Differential dispersal and survival of an African mistletoe: does host size matter

摘要

Mistletoes typically grow on tall old trees. Does this positive size-prevalence relationship result simply from the accumulation of infections as trees age, or do other factors related to tree size lead to differential dispersal, germination, establishment or survival of mistletoes? We examined patterns of infection prevalence and intensity of the mistletoe Phragmanthera dschallensis on its main host, Acacia sieberana, in a savannah in Zambia. The probability that an A. sieberana tree was infected increased with tree size, although infection intensity did not. In addition, infected trees were significantly taller than non-infected trees, even after removing the effect of tree age, using trunk diameter as a proxy. To determine whether differential dispersal occurred, we observed the feeding behaviour of the three main avian dispersers of this mistletoe, Pogoniulus chrysoconus, Lybius torquatus and Cinnyricinclus leucogaster, in relation to tree size. All three preferred perching in taller trees, whether trees were parasitized or not. To test whether differential germination of seeds or establishment of seedlings occurred, we planted mistletoe seeds on small and large A. sieberana trees and recorded germination, establishment and survival of seedlings. All seeds germinated, and seedling establishment at 7 months and seedling survival to 14 months was no different on small and large trees. Thus, host size did not affect germination and establishment. Finally, we measured the survival of established mistletoes on trees of different sizes over a 3-year period. Mistletoes survived significantly better on taller hosts. Our results support the hypotheses that higher mistletoe infection prevalence in taller trees results from differential dispersal of mistletoe seeds to tall trees as well as differential survival of established mistletoes on tall trees, and is not simply an accumulation of infections as trees age.