Green, A.K.*; Ward, D.: Dispersal of hemiparasitic mistletoe (Plicosepalus acaciae) by the Yellow-vented Bulbul (Pycnonoyus xanthopygos) among Acacia trees in Israel

Infection by the mistletoe (Plicosepalus acaciae) may exacerbate the already low and infrequent recruitment of Acacia raddiana, A. tortilis, and A. gerrardii negevensis (<1%) trees in ephemeral river valleys (wadis) of the Arava valley, Israel. Yellow-vented Bulbuls (Pycnonotus xanthopygos) were shown to be the primary, if not sole, dispersal vectors of this mistletoe. Bulbuls removed the fruit’s protective exocarp, swallowed the pulp-covered seed whole, and defecated the viable, viscin-covered seed (% germination of defecated seeds=50.0%; n=66 seeds). Foraging observations during summer, fall, and winter indicated that mistletoe fruits were consumed when present, but were a substantial portion of the diet (>70% of foraging attempts) only when fruits were abundant in the late fall. Bulbuls spent most of their time perched in Acacia trees (range: 55%-92%). We combined data on bulbul movements between Acacia host trees with data on transit times of mistletoe seeds (range: 4-55 min; Mean�S.D. = 19�4 min; n=10 bulbuls) to create a probability distribution of seed deposition as a function of distance from the parent mistletoe plant (seed shadow). The seed shadow indicated that a mistletoe seed is deposited on the same host tree as the parent plant only 4% of the time. The modal dispersal distance is 40-50m (>13% probability), and the maximum distance is 270-280m (< 1% probability). These data show that the bulbul provides quality, directed dispersal for mistletoe. The increasing population of bulbuls following the increase in agriculture and human settlements in this region could further spread P. acaciae and enhance the threat to the long-term survival of Acacia populations in the Arava valley of Israel.