Large herbivores facilitate an insect herbivore by modifying plant community composition in a temperate grassland.

Xiaofei Li,Zhiwei Zhong,Moshe Inbar,Chelse Prather,Petri Nummi,Deli Wang,Ho Yi Wan,Hui Zhu,Qiang Gao,Shengnan Wang

doi:10.1002/ece3.8327

Abstract

Large herbivores often co‐occur and share plant resources with herbivorous insects in grassland ecosystems; yet, how they interact with each other remains poorly understood. We conducted a series of field experiments to investigate whether and how large domestic herbivores (sheep; Ovis aries) may affect the abundance of a common herbivorous insect (aphid; Hyalopterus pruni) in a temperate grassland of northeast China. Our exclosure experiment showed that 3 years (2010–2012) of sheep grazing had led to 86% higher aphid abundance compared with ungrazed sites. Mechanistically, this facilitative effect was driven by grazing altering the plant community, rather than by changes in food availability and predator abundance for aphids. Sheep significantly altered plant community by reducing the abundance of unpalatable forbs for the aphids. Our small‐scale forb removal experiment revealed an “associational plant defense” by forbs which protect the grass Phragmites australis from being attacked by the aphids. However, selective grazing on forbs by sheep indirectly disrupted such associational plant defense, making P. australis more susceptible to aphids, consequentially increasing the density of aphids. These findings provide a novel mechanistic explanation for the effects of large herbivores on herbivorous insects by linking selective grazing to plant community composition and the responses of insect populations in grassland ecosystems.

Highlights

In grasslands around the world, two ubiquitous functional groups of herbivores, large vertebrates and insects, commonly coexist and interact with each other
Our small-scale mechanical removal experiment revealed that in our system, we found that food availability (P. australis) and the abundance of unpalatable neighboring plants, rather
These studies commonly focused on the interactions between one herbivore species and plant communities, largely ignoring the fact that herbivores can themselves alter the properties of plant community

Summary

| INTRODUCTION

In grasslands around the world, two ubiquitous functional groups of herbivores, large vertebrates and insects, commonly coexist and interact with each other. In a community with diverse herbivore assemblages, the consumptive and non-consumptive activities (e.g., trampling) of large vertebrate herbivores may eliminate or reduce the abundance of these unpalatable neighbors (Augustine & McNaughton, 1998; Liu et al, 2015; Olff & Ritchie, 1998), potentially dissolving associational plant defenses and benefiting co-occurring herbivorous insects (Zhong et al, 2014) Despite these suggestive evidence, until more recently few empirical studies have investigated the potential consequences of large-herbivore-induced changes in plant community context on other co-occurring herbivorous insects, limiting our full understanding of the mechanisms of species co- existence and community assemblages of herbivore species. We conducted an additional, small-scale field experiment to explore the possible mechanisms by which sheep grazing may affect aphid abundance

| METHODS

Findings

| DISCUSSION