Nitrogen deposition mediates the cascading effects of bears on plants
Humans have dramatically altered earth’s nitrogen (N) cycle, resulting in elevated atmospheric N deposition over large areas that have historically been limited by N availability. In general, greater N deposition is thought to increase plant productivity and quality for herbivores, which can cause herbivore damage to intensify. However it is unclear whether N deposition influences species interactions at higher trophic levels. Predators often indirectly benefit plants by suppressing herbivores, and the strength of these trophic cascades may change across N deposition levels. I previously found that black bears benefit rabbitbrush plants by consuming ants that are mutualists of herbivorous treehoppers. Here, I asked whether the strength of this cascade depends on N deposition and which of the component species interactions changed under elevated N. In a meadow with low N deposition, I conducted a field experiment simulating elevated N deposition in half of 36 ant nest enclosures. I added N and monitored bear damage to ant nests for 3 summers and, in the third summer, manipulated ants and treehoppers on plants within enclosures. I measured plant performance and arthropod abundance responses and used generalized linear mixed models to analyze the effects of N, bears, ants and treehoppers.
Simulated N deposition weakened the cascade from bears to plants. When N was low, bear damage to ants indirectly benefited plant seed production as expected; but when N was high, bears had no affect on seed production. Elevated N changed the strength of interactions between treehoppers, ants, and other arthropods, which contributed to weakening the net effect of bears on plants. Additionally, plants had greater leaf carbon concentrations when N was added, which was independent of the effects of bears, ants, and treehoppers. This result suggests that N deposition stimulated photosynthesis, and also that the cascade may have become weaker if plants were more tolerant of herbivores when resources were abundant. Overall, these results indicate that current rates of N deposition mediate the strength of predator effects on plants through multiple mechanisms. Trophic cascades are important for determining ecological community stability, and N deposition effects on cascades could potentially destabilize communities and disrupt ecosystem functions and services. As elevated N deposition continues to fertilize vast expanses of our planet, we urgently need further study to anticipate how species interactions and trophic cascades will respond.