Root hemiparasites are photosynthetically competent green plants that form vascular bridges to neighbor’s roots to acquire host resources. Hemiparasitism strongly suppresses host growth and reproduction, and when dominant species are more susceptible, hemiparasites can increase community diversity. Their potential as keystone species makes hemiparasites of particular interest to managers seeking to increase or maintain grassland community biodiversity. Much of our understanding of the role of hemiparasites comes from studies of annual species in European grasslands. Relatively few studies of North American grasslands have examined hemiparasites, many of which are perennials. It is not clear whether general patterns regarding hemiparasite abundance and community metrics exist. By analyzing vegetation surveys from prairies in Illinois, I tested the hypothesis that hemiparasitic species increase measures of biodiversity. Cover data from vegetation surveys of eight prairies and one old field with known populations of hemiparasites were analyzed with MANCOVA. Species number and evenness3 in sampled plots were response variables. Hemiparasite cover and cover2 were continuous variables, and site was a main effect.
Results/Conclusions
Perennial hemiparasites Pedicularis canadensis (6 sites, maximum cover = 80%) and Comandra umbellata (4 sites, maximum cover = 37.5%) were the most common hemiparasites. Hemiparasite cover (P=0.0051) and site (P<0.0001) significantly affected the multivariate expression of community response. Hemiparasite cover more strongly affected evenness3 whereas variation in species richness was explained by site. Hemiparasite cover2 was marginally significant in the overall analysis (P= 0.0732). Interactions of site with cover and cover2 were not significant, indicating that hemiparasite cover had consistent effects across these nine sites. In a univariate analysis, evenness3 was nonlinearly related to hemiparasite cover (P=0.0226). The non-monotonic relationship between hemiparasite cover and relative abundances over the range of hemiparasite cover suggests that an optimal level of hemiparasite cover maximizes evenness. These data indicate that as hemiparasites reduce productivity they alter relative abundances and evenness without promoting a more species-rich community.