Grassland Arthropods Are Controlled by Direct and Indirect Interactions with Cattle but Are Largely Unaffected by Plant Provenance.

Kelly Anne Farrell,Claudia Stein,Elizabeth T Borer,W Stanley Harpole,Katharine N Suding,Erjun Ling

doi:10.1371/journal.pone.0129823

Abstract

Cattle grazing and invasion by non-native plant species are globally-ubiquitous changes occurring to plant communities that are likely to reverberate through whole food webs. We used a manipulative field experiment to quantify how arthropod community structure differed in native and non-native California grassland communities in the presence and absence of grazing. The arthropod community was strongly affected by cattle grazing: the biovolume of herbivorous arthropods was 79% higher in grazed than ungrazed plots, whereas the biovolume of predatory arthropods was 13% higher in ungrazed plots. In plots where non-native grasses were grazed, arthropod biovolume increased, possibly in response to increased plant productivity or increased nutritional quality of rapidly-growing annual plants. Grazing may thus affect plant biomass both through the direct removal of biomass, and through arthropod-mediated impacts. We also expected the arthropod community to differ between native and non-native plant communities; surprisingly, arthropod richness and diversity did not vary consistently between these grass community types, although arthropod abundance was slightly higher in plots with native and ungrazed grasses. These results suggest that whereas cattle grazing affects the arthropod community via direct and indirect pathways, arthropod community changes commonly associated with non-native plant invasions may not be due to the identity or dominance of the invasive species in those systems, but to accompanying changes in plant traits or functional group composition, not seen in this experiment because of the similarity of the plant communities.

Highlights

Through the intentional and incidental introduction of non-native plant species into grasslands, coupled with heavy agriculture and livestock grazing, many of the world’s grasslands have experienced significant changes in plant composition and diversity [1,2]
Grazing resulted in increased biomass of herbivorous arthropods and decreased biomass of predatory arthropods
These results suggest that grazing may alter plant biomass and composition directly via vegetation removal and indirectly via an altered arthropod consumer community

Summary

Introduction

Through the intentional and incidental introduction of non-native plant species into grasslands, coupled with heavy agriculture and livestock grazing, many of the world’s grasslands have experienced significant changes in plant composition and diversity [1,2]. Understanding the effects of grazing and grassland invasion on arthropods represents a critical gap because of the key role arthropods play in the composition and function of grasslands. Arthropod diversity often covaries with plant diversity [7,8], plant functional group diversity [8], plant community composition [12], or plant productivity [13,14]. Because of this relationship between arthropod and plant communities, changes in plant community due to grazing or non-native plants may precipitate altered arthropod community composition and to altered ecosystem function. We currently have little understanding of the dominant mechanisms and outcomes of these perturbations or whether their effects may synergistically impact grassland arthropods

Methods

Results

Conclusion