Elsevier

Biological Conservation

Volume 102, Issue 3, December 2001, Pages 227-234
Biological Conservation

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

https://doi.org/10.1016/S0006-3207(01)00107-0Get rights and content

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.

Introduction

Quaking aspen is the most widely distributed tree in North America. It is native to Yellowstone National Park’s (YNP) northern range, a 100,000-ha area including the valleys of the Yellowstone, Lamar, and Gardiner rivers. It is estimated that aspen historically covered 4–6% of the northern range (Houston, 1982) but that percentage has declined to approximately 1% of the landscape (Larsen and Ripple, 1998, unpublished data). The character of aspen stands has also changed from a previous condition of variable age classes to its current state of mature/declining stands of older stems (45, 24). On some northern range sites, aspen clones also exist in a perennial shrub or herb form (6, 29). Aspen seedling establishment is considered rare but once a root system is established it may persist for centuries (Knight, 1994). Aspen usually occur in clonal stands of genetically identical stems. The individual stems originate as suckers growing from a common root system, so the stems making up a clonal stand are interconnected and can transfer water and solutes among themselves (Jones and DeByle, 1985).

The decline of aspen biomass on the northern range has become a resource issue of great debate. Much research has revolved around identifying the causes of the decline with the relative roles of climate fluctuation, mammalian predation on elk, fire suppression, and ungulate (elk) browsing being considered (15, 16, 33, 48, 24, 38).

Herbivory by elk (Cervus elaphus) has been identified as a proximal factor in the decline of aspen in YNP (YNP, 1997). Elk eat the smooth white bark of aspen trunks, resulting in growths of thick corky bark reaching as high as the elk can reach. This practice can stress aspen and allows cankers and fungi to attack mature stems (Romme et al., 1995). Elk also heavily browse new aspen sprouts emerging from the clonal root system, inhibiting stem growth and maintaining some aspen stands in a shrub form. Ungulate browsing has been identified as preventing the recruitment of younger stems into the overstory and inhibiting clonal expansion, but there is uncertainty over why browsing has a different influence now than it has had historically (YNP, 1997). Elk numbers were manipulated by National Park Service removals (until 1968) and reduced to approximately 4000 animals during the 1960s, but aspen increment core data show that reductions in elk herd size have not led to new cohorts of ramets joining YNP’s aspen overstory (33, 30).

Ripple and Larsen (2000) provide evidence that YNP aspen successfully regenerated overstory stems from the middle to late 1700's–1930. After 1930, aspen overstory regeneration in YNP ceased, except in a few sites protected from browsing. Fallen conifers from the 1988 Yellowstone fires are currently providing refugia for aspen stems, a process that may assist limited aspen persistence on the northern range (Ripple and Larsen, 2001).

Wolves were extirpated as a source of elk predation in YNP between 1914–1926, with at least 136 wolves killed in that period (Weaver, 1978). Ripple and Larsen (2000) hypothesized that aspen decline in YNP may be related to the removal of a trophic cascades interaction where wolves modified elk movements, density, and foraging behavior. A trophic cascade has been defined as “the progression of indirect effects by predators across successively lower trophic levels” (Estes et al., 2001). Wolves may influence lower trophic levels both by killing elk and altering their behavior. It was estimated that wolf reintroduction might reduce the elk population in YNP from 5–20% (3, 22). Changes in prey behavior due to the presence of predators are referred to as predation risk effects (Schmitz et al., 1997). These behavioral modifications include changes in diet, temporal alterations of feeding patterns, and spatial changes regarding habitat use, patch selection and choices of feeding sites (21, 23). The presence of wolves may have been crucial in maintaining northern range aspen stands both through predation and especially through predation risks affecting elk movement and herbivory patterns. When wolves were present, ungulate antipredator strategies may have included the avoidance of high wolf use areas (25, 32, 1, 7, 10). Since wolf recolonization in the early 1970s, researchers in Canada’s Banff and Jasper National Parks have found higher elk densities in low wolf predation areas (5, 47). In addition, White et al. (1998) reported that a new cohort of aspen has grown into trees 3–5 m tall since wolves recolonized Jasper National Park, with particularly vigorous regeneration near wolf trails and other areas of heavy wolf use.

Wolves were reintroduced into YNP in 1995 (Bangs and Fritts, 1996). By the end of 1998, 112 wolves lived in 11 packs in the greater Yellowstone ecosystem (Smith et al., 1999). Four packs had established themselves on the northern range. In 1999, we established permanent transects (plots) in aspen stands to test whether reintroduced wolves may be influencing elk use, browse patterns, and aspen response. Our objective was to compare aspen stands in high and low wolf-use areas to determine if there were significant differences between the two areas in the number of elk pellet groups, aspen sucker heights and the percentage of suckers being browsed. No previous research has established any spatial pattern for elk browsing in northern range aspen stands but the uniform lack of overstory recruitment suggests that all aspen stands available to ungulates were heavily browsed. To rectify this situation and monitor possible ecological change over time, we sought to establish baseline data for the three trophic levels involving wolves, elk, and aspen on YNP’s northern range.

Section snippets

Methods

An inventory of YNP northern range aspen stands was created from a set of 1:24,000 color infrared (CIR) aerial photographs taken in October 1988, at the conclusion of the fire season. A grid of 96 rectangular cells (1×1.5 cm, 240×360 m on the ground) was overlain on each CIR aerial photograph. A scanning stereoscope was used to identify grid cells containing large-stem aspen and a comprehensive list of cells containing aspen was compiled from the photographs to produce the inventory.

We used

Results

Using the telemetry data, the Kernel HR software and a geographic information system (GIS), high wolf-use areas were defined as those areas inside the 50% fixed kernel estimates for the Druid, Rose Creek, and Leopold wolf packs (Fig. 1). The Druid pack high-use territory (<50% fixed kernel estimate) consisted of 4337 ha in the Lamar River basin approximately centered at the Soda Butte Creek confluence. The Leopold pack had a high-use territory of 3390 ha on YNP’s Blacktail Plateau. The Rose

Discussion

There is mounting evidence that predators such as the gray wolf may influence their communities in a cascade of interactions extending through several trophic levels (8, 36, 17, 27, 43, 9). Since wolves were reintroduced to YNP in 1995, elk have been their preferred prey and annually comprise >80% of observed kills (28, 41, 40). In addition to the killing of elk, wolf predation may also cause modifications in elk behavior that may benefit northern range aspen. This hypothesis is based on the

Acknowledgements

The authors thank William H. Romme for reviewing an early draft of this manuscript. Mark L. Hanus provided advice on forest mensuration topics. Thanks to Christie Hendrix for assistance in the field. The YNP Center for Resources provided assistance in housing and computer use. Kobe Harkins, Elizabeth M. Glenn, and Andrea S. Laliberte provided GIS assistance. The permanent plot data is archived at the Yellowstone Center for Resources.

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