Quantitative Approaches to the Analysis of Stable Isotope Food Web Data

Abstract

Ecologists use stable isotopes (delta13C, delta15N) to better understand food webs and explore trophic interactions in ecosystems. Traditionally, delta13C vs. delta15N bi-plots have been used to describe food web structure for a single time period or ecosystem. Comparisons of food webs across time and space are increasing, but development of statistical approaches for testing hypotheses regarding food web change has lagged behind. Here we present statistical methodologies for quantitatively comparing stable isotope food web data. We demonstrate the utility of circular statistics and hypothesis tests for quantifying directional food web differences using two case studies: an arthropod salt marsh community across a habitat gradient and a freshwater fish community from Lake Tahoe, USA, over a 120-year time period. We calculated magnitude and mean angle of change (theta) for each species in food web space using mean delta13C and delta15N of each species as the x, y coordinates. In the coastal salt marsh, arthropod consumers exhibited a significant shift toward dependence on Spartina, progressing from a habitat invaded by Phragmites to a restored Spartina habitat. In Lake Tahoe, we found that all species from the freshwater fish community shifted in the same direction in food web space toward more pelagic-based production with the introduction of nonnative Mysis relicta and onset of cultural eutrophication. Using circular statistics to quantitatively analyze stable isotope food web data, we were able to gain significant insight into patterns and changes in food web structure that were not evident from qualitative comparisons. As more ecologists incorporate a food web perspective into ecosystem analysis, these statistical tools can provide a basis for quantifying directional food web differences from standard isotope data.

Nitrogen deposition and prey nitrogen uptake control the nutrition of the carnivorous plant Drosera rotundifolia

“…This approach has been used extensively for food-web studies (Schmidt et al 2007), in plants for quantifying N 2 fixation (Shearer and Kohl 1986) and to quantify prey N uptake in carnivorous plants (Brearley 2011). The limitations of this approach have been discussed previously (Millett et al 2012a(Millett et al , 2012b with the general consensus being that this approach provides a good semi-quantitative estimate of the contribution of the two sources.…”

Stable isotope evidence for trophic overlap of sympatric Mexican Lake Chapala silversides (Teleostei: Atherinopsidae: Chirostoma spp.)

“…in Lake Chapala. We then focused on Chirostoma spp., using methods in Schmidt et al (2007) and Turner et al (2010) to investigate isotopic differences among species within the genus, and between "pescados blancos" and "charales".…”

“…They can also provide key guidelines for ecosystem management, as they inform how species interact with other ecosystem components, and allow a synthesis of multiple ecological processes (Vander Zanden et al, 2003;Fry, 2006). In recent years, food web studies have benefited from the utilization of stable isotope techniques as tools for elucidating trophic structure and inferring pathways of energy/mass flow in food webs (Peterson & Fry, 1987;Fry, 2006;Schmidt et al, 2007). Although studies have addressed the dietary content and trophic role of individual species in Mexican freshwater Trophic ecology of sympatric Mexican Lake Chirostoma species 390 ecosystems (e.g., Navarrete et al, 1996;Moncayo-Estrada et al, 2007;Ramírez-Herrejón et al, 2013), those that use stable isotope techniques to describe food webs or food web interactions in freshwater ecosystems are relatively few Zambrano et al, 2010aZambrano et al, , 2010bHelmus et al, 2013).…”

Habitat corridors alter relative trophic position of fire ants

“…Stable isotopes are frequently used to characterize food webs and estimate trophic position of consumers (Layman et al 2007b, Newsome et al 2007, Schmidt et al 2007. d 15 N is particularly useful because nitrogen from the tissue of consumers is enriched in 15 N relative to that of prey, which means that d 15 N is positively correlated with a consumer's trophic position (Gannes et al 1998, Post 2002 (Woodcock et al 2012).…”