Impact of Insect Herbivores on Competition Between Birds and Mammals for Pinyon Pine Seeds

Abstract

Few studies have considered impacts of insect cone herbivores on bird and mammal seed consumers. Harvests of pinyon pine (Pinus edulis) cones by insects, birds, and mammals were negatively correlated both within and across sites. Birds and mammals significantly increased their cone harvest when the other group was experimentally prevented access to trees, and mammals harvested more cones when insects were removed from trees with an insecticide. These results suggest that stem— and cone—boring insects (primarily Dioryctria albovittella), birds (Clark's Nutcrackers, Pinyon Jays, and Scrub Jays), and mammals (cliff chipmunks and rock squirrels) compete for pinyon pine seeds. Additionally, insects affected the interaction between birds and mammals because the ratio of mammal to bird harvest increased up to 350—fold as insect cone consumption increased. Because insect—infected and uninfested pinyon pines are genetically different, these observations and experiments suggest that genetic interactions between plants and herbivores can affect the structure of a seed—consumer community.