SEED DISPERSERS AS DISEASE VECTORS - BIRD TRANSMISSION OF MISTLETOE SEEDS TO PLANT HOSTS

The relationship between mistletoes and birds has been studied from th e perspectives of mutualism and seed dispersal. Here, we emphasize the role that avian dispersers play as agents of mistletoe seed transmiss ion to plant hosts. We describe the patterns of transmission of the se eds of Tristerix aphyllus, an endophytic Chilean mistletoe, on two of its columnar cacti hosts (Eulychnia acida and Echinopsis skottsbergii) by the Chilean Mockingbird Mimus thenca. In north-central Chile, thes e cacti grow in relatively discrete subpopulations on north-facing slo pes. We measured variation in seed transmission within 10 subpopulatio ns varying in species composition, host density, parasite density, par asite prevalence (defined as the percentage of hosts infested in a giv en population), and disperser abundance. Seed transmission was indepen dent of species, but was strongly dependent on prior parasitism. Paras itized individuals received seeds much more frequently than expected f rom their relative abundance. We found no correlation between the dens ity of hosts and seed transmission. We found strong positive correlati ons, however, between parasite prevalence and seed transmission to bot h parasitized and nonparasitized hosts. Seed transmission of T. aphyll us seeds by M. thenca appeared to be frequency- rather than density-de pendent. Seed transmission was also tightly and positively correlated with the abundance of seed-dispersing birds at each site. Because bird abundance and parasite prevalence were correlated, we conducted path analysis to disentangle their relative effect on seed transmission. A model including only the direct effect of bird abundance and the indir ect effect of parasite prevalence through bird abundance explained rou ghly the same variance as a full model including both the direct and i ndirect effects of bird abundance and prevalence on seed transmission. Apparently, variation in bird abundance was the main determinant of v ariation in transmission. We suggest that mistletoes, host plants, and the birds that disperse mistletoe seeds are systems well suited for s tudies of the ecological and evolutionary dynamics of disease transmis sion.