The rapid proliferation and spread of invasive species may be driven by a more favorable climate, release from natural enemies, or other ecological and genetic differences between native and introduced populations. Alternatively, ecological conditions and genetic variation may be similar in both ranges, such that introduced populations ultimately reach an equilibrium in abundance and distribution that mirrors the native range. Testing these hypotheses has been difficult without comprehensive field data. We initiated a coordinated-distributed field survey to measure individual- and population-level performance, as well as herbivore and pathogen damage in the invasive plant Alliaria petiolata (garlic mustard). From 2009 to 2013 we measured population extent, density, demographic structure, and incidence of herbivores and fungal pathogens at 395 locations throughout Europe and North America. Within each location, we measured height, number of fruits, leaf number, and damage by herbivores and pathogens on up to 50 mature adult plants. We also recorded habitat characteristics and interpolated bioclim climatic variables from the CliMond.org dataset. We used hierarchical model selection with spatially autocorrelated error terms to compare the relative influence of abiotic environment, natural enemies, and continent of origin on performance of A. petiolata throughout Europe and North America.
Results/Conclusions
We found considerable variation among populations within each continent, but on average introduced populations were almost 10x larger than native populations. Additionally, 14 of 395 populations contained an estimated 1,000,000+ individuals, all in North America. By contrast, the largest European populations did not exceed 500,000 individuals. Introduced populations also had a higher average ratio of juvenile rosettes to mature adults, suggesting increased germination and seedling survival and higher population growth rates. Abiotic environment, natural enemies, and continent of origin were all significant predictors of population size and demographic structure. Together, these three models accounted for up to 37% of the variation in performance among populations. However, abiotic models alone explained up to 36%of the variation. Our coordinated distributed field study provides strong evidence that invasive populations receive a performance boost, but climate is a much stronger predictor of performance than enemy release or continent of origin. Our work also demonstrates the value of taking a coordinated-distributed approach for studying species with large geographical distributions.