Stable Isotope Evidence for Dietary Overlap between Alien and Native Gastropods in Coastal Lakes of Northern KwaZulu-Natal, South Africa

Nelson A F Miranda,Renzo Perissinotto,Dirk Steinke

doi:10.1371/journal.pone.0031897

Abstract

Background Tarebia granifera (Lamarck, 1822) is originally from South-East Asia, but has been introduced and become invasive in many tropical and subtropical parts of the world. In South Africa, T. granifera is rapidly invading an increasing number of coastal lakes and estuaries, often reaching very high population densities and dominating shallow water benthic invertebrate assemblages. An assessment of the feeding dynamics of T. granifera has raised questions about potential ecological impacts, specifically in terms of its dietary overlap with native gastropods.Methodology/Principal FindingsA stable isotope mixing model was used together with gut content analysis to estimate the diet of T. granifera and native gastropod populations in three different coastal lakes. Population density, available biomass of food and salinity were measured along transects placed over T. granifera patches. An index of isotopic (stable isotopes) dietary overlap (IDO, %) aided in interpreting interactions between gastropods. The diet of T. granifera was variable, including contributions from microphytobenthos, filamentous algae (Cladophora sp.), detritus and sedimentary organic matter. IDO was significant (>60%) between T. granifera and each of the following gastropods: Haminoea natalensis (Krauss, 1848), Bulinus natalensis (Küster, 1841) and Melanoides tuberculata (Müller, 1774). However, food did not appear to be limiting. Salinity influenced gastropod spatial overlap. Tarebia granifera may only displace native gastropods, such as Assiminea cf. ovata (Krauss, 1848), under salinity conditions below 20. Ecosystem-level impacts are also discussed.Conclusion/SignificanceThe generalist diet of T. granifera may certainly contribute to its successful establishment. However, although competition for resources may take place under certain salinity conditions and if food is limiting, there appear to be other mechanisms at work, through which T. granifera displaces native gastropods. Complementary stable isotope and gut content analysis can provide helpful ecological insights, contributing to monitoring efforts and guiding further invasive species research.

Highlights

Alien invasive species (AIS) can cause disruptions to ecosystems
Haminoea natalensis was the only native gastropod not to be found in the middle of a T. granifera patch
Gastropod densities as well as available microphytobenthic (MPB) biomass varied within transects (Fig. 2A–6A; Table S1).Generally, native gastropods had lower population densities when compared to T. granifera (Table S1)

Summary

Introduction

Alien invasive species (AIS) can cause disruptions to ecosystems. Quantifying their impacts is problematic due to the complexity of ecological interactions [1,2]. Stable isotope analysis provides a time-integrated view of ecological processes, whereas gut content analysis reveals snapshots of feeding activity [6]. Stable isotope and gut content analyses are complementary, provide insight into diets and trophic dynamics in ecosystems [7,8] and have successfully been used in several studies addressing the impacts of AIS on aquatic ecosystems [8,9,10,11,12]. An assessment of the feeding dynamics of T. granifera has raised questions about potential ecological impacts, in terms of its dietary overlap with native gastropods

Objectives

Methods

Results

Conclusion