7 Mistletoes as Parasites and Seed - dispersing Birds as Disease Vectors : Current Understanding , Challenges and Opportunities

Most mistletoes are vector-borne parasites whose vectors are their avian seed-dispersers (Martínez del Rio et al., 1996). In most vector-borne parasites and diseases, the vector maintains a parasitic or, at best, a commensal relationship with the parasite (Price, 1980). Mistletoes are unique among vector-borne parasites because they maintain a mutualistic interaction with their vectors (Martínez del Rio et al., 1996; Fig. 7.1). Birds obtain nutrients, energy and, in the desert, water from mistletoes (Walsberg, 1975; Reid, 1991). In turn, mistletoes receive directed movement of their propagules into safe germination sites (Reid, 1991). Because of the apparently specialized nature of the interaction between mistletoes and birds, the dispersal of mistletoes has received considerable attention (Cowles, 1936; Reid, 1991; Overton, 1994; Martínez del Rio et al., 1995; Sargent, 1995; Larson, 1996). Here we attempt to place the interaction between birds, mistletoes and host plants in a broad context. We argue that mistletoes present unique opportunities to integrate seeddispersal ecology with several other, seemingly disparate, areas of biology, such as plant physiology, parasitology and metapopulation ecology. We also contend that the biology of mistletoes makes them well suited for developing and testing models of how seed dispersal shapes the spatial and temporal dynamics of plant populations. To emphasize the connection between seed dispersal and parasitism, we use parasitology terminology, such as ‘prevalence’ and ‘intensity’ of infection to refer to mistletoe infection frequency and the number of mistletoes per host, respectively (Price, 1980). The ideas presented here were shaped by our research on two desert mistletoes: Tristerix aphyllus (Loranthaceae) and Phoradendron californicum (Viscaceae). The natural history of these two species is described in detail in Martínez del Rio et al. (1995) and Larson (1996). Briefly, T. aphyllus infects several species of columnar cacti in semiarid regions of Chile. Its seeds are dispersed primarily by the Chilean mockingbird (Mimus thenca).