Yellowstone Wolf Trophic Cascade

[On this post I have summarized: Ripple WJ & Beschta RL, 2012. Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction. Biological Conservation 145, 205-213.]

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.

In a system with three trophic levels involving predators, prey, and plants, predators can indirectly affect plant communities by influencing prey behavior and density, thus releasing plants from herbivory. 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. 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).

While more widespread aspen recruitment would suggest an increasing influence from a density-mediated trophic cascade because elk numbers have trended significantly lower since wolf reintroduction, it is difficult to separate density effects from behavioral effects because predation risk can be temporally dynamic and exist at multiple spatial scales, from a few meters to very large landscapes. Sometimes large-scale shifts in behavior due to predation risk may locally appear to be density effects. For example, in recent years elk have reduced their use of the high elevation winter range in and around the Lamar Valley compared to low-elevation winter range both in and out of the park. This may be explained by elks being more vulnerable to wolf attacks at higher elevations in winter due to relatively deep snowpacks. In addition, researchers have documented major behavioral effects whereby elk in YNP, under the risk of predation by wolves, have altered their habitat use, movements, group sizes, vigilance, and other traits.

Even with the occurrence of increased recruitment within existing aspen stands, full recovery of aspen to historical conditions may not be possible during the next few decades because most stands (approximately 2/3rds) have already died out and were lost due to heavy elk herbivory during the seven-decade period of wolf absence. Furthermore, a recent modeling study has predicted there will be an aspen snag deficit during the second half of the 21st century corresponding to the aspen recruitment gap that was created during the long wolf-free period of the 20th century. It should be noted that future aspen restoration is also possible with sexual reproduction now that wolves are again present in the Yellowstone ecosystem. For example, aspen may again regenerate following fire from seed sources rather than from extant clones.

From the 1920s to the mid 1960s, when wolves were absent, the park service tried to attain improved recruitment of aspen and other woody browse species with decreased elk densities through culling of elk. In retrospect that experiment failed. While recruitment of multiple woody browse species appears to have begun in some areas of Yellowstone National Park in the presence of wolves, we might contemplate what will be required for a more complete and widespread recruitment of upland aspen in the coming years. The combination of behavioral and density effects from wolf presence (press disturbance) with periodic fire (pulse disturbance) may realistically portray how this system functioned historically.

The effects of new recruitment of woody browse species does not stop with plant communities, but continues to ripple through an ecosystem potentially changing biogeochemical processes, as well as habitat and food-web support for a host of vertebrates and invertebrates with potential consequences for increased biodiversity.

The predicted aspen snag deficit described above will likely affect populations of cavity nesting birds on the northern range for decades to come (Hollenbeck and Ripple 2008). Conversely, small herbivores such as rodents and lagomorphs may already be benefiting from decreases in coyotes (Canis latrans) and additional cover and forage due to decreases in elk herbivory and changes to plant communities (Ripple et al. 2011). Any increase in small herbivores could significantly affect the prey base for both avian and mammalian predators [e.g., red foxes (Vulpes vulpes), and badgers (Taxidea taxus)] that subsist on these smaller mammals. Direct and indirect effects of wolves on other animals in Yellowstone have also been suggested for scavengers such as ravens (Corvus corax), bald eagles (Haliaeetus leucocephalus), and black-billed magpies (Pica hudsonia) due to subsidies from wolf-killed carcasses (Wilmers et al. 2003), and on smaller carnivores due to the killing of coyotes by wolves (Ripple et al. 2011). Wolves could have a positive effect on the diets of birds and bears thorough a decrease in ungulate browsing on berry-producing shrubs, resulting in higher berry production and more food for these taxa. In turn, birds and bears, can affect berry-producing shrub establishment by dispersing seeds after consuming the berries. Wolves and bears may provide multiple and linked positive feedback loops in their sympatric predation effects such that wolves provide subsidies to bears through scavenging opportunities on wolf-killed carrion, thus supporting higher bear densities and increased predation by bears on neonatal elk, further lowering elk densities. In recent years, the bear population on the northern range has increased and in 2003–2005 bears killed more elk calves than wolves, coyotes, and cougars combined (Barber-Meyer et al. 2008).

Beaver have also increased since wolf reintroduction; from one colony in 1996 to 12 in 2009. Although beaver were reintroduced into the national forest just north of the park between 1986 and 1999, the park increase in beaver is likely due, at least in part, to the resurgence of willow communities, because beaver on the northern range have been almost exclusively feeding on the newly released willow. Increases in beaver populations have tremendous implications for riparian hydrology and biodiversity. Beaver have important roles in the hydrogeomorphic processes of decreasing streambank erosion, increasing sediment retention, raising wetland water tables, modifying nutrient cycling, and ultimately influencing plant, vertebrate, and invertebrate diversity and abundance in riparian ecosystems. Concurrent with the declining elk population, the bison population has been increasing on the northern range. Wolves may be allowing the bison population to increase through a decrease of inter-specific competition with lower elk numbers. Increased bison herbivory appears to be impacting young woody plants (e.g. willow, cottonwoods) on the northern range especially in the Lamar Valley where there is a relatively high year-round population (Painter and Ripple 2012). Increased bison herbivory may explain why most cottonwood recruitment observed in recent studies has been on Soda Butte Creek and the extreme east end of the Lamar Valley, with little recruitment in the rest of the valley. These secondary cascading effects may represent an example by which predators can influence multiple trophic levels through mediating the competitive interaction between the two prey species, elk and bison.

Frank (2008) even suggested that a wolf-triggered trophic cascade on elk likely altered soil nitrogen mineralization in northern range grasslands.

Based on studies of aspen, willow, and cottonwood in recent years, it appears that wolves have initiated a restructuring of northern Yellowstone’s ecosystems via passive restoration.

Taken collectively, the evidence provided by recent studies of top-down forcing and tri-trophic cascades caused by large predators with interacting bottom-up forces is becoming increasingly persuasive. Predation and predation risk associated with large predators appear to represent powerful ecological forces capable of affecting the interactions of numerous animals and plants, as well as the structure and function of ecosystem.

Ripple WJ & Beschta RL, 2012. Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction. Biological Conservation 145, 205-213.

Barber-Meyer, S.M., Mech, L.D., White, P.J., 2008. Elk Calf Survival and Mortality Following Wolf Restoration to Yellowstone National Park. Wildlife Monograph No. 169.

Frank, D.A., 2008. Evidence for a top predator control of a grazing ecosystem. Oikos 117, 1718–1724.

Hollenbeck, J.P., Ripple, W.J., 2008. Aspen snag dynamics, cavity-nesting birds, and trophic cascades in Yellowstone’s northern range. Forest Ecology and Management 255, 1095–1103.

Painter, L.E., Ripple, W.J., 2012. Effects of bison on willow and cottonwood in northern Yellowstone National Park. Forest Ecology and Management. 264, 150–158.

Ripple, W.J., Wirsing, A.J., Beschta, R.L., Buskirk, S.W., 2011. Can restoring wolves aid in lynx recovery. Wildlife Society Bulletin 35: 514-518.

Wilmers, C.C., Crabtree, R.L., Smith, D.W., Murphy, K.M., Getz, W.M., 2003. Trophic facilitation by introduced top predators: grey wolf subsidies to scavengers in Yellowstone National Park. Journal of Animal Ecology 72, 909–916.