Abstract: Although niche theory provides a broad framework for addressing basic and applied ecological questions, the challenge of empirically quantifying the niche in situ has limited our ability to test theory and improve ecological forecasting. Here, we integrate demographic ( >4,500 individuals) and environmental (160 plots) data to quantify the niche of Alliaria petiolata (“garlic mustard”) across southern New England. By controlling for interspecific competition, we are able to examine fundamental and realized niches, and thereby quantify biotic resistance to invasion across both niche space the life cycle. To address the limitations of examining asymptotic population growth rates, we incorporate density-dependent feedbacks and evaluated modeled populations in terms of self-stabilized densities. Moreover, we quantify density dependence in terms of total A. petiolata biomass, which represents a more comprehensive metric of the intraspecific competitive environment than the number of individuals. Thus, the full demographic vital rate models that underly integral projection models (IPMs) include effects of individual plant size and plot-level values of soil moisture, light availability, soil mineral nitrogen, and conspecific aboveground biomass.
Overall, the realized niche was greatly constrained compared to the fundamental niche, with modeled A. petiolata densities reduced between 50% and 99% in the presence of interspecific competitors, i.e. strong biotic resistance to invasion. However, the influence of environmental factors depended on competition. Without competitors, A. petiolata density was primarily determined by soil moisture, achieving the highest levels at moderate moisture. The presence of competitors increased the population-level influence of all environmental factors, such that densities were greatest in high light, high nitrogen, and moderate moisture environments. These differential environmental effects in the presence and absence of competitors resulted in overall biotic resistance that primarily varied with light availability and was strongest in dark environments. Moreover, the population-level consequence of biotic resistance was negligible for early A. petiolata life cycle stages and was most pronounced for overwinter survival. These results indicate that measures of biotic resistance in a single environment or life cycle stage may be difficult to generalize and highlight the utility of tightly integrating environmental and demographic data.
