Subdividing the spectrum: quantifying host specialization in mistletoes

Parasites necessarily depend on their hosts, but the number of host species used by a parasite varies from one to hundreds. Estimating host range and identifying the preferred host species that influence distributional boundaries and confer greater advantage to the parasite has proven elusive. As well as the confounding effects of sampling effort, characterising host specificity and preference has been hindered by considering host-use without accounting for availability. We selected three mistletoe species (Lysiana exocarpi, Amyema quandang, and Amyema lucasii) and sampled mistletoe-host interactions and host availability free from sampling bias. To quantify host specificity and identify preferred host species we applied specialist/generalist scores (G) and resource selection ratios (omega) respectively. Host specificity and preference were assessed at four scales. The generalist L. exocarpi was found to parasitise 31 plant species. Even at small scales, G values and host species turnover were high, with eight preferred hosts identified. Amyema quandang had a low G score with significant preference for half of its Acacia hosts. Amyema lucasii significantly preferred one host, consequently having low G values at all scales. By collecting potential host data and applying G scores and omega, the parasite host spectrum can be quantitatively estimated rather than qualitatively described.