Nonhost diversity and density reduce the strength ofparasitoid-host interactions.

Rachel Kehoe,Flurin Jecker,Frank Van Veen,Enric Frago,Catherin Barten,Dirk Sanders

doi:10.1002/ece3.2191

Abstract

The presence of nonprey or nonhosts is known to reduce the strength of consumer– resource interactions by increasing the consumer's effort needed to find its resource. These interference effects can have a stabilizing effect on consumer–resource dynamics, but have also been invoked to explain parasitoid extinctions. To understand how nonhosts affect parasitoids, we manipulated the density and diversity of nonhost aphids using experimental host–parasitoid communities and tested how this affects parasitation efficiency of two aphid parasitoid species. To further study the behavioral response of parasitoids to nonhosts, we tested for changes in parasitoid time allocation in relation to their host‐finding strategies. The proportion of successful attacks (attack rate) in both parasitoid species was reduced by the presence of nonhosts. The parasitoid Aphidius megourae was strongly affected by increasing nonhost diversity with the attack rate dropping from 0.39 without nonhosts to 0.05 with high diversity of nonhosts, while Lysiphlebus fabarum responded less strongly, but in a more pronounced way to an increase in nonhost density. Our experiments further showed that increasing nonhost diversity caused host searching and attacking activity levels to fall in A. megourae, but not in L. fabarum, and that A. megourae changed its behavior after a period of time in the presence of nonhosts by increasing its time spent resting. This study shows that nonhost density and diversity in the environment are crucial determinants for the strength of consumer–resource interactions. Their impact upon a consumer's efficiency strongly depends on its host/prey finding strategy as demonstrated by the different responses for the two parasitoid species. We discuss that these trait‐mediated indirect interactions between host and nonhost species are important for community stability, acting either stabilizing or destabilizing depending on the level of nonhost density or diversity present.

Highlights

Interactions between species define the network structure of communities and are key components that drive ecological and evolutionary processes (Fontaine et al 2011)
Our system is comprised of plant–aphid–parasitoid communities consisting of Vicia faba L., three aphid species Megoura viciae (Buckton 1876), Myzus persicae (Sulzer), and Aphis fabae (Scopoli), and two parasitoids, A. megourae (Stary, 1965) and L. fabarum (Marshall, 1896), parasitizing M. viciae and A. fabae, respectively
The parasitoid A. megourae responded more strongly to an increase in nonhost diversity than in nonhost density, with the opposite pattern being displayed by the parasitoid L. fabarum

Summary

Introduction

Interactions between species define the network structure of communities and are key components that drive ecological and evolutionary processes (Fontaine et al 2011). Indirect interactions are considered to be of equal importance to direct interactions in explaining community structure and functions, they are often hard to observe, and their impact on ecological processes remain difficult to predict (Janssen et al 1998). Indirect effects can be mediated by changes in the density of an intermediate species or by changes in their traits (physical, chemical or behavioral) (Wootton 1993; Menge 1995). An example of a well-known trait-mediated indirect interaction is the impact of predators on plant biomass through their effect on herbivore feeding behavior (Beckerman et al 1997).

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Conclusion