Intraguild Predation and Competition Among Desert Scorpions

Abstract

Interactions among four species of desert scorpion were analyzed over a 9—yr period at two sites in the Coachella Valley, California. Although potential competitors that feed on similar anthropod prey, these species also eat one another (i.e., engage in intraguild predation); heterospecific scorpions were found to form 8—21.9% of the diet. Size differences determine scorpion—scorpion predation. Larger individuals of any species were always the predator, and prey scorpions were most frequently males and individuals that were in motion. Intraguild predation was most frequent when food availability was low. The numerically dominant scorpion, Paruroctonus mesaensis (>95% of all individuals), killed and ate 8 and 6% of two smaller scorpion species (Paruroctonus luteolus and Vaejovis confusus, respectively) and 10% of all the observed newborn of another scorpion species, Hadrurus arizonensis. Mortality due to P. mesaensis was generally an inverse function of the density of P. luteolus and of V. confusus. During a 29—mo experiment, >6000 P. mesaensis were removed from 300 100—m2 quadrats. There were significant increases in the populations of P. luteolus and V. confusus (but not of Hadrurus) in the removal quadrats, compared to control quadrats. The hypothesis that species compete exploitatively for food was also tested. Although there was extensive evidence that food limited feeding, growth, adult size, and reproduction, there was no evidence that food use by one scorpion depleted the availability of prey to other scorpions: abundance of prey (determined by trapping), percent feeding (number of individuals observed with prey ° total number of individuals observed), and mean body sizes were not greater in the removal vs. control quadrats. We conclude that intraguild predation rather than exploitation competition is the major factor structuring the observed patterns of distribution and abundance of these scorpions. We suggest that aspects of foraging behavior and the spatial and temporal patterns of surface activity of smaller scorpion species and age classes have coevolved largely to avoid predation by P. mesaensis. Many assemblages of scorpion species exhibit intraguild predation, and we speculate that this process produces behavioral and activity patterns similar to those observed in this study.