PARASITE IMPACTS ON HOST COMMUNITIES: PLANT PARASITISMIN A CALIFORNIA COASTAL PRAIRIE

Abstract

Generalized parasites may strongly influence host communities by differentially impacting host species. However, the community effects of an important and widespread group of generalized parasites, the parasitic plants, have been poorly studied. I performed removal experiments to assess the role of a small annual hemiparasite, Triphysaria pusilla (Scrophulariaceae), in a California coastal prairie community. Removal of Triphysaria led to increased total biomass of the host assemblage; this effect was largely attributable to a substantial increase in the biomass of graminoids. In addition, host suitability differed dramatically among three annual taxa: Triphysaria parasitizing grass hosts produced 3.2 and 6.4 times more fruits, on average, than those attacking two dicot hosts, Hypochaeris glabra Lupinus nanus respectively. Availability of multiple host species failed to improve parasite performance beyond that achieved on grasses alone. Triphysaria impacts also differed greatly among host species: natural densities of Triphysaria significantly reduced the dry mass of grasses, while Hypochaeris Lupinus were unharmed. In addition to these direct effects, I tested whether Triphysaria attack influenced competitive interactions among host taxa. Natural densities of grasses significantly reduced growth and fecundity of Hypochaeris Lupinus suggesting that the presence of Triphysaria in mixed stands of hosts might allow competitive release of these dicot species. However, the combination of grasses Triphysaria caused a more severe reduction in Lupinus performance than that caused by grasses alone. Thus, observations of direct effects among pairs of species were not sufficient to explain either the outcome or intensity of multi-species interactions. Taken together, these experiments demonstrate that a generalist plant parasite can influence the structure of a natural plant community, and that interactions between a generalist parasite and its multiple host species can be difficult to predict from studies that consider only single host taxa.