Interactions between Abert's Squirrel and Ponderosa Pine: The Relationship between Selective Herbivory and Host Plant Fitness

Abstract

Abert's squirrels are highly specialized and selective herbivores and are ecologically dependent on their host species, ponderosa pine. Ponderosa pine trees defoliated as a result of inner bark feeding by Abert's squirrels (target trees) suffered significant reductions in several fitness components including incremental growth, male strobilus production, female cone production, and seed quality. Reproductive output was also depressed in trees defoliated experimentally to mimic squirrel herbivory. In previous work, I have shown that target tree selection is mediated by a suite of host characteristics, some of which are under genetic control. Abert's squirrels may be important agents of natural selection in host ponderosa pine populations because they preferentially attack trees with specific genetically determined traits and because target trees suffer significant reductions in several fitness components.