations suggest that the speed of evolutionarily accelerated invasions is highly dependent on the genetic correlation between traits. Positive correlations (i.e., farther-than-average dispersers have higher-than-average per-capita growth rates) result in invasions that are faster, on average, than invasions where traits are uncorrelated. Conversely, negative correlations (tradeoffs) result in invasions that are slower on average than invasions where traits are uncorrelated. In all cases, evolving invasions spread faster than non-evolving invasions.
In addition to increasing the average invasion speed, spatial evolutionary processes have been shown to generate increased among-replicate variance in invasion speed. Here, we find that this increase in variance is also dependent on the genetic correlation between dispersal and density-dependent growth. When the correlation is positive, among-replicate variance in invasion speed increases; when the correlation is negative, among-replicate variance decreases. Thus, neglecting to account for correlations between traits misses a wide range of potential invasion outcomes. Furthermore, researchers might expect to be able to make more accurate invasion forecasts for species where individuals have (or are expected to have) tradeoffs between dispersal and their density-dependent growth rate.