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

Abstract

<div><h3>Background</h3><p><em>Tarebia granifera</em> (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, <em>T. granifera</em> 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 <em>T. granifera</em> has raised questions about potential ecological impacts, specifically in terms of its dietary overlap with native gastropods.</p> <h3>Methodology/Principal Findings</h3><p>A stable isotope mixing model was used together with gut content analysis to estimate the diet of <em>T. granifera</em> and native gastropod populations in three different coastal lakes. Population density, available biomass of food and salinity were measured along transects placed over <em>T. granifera</em> patches. An index of isotopic (stable isotopes) dietary overlap (IDO, %) aided in interpreting interactions between gastropods. The diet of <em>T. granifera</em> was variable, including contributions from microphytobenthos, filamentous algae (<em>Cladophora</em> sp.), detritus and sedimentary organic matter. IDO was significant (>60%) between <em>T. granifera</em> and each of the following gastropods: <em>Haminoea natalensis</em> (Krauss, 1848), <em>Bulinus natalensis</em> (Küster, 1841) and <em>Melanoides tuberculata</em> (Müller, 1774). However, food did not appear to be limiting. Salinity influenced gastropod spatial overlap. <em>Tarebia granifera</em> may only displace native gastropods, such as <em>Assiminea</em> cf. <em>ovata</em> (Krauss, 1848), under salinity conditions below 20. Ecosystem-level impacts are also discussed.</p> <h3>Conclusion/Significance</h3><p>The generalist diet of <em>T. granifera</em> 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 <em>T. granifera</em> 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.</p> </div