A continent-scale field survey reveals increases in population size and density, and altered demographic structure in the introduced range of a global plant invader
Understanding what makes species successful invaders requires first quantifying the key drivers of population performance over large spatial scales, as well as whether and how populations in the native and introduced regions differ. Even for the world’s most notorious invasive species, however, such data are sparse. Rigorous comparisons of individual performance and population demography between native and introduced ranges of a species require data collection from multiple populations at a similar phenological stage and representing diverse climates and habitats. A comprehensive sampling regime like this is quite difficult for a single lab group to implement. We initiated a coordinated, distributed field survey to collect data on a single species, garlic mustard (Alliaria petiolata), across its native and introduced ranges. We chose this species based on its ecological impacts, biennial life history, well-studied biology, and genetic similarity to several model organisms. Data were collected by 164 participants from 383 populations, focused on individual performance traits and population demography. We incorporated climate and weather data from online databases to model abiotic effects on plant performance and demography. Data were also collected on fungal infection rates and herbivory, as differences in enemy attack is an important driver of some plant invasions
Performance of individual garlic mustard plants did not differ between the native and introduced range. However, introduced populations reach higher density than native populations. The difference in density is due to an increase in rosette density as adult densities are similar. Thus, introduced populations have an age distribution characteristic of growing populations, with more juvenile than mature individuals. Additionally, introduced populations cover larger contiguous areas, on average, than native populations. The combined differences in density and extent lead to a dramatic difference in total population size, with introduced population being on average more than an order of magnitude larger than native populations – a mean increase of about 500,000 individuals. As predicted by the Enemy Release hypothesis, the introduced populations have reduced incidence of fungal infection, which may drive differences in population performance and juvenile densities. In conclusion, the coordinated field survey has provided rigorous data from across a broad geographic distribution that would be essentially impossible for a single lab, or even a typical group of collaborators to collect. The data reveal that population, but not individual, characteristics differ between ranges. Using seeds collected as part of the survey, we can now study the ecological and molecular genetics underlying these differences.