Can crabs kill like a keystone predator? A field-test of the effects of crab predation on mussel mortality on a northeast Pacific rocky shore.

Wesley W Hull,Paul E Bourdeau,Geerat J Vermeij

doi:10.1371/journal.pone.0183064

Abstract

Predation can strongly influence community structure and ecosystem function, so the loss of key predators can have dramatic ecological consequences, unless other predatory species in the system are capable of playing similar ecological roles. In light of the recent outbreak of sea star wasting disease (SSWD) and subsequent depletion of west coast sea star populations, including those of the keystone predator Pisaster ochraceus, we examined whether large mobile crabs could play a role as predators on mussels (Mytilus californianus) on a rocky shore in Northern California. Using a combination of sea star removal and predator exclusion cages we found that mussel mortality was 43–294 times greater in uncaged treatments versus caged treatments. Mortality on uncaged mussels at low tidal elevations was due to predation by large mobile crabs (Cancer productus and Romaleon antennarium); confirmed by the presence of mussel shell fragments and documented attacks on wax snail replicas. Laboratory feeding assays indicated that crabs, on a per unit biomass basis, can consume almost twenty-five times as many mussels per day than sea stars, which together with the results of our field experiment, suggest that large predatory crabs could play an important role in maintaining ecosystem function through their predation on mussels on rocky shores where P. ochraceus are rare, absent, or have been depleted by SSWD.

Highlights

Understanding the role of species redundancy and/or compensation within ecosystems will be critical for predicting the impacts of increasing local extinctions in a rapidly changing global environment
One potential consequence of the recent sea star wasting disease (SSWD) epidemic is that loss of the keystone predator P. ochraceus may shift the low intertidal zone of Pacific Coast rocky shores to an alternate community state dominated by mussels
Recently Menge et al [17] hypothesized that if alternative predators can compensate for the loss of sea stars by removing mussels at some locations, the low intertidal zone may not become mussel-dominated at all locations

Summary

Introduction

Understanding the role of species redundancy and/or compensation within ecosystems will be critical for predicting the impacts of increasing local extinctions in a rapidly changing global environment. Because individual species can play critical ecological roles, e.g., keystone species [1,2,3,4], the loss of these species via extinction can cause major changes in ecosystems [5,6,7]. Structure and function may be maintained in ecosystems that have recently lost key species through the actions of so-called redundant and/or compensatory species; those that perform a similar ecological role [8] or can partially or solely compensate for the ecological role of declining or locally extinct species [9, 10], respectively. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript

Objectives

Methods

Results

Conclusion