Porcupines, pinyon pines, and pine engraver beetles: Multitrophic interactions in Pinyon-Juniper woodlands of Texas

Scope and method of study. The primary objective of this study was to evaluate the hypothesis that North American porcupine ( Erethizon dorsatum) bark-feeding activity predisposes the papershell pinyon pine (Pinus remota), a Pleistocene-relict, to infestation by pine engraver beetles (Ips hoppingi). Research was conducted at Kickapoo Caverns State Park in the southwestern Edwards Plateau of Texas. Radio-telemetry was used to monitor 39 animals and to describe their demographic characteristics. Morphological and physiological data were collected on 183 pairs (porcupine-damaged and undamaged) of trees on 20 transects. Data were analyzed using paired t-tests and ANOVA to determine the characteristics of trees selected by porcupines and by pine engraver beetles. Chi-square analyses were used to determine if there was an association between porcupine-damaged trees and subsequent bark beetle activity. Dendrochronlogical analysis was used to determine when porcupine populations became established in this region.; Findings and conclusions. Porcupines expanded their range into this region approximately 30 years ago, but were rare until the last 20 years. The estimated population growth of 0.034 indicated the study population continues to grow. Porcupines selected pinyon pines over more abundant species (P < 0.001) and were selective at the level of tree morphology whereas beetles selected at levels of morphology and physiology. The distribution of bark beetle infestations varied (χ2 = 75.3, d.f = 3, P < 0.01) between damaged and undamaged trees. Although attacks by beetles were evident on both types of trees, successful colonization was greater on pines damaged by porcupines. Intensity of porcupine attack, indexed by number of feeding scars and amount of bark removed, was also associated with greater colonization rates by beetles. I propose that release of volatile terpenes as a result of porcupine feeding and reallocation of carbon resources as a response to stress explain these observations.