Doctor of Philosophy
Date of Defense
Robert J. Marquis, PhD
This study examines direct and indirect relationships between three trophic levels to determine effects on plant damage, herbivore abundance and community structure, and bird distribution in forest ecosystems. Exclusion experiments on white oak (Quercus alba) revealed that bird predation effects to not vary spatially despite variation in abundance of both birds and insects. Using a leaf quality manipulation, I demonstrated that bird impacts do not differ with host plant quality. Rather, birds and plant traits had additive effects on herbivore damage. Bottom-up effects of leaf quality were also more important than top-down effects of birds in structuring the insect herbivore community on white oak. Leaf quality influenced the total abundance and richness of herbivores as well as the abundance of different feeding guilds. These effects of leaf quality were strongest at the end of the growing season, when leaf quality is presumably lowest overall. Bottom-up effects may also be modified by the physical environment in which a plant grows. I studied abundance and distribution of a specialist oak herbivore and showed that individuals choosing a host plant may face a trade off between the optimal physical environment and suitable plant traits. Finally, I demonstrated a bottom-up effect of invasive prey on insectivorous birds: outbreaking gypsy moths alter the annual distribution of native cuckoos at a regional scale. This study indicates that complex interactions exist beyond a simple, unidirectional consumption model of plants, herbivores, and avian predators. The indirect positive effect of birds on plants appears robust to variation in the abundance and traits of the three trophic levels, but the mechanism for this effect may vary through time and space. The impact of birds, however, did not vary with plant characteristics. These characteristics, which can depend on environmental context, likely play a larger role in determining the abundance, structure, and impacts of herbivores than do insectivorous bird predators.
Barber, Nicholas Anthony, "Tritrophic interactions in forests: Direct and indirect interactions between birds, insect herbivores, and oaks" (2009). Dissertations. 520.