Ecosystem engineering strengthens bottom-up and weakens top-down effects via trait-mediated indirect interactions.

Zhiwei Zhong,Dirk Sanders,Yu Zhu,Ling Wang,Xiaofei Li,Deli Wang,Dean Pearson

doi:10.1098/rspb.2017.0894

Abstract

Trophic interactions and ecosystem engineering are ubiquitous and powerful forces structuring ecosystems, yet how these processes interact to shape natural systems is poorly understood. Moreover, trophic effects can be driven by both density- and trait-mediated interactions. Microcosm studies demonstrate that trait-mediated interactions may be as strong as density-mediated interactions, but the relative importance of these pathways at natural spatial and temporal scales is underexplored. Here, we integrate large-scale field experiments and microcosms to examine the effects of ecosystem engineering on trophic interactions while also exploring how ecological scale influences density- and trait-mediated interaction pathways. We demonstrate that (i) ecosystem engineering can shift the balance between top-down and bottom-up interactions, (ii) such effects can be driven by cryptic trait-mediated interactions, and (iii) the relative importance of density- versus trait-mediated interaction pathways can be scale dependent. Our findings reveal the complex interplay between ecosystem engineering, trophic interactions, and ecological scale in structuring natural systems.

Highlights

Ecosystem engineers are defined as ‘organisms that modulate the availability of resources to other species by causing physical state changes in biotic or abiotic materials’ [1,2]
Plant height mixed effect models with grazing treatment treated as a fixed effect and replicate sites as a random effect to assess the impact of sheep grazing on plant cover, density, and height for each plant group and across all groups combined (L. chinensis, other grasses, forbs), air temperature and relative humidity, densities of A. bruennichi and Euchorthippus, A. bruennichi web area and height, and A. bruennichi predation successes
Ecosystem engineering is a ubiquitous and powerful phenomenon which influences a wide range of ecological mean A. bruennichi web area

Summary

Introduction

Ecosystem engineers are defined as ‘organisms that modulate the availability of resources to other species by causing physical state changes in biotic or abiotic materials’ [1,2]. Mixed effect models (lme) with grazing treatment treated as a fixed effect and replicate sites as a random effect to assess the impact of sheep grazing on plant cover, density, and height for each plant group and across all groups combined (L. chinensis, other grasses, forbs), air temperature and relative humidity (at ground surface and 30 cm above), densities of A. bruennichi and Euchorthippus, A. bruennichi web area and height, and A. bruennichi predation successes (mean number of grasshoppers caught per spider web). These were done using the function lme from the package nlme. We used VarIdent to account for variance heterogeneity in effect sizes between treatment groups

Results

Findings

Discussion