In ecosystems worldwide, predators generate ‘trophic cascades’ by interacting with herbivores and indirectly affecting plants. These dynamics alter plant abundances and biomass, and can modify plant traits as well. In particular, studies have shown that predators indirectly affect plant chemistry by altering herbivore abundances and changing herbivore physiology and behavior, which impact resource consumption by herbivores. These cascades occur via chains of antagonistic, predator-prey types of relationships, but can also occur via interaction chains that contain mutualistic relationships. For example, mutualisms between predatory ants and herbivores can cause lower nitrogen (N) concentration in plants. Higher-level predators, such as predators of ants, may affect plant chemistry by altering mutualistic interactions, but it is unclear whether such cascades exist. Here, I asked whether ant-eating black bears indirectly affect the carbon:nitrogen (CN) stoichiometry of plants that are consumed by herbivores that are mutualists of ants. Within 23 ant enclosures, I monitored bear damage to ant nests and manipulated mutualist herbivore and ant presence on rabbitbrush plants. Then I measured rabbitbrush and herbivore CN and insect abundance responses to the presence of bear disturbance and mutualist herbivores and ants, as well as ant stable isotope (δ13C and δ15N) responses to bear damage to nests.
Results/Conclusions
I found that bear damage to ant nests indirectly increased the foliar nitrogen concentration of nearby plants, thereby lowering plant CN ratio. Though bears did not impact herbivore whole-body stoichiometry, bear damage decreased the abundances of both tending ants and mutualist herbivores. Bear damage to ants also altered the relationship between plant and herbivore stoichiometry. In the presence of ants that were undisturbed by bears, there were negative relationships between plants and herbivores for both N concentration and CN ratio. However, when bears damaged ant nests, these relationships became positive (N) or neutral (CN). Plant and herbivore C concentration were not significantly affected by the experimental factors and had little effect on the CN results. Furthermore, stable isotope analysis showed that ant worker δ13C values increased in response to bear damage, suggesting that ants shifted foraging to different plants species, while ant δ15N values were unaffected by bears. Altogether, these results indicate that bears can indirectly affect plant chemistry by altering ant foraging, herbivore abundances, and the interaction between plants and herbivores that are mutualists of ants. This cascade is likely to influence N cycling and other ecosystem functions, which suggests that black bears should be a conservation priority.