Bears benefit plants via a cascade with both antagonistic and mutualistic interactions

Abstract

Predators can influence primary producers by generating cascades of effects in ecological webs. These effects are often non-intuitive, going undetected because they involve many links and different types of species interactions. Particularly, little is understood about how antagonistic (negative) and mutualistic (positive) interactions combine to create cascades. Here, we show that black bears can benefit plants by consuming ants. The ants are mutualists of herbivores and protect herbivores from other arthropod predators. We found that plants near bear-damaged ant nests had greater reproduction than those near undamaged nests, due to weaker ant protection for herbivores, which allowed herbivore suppression by arthropod predators. Our results highlight the need to integrate mutualisms into trophic cascade theory, which is based primarily on antagonistic relationships. Predators are often conservation targets, and our results suggest that bears and other predators should be managed with the understanding that they can influence primary producers through many paths.

Advances in restoration ecology: rising to the challenges of the coming decades

“…The importance of key species has been highlighted by studies showing the influence of top predators, such as wolves or bears, in structuring plant communities through their effects on herbivore populations and behavior via antagonistic and mutualistic relationships (as described eloquently in Leopold's ''Thinking Like a Mountain''; Leopold 1949, Ripple and Beschta 2007, Grinath et al 2015. Overabundance of herbivores, such as deer, has important effects on plant community dynamics, often, for instance, preventing tree regeneration (Côté et al 2004, Hobbs 2009).…”

Short‐term, low‐level nitrogen deposition dampens a trophic cascade between bears and plants

“…Aboveground plant growth was obtained by calculating the difference between plant mass estimates on July 11 and August 3. These estimates were found nondestructively by applying measures of plant height and cover area to an allometric model developed previously for rabbitbrush at this site (Grinath et al, ). Cumulative abundances were measured to evaluate insect herbivore (treehoppers, beetles, caterpillars, and leafhoppers) and foraging ant (which persisted in exclosures after ant nests became inactive) responses across the duration of the experiment.…”

“…I included paths from flower buds to seed production and to herbivores to further evaluate changes in plant–herbivore relationships. As in previous SEM analyses (Grinath et al, , ), “change in treehoppers” (treehopper abundances in the final survey minus those at the start of the experiment) was used as a response variable representing treehopper survival because initial abundances were manipulated. Prior to analysis, I visually examined bivariate relationships among the SEM variables to determine if there were extreme outlying observations.…”

“…Across western North America, the thatch ant Formica obscuripes (Figure ) constructs large, mounded nests to house their colonies (Jergensen, Storer, & Risch, ; Weber, ). A prior study (Grinath, Inouye, & Underwood, ) showed that black bears in Colorado tear apart F. obscuripes nests to consume the immature and adult ants within, leading to nest inactivity and a cascade of effects on surrounding plants (Figure ). The ants have a mutualistic (positive) relationship with a dominant herbivore, a sap‐sucking treehopper ( Publilia modesta : Figure ) which provides sugary honeydew as food in exchange for ant protection from other arthropod predators, such as lady beetles and spiders (Grinath et al, ).…”

“…A prior study (Grinath, Inouye, & Underwood, ) showed that black bears in Colorado tear apart F. obscuripes nests to consume the immature and adult ants within, leading to nest inactivity and a cascade of effects on surrounding plants (Figure ). The ants have a mutualistic (positive) relationship with a dominant herbivore, a sap‐sucking treehopper ( Publilia modesta : Figure ) which provides sugary honeydew as food in exchange for ant protection from other arthropod predators, such as lady beetles and spiders (Grinath et al, ). Ant protection for treehoppers reduces predator abundances, which facilitates other herbivores that are unmolested by ants, such as caterpillars and leafhoppers (Grinath et al, ).…”