Trophic cascades among wolves, elk and aspen on Yellowstone National Park’s northern range

Abstract

Quaking aspen (Populus tremuloides) biomass has declined in Yellowstone National Park (YNP) in the past century. We installed permanent belt transects (plots) for long-term monitoring of aspen stands both within and outside of established wolf pack territories on YNP's northern range to determine if reintroduced wolves are influencing elk browsing patterns and aspen regeneration through a trophic cascades interaction. Wolves may have an indirect effect on aspen regeneration by altering elk movements, browsing patterns, and foraging behavior (predation risk effects). Elk pellet groups, aspen sucker heights, and the percentage of browsed suckers were the variables used to measure differences in aspen stands in high and low wolf-use areas of the northern range. The aspen stands in the high wolf-use areas had significantly lower counts of elk pellet groups in the mesic upland steppe and the combined mesic upland steppe and riparian/wet meadow habitat types. Based on our pellet group results, it appears that elk foraging behaviors may have been altered by the increased risk of predation due to the reintroduction of the wolf. In the riparian/ wet meadow habitat type, mean aspen sucker heights were significantly higher in the high wolf-use areas than in the low wolf-use areas. The percentage of browsed suckers in high and low wolf-use areas showed no significant differences in any of the habitat types. Considering the high browsing pressure in YNP aspen stands, it is uncertain whether the taller aspen suckers measured in the high wolf-use areas will eventually join the aspen overstory. These permanent plots represent a valuable baseline data set to assess any current and future aspen regeneration responses to the reintroduction of wolves in YNP. #

Vegetation dynamics of predator‐free land‐bridge islands

“…Increasing numbers of studies in North America are demonstrating that wolves exert a strong top-down force that extends to the plant community (Mclaren & Peterson 1994;Ripple et al 2001). In the absence of wolves, beaver, white-tailed deer and other ungulates become superabundant and begin to alter the composition of plant communities (Alverson et al 1988;McShea et al 1997;Pedersen & Wallis 2004).…”

Lichen community change in response to succession in aspen forests of the southern Rocky Mountains

“…While aspen suckers may continue to emerge within small canopy openings, their proliferation becomes increasingly limited without larger disturbance. Survival of aspen regeneration in conifer-dominated stands is often low due to increasing resource limitations and the well documented impacts of ungulate browsing (Baker et al, 1997;Hessl and Graumlich, 2002;Kay and Bartos, 2000;Ripple et al, 2001). Moreover, reduced aspen cover has resulted in limitations on further asexual reproduction: the fewer healthy trees above ground, the less likely new suckers will emerge either on a continuous basis or in a flush following disturbance (Shepperd et al, 2006).…”

“…Recently this term has been applied to aspen in the western US (Campbell and Bartos, 2001;Manley et al, 2000). Ripple et al (2001) traced the trophic interactions of wolves, elk, and aspen survival in Yellowstone National Park emphasizing the critical nature of carnivores in regulating large ungulates that browse on aspen regeneration. Without successful regeneration following large-scale disturbance, future aspen forests, dependent on vegetative suckering to persist, will dwindle on the landscape.…”

“…From this perspective succession in aspen communities plays a crucial role not only in the development and potential conversion of aspen to other types, but as a catalyst for change in associated species. There is strong support for the notion of aspen's unique contribution to biodiversity of western North American landscapes (DeByle, 1985;Mueggler, 1988;Ripple et al, 2001;Shepperd et al, 2006) and some have highlighted aspen as a ''keystone'' type, denoting their amplified role in supporting entire ecosystems (Campbell and Bartos, 2001;Manley et al, 2000).…”

Rethinking predators: Legend of the wolf

“…In the same year, Ripple, an ecologist at Oregon State University in Corvallis, published a key paper 4 on the most famous, and probably the best-studied, example of a terrestrial carnivore structuring an ecosystem: Yellowstone's wolves. The ecosystem offered a natural experiment because the US National Park Service had the park's exterminated wolves (Canis lupus) by 1926 and then reinstated them in the 1990s, after public sentiment and ecological theory had shifted.…”