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

Abstract

Human activities have substantially increased atmospheric nitrogen (N) deposition in ecosystems worldwide, often leading to higher plant quality for herbivores and greater herbivory. Predators frequently suppress herbivores and indirectly benefit plants via “trophic cascades”, and the strength of these interactions can also depend on N availability. However, the evidence for N deposition effects on cascades primarily comes from studies of high‐level N deposition. Most terrestrial ecosystems currently receive elevated, but low‐level N deposition, and it is unclear whether this subtle N enrichment has any effect on cascades. Here, I asked whether low‐level N deposition alters a trophic cascade from black bears to plants in Colorado. In this ecological network, bears indirectly benefit plants by consuming ants and suppressing positive effects of ants on herbivores. Using a three year N enrichment experiment, I assessed changes in this cascade by measuring plant and arthropod responses to simulated N deposition, bear damage to ant nests, and the presence of mutualist herbivores and ants. I found that low‐level N enrichment and bears had interacting effects on plant reproduction. In ambient N conditions, bears indirectly increased plant reproduction by causing ant nests to become inactive and suppressing positive ant effects on herbivores that were detrimental for plants. Yet, bear‐induced ant nest inactivity had no effect on plant reproduction in N‐enriched conditions. When N was added, ants had greater positive effects on herbivores, but herbivores had weak effects on plants, potentially because plants were more resistant to herbivores. Ultimately, the results indicate that N enrichment strengthened resource control of the community and weakened plant–herbivore interactions and the cascade from bears to plants. This study suggests that common rates of low‐level N deposition are changing the strength of trophic cascades and may have already altered resource versus consumer control of ecological community structure in many ecosystems.

Effects of Human Disturbance on Terrestrial Apex Predators

“…For example, predation on ants by American black bears (Ursus americanus) reduced the protection that ants provide to herbivorous insects through a mutualistic relationship, allowing arthropod predators to suppress the herbivores, which benefited plants [72]. Grinath [73] followed up this line of research and suggested that conservation of bears and other omnivores and predators that generate trophic cascades may be essential for managing the long-term effects of nitrogen deposition in ecosystems caused by human activities.…”

Grasshopper consumption by grey wolves and implications for ecosystems

“…). Unfortunately, this view recapitulates the fallacy that vertebrate and invertebrate food webs are separate entities, which they clearly are not (e.g., Grinath ). Our observation of nearly 200 partially digested grasshoppers within the scat of North America's iconic large carnivore graphically illustrates that nature does not heed to the taxonomic boundaries imposed by ecologists.…”