Monday, December 10, 2007 - 10:05 AM

Arthropod communities in a temperate forest canopy: Does predation pressure vary in vertical space?

Kathleen R. Aikens, and Christopher M. Buddle. McGill University, Natural Resource Sciences, 21,111 Lakeshore Road, Ste Anne-de-Bellevue, QC, Canada

In many forest systems, predator-prey interactions have important consequences for arthropod communities. Resource quality/availability differ through vertical space in the forest, and it is expected that prey species distribute themselves in relation to these different resources, while also avoiding predators. Enemy-free space is defined as ways of living that reduce vulnerability to natural enemies, and as such, it provides a conceptual framework with which to examine the distribution of arthropods in the forest canopy. Different predation rates in different forest strata would suggest that some strata offer enemy-free or “enemy-reduced” space. Important canopy predators of arthropods include both invertebrates (e.g. spiders, ants, assassin bugs) and vertebrates, primarily birds. Our objective is to determine whether the ability of predators to manipulate prey density translates into a difference in distribution of arthropods in vertical space from the understory to upper canopy. To examine differences in arthropod density and predation pressure, we considered four strata in a sugar maple forest. We combined foliage-beating techniques with bird-exclusion experiments to determine how the impact of avian predators varies in vertical space. The magnitude of effect of bird exclusion differed across strata, suggesting that avian predators more effectively depress arthropod communities in upper canopy foliage.