Community ecology of parasitic plants

Parasites are major components of most ecological communities that can exact strong effects on their hosts. Although it is widely accepted that parasites can alter the structure of plant and animal communities, rigorous tests of the community effects of parasites are lacking. My dissertation research examines the community level interactions and indirect effects of two parasitic plant species, Castilleja wightii (Scrophulariaceae) and Triphysaria pusilla (Scrophulariaceae). I examined the effects of these parasites on the growth and fecundity of both single and multiple host species. I also examined whether parasite performance was affected by their hosts, and whether hosts indirectly influenced the reproduction of herbivores feeding on plant parasites. Finally, I reviewed studies examining parasite performance on different hosts to emphasize the consequences of host requirements for conservation of rare parasitic plants.; Greenhouse and field studies indicated that host taxa differ dramatically in their suitability for parasite growth and reproduction. Further, parasites benefited by simultaneously attacking multiple host species.; Host plants also strongly affected the performance of an aphid, Nearctaphis kachena, that feeds on C. wightii. Aphid performance was positively correlated with parasite nitrogen content, which in turn depended on the host species or combination of hosts attacked. Thus, interactions between parasites and hosts can markedly affect the tri-trophic interactions among plant hosts, parasitic plants, and herbivores.; Parasitic plants strongly and differentially impacted the performance of their host species. In addition to direct effects, I tested whether parasitic attack influenced competitive interactions among hosts. Based on studies of single host species and of competitive relationships among hosts, I predicted that competitively inferior hosts would benefit from the presence of parasites in mixed host assemblages. However, parasites harmed both competitively superior and inferior hosts, indicating that indirect effects can play an important role in multi-species interactions involving parasitism.; My experiments demonstrate that parasitic plants can strongly affect their surrounding plant and herbivore communities. More generally, my research demonstrates that studies of species pairs may be insufficient to correctly predict the outcome of multi-species interactions, thus emphasizing the importance of indirect effects.