Plant-Feeding by Arthropod Predators Contributes to the Stability of Predator-Prey Population Dynamics

Abstract

Plant-feeding is common in many taxa of arthropod predators, particularly in the predaceous Heteroptera (Cohen 1996, Naranjo and Gibson 1996, Coll 1998). Plant-feeding predators fit the classic definition of omnivores in the sense that they feed at more than one trophic level (Pimm and Lawton 1978). However, several features distinguish these plant-feeding predatory Heteroptera from omnivorous taxa that solely feed on animal prey from different trophic levels. First, the relative size of the plant is large compared to the predator, and, as a result, each plant-feeding event removes only a small proportion of the plant's body mass. Second, the plant constitutes the habitat for the herbivorous prey species, and thus defines the arena for prey search by the predator. Under these circumstances, the encounter rate of the predator with plant food is usually high compared to that with its prey, and does not change after plant feeding occurs (Coll 1996, Coll and Izraylevich 1997). Third, differences in the chemical composition, nutritional value and feeding dynamics (e.g. handling time) of plant vs animal food are much greater than those between different animal prey species (Coll 1996, Coll and Izraylevich 1997, Cohen 1998). Plant-feeding predators, so called zoophytophages, have come under increasing scrutiny for use as agents for biological control of agricultural pests (Alomar and Wiedenmann 1996, Coll and Ruberson 1998). Plantfeeding by predators is thought to contribute to the success of biological control because the presence of the plant as an alternative food source allows predators to colonize crops before the arrival of prey, and to persist during periods of prey scarcity (Naranjo and Gibson 1996, Wiedenmann et al. 1996). In addition, the nutritional benefits of plant feeding may lead to improvements in life-history traits (e.g. increases in development rate, survival, and fecundity) that, in turn, result in increases in predator reproduction and rates of predation (Naranjo and Gibson, 1996). Although the stability of predator-prey population dynamics may also be critical to the success of biological control, only one previous study has addressed the influence of plant-feeding on population stability in systems with zoophytophagous predators (Coll and Jzraylevich 1997). In a model developed by Coll and Izraylevich (1997), increases in the degree of plant-feeding in a population of predators resulted in decreases in stability and increasingly chaotic dynamics. Similarly, in classic work on the influence of omnivory on population stability, systems with more omnivory were also found to be less stable (Pimm and Lawton 1978). This theoretical result and the results of early surveys for omnivory led to the conclusion that omnivory is relatively rare in nature (Pimm and Lawton 1978, Pimm 1982). Recently, this idea has been challenged. More detailed analysis of feeding relationships in food webs shows that omnivory is much more common than was previously thought (Polis et al. 1989, Polis 1991, Polis and Holt 1992, Diehl 1993, Polis and Strong 1996). In addition, recent theoretical analyses and empirical tests of the effects of omnivory on food webs have revealed that omnivory can increase stability (Lawler and Morin 1993, Fagan 1997, McCann and Hastings 1997, Holyoak and Sachdev 1998). Here, we expand on the analysis of Coll and Izraylevich (1997) and further explore the influence of plantfeeding on the stability of predator-prey dynamics. Plant feeding by omnivorous arthropod predators is apparently common in nature, although the results of