The Spatial Demography of Mistletoe Parasitism on a Yemeni Acacia

The frequency of parasitism of a mistletoe, Plicosephalus curviflorus, on Acacia tortilis as a function of host density and host size was investigated. The influence of host size and parasite frequency on flowering success of the parasite was also investigated. Parasitism was contagiously distributed but occurred most intensely in low-density Acacia stands. The distribution of Acacia size classes showed spatial autocorrelation, with smaller, more shrublike trees occurring in high-density clusters. Parasitism frequency and intensity were highest on larger trees, indicating that the spatial patterns of parasite distribution could be explained in part by the spatial patterns of the host size classes. Flowering success of mistletoes was not influenced by tree size but was negatively correlated with the number of mistletoes on the host. The late onset of parasitic attack with respect to the age or size of the host and the negative conspecific density effects on flowering success by the parasite could reduce adverse fitness effects on the host and thereby increase the probability of stable coexistence of A. tortilis and P curviflorus. The size structure of the host population, by influencing the distribution of parasites among the hosts, may influence the population dynamics of the parasite and the stability of the host-parasite association.