Experimental evidence linking overabundant deer to exotic invasion and native decline: Demographic and physiological mechanisms and consequences
Overabundant ungulates often drive declines in native plant abundances while increasing invasive species. Invaders can further threaten native plants through their effects on belowground processes, including the disruption of root fungal symbionts (RFS). As a result, various aboveground and belowground mechanisms link the performance of ungulates, invaders, and natives. Here, we utilize physiological and demographic approaches to assess the mechanisms underlying these linkages and quantify the short and long-term consequences for native plants. Our system focuses on the interactions between white-tailed deer, the allelopathic invader Alliaria petiolata, and the native Maianthemum racesmoum, which is a preferred browse species and highly dependent on RFS.
First, we analyze a decade of data on individually tagged Maianthemum in a full factorial deer exclusion/Alliaria removal experiment. We compare vital rates among our four treatments after seven years of Alliariaremoval and 11 years of deer exclusion using response- and odds-ratio effect sizes (RR, OR).
Second, in a greenhouse experiment we test the potency of Alliaria’s allelochemicals and determine the consequences of disrupting Maianthemum’s belowground symbionts. After applying one of three treatments to plants (Alliaria or non-allelopathic Hesperis matrionalisleaves, or a fungicide) we measure physiological responses, growth and carbohydrate storage over the growing season.
Plant growth (p <0.01, RR = 1.7), flowering (p < 0.01, OR = 5.6), and survival (OR = 1.26) increase due to deer exclusion, while dormancy decreases (OR=1.8). Quantile regression reveals that Alliaria removal benefits growth among smaller plants (increase in 25th percentile; p < 0.01, RR = 1.3) but not larger individuals (mean RR = 1.1, median RR = 1.1, 75th percentile RR = 1.1; all p > 0.3). Additionally, flowering increases (p = 0.04, OR = 1.8) and prolonged dormancy decreases (OR = 1.22) with Alliariaweeding.
Effects of allellochemicals delivered through Alliaria leaf treatments are nearly identical to fungicide treatment. Specifically, our Alliaria treatment reduces RFS abundance in the soil (p = 0.03), Maianthemum physiological rates (photosynthesis, transpiration, conductance; all p < 0.05), and carbon allocation (storage, root biomass, asexual reproduction; all p < 0.05) relative to the Hesperiscontrol.
Together, these experiments provide demographic and physiological mechanisms that account for the effects of both overabundant ungulates and invasive species on native plants at the individual and population level. Our results highlight the need for co-management of overabundant ungulates and invaders in ecosystems worldwide to conserve native plant biodiversity.