Effects of time-averaged sampling on the inferred strength and temporal consistency of intraspecific diet specialization

Background/Question/Methods

Animal populations often consist of individuals that vary substantially in their diets.  Quantification of the strength and temporal consistency of such intraspecific diet specialization is needed to understand its effects on predator-prey dynamics and community structure.  Two approaches – stable isotope ratios and gut contents analysis – predominate the literature but are limited by potential biases that may alter estimates of specialization.  The extent to which these approaches reflect instantaneous to highly time-averaged measures of an individual’s diet is of particular concern.  We used direct foraging observations of southern sea otters (Enhydra lutris nereis) to avoid these limitations and thereby quantify the strength and consistency of individual diet specialization over time. 

Results/Conclusions

Our study indicates limited short-term predictability but remarkably high long-term consistency of individual sea otter diets.  For an average individual, and when prey identity alone is considered, the diet similarity of two foraging bouts is only 0.46 (on a scale of 0 to 1) for back-to-back bouts, but remains more than half as similar for over 900 elapsed days thereafter.  The back-to-back similarity of the average individual’s diet is reduced to 0.42 but remains similarly consistent in the long-term (> 500 elapsed days) when relative prey frequencies are also considered.  Individual specialization remains distinguishable from the population’s between-individual variation in diet choices for periods of 500-3000 days. Analyses of observations aggregated over hourly to annual intervals of time reveal a substantial bias associated with time-averaged approaches.  Time averaging affects a marked increase in the inferred strength of an individual’s short-term diet similarity.  However, it does not affect the inferred strength of long-term diet consistency. Our study thus provides quantitative insights into the utility of instantaneous and time-averaged approaches for understanding intraspecific diet variation and its effects on populations and community dynamics.