Parasitic plants and community composition: how Castilleja levisecta affects, and is affected by, its community

Abstract

Parasitic plants are native to many ecosystems around the world. Their effects on their environment are not always negative, and in some cases their presence can increase diversity in an ecosystem. We used Castilleja levisecta, a parasitic angiosperm native to the Pacific Northwest, to investigate both the effects of the parasite on the community, and the host plants' (community's) effect on the parasite. First, we examined host plant effects on the parasite by outplanting C. levisecta in host-parasite pairs, using eleven different host species, and monitored a variety of growth and reproductive traits. Second, we investigated the mechanism behind host effects by adding stable isotopes of carbon and nitrogen to host plants parasitized by C. levisecta, and tracked movement of these elements into the parasite. Finally, we used an existing study of prairie restoration methods to statistically test the effect of Castilleja plant density on the surrounding plant community. We found that the identity of the parasite's host plant did make a difference in parasite performance, in survival, growth, and reproduction. We also found that C. levisecta received differing levels of nutrition (measured in heavy isotope levels) from some host species. However, when looking at the reverse effect, we found inconclusive evidence of the Castilleja’s influence on the community. In some cases, the parasite did affect community composition, but not in a consistent pattern. In summary, the relationship between this parasite and the surrounding plant community is complex: the community has influence on, and is sometimes influenced by, Castilleja levisecta.