The influence of top-down effects is a subject of great interest to community ecology. However, little work has been done to experimentally test trophic cascade hypotheses in regards to large, terrestrial predators. I investigated whether a large mammalian predator affects forest community structure and diversity. I asked: 1) How do predators affect plants through changes in prey density/behavior?, 2) How do predators affect nonadjacent plant/animal communities through changes in the density/behavior of their competitors?, and 3) How do these changes affect soil nutrients?
I performed natural and manipulative tests in a northern mesic forest mosaic from 2008-2013. Twelve permanent sites were established in forest patches scattered throughout high- and low- wolf use areas (six each). I manipulated deer access, hare access, and rodent access with exclosures at each site. I measured sapling height, forb diversity, seedling abundance (rodent exclosures only), deer behavior, hare herbivory, rodent abundance, and soil nitrogen levels at these sites. I also surveyed mammalian carnivore distributions throughout the forest.
My research suggests wolves play a significant role in determining community structure in this forest. Deer and coyote use were significantly lower in high- compared to low-wolf use areas. Foxes meanwhile used high-wolf use areas almost exclusively. Snowshoe hare browsed high-wolf use sites more intensely, whereas deer mice abundances were significantly lower in high-wolf use.
Wolf-deer interactions appeared to generate a cascade. Deer had no significant impacts on plants in high-wolf use areas based on exclosure results. Maple saplings were less browsed and significantly taller in high-wolf use areas, and forb species richness was greater. Meanwhile we did not observe evidence for significant hare or rodent impacts on maple growth regardless of wolf use, suggesting no cascading effects.
We also monitored soil nitrogen levels to see if wolf-deer interactions would produce food web effects on ecosystem-level processes. Neither deer nor wolves had an effect on soil nitrogen, suggesting food web effects will not cascade to soil nutrients.
Taken together, this research suggests that mammalian top predators can have significant effects on other mammalian consumers and plants, but that this does not affect soil nutrients.