Influence of bird predation on herbivore abundance and understory tree growth in a northern forest ecosystem

Ecologists have long debated the importance of predators in limiting herbivore abundance and the conditions under which predator effects cascade through to primary productivity. We investigated the impact of insectivorous birds on herbivorous insects and tree growth across an environmental gradient by experimentally excluding birds from striped maple (Acer pensylvanicum) saplings at three elevations in the Hubbard Brook Experimental Forest (HBEF), New Hampshire from 2004 to 2006. The findings suggest that, although herbivorous insects are a major item in the diet of birds at HBEF, herbivore abundance was not tightly coupled to avian predation during the study period. Furthermore, environmental factors such as seasonal light availability were more important in determining tree growth than herbivore abundance and consumption of leaf tissue. A complex food web structure, with birds representing only a subset of the predator community, and foliage quality that ordinarily constrains herbivore abundance may be factors in preventing strong trophic cascades initiated by birds from developing in this forested ecosystem.