Species interactions are classified by the net benefit or cost incurred by interacting organisms. However, interaction outcomes are context dependent and may change in response to biotic and abiotic conditions. Parasitism is an interaction where one species benefits at the expense of the other. Although parasitism is often considered to be one of the most widespread interactions on the planet, how different biotic and abiotic factors influence the outcome of this interaction is still poorly understood. To fill this knowledge gap, we investigated the effect of nutrient availability on host-parasite interactions using a potted plant experiment with the parasitic plant (Agalinis tenuifolia) and two host plants, Koeleria marcrantha (grass) or Ratibida pinnata (forb). Plants were grown under four interaction treatments: interspecific competition, intraspecific competition, parasitism, and grown alone (control). Each interaction treatment was then crossed with one of two nutrient treatments, high or low nitrogen, in a full factorial design with 40 replicates. All plants in the experiment were grown for three months before focal plants were harvested. We hypothesized that parasitism would have a greater negative effect on host-plant biomass when nutrients are limited, and that the effects of parasitism would be at least as great as interspecific competition.
Results/Conclusions
Overall, we find evidence that the relative outcomes of parasitism change under different nutrient conditions. For grasses under high nutrient conditions, aboveground biomass was lowest in the presence of parasite, and greatest when grown alone; biomass was higher when grown with an intraspecific competitor than when grown with an interspecific competitor. This general pattern was maintained in the low nutrient treatment, although the overall effect was reduced in magnitude. In contrast, for forbs under high nutrient conditions, biomass was lowest in the presence of intraspecific competition and highest when grown alone; biomass was higher for forbs grown in the presence of interspecific competition or with a parasite, and these two treatments were not significantly different from one another. Taken together, our results illustrate how nutrient availability can strongly mediate the outcomes of host-parasitism interactions.