Testing intraguild predation theory in a field system: does numerical dominance shift along a gradient of productivity?

Abstract

Although ecological theory exists to predict dynamics in communities with intraguild predation (IGP), few empirical tests have examined this theory. IGP theory, in particular, predicts that when two competitors interact via IGP, with increasing resource productivity: (1) the IG predator will increase in abundance as the IG prey declines, and (2) increasing dominance of the IG predator will cause resource density to increase. Here, we provide a first test of these predictions in a field community consisting of a scale insect and its two specialist parasitoids, Aphytis melinus (the IG predator) and Encarsia perniciosi (the IG prey). The shared resource, California red scale, is a pest of citrus, and its productivity varies across a threefold range among citrus cultivars. We examined both absolute and relative densities of parasitoids along this natural gradient of scale productivity in three citrus cultivars (orange, grapefruit and lemon). Although both parasitoid species were found in all three cultivars, their abundances reflected those predicted by IGP theory: the IG prey species dominated at low productivity and the IG predator dominated at high productivity. This relationship was caused by an increase in Aphytis density with productivity. In addition, the density of scale increased with the dominance of the IG predator. These results from a field system demonstrate the important dynamic outcomes for food webs with IGP.

How functional response and productivity modulate intraguild predation

“…Following the publication of a general theory of IGP by Holt and Polis (1997), several models and empirical studies have investigated the nature and prevalence of IGP in diverse terrestrial and aquatic communities. An important component of this theory is the effect of environmental productivity on the coexistence of competing organisms (Holt and Polis 1997, Morin 1999, Diehl and Feissel 2000, 2001, Mylius et al 2001, Borer et al 2003. According to the theory, only the intermediate predator (i.e., the intraguild prey; IG prey) should persist at low productivity because it has a better capacity to exploit resources than the top predator (i.e., the intraguild predator; IG predator), whereas the latter should drive the IG prey to extinction at high productivity through a combined effect of competition and predation.…”

“…According to the theory, only the intermediate predator (i.e., the intraguild prey; IG prey) should persist at low productivity because it has a better capacity to exploit resources than the top predator (i.e., the intraguild predator; IG predator), whereas the latter should drive the IG prey to extinction at high productivity through a combined effect of competition and predation. Therefore, IGP theory predicts that coexistence can only occur at intermediate productivity levels (Holt and Polis 1997, Diehl and Feissel 2000, Mylius et al 2001, Borer et al 2003, Borer et al 2007), but few empirical studies have validated this prediction Feissel 2000, 2001). In contrast to the theory, many field and laboratory experiments have reported cases of coexistence over a large gradient of productivity (Lawler and Morin 1993, Morin 1999, Amarasekare 2000, Borer et al 2003, Amarasekare 2007, Lucas and Rosenheim 2011.…”

“…Therefore, IGP theory predicts that coexistence can only occur at intermediate productivity levels (Holt and Polis 1997, Diehl and Feissel 2000, Mylius et al 2001, Borer et al 2003, Borer et al 2007), but few empirical studies have validated this prediction Feissel 2000, 2001). In contrast to the theory, many field and laboratory experiments have reported cases of coexistence over a large gradient of productivity (Lawler and Morin 1993, Morin 1999, Amarasekare 2000, Borer et al 2003, Amarasekare 2007, Lucas and Rosenheim 2011. Moreover, IGP generally tends to decrease as the density of the shared or extraguild (EG) prey increases (Lucas and Rosenheim 2011), suggesting that at high productivity (typically characterized by high EG prey density) IGP may not be common and coexistence possible.…”

“…Similarly, Lucas and Rosenheim (2011) hypothesized that the shape of the predators' functional response contributes to explain why IGP is less frequent when EG prey density increases. Most interestingly, theoretical model predictions differ depending on the type of functional response: IGP models assuming a linear functional response for the IG predator lead to predictions of high extinction probability of the IG prey under conditions of high productivity (Holt and Polis 1997, Diehl and Feissel 2000, Borer et al 2003, Briggs and Borer 2005, Borer et al 2007, Hall 2011) whereas, with nonlinear functional response models, predation rate reaches a maximum at high prey densities resulting in a weaker interaction that could decrease IGP intensity and thereby the probability of IG prey extinction (McCann and Hastings 1997, McCann et al 1998, Uchida et al 2007, Rall et al 2008, Williams 2008, Abrams and Fung 2010. Therefore, testing assumptions of theoretical models and evaluating differences between the linear and nonlinear functional response models is crucial for improving these models and deepening our understanding of multiple predator interactions across a productivity gradient.…”

Syrphid aphidophagous predators in a food-web context

“…2001); herbivore defences vary on different plants (de Moraes & Mescher, 2004); intraguild predation modifies natural enemy impact (Borer et al, 2003); trophic cascades occur in aphidophage communities (Matsumoto et al, 2003); effects occur over more than three trophic levels (Harvey et al, 2003); and community level effects are assuming greater and greater importance as our knowledge of them expands (e.g. Rutledge et al, 2003).…”