Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction
Highlights
► The reintroduction of wolves into Yellowstone has triggered a tri-trophic cascade. ► Woody plants are now growing taller in portions of the northern range. ► Wolf reintroduction may represent an effective approach for passively restoring ecosystems.
Introduction
In a system with three trophic levels (tri-trophic) involving predators, prey, and plants, predators can indirectly affect plant communities by influencing prey behavior and density, thus releasing plants from herbivory (Strong and Frank, 2010). In Yellowstone, wolves (Canis lupus) were extirpated from the park by the mid-1920s, absent for a period of seven decades, and reintroduced in the winters of 1995/1996. The historical presence, then absence, and now presence of wolves in Yellowstone National Park (YNP) represents a natural experiment through time and an opportunity to study cascading trophic interactions. During the seven-decade wolf-free period, the collapse of a tri-trophic cascade allowed elk (Cervus elaphus) to significantly impact wildlife habitat, soils, and woody plants. For example, species such as aspen (Populus tremuloides) and willows (Salix spp.) were generally unable to successfully recruit young stems into the overstory on Yellowstone’s northern winter ranges, except in fenced exclosures (Grimm, 1939, Lovaas, 1970, NRC, 2002, Barmore, 2003).
Recent studies of aspen and cottonwood (Populus spp.) age structures, based on assessments of tree rings and diameter classes, have shown that the extirpation of wolves and subsequent increase in elk herbivory was linked to the long-term decline in the recruitment of these deciduous species (Ripple and Larsen, 2000, Beschta, 2005, Halofsky and Ripple, 2008b). With wolves now back on the Yellowstone landscape for 15 years, we ask the question: How has the reintroduction of wolves affected the recruitment of woody browse species? Our objectives were to (1) collect new data on the recruitment status of both aspen and cottonwood in the Lamar River catchment on the northern winter range of YNP, and (2) synthesize the existing body of work on tri-trophic cascades (i.e., wolves, elk, and changes in woody plants) in Yellowstone since wolf reintroduction 15 years ago.
Section snippets
Methods
In September of 2010, we repeated an aspen recruitment survey originally conducted in 2006 in the Lamar catchment of YNP’s northern range where riparian and adjacent upland aspen stands had been surveyed along four streams; the Lamar River and Slough, Crystal, and Rose Creeks (Ripple and Beschta, 2007b). This pairing originally allowed us to compare browsing levels and aspen heights between upland and riparian (stream side) areas. We returned to the same 98 stands measured in 2006 using a
Results
Between 1995 and 2003, northern range wolf numbers increased from 0 to 98 (Smith et al., 2011). However, since 2003 the population has generally declined, but has fluctuated substantially (Fig. 1A). According to annual elk census data, the northern range elk counts decreased from highs >15,000 individuals during the early 1990s before wolf reintroduction to approximately 6,100 in 2010 (Fig. 1B) (White and Garrott, 2005; unpublished data – Yellowstone National Park).
Based on plant architecture
Discussion
Since wolf reintroduction, Yellowstone northern ecosystems have responded as predicted by classic ecological theory with alternating biomass levels across adjacent trophic levels (i.e., more wolves, fewer elk with altered behavior, more plant biomass). Over a 13-year period from 1998 to 2010, we found a strong inverse relationship between browsing intensity and heights of young aspen in that as browsing decreased, aspen height increased (Fig. 1C and D). Browsing on aspen has been declining in
Conclusions
Integrating top-down and bottom-up processes is a formidable challenge for ecologists and will require additional research on how potential bottom-up factors interact with top-down forces affect woody plant growth in Yellowstone. Examples of bottom-up factors include snow depth, precipitation (drought), and temperature (climate warming). More studies that sample multiple plant species across space and time are needed especially those that incorporate ungulate browsing levels and measures of
Acknowledgements
We thank C. Eisenberg, J. Halofsky, J. Hollenbeck, E. Larsen, and L. Painter for reviewing an early draft of this manuscript. We appreciate the animal data for Yellowstone National Park provided by D. Smith and P. J. White as well as comments and suggestions provided by three anonymous reviewers.
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