Predicting geographic invasion patterns based on experimental demography: A comparison of two invasive plants and their native analogs in New England

Background/Question/Methods

Invasive species’ geographic distributions are often not at equilibrium in their invasive ranges. Therefore, invasion risk based on current occurrence patterns may lead to biased estimates of the locations at risk of invasion. We took an experimental biogeographical approach to investigating the underlying demographic processes that drive population dynamics across a range of potentially invasible environmental conditions. By linking demographic rates (growth, survival, fecundity) to explanatory environmental variables we obtained a mechanistic basis for determining species range limits. These mechanisms are particularly important for understanding invasive species distributions to improve early detection abilities for species with nonequilibrium distributions.  We investigated population dynamics of two invasive species: a monocarpic biennial (Alliaria petiolata; garlic mustard) and a woody shrub (Berberis thunbergii; Japanese barberry) and two native ecological analogs using Hierarchical Bayesian Integral Projection Models (IPMs) to predict establishment-phase population growth rates (λ) across New England. 

Results/Conclusions

The population growth rates varied widely by environment, however the invasive species had higher values than native analogues in all cases. Better performance by the invasive biennial was driven by higher survival, earlier reproduction, and higher seed production than its native analogue. Better performance by the invasive woody shrub was driven by higher germination success and seed production than its native analogue. The predicted geographic distribution of both native species, based on the ability to establish, showed that the species were primarily restricted to southern New England, matching their known distributions. In contrast, the invasive analogues were not limited by climate in New England, and could readily establish given suitable local habitat conditions. These results suggest that the distributions of both A. petiolata and B. thunbergii have been limited by dispersal and that the species are an invasion risk in northern New England where they do not currently occur. The links that we illustrate between environment, demographic rates, and population dynamics are critical for understanding the potential distributions of range-shifting species to gain confidence in extrapolation to regions at risk of invasion.