Trophic cascades: Linking ungulates to shrub-dependent birds and butterflies

Abstract

Top predators are known to regulate freshwater, marine, and terrestrial ecosystems. However, few studies have demonstrated trophic cascades in productive and biologically diverse terrestrial ecosystems. Elk Island National Park and surrounding protected areas have a wide range in the intensity of use by ungulates (IUU) (2.3 to 53.4 units/km2) due to the functional loss of top predators, management for high ungulate numbers and variable hunting pressure. To evaluate whether high IUU results in a trophic cascade, I examined vegetation characteristics and the abundance of shrub-dependent yellow warblers (Dendroica petechia) and Canadian tiger swallowtails (Papilio canadensis). Areas with high IUU resulted in loss of horizontal shrub cover that resulted in reductions of yellow warblers. Abundance of Canadian tiger swallowtail was related to reductions in larval host plant density, particularly chokecherry (Prunus virginiana). This study provides evidence of a species-level trophic cascade, initiated by a combined effect of the loss of top predators and management for high ungulate densities.