Can omnivores be considered keystone species? Bears as a study case

The functional roles of herbivores and carnivores in ecosystems have been relatively well studied: both can shape ecosystems through strong direct and indirect top-down controls on other guilds. By contrast, the role of omnivores has been little explored, although their diet spans several trophic levels with potential numerous effects on ecosystems, through trophic and non-trophic interactions. To gain a better understanding of the potential key ecosystemic role of omnivores interactions, we performed a literature review for Bears (Ursidae): a specific group of terrestrial omnivores. We submitted equation searches on the Web of Science for each type of interactions and omnivore bear species. We kept 411 references highlighting a wide range of trophic and non-trophic interactions in various ecosystems. Our review sheds light on their large number, diversity and significance of interactions. Moreover, we highlight a great variability of interactions across bear species but also across ecosystems for a given bear species. Beyond bears, our review shows that omnivores do play a pivotal role in ecosystem functioning through the strong interactions they have with a reduced set of other species but also through their multiple and weaker interactions with a diversity of taxa and trophic levels. Here, we suggest that omnivore may be considered as keystone species, even if further studies is needed, and in any case may represent worthwhile conservation targets