Indirect effect of black bears on sunflowers in nitrogen-polluted and pristine steppe
Abstract
Both antagonistic and mutualistic types of interactions between species occur in most ecosystems, and these different types of interactions can contribute to indirect effects between species, such as trophic cascades. These interactions can be altered by humans, and one major way this is occurring is via atmospheric nitrogen (N) pollution. Though research has been conducted to study how N pollution affects cascades, results have not been consistent. The objective of this experiment is to determine the indirect effect of black bears on mule's ear sunflowers through a cascade of antagonistic and mutualistic interactions, and whether N pollution could alter this cascade. This sunflower is a primary host for sap-sucking treehoppers, which can complete their entire life cycle on the plant, potentially harming plant performance. The treehoppers have a mutualistic relationship with thatch ants, which provide protection against predators while the treehoppers provide sugary food for the ants. These ants are a common food for black bears, and this predation has been shown to indirectly affect rabbitbrush plants. Building on a 15-year N enrichment study, I tested whether black bears and ants indirectly affect mule’s ears sunflowers (Wyethia magna), and if long-term N deposition could affect this cascade. I manipulated initial treehopper nymph densities on individual plants in plots with added or ambient N and with either active or inactive (due to bear predation) thatch ant nests, and measured responses of insects and plants to these experimental conditions. I found that while treehoppers negatively affected plant water content, there was no evidence for a trophic cascade from bears to mule’s ears sunflowers. However, N addition did alter ant abundance responses to treehopper density, suggesting that N pollution may weaken the mutualistic interaction between these animals. Further work is needed to account for the effects of cattle grazing and other factors to gain a clearer understanding for the effects of predators and N pollution in this ecological web.
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References (29)
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