Variation in biotic resistance to Alliaria petiolata invasion is mediated by density and the environment

Alden B. Griffith, Environmental Studies, Wellesley College, Wellesley, MA and Elizabeth Bouchard, Graduate Program in Ecology and Evolution, Rutgers University, New Brunswick, NJ

Background/Question/Methods
The ecological niche provides a broad theoretical framework for addressing questions in both basic (e.g. species coexistence) and applied (e.g. biological invasions) questions in ecology. As niche space is ultimately determined by the persistence of populations, demography provides particularly relevant methods for empirically quantifying the niche. Moreover, this approach allows for a population-level assessment of biotic resistance to invasion within the broader community ecology framework of fundamental vs. realized niches. In this study, we integrate demographic and environmental sampling to examine biological resistance to invasion of Alliaria petiolata (“garlic mustard”) across niche space. We established a total of 160 plots at 16 sites across Massachusetts, Connecticut, and New York invaded by A. petiolata. We measured the size, survival, and fecundity of over 4,500 A. petiolata individuals during eight censuses between June 2018 and July 2020. In addition, we quantified light availability, soil nitrogen, soil moisture. We experimentally removed understory competitors from half of the plots in order to assess biotic resistance and quantify both fundamental and realized niches. Environmentally-driven integral projection models (IPMs) were constructed to quantify the niche with respect to population growth rate in the presence and absence of interspecific competition and across a range of intraspecific densities.

Results/Conclusions
The magnitude of biotic resistance by the resident understory plant community varied substantially across environmental niche space. In environments with relatively low light and soil moisture, invader population growth rates (λ) at low density were reduced by up to 50%. Similarly, strong biotic resistance was also observed in soils with relatively high mineral nitrogen. However, biotic resistance was minimal in high moisture environments, with similar invasive λ values in the presence or absence of interspecific competitors. The magnitude of biotic resistance was greater and more consistent across all environments for high-density invasive populations, indicating a lower sensitivity of biotic resistance to variation in environmental conditions at high density. The presence of interspecific competitors was also associated with strong negative density dependence, suggesting that releasing invading populations from interspecific competition also releases them from intraspecific competition. This effect was to such a degree that the maximum densities observed in the field in the absence of biotic resistance were not enough to constrain projected population sizes. Overall, these results highlight the utility of demographic methods in examining the complex intersection of both abiotic and biotic drivers of invasibility.