Within an organism, lipids are depleted in (13)C relative to proteins and carbohydrates (more negative delta(13)C), and variation in lipid content among organisms or among tissue types has the potential to introduce considerable bias into stable isotope analyses that use delta(13)C. Despite the potential for introduced error, there is no consensus on the need to account for lipids in stable isotope analyses. Here we address two questions: (1) If and when is it important to account for the effects of variation in lipid content on delta(13)C? (2) If it is important, which method(s) are reliable and robust for dealing with lipid variation? We evaluated the reliability of direct chemical extraction, which physically removes lipids from samples, and mathematical normalization, which uses the carbon-to-nitrogen (C:N) ratio of a sample to normalize delta(13)C after analysis by measuring the lipid content, the C:N ratio, and the effect of lipid content on delta(13)C (Deltadelta(13)C) of plants and animals with a wide range of lipid contents. For animals, we found strong relationships between C:N and lipid content, between lipid content and Deltadelta(13)C, and between C:N and Deltadelta(13)C. For plants, C:N was not a good predictor of lipid content or Deltadelta(13)C, but we found a strong relationship between carbon content and lipid content, lipid content and Deltadelta(13)C, and between and carbon content and Deltadelta(13)C. Our results indicate that lipid extraction or normalization is most important when lipid content is variable among consumers of interest or between consumers and end members, and when differences in delta(13)C between end members is <10-12 per thousand. The vast majority of studies using natural variation in delta(13)C fall within these criteria. Both direct lipid extraction and mathematical normalization reduce biases in delta(13)C, but mathematical normalization simplifies sample preparation and better preserves the integrity of samples for delta(15)N analysis.

Much research has focused on identifying species that are susceptible to extinction following ecosystem fragmentation, yet even those species that persist in fragmented habitats may have fundamentally different ecological roles than conspecifics in unimpacted areas. Shifts in trophic role induced by fragmentation, especially of abundant top predators, could have transcendent impacts on food web architecture and stability, as well as ecosystem function. Here we use a novel measure of trophic niche width, based on stable isotope ratios, to assess effects of aquatic ecosystem fragmentation on trophic ecology of a resilient, dominant, top predator. We demonstrate collapse in trophic niche width of the predator in fragmented systems, a phenomenon related to significant reductions in diversity of potential prey taxa. Collapsed niche width reflects a homogenization of energy flow pathways to top predators, likely serving to destabilize remnant food webs and render apparently resilient top predators more susceptible to extinction through time.

SummaryCalorie restriction (CR) and late-onset CR enhance longevity in many organisms. Resource allocation theory suggests that longevity is enhanced by increasing somatic storage, at the expense of current reproduction. Phytophagous insects accumulate amino acids as hemolymph storage proteins for major developmental events. We hypothesized that protein storage is involved in life extension from CR. In a longitudinal experiment, we tested whether CR altered protein storage in female grasshoppers. Individuals on CR (60% or 70%) or late-onset CR had at least 60% greater longevity than ad libitum individuals. Age at first oviposition, dry mass of the first clutch, or lifetime fecundity were not affected by CR, but CR did increase the number of clutches produced. Most important, females on life-extending CR and late-onset CR did not differ in the concentration of hemolymph storage of proteins in comparison to ad libitum females. Protein storage changed with time in all groups, demonstrating sufficient sensitivity in our methods. Previous experiments have shown that severe CR (~30% of ad libitum) can reduce hemolymph storage. Therefore, the reduction in intake needed to extend lifespan is not sufficient to reduce protein storage in the hemolymph. These results do not support the hypothesis that protein storage is involved in life extension from CR.