INDIRECT HERBIVORE MEDIATION OF AVIAN SEED DISPERSAL IN PINYON PINE

During a 4-yr study, we examined how an insect herbivore indirectly influenced pinyon pine by affecting its avian seed dispersal agents. Colorado pinyon pine, Pinus edulis, suffered reduced cone initiation and increased cone mortality primarily from infestation by the stem- and cone-boring moth Dioryctria albovitella. Because avian dispersal agents selectively foraged where cone crops were highest, individual trees and stands of trees with greater insect abundances were avoided. Even herbivore-resistant trees with substantial cone crops did not receive dispersal services if they were surrounded by susceptible trees because the birds often ignored entire stands of trees. A cone removal experiment showed that avian seed dispersers also ignored trees with artificially reduced crops even though many cones remained. We argue that masting, the production of large cone crops at irregular intervals, may have eolved to insure dispersal success. The decline in attractiveness of entire stands of trees to dispersal agents due to infestation may functionally alter the sex expression of uninfested individuals.