Resource availability and herbivory in Larrea tridentata

Abstract

L. tridentata is a widespread, dominant shrub throughout the warm desert regions of N. America, and is abundant in the Mediterranean-climate arid regions of California. As a long-lived shrub, L. tridentata can serve as a model system to study the interactions of resource availability and herbivore activity in evergreen sclerophyll species. Resinous compounds which coat the leaves of L. tridentata deter virtually all vertebrate herbivory. However, for herbivores which can tolerate these chemical defences, variability of resource quality in leaf tissue significantly affects herbivore activity. The primary herbivores of Larrea are acridid grasshoppers with highly specialized feeding habits. The competitive interactions of these herbivores has led to the evolution of specific patterns of insect behaviour, particularly with respect of territoriality. Experimental changes in availability of water and/or nitrogen resources to host plants did not alter quantitative patterns of chemical defence, but significantly affected plant productivity and resource allocation. When highly-productive shrubs receiving water and nitrogen were compared with control shrubs, the biomass of leaves consumed by herbivores on productive shrubs was larger than on control shrubs but herbivores nevertheless consumed a greater percentage of the leaves on control plants.