Testing the niche variation hypothesis with a measure of body condition

Abstract

Individual variation and fitness are cornerstones of evolution by natural selection. The niche variation hypothesis (NVH) posits that when interspecific competition is relaxed, intraspecific competition should drive niche expansion by selection favoring use of novel resources and that among‐individual variation should confer a selective advantage. Population‐level niche expansion could be achieved by all individuals using all available resources, or by each individual using a unique combination of resources, thereby increasing among‐individual dietary niche variation. Although individual variation can lead to species‐level evolutionary and ecological change, observed variation does not ensure a beneficial outcome. We used carbon and nitrogen stable isotope analysis of claw keratin and a Bayesian stable isotope mixing model to estimate the summer (July–September) assimilated diet of individual female black Ursus americanus and brown U. arctos bears. We quantified variation in dietary niche in both populations, and assessed diet relative to percentage body fat. We hypothesized that if the NVH held, percentage body fat would be similar for individuals of the same species across much of the dietary range of observed proportional salmon contributions to individual bear diets. Although we found greater differences in dietary niches between than within species, we observed greater among‐individual dietary variation in the brown bear population. Moreover, we found that within each species individual female bears achieved similar ranges of percentage body fat at various levels of salmon in the diet. Our results provide support for the NVH. Linking individual dietary niches to measures of physiological condition related to fitness can offer new insights into eco‐evolutionary processes related to food resource use.

Body size and lean mass of brown bears across and within four diverse ecosystems

“…This partitioning can lead to differences in body size though not necessarily proportional body composition (i.e. percent body fat) (Ben‐David, Titus & Beier, ; Belant et al ., , ; Lafferty et al ., ; Mangipane et al ., ). Our study areas all showed broad variation in body size despite different levels of available nutritional resources, relative densities, and management regimes.…”

“…The observed phenotypic plasticity within populations in both sexes is likely due to a combination of dietary specialization, individual behaviors related to risk tolerance or aversion, learned behavior, reproductive strategies, diversity and abundance of available resources, and population density (e.g. Lafferty et al, 2015;Mangipane et al, 2017). Generally, a population comprised of a diversity of specialized individuals may be more stable and thus resilient to perturbations that may occur at higher densities by minimizing intra-specific competition (Bolnick et al, 2003).…”

“…Despite dietary plasticity indicating broad niche variation in nutritional resource utilization, the proportional fall body composition (i.e. percent body fat) within a population has been shown to be largely consistent indicating that different strategies can lead to the same outcome relative to individual fitness (Lafferty, Belant & Phillips, ; Mangipane et al ., ). Larger body size is positively associated with increased individual fitness and thus, population vigor and density (Hilderbrand et al ., ; Zedrosser et al ., ).…”

Sex and occupation time influence niche space of a recovering keystone predator

“…Recently, nutrition, physiology, and behaviour have also been used to better characterize niche breadth and predict a species’ trophic interactions (Machovsky‐Capuska et al, ). Although once viewed as static, niche space is no longer seen as a fixed property, but one that can change as a function of intraspecific variation (Baudrot, Perasso, Fritsch, Giraudoux, & Raoul, ; Ingram, Costa‐Pereira, & Araujo, ; Lafferty, Belant, & Phillips, ; Newsome et al, ). Factors such as life stage and habitat use (Polis, ), intraspecific competition (Newsome et al, ), and sex (Shine, ) all affect niche variation within species or populations.…”

Hitting the moving target: modelling ontogenetic shifts with stable isotopes reveals the importance of isotopic turnover

“…This suggested that local, smaller-scale variation was underlying much of the overall variation. Individual-level diet specialization could cause these differences (Ara ujo et al 2007), and this could be the result of niche partitioning due to intraspecific competition (Lafferty, Belant & Phillips 2015). Contrastingly, juvenile Chinook salmon may be using similar resources, but with slightly different isotopic ratios due to local differences in nutrient utilization (Rau, Ohman & Pierrot-Bults 2003) or productivity of phytoplankton (Laws et al 1995;Miller, Brodeur & Rau 2008).…”