Population-Level Metrics of Trophic Structure Based on Stable Isotopes and Their Application to Invasion Ecology

Abstract

Biological invasions are a significant driver of human-induced global change and many ecosystems sustain sympatric invaders. Interactions occurring among these invaders have important implications for ecosystem structure and functioning, yet they are poorly understood. Here we apply newly developed metrics derived from stable isotope data to provide quantitative measures of trophic diversity within populations or species. We then use these to test the hypothesis that sympatric invaders belonging to the same functional feeding group occupy a smaller isotopic niche than their allopatric counterparts. Two introduced, globally important, benthic omnivores, Louisiana swamp crayfish (Procambarus clarkii) and carp (Cyprinus carpio), are sympatric in Lake Naivasha, Kenya. We applied our metrics to an 8-year data set encompassing the establishment of carp in the lake. We found a strong asymmetric interaction between the two invasive populations, as indicated by inverse correlations between carp abundance and measures of crayfish trophic diversity. Lack of isotopic niche overlap between carp and crayfish in the majority of years indicated a predominantly indirect interaction. We suggest that carp-induced habitat alteration reduced the diversity of crayfish prey, resulting in a reduction in the dietary niche of crayfish. Stable isotopes provide an integrated signal of diet over space and time, offering an appropriate scale for the study of population niches, but few isotope studies have retained the often insightful information revealed by variability among individuals in isotope values. Our population metrics incorporate such variation, are robust to the vagaries of sample size and are a useful additional tool to reveal subtle dietary interactions among species. Although we have demonstrated their applicability specifically using a detailed temporal dataset of species invasion in a lake, they have a wide array of potential ecological applications.

Angling baits and invasive crayfish as important trophic subsidies for a large cyprinid fish

“…Other studies on invasive crayfish have highlighted their importance as a food resource for many predatory fish (Blake and Hart 1995;Garvey et al 2003;Hein et al 2006;Nystr¨om et al 2006;Jackson et al 2012), which could potentially help in controlling the invasive crayfish abundance (Hein et al 2006;Nystro¨m et al 2006). Our results indicate that invasive crayfish represent an important food source for adult B. barbus, even in the presence of an abundant allochthonous food resource such as fishmeal pellets.…”

“…These disturbances alter the supply of resources and, therefore, often cause changes in the diet of resident species (Harmon et al 2009). Evidence suggests that food webs in freshwater ecosystems shift under the influence of invasion and fishery activities through inputs of novel resources (Vander Zanden et al 1999;Britton et al 2010;Jackson et al 2012). Fishery activities associated with angling and aquaculture can magnify the input of allochthonous resources to freshwater ecosystems via the introduction of energy rich foods, such as pelletized fishmeal, and introductions of invasive species; both of which can supplement the diet of native species (Grey et al 2004;.…”

Contrasting Responses in the Niches of Two Coral Reef Herbivores Along a Gradient of Habitat Disturbance in the Spermonde Archipelago, Indonesia

“…Metrics included nitrogen (δNR) and carbon (δCR) ranges detailing the total range of nitrogen and carbon values of exploited sources; mean distance to the centroid (CD), providing a description of trophic diversity; standard deviation of nearest neighbor distance (SDNND), which provides a measure of trophic evenness; and standard ellipse area (SEA), which provides a bivariate measure of the core isotopic niche (Layman et al, 2007;Jackson et al, 2011). The calculation of SEA allows for subsequent analysis of the degree of niche overlap (%) which can then be used as a quantitative measure of dietary similarity among sites (Jackson et al, 2012;Jackson and Britton, 2013). A small sample size correction (SEA C ) for improving accuracy of SEA values was applied as suggested in Jackson et al (2011).…”

Sympatric elasmobranchs and fecal samples provide insight into the trophic ecology of the smalltooth sawfish

“…Standard ellipse area (SEA) was calculated using the variance and covariance of bivariate isotope data, bootstrapped to 10 000 iterations (Jackson et al 2012). The SEAs contain approximately 40% of the data and represent a core isotopic niche for size classes of each species.…”