The interaction of intraspecific competition and habitat on individual diet specialization: a near range-wide examination of sea otters

Abstract

The quantification of individuality is a common research theme in the fields of population, community, and evolutionary ecology. The potential for individuality to arise is likely context-dependent, and the influence of habitat characteristics on its prevalence has received less attention than intraspecific competition. We examined individual diet specialization in 16 sea otter (Enhydra lutris) populations from southern California to the Aleutian Islands in Alaska. Because population histories, relative densities, and habitat characteristics vary widely among sites, we could examine the effects of intraspecific competition and habitat on the prevalence of individual diet specialization. Using observed diet data, we classified half of our sites as rocky substrate habitats and the other half containing a mixture of rocky and unconsolidated (soft) sediment substrates. We used stable isotope data to quantify population- and individual-level diet variation. Among rocky substrate sites, the slope [±standard error (SE)] of the positive significant relationship between the within-individual component (WIC) and total isotopic niche width (TINW) was shallow (0.23 ± 0.07) and negatively correlated with sea otter density. In contrast, the slope of the positive WIC/TINW relationship for populations inhabiting mixed substrate habitats was much higher (0.53 ± 0.14), suggesting a low degree of individuality, irrespective of intraspecific competition. Our results show that the potential for individuality to occur as a result of increasing intraspecific competition is context-dependent and that habitat characteristics, which ultimately influence prey diversity, relative abundance, and the range of skillsets required for efficient prey procurement, are important in determining when and where individual diet specialization occurs in nature.

Drivers of individual niche variation in coexisting species

“…Surprisingly, the diversity of available resources did not explain variations in the degree of individual specialization. This contrasts with previous studies revealing an overall positive effect of ecological opportunity on diet variation because higher prey diversity creates more opportunity for individual diet variation by expanding individuals’ foraging options (Costa‐Pereira et al., ; Evangelista et al., ; Newsome et al., ). This discrepancy may simply arise because thin‐toed frogs are generalists which consume a wide variety of species.…”

“…We tested whether each observed V value could be generated if individuals sample prey stochastically from the population diet distribution (i.e., the significance of V indexes) using Monte Carlo bootstrap simulations (1,000 replicates) (Zaccarelli, Bolnick, & Mancinelli, ). As measures of individual specialization based on gut contents can also be influenced by the degree of taxonomic resolution (Newsome et al., ), we calculated the V index using both Order and Family level identifications separately. As we found a high correlation between these indexes ( r = 0.84, p < 0.001), we only used family‐level data in the analyses.…”

“…Furthermore, the majority of these studies focused on the specific effect of only one of these four ecological factors (e.g., intraspecific competition, Svanbäck & Persson, ; Svanbäck & Bolnick, ; Tinker, Bentall, & Estes, ). Only recently have observational studies started to provide some insights on how two ecological factors can interact to determine the degree of individual niche variation in populations (Costa‐Pereira, Tavares, de Camargo, & Araújo, ; Evangelista, Boiche, Lecerf, & Cucherousset, ; Newsome et al., ). However, under natural conditions, populations are usually facing all four factors simultaneously to varying degrees and the effects of these factors may not be independent.…”

The dimensionality of individual niche variation

“…These quantities are straightforward to define as variances in the case of a continuous niche axis such as prey size (Roughgarden 1972, 1974, Bolnick et al 2002, or can be calculated using Shannon diversity indices as a proxy for variance when resource use is measured as frequencies of utilization of discrete food or habitat types (Roughgarden 1979, Bolnick et al 2002. Measures of individual specialization based on resource categories typically do not consider quantitative differences between categories (e.g., prey size, nutritional value or trophic level), and can be influenced by the degree of taxonomic resolution (Newsome et al 2015). Measures of individual specialization based on resource categories typically do not consider quantitative differences between categories (e.g., prey size, nutritional value or trophic level), and can be influenced by the degree of taxonomic resolution (Newsome et al 2015).…”

Foraging strategies of a generalist marine predator inhabiting a dynamic environment

“…This variability may arise as a result of intrinsic factors such as individual differences in physiology, morphology, or behavior (Killen et al 2011;Patrick and Weimerskirch 2014;Hoskins et al 2015), or from external factors such as prey and habitat availability (Svanbäck and Bolnick 2005;Kernaléguen et al 2015;Newsome et al 2015;Rosenblatt et al 2015). Population-level descriptions largely overlook this individual variability, which may result in incomplete descriptions of foraging behavior and obscure the responses of individuals and populations to environmental variability.…”