IMPACT OF INSECT HERBIVORES ON COMPETITION BETWEEN BIRDS AND MAMMALS FOR PINYON PINE SEEDS

Few studies have considered impacts of insect cone herbivores on bird and mammal seed consumers. Harvests of pinyon pine (Pinus edulis) cone s by insects, birds, and mammals were negatively correlated both withi n and across sites. Birds and mammals significantly increased their co ne harvest when the other group was experimentally prevented access to trees, and mammals harvested more cones when insects were removed fro m trees with an insecticide. These results suggest that stem- and cone -boring insects (primarily Dioryctria albovittella), birds (Clark's Nu tcrackers, Pinyon Jays, and Scrub Jays), and mammals (cliff chipmunks and rock squirrels) compete for pinyon pine seeds. Additionally, insec ts affected the interaction between birds and mammals because the rati o of mammal to bird harvest increased up to 350-fold as insect cone co nsumption increased. Because insect-infested and uninfested pinyon pin es are genetically different, these observations and experiments sugge st that genetic interactions between plants and herbivores can affect the structure of a seed-consumer community.