Predicting the ecological impacts of a new freshwater invader: functional responses and prey selectivity of the ‘killer shrimp’, Dikerogammarus villosus, compared to the native Gammarus pulex

Jennifer A Dodd,Mhairi E Alexander,Jaimie T A Dick,Alison M Dunn,Calum Macneil,David C Aldridge

doi:10.1111/fwb.12268

Abstract

Summary The ability to predict the likely ecological impacts of invasive species in fresh waters is a pressing research requirement. Whilst comparisons of species traits and considerations of invasion history have some efficacy in this respect, we require robust methods that can compare the effects of native and invasive species. Here, we utilise comparative functional responses and prey selectivity experiments to understand and predict the ecological impact of an invader as compared to a native. We compared the predatory functional responses of an emerging invasive species in Europe, the ‘killer shrimp’, Dikerogammarus villosus, and an analogous native species, Gammarus pulex, towards three representative prey species: Asellus aquaticus, Daphnia magna and Chironomus sp. Furthermore, as ecological impact may be greater for invasive species with more indiscriminate feeding habits, we compared the selectivity for the three prey types between the invasive and native species. In both the presence and absence of experimental habitats, large D. villosus, and those matched for body size with G. pulex, generally showed higher (Type II) functional responses than G. pulex, with the invasive species exhibiting higher maximum feeding rates. Further, D. villosus exhibited significantly more indiscriminate prey selection compared with G. pulex, a trait that became more evident as the invader increased in size. Differences in functional responses and prey selectivity were prey species specific, with higher to lower predicted impacts in the order A. aquaticus, D. magna and Chironomus sp. This is in accord with the impact of this invasive species on macroinvertebrates in the field. We thus provide understanding of the known ecological impact of D. villosus and discuss the utility of the phenomenological use of comparative functional responses and resource use as a tool through which the potential ecological impacts of invasive species may be identified.

Highlights

The ability to predict the ecological consequences of species introductions is becoming increasingly important as biological invasions continue to be one of the main drivers of global biodiversity loss (Sala et al, 2000; Ricciardi, 2007; Davis, 2009; Leung et al, 2012; Strayer, 2012; Simberloff et al, 2013)
As ecological impact may be greater for invasive species with more indiscriminate feeding habits, we compared the selectivity for the three prey types between the invasive and native species
We investigated the relative predatory capacity of the invader, D. villosus, and an analogous native species, Gammarus pulex, by examining the functional responses and prey selectivity towards three common and representative prey types found in freshwater systems: an isopod, Asellus aquaticus, a cladoceran, Daphnia magna and a dipteran larva, Chironomus sp

Summary

Introduction

The ability to predict the ecological consequences of species introductions is becoming increasingly important as biological invasions continue to be one of the main drivers of global biodiversity loss (Sala et al, 2000; Ricciardi, 2007; Davis, 2009; Leung et al, 2012; Strayer, 2012; Simberloff et al, 2013). A potential approach for assessing the ecological impacts of an invading species on a community is the comparison of its rate of resource uptake with that of a trophically analogous native species (Bollache et al, 2008; Dick, Alexander & MacNeil, 2012; Dick et al, 2013a,b) Such predatory capacity, and more generally the use of resources by consumers (see Dick et al, 2013a), can be quantified by measuring the ‘functional response’ (Solomon, 1949; Abrams, 1990), the relationship between resource consumption rate (e.g. predation rate) and resource density (e.g. prey availability; Holling, 1966; Juliano, 2001). Measurements of functional responses have been suggested as a mechanism through which species with the potential to become damaging invaders could be identified (e.g. Bollache et al, 2008; Dick et al, 2010, 2013a,b), as the type of functional response (Type II or III) has potential implications for resource (such as prey) population stability, as Type III functional responses are likely to be more stabilising towards prey populations, whereas Type II responses can be destabilising and lead to local extinctions of prey over certain ranges of density (Murdoch & Oaten, 1975; Juliano, 2001)

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