Mixed-species groups of Serengeti grazers: a test of the stress gradient hypothesis

Lydia Beaudrot, Meredith S. Palmer, T. Michael Anderson, Craig Packer

Research output: Contribution to journalArticlepeer-review

6 Scopus citations

Abstract

Understanding the role of species interactions within communities is a central focus of ecology. A key challenge is to understand variation in species interactions along environmental gradients. The stress gradient hypothesis posits that positive interactions increase and competitive interactions decrease with increasing consumer pressure or environmental stress. This hypothesis has received extensive attention in plant community ecology, but only a handful of tests in animals. Furthermore, few empirical studies have examined multiple co-occurring stressors. Here we test predictions of the stress gradient hypothesis using the occurrence of mixed-species groups in six common grazing ungulate species within the Serengeti-Mara ecosystem. We use mixed-species groups as a proxy for potential positive interactions because they may enhance protection from predators or increase access to high-quality forage. Alternatively, competition for resources may limit the formation of mixed-species groups. Using more than 115,000 camera trap observations collected over 5 yr, we found that mixed-species groups were more likely to occur in risky areas (i.e., areas closer to lion vantage points and in woodland habitat where lions hunt preferentially) and during time periods when resource levels were high. These results are consistent with the interpretation that stress from high predation risk may contribute to the formation of mixed-species groups, but that competition for resources may prevent their formation when food availability is low. Our results are consistent with support for the stress gradient hypothesis in animals along a consumer pressure gradient while identifying the potential influence of a co-occurring stressor, thus providing a link between research in plant community ecology on the stress gradient hypothesis, and research in animal ecology on trade-offs between foraging and risk in landscapes of fear.

Original languageEnglish (US)
Article numbere03163
JournalEcology
Volume101
Issue number11
DOIs
StatePublished - Nov 1 2020

Bibliographical note

Funding Information:
We thank the Zooniverse staff and > 30,000 volunteers who contributed to Snapshot Serengeti classifications (https://www.snapshotserengeti.org/#/authors), the Serengeti Lion Project field staff, particularly D. Rosengren and I. Munuo, and the Minnesota Supercomputing Institute for providing resources that contributed to the data storage. Research clearance was provided by the Tanzania Wildlife Research Institute and Tanzania National Parks. The study was funded in part by NSF grant DEB 1020479 and an NGS Explorer grant to C. Packer. Thanks to the National Geographic Society for providing funding to support camera trapping through grants WW-024R-17, WW-025R-17, and NGS-52921R-18 to T. M. Anderson at Wake Forest University. L. Beaudrot thanks the University of Michigan Department of Ecology & Evolutionary Biology and the Michigan Society of Fellows. We also thank the organizers of the 2018 Gordon Research Conference on Predator-Prey Interactions where the first two authors met, and Norman Owen-Smith, Matt Hayward, Adrian Shrader, and several anonymous reviewers for comments on previous versions. Authorship statement: L. Beaudrot and M. S. Palmer conceived the project idea. L. Beaudrot analyzed the data and led the writing. M. S. Palmer and T. M. Anderson collected camera trap data; M. S. Palmer processed the camera trap data, calculated covariates, and contributed substantially to the writing. M. S. Palmer and C. Packer guided the analysis. C. Packer and T. M. Anderson provided long-term insight into the system.

Funding Information:
We thank the Zooniverse staff and > 30,000 volunteers who contributed to Snapshot Serengeti classifications ( https://www.snapshotserengeti.org/#/authors ), the Serengeti Lion Project field staff, particularly D. Rosengren and I. Munuo, and the Minnesota Supercomputing Institute for providing resources that contributed to the data storage. Research clearance was provided by the Tanzania Wildlife Research Institute and Tanzania National Parks. The study was funded in part by NSF grant DEB 1020479 and an NGS Explorer grant to C. Packer. Thanks to the National Geographic Society for providing funding to support camera trapping through grants WW‐024R‐17, WW‐025R‐17, and NGS‐52921R‐18 to T. M. Anderson at Wake Forest University. L. Beaudrot thanks the University of Michigan Department of Ecology & Evolutionary Biology and the Michigan Society of Fellows. We also thank the organizers of the 2018 Gordon Research Conference on Predator‐Prey Interactions where the first two authors met, and Norman Owen‐Smith, Matt Hayward, Adrian Shrader, and several anonymous reviewers for comments on previous versions. Authorship statement: L. Beaudrot and M. S. Palmer conceived the project idea. L. Beaudrot analyzed the data and led the writing. M. S. Palmer and T. M. Anderson collected camera trap data; M. S. Palmer processed the camera trap data, calculated covariates, and contributed substantially to the writing. M. S. Palmer and C. Packer guided the analysis. C. Packer and T. M. Anderson provided long‐term insight into the system.

Publisher Copyright:
© 2020 The Authors. Ecology published by Wiley Periodicals LLC on behalf of Ecological Society of America

Keywords

  • associational defense
  • facilitation
  • group defense
  • habitat amelioration
  • heterospecific group
  • interspecific group
  • landscape of fear
  • polyspecific association
  • predation

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