Timescales alter the inferred strength and temporal consistency of intraspecific diet specialization

Abstract

Many populations consist of individuals that differ substantially in their diets. Quantification of the magnitude and temporal consistency of such intraspecific diet variation is needed to understand its importance, but the extent to which different approaches for doing so reflect instantaneous vs. time-aggregated measures of individual diets may bias inferences. We used direct observations of sea otter individuals (Enhydra lutris nereis) to assess how: (1) the timescale of sampling, (2) under-sampling, and (3) the incidence- vs. frequency-based consideration of prey species affect the inferred strength and consistency of intraspecific diet variation. Analyses of feeding observations aggregated over hourly to annual intervals revealed a substantial bias associated with time aggregation that decreases the inferred magnitude of specialization and increases the inferred consistency of individuals’ diets. Time aggregation also made estimates of specialization more sensitive to the consideration of prey frequency, which decreased estimates relative to the use of prey incidence; time aggregation did not affect the extent to which under-sampling contributed to its overestimation. Our analyses demonstrate the importance of studying intraspecific diet variation with an explicit consideration of time and thereby suggest guidelines for future empirical efforts. Failure to consider time will likely produce inconsistent predictions regarding the effects of intraspecific variation on predator–prey interactions.

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Acknowledgments

We are grateful to the many people who contributed to field research and data collection, especially C. Alfano, J. Ames, G. Bentall, J. Bodkin, B. Cummings, G. Esslinger, J. Estes, M. Harris, B. Hatfield, D. Jessup, A. Kage, M. Kenner, C. Lin, D. Monson, M. Murray, T. Nicholson, J. Perry, M. Staedler, J. Stewart, S. Wolrab and L. Yeates. We also thank D. Bolnick, P. Guimarães, Jr., K. Laidre, J. Yee and two anonymous reviewers for suggestions made on the manuscript. Support for field work was provided by the US Geological Survey, the California Department of Fish and Game, the Monterey Bay Aquarium, the Kenneth S. Norris Rancho Marina Reserve, and a grant from the Minerals Management Service. M. N. received partial support from National Science Foundation OCE- 1041454. Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the US government.

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Correspondence to Mark Novak.

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Communicated by Craig A. Layman.

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The following appendices are available for this article online:

S1. Additional details for the four similarity indices.

S2. Summary of observational data, fitting example, and Akaike model weights.

S3. An alternative measure of diet specialization.

S4. Consequences of classifying prey into functional categories.

S5. Sample-size effects on estimates of between- and within-individual similarity.

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Novak, M., Tinker, M.T. Timescales alter the inferred strength and temporal consistency of intraspecific diet specialization. Oecologia 178, 61–74 (2015). https://doi.org/10.1007/s00442-014-3213-2

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Keywords

  • Individual variation
  • Predation
  • Prey switching
  • Seasonal prey selection
  • Time-averaging