Predator size-structure and species identity determine cascading effects in a coastal ecosystem.

John N Griffin,Brian R Silliman

doi:10.1002/ece3.4571

John N Griffin, Brian R Silliman

Open Access

https://doi.org/10.1002/ece3.4571

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Abstract

Cascading consequences of predator extinctions are well documented, but impacts of perturbations to predator size‐structure and how these vary across species remain unclear. Body size is hypothesized to be a key trait governing individual predators' impact on ecosystems. Therefore, shifts in predator size‐structure should trigger ecosystem ramifications which are consistent across functionally similar species. Using a US salt marsh as a model system, we tested this hypothesis by manipulating size class (small, medium, and large) and size diversity (combination of all three size classes) within two closely related and functionally similar predatory crab species over 4 months. Across treatments, predators suppressed densities of a dominant grazer and an ecosystem engineer, enhanced plant biomass, and altered sediment properties (redox potential and saturation). Over the metabolically equivalent experimental predator treatments, small size class predators had stronger average impacts on response variables, and size class interacted with predator species identity to drive engineer suppression. Within both predator species, size diversity increased cannibalism and slightly weakened the average impact. These results show that predator impacts in a salt marsh ecosystem are determined by both size class and size diversity; they also highlight that size class can have species‐dependent and response‐dependent effects, underlining the challenge of generalizing trait effects.

Highlights

Human activities have driven functional, or total, extinctions of predators in many ecosystems (Dirzo et al, 2014; McCauley et al, 2015)
Our results show that resident predatory crabs strongly and broadly affect ecosystem functioning in our southeastern U.S salt marsh study system and that predator size‐structure and identity have the potential to modify these effects
One of our most striking results was that the strong effect of predator size class on trophic control of a func‐ tionally important prey species, and the fact this was contingent on predator identity

Summary

| INTRODUCTION

Human activities have driven functional, or total, extinctions of predators in many ecosystems (Dirzo et al, 2014; McCauley et al, 2015). To better understand the relative impacts of predator body size, size diversity and taxonomic identity on ecosystem function‐ ing, we experimentally manipulated these predator variables, using a southeastern U.S salt marsh as a model system These marshes are characterized by vast stands of smooth cordgrass, Spartina alterniflora, and a relatively simple food web with strong trophic feedbacks. We manipulated the system’s two species of resident, predatory crab (Eurytium limosum and Panopeus obesus) which are closely related and occur within the mud crab (Panopeidae) family (see Griffen & Mosblack, 2011 for phylogeny) They are functionally similar, sharing sit‐and‐wait hunting modes, benthic/burrow microhabitats, and body size ranges. Based on the hypothesis that predator size is an important functional trait determining the trophic and non‐trophic interac‐ tions of predators, we tested the following predictions in our field experiment: (a) mean size (i.e., size class) will govern the multifunc‐ tional impacts of predators; (b) size diversity will foster cannibal‐ ism, reduce predator survivorship, and weaken these impacts; and, given the trait similarity of predator species, (c) the effects of size class and diversity on collective functional impacts will hold across species

| MATERIALS AND METHODS

Findings

| DISCUSSION