Ripple, Laundre and Hernandez landmark research--THE LANDSCAPE OF FEAR
Luke E. Painter et al, Aspen recruitment in the Yellowstone region linked to reduced herbivory after large carnivore restoration,
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Abstract
Quaking aspen (Populus tremuloides) recruitment during the 1980s–90s was suppressed by Rocky Mountain elk (Cervus canadensis) herbivory on winter ranges in the Yellowstone region, and saplings (young aspen taller than 2 m) were rare. Following the 1995–96 reintroduction of gray wolves (Canis lupus), browsing decreased and sapling recruitment increased in Yellowstone National Park.
We compared aspen data from inside the park to data collected in three winter ranges outside the park. For most areas, the percentage of young aspen browsed annually was 80–100% in 1997–98, decreasing to 30–60% in 2011–15. Sapling recruitment was inversely correlated with browsing intensity, and increased despite climate trends unfavorable for aspen. Browsing decreased with decreasing elk density, a relationship suggesting that densities greater than about 4 elk/km2 resulted in consistently strong browsing effects likely to suppress aspen recruitment.
Changes in elk density and distribution were influenced by predators, as well as human hunters. Most evidence for trophic cascades involving large terrestrial mammals has been from protected areas within national parks. This study provides evidence of widespread changes in plant communities resulting from large carnivore restoration, extending outside a protected national park to areas with hunting, livestock grazing, and other human activities.
EXCERPTS FROM FULL RESEARCH PAPER
The ecosystem of Yellowstone National Park (YNP) provides an example of the relationships between aspen, elk, and large carnivores. In the late 1800s and early 1900s, aspen stands covered much more area than they do now in the valleys of northern Yellowstone (YNP 1997, NRC 2002). After wolves were extirpated in the 1920s and other predators were greatly reduced, browsing by elk was intensive and aspen stands in the northern Yellowstone elk winter range (northern range) failed to recruit new trees (Romme et al. 1995, Ripple and Larsen 2000).
Although elk in the park were culled to reduce their impacts, young aspen rarely grew above the reach of elk (YNP 1997, Barmore 2003, Wagner 2006). Culling in the park ended in 1968, and elk herds grew to record numbers in the 1980s–90s (Coughenour and Singer 1996), expanding beyond the park boundary and suppressing aspen recruitment in winter ranges both inside and outside the park (St. John 1995, Kay 2001, Larsen and Ripple 2003, 2005, Halofsky and Ripple 2008).
Although elk in the park were culled to reduce their impacts, young aspen rarely grew above the reach of elk (YNP 1997, Barmore 2003, Wagner 2006). Culling in the park ended in 1968, and elk herds grew to record numbers in the 1980s–90s (Coughenour and Singer 1996), expanding beyond the park boundary and suppressing aspen recruitment in winter ranges both inside and outside the park (St. John 1995, Kay 2001, Larsen and Ripple 2003, 2005, Halofsky and Ripple 2008).
During the first two decades after the return of wolves in 1995–96, elk densities decreased in northern YNP, and an increasing number of aspen stands produced saplings (young aspen >2 m in height), an indication that some aspen were escaping from browsing (Ripple and Beschta 2007, 2012, Painter et al. 2014, 2015, Klaptosky 2016, Beschta et al. 2018).
These changes were associated with reduced herbivory, consistent with a trophic cascade whereby wolves, combined with bears (Ursus spp.) and cougars (Puma concolor), benefited aspen through effects on elk density or behavior (Hernandez and Laundre 2005, Hamlin et al. 2009, Peterson et al. 2014, Beschta and Ripple 2016).
Human hunters outside the park also took many elk before 2006 (Vucetich et al. 2005, Eberhardt et al. 2007), but similar harvests previously had not been sufficient to limit the northern Yellowstone herd (Lemke et al. 1998, White and Garrott 2005).
We examined evidence for trophic cascades involving aspen, elk, and wolves in the Yellowstone region, using surveys of aspen stands in elk winter ranges north, west, and east of the park, as well as inside the park boundary. Young aspen in these ranges were suppressed by browsing in the 1980s–90s, but after the return of wolves in the late 1990s, elk densities and distributions changed (Hamlin et al. 2009, Christianson and Creel 2014). We investigated the possibility that browsing intensity decreased and aspen recruitment increased in winter ranges outside the park, as has occurred in northern YNP (Painter et al. 2014, 2015), and that browsing intensity was linked with recent changes in elk abundance and distribution.
Large carnivore restoration has indirectly benefited aspen recruitment in the Yellowstone region through a trophic cascade. This hypothesis was supported by evidence from aspen surveys and elk counts. In winter ranges in and near the park, browsing generally decreased and aspen recruitment increased. Yet, in some places such as Daly Creek, intensive browsing by elk continued and aspen were still suppressed. Elk use of the Dome Mountain WMA area has increased, beginning to suppress young aspen there. These exceptions to the general pattern demonstrate the importance of placing local phenomena into a broader context. As elk densities have decreased more in some areas than others, spatial variation in browsing has increased, resulting in a patchy increase in aspen recruitment.
Following the return of wolves to Yellowstone, elk herds that formerly were limited by forage or hunting have become limited by predators, and the proportion of the elk population wintering in or near the park has decreased. Surveys of aspen both in and out of the park found reduced herbivory and increased aspen recruitment within the last decade, despite climate trends unfavorable for aspen.
This decrease in herbivory of aspen has followed changes in elk densities and distribution, suggesting that relatively low elk densities are required for widespread aspen recruitment (Fig. 3b). If predation has been a significant cause of these changes, as the evidence supports, then the new aspen recruitment we observed was, at least in part, an indirect effect of wolf restoration.
This decrease in herbivory of aspen has followed changes in elk densities and distribution, suggesting that relatively low elk densities are required for widespread aspen recruitment (Fig. 3b). If predation has been a significant cause of these changes, as the evidence supports, then the new aspen recruitment we observed was, at least in part, an indirect effect of wolf restoration.
In addition to direct mortality of elk, behavioral responses to predators and human hunters may have influenced elk range selection, foraging, and use of habitat. These results provide evidence of a trophic cascade involving wolves outside of a protected park, in a complex landscape with hunting, livestock, and other human influences. The consequences of large carnivore restoration in Yellowstone continue to unfold, with cascading effects over large scales, as prey balance the pressures of predation and hunting with other environmental factors.
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