Although the ecological niche provides a broad theoretical framework for addressing questions in both basic and applied ecology, the challenge of empirically quantifying the niche in situ has limited our ability to test theory and improve ecological forecasting. Here, we integrate demographic and environmental data to examine the ecological niche of Alliaria petiolata (“garlic mustard”), an aggressive invasive plant in forest understories of the Eastern U.S. Beginning in spring 2018, we conducted repeated demographic censuses (4,423 individuals) within 160 plots at 16 locations that span 250 km across Southern New England. At each plot we also quantified light availability, soil moisture, and mineral nitrogen. 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 and persistence probability in the presence and absence of interspecific competition.
Results/Conclusions
Model selection results for the 9 vital rate models (used to construct the IPMs) provided strong support for the incorporation of environmental drivers, confirming the utility of our methodological framework. Overall we found high biotic resistance to invasion, such that under some environmental conditions the population growth rate was nearly halved in the presence of interspecific competitors. However, differences in vital rates (with and without competitors) during the overwinter transition predominantly contributed to this population-level effect. Biotic resistance was highly variable across abiotic niche space, primarily with respect to soil moisture. For example, biotic resistance was substantially reduced in wet environments compared to consistently high resistance in environments with average to low moisture. Increased light availability was associated with higher population growth rates, although this effect was consistent across both fundamental and realized niches and thus had little influence on biotic resistance. Overall, these results highlight the utility of demographic methods in examining the complex intersection of both abiotic and biotic drivers of invasibility.