Top-down control by insectivorous birds during primary succession on Mount St. Helens

Background/Question/Methods

Sitka willow (Salix sitchensis) is a foundation species on regions of Mount St. Helens undergoing primary succession, where it provides the only significant source of vertical structure. On the Pumice Plain, S. sitchensis is frequently suppressed by insect herbivores, particularly in upland areas, which in turn limits colonization by certain assemblages of animal species, including birds and small mammals. Through an exclusion experiment, we assessed the effects of top-down control by birds on S. sitchensis herbivore damage and biomass in plots of high and low willow density. Because plots with high densities of willow support a larger diversity of birds than their low-density counterparts and frequently occur in wet areas of higher resource availability, we hypothesized that high density plots would have greater potential for herbivore suppression by birds. One high- and 1 low-density plot were chosen from each of 2 sites on the Pumice Plain. Within each plot, 16 S. sitchensis plants were selected and assigned at random as bird exclosures, shade controls, open controls, or pesticide treatments. After 8 weeks, we measured leaf damage and stem diameters of each plant. We also used sticky traps to assess arthropod abundance and diversity.

Results/Conclusions

After 8 weeks, the probability of leaf damage by insect herbivores was greater in low-density plots. Further, this effect was strongly increased with exclusion of birds. However, for attacked leaves, no significant effects of density or bird exclusion were seen in the magnitude of leaf damage. This suggests that birds decreased the likelihood that an herbivore would feed on a given leaf but did not affect the amount of damage to the leaf once feeding had commenced. The lack of an effect of birds in high-density plots could reflect increased food web complexity. Interactions within a guild, such as competition or intraguild predation, could reduce cascade strength due to weaker and more diffuse interactions among trophic levels. Assessment of treatment effects on arthropod abundance and diversity should provide further insight into the mechanisms that drive top-down control by birds during primary succession.