Interaction patterns within a multi-herbivore assemblage derived from stable isotopes

Abstract

The ecological effects of consumer guilds are strongly driven by the relative resource use of included species. Here we use stable isotopes in faecal samples from 6 cooccurring mammalian herbivores in an African savanna to identify species-specific trophic niches and detect patterns of interactions between herbivores and their feeding resources. Species-specific foraging strategies were reflected in the faecal  13 C values, with species aligning along both a browser-grazer gradient and a niche breadth gradient. Within the broad foraging strategies outlined by  13 C,  15 N indicated a seasonal shift in resource use for some herbivores. We expected that 13 C isotope niches would overlap in a nested fashion, with the niches of grazers and browsers being included in those of mixed feeders, and that 15 N niches would separate into discrete modules reflecting leguminous and protein content of respective diets. However, the observed structure was more complex, with combined modular and nested patterns of interactions between herbivores and 13 C isotopes. We identified an isolated module comprising eland and its exclusive δ 13C values, and a series of nested structures with a set of specialist herbivores (hartebeest and sable) which δ 13 C values were nested within those of more generalist herbivores (impala, waterbuck and zebra). Networks based on δ 15 N, however, reflected a higher level of overlap in resource use with random patterns in herbivore interactions with resources, and only a significant modular interaction pattern during the dry season. We suggest that the combined modular and nested pattern of δ 13 C interactions reflect the simultaneous mutualistic and antagonistic characteristics of plant-herbivore interactions. We argue that such interaction patterns could stabilize ecosystems by constraining effects of perturbations to specific modules and by increasing functional redundancy through nested interactions.

Landscape Level Effects of Lion Presence (Panthera leo) on Two Contrasting Prey Species

“…Stable carbon and nitrogen isotope measurements can be good indicators of the foraging ecology and habitat utilization of African ungulates (Codron et al, 2007;Miranda et al, 2014). Three main foraging strategies can be identified among African herbivores; grazers, browsers and mixed-feeders (Hofmann and Stewart, 1972).…”

“…Stable nitrogen isotope ratios can be good indicators of habitat utilization (Ambrose and De Niro, 1986), although they may also reflect physiological mechanisms of rumination (Ambrose, 1991). Combining carbon and nitrogen isotope data may therefore offer complementary information regarding the ecology of African ungulates (Miranda et al, 2014).…”

Network topology of stable isotope interactions in a sub-arctic raptor guild

“…However, topological approaches have rarely been applied to stable isotope data (but see Miranda et al 2014). This is unfortunate, because isotope data contain information about resources assimilated over time.…”

“…We conducted our analyses on separate age classes partly since certain prey may have been favoured to bring back to the nests (e.g., Lindström 1994;Catry et al 2016), and partly because the isotopes in adult and juvenile feather reflected different time periods. We first generated a bipartite network for each element, in which the consumer nodes represented the species and age class categories and the resource nodes a categorized representation of respective δ 13 C and δ 15 N values (Miranda et al 2014). We set all isotope categories within each element to be of equal width.…”

“…However, despite its applicability to predation processes, topological approaches to study trophic interactions within predator assemblages have been surprisingly neglected (Miranda et al 2013). In an ecological context, a trophic network typically consists of two sets of discrete nodes that are connected through links representing resource use (Elton 1927), where the nodes usually represent species or other recognizable taxonomic units, although exceptions exists (e.g., Miranda et al 2014). Characterizations of trophic network topology allow for evaluations of interspecific patterns in resource use (Sugihara 1984), where nested and modular patterns may be particularly relevant for ecosystem properties (May 1973, Bascompte et al 2003Newman 2006;Thébault and Fontaine 2010;Stouffer and Bascompte 2011).…”