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

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.

Chemical Cues Released by an Alien Invasive Aquatic Gastropod Drive Its Invasion Success

“…Generally it has been assumed that T. granifera displaces native gastropods through exploitative competition. However, as food resources are not limiting in iSimangaliso [28], it was hypothesized that chemical cues are involved in displacement interactions between T. granifera and native gastropod species.…”

Stable Isotopes of C and N Reveal Habitat Dependent Dietary Overlap between Native and Introduced Turtles Pseudemys rubriventris and Trachemys scripta

“…Factors affecting δ 13 C and δ 15 N isotope assimilation include tissue metabolism, trophic level, temperature, C∶N ratios in items consumed, taxonomy, body size, and an organism's form of eliminating nitrogenous waste [25],[26]. Stable isotopes have been used in determining the C and N sources in organisms' diets [27], trophic position in food webs [28] and in comparative studies of species feeding ecology between study sites [29]–[31].…”

Ecological strategies of Macarorchestia remyi compared to two sympatric talitrids of a Tyrrhenian beach

“…Plots using 95%, 75% and 25% credibility intervals obtain with SIAR showed the proportional contribution of each food item to the diet of each amphipod species. Isotopic dietary overlap (IDO) between species (Miranda and Perissinotto, 2012) was calculated from mean isotopic contribution of the different food items, according the following equation: IDO jk ¼ Smin(p ij ,p ik )$100, where p is the proportion of source i resulting from the mixing models for species j and k, respectively. The IDO was calculated from the sum of the minimum proportions of each food item.…”