Beschta & Ripple (2009): Large predators and trophic cascades in terrestrial ecosystems of the western United States. DOI: 10.1016/j.biocon.2009.06.015.
Eisenberg et al. (2014a): Effects of predation risk on elk (Cervus elaphus) landscape use in a wolf (Canis lupus) dominated system. DOI: 10.1139/cjz-2014-0138.
Large predators potentially can help shape the structure and functioning of terrestrial ecosystems, yet strong evidence of top-down herbivore limitation has not been widely reported in the scientific literature. Herein we synthesize outcomes of recent tri-trophic cascades studies involving the presence and absence of large predators for five national parks in the western United States, including Olympic, Yosemite, Yellowstone, Zion, and Wind Cave. Historical observations by park biologists regarding woody browse species and recently compiled age structure data for deciduous trees indicate major impacts to woody plant communities by ungulates following the extirpation or displacement of large predators. Declines in long-term tree recruitment indexed additional effects to plant communities and ecological processes, as well as shifts towards alternative ecosystem states. The magnitude and consistency of vegetation impacts found within these five parks, in conjunction with other recent North American studies, indicate that broad changes to ecosystem processes and the lower trophic level may have occurred in other parts of the western United States where large predators have been extirpated or displaced. Thus, where ungulates have significantly altered native plant communities in the absence of large predators, restoration of native flora is urgently needed to recover former ecosystem services. Following the reintroduction of previously extirpated gray wolves Canis lupus into Yellowstone National Park, a spatially patchy recovery of woody browse species (e.g., aspen Populus tremuloides, willow Salix spp., cottonwood Populus spp.) has begun, indicating that large predator recovery may represent an important restoration strategy for ecosystems degraded by wild ungulates.
Eisenberg et al. (2014b): Context dependence of elk (Cervus elaphus) vigilance and wolf (Canis lupus) predation risk. DOI: 10.1139/cjz-2014-0049.
Food acquisition and predation avoidance are key drivers of herbivore behaviour. We investigated the interaction of top-down (predator) and bottom-up (food, fire, thermal) effects by measuring the relationship between wolf (Canis lupus L., 1758) predation risk perceived by elk (Cervus elaphus L., 1758) and elk landscape use. We conducted fecal pellet and wolf scat surveys in three valleys with three wolf population levels (Saint Mary: low; Waterton: moderate; North Fork: high). In the North Fork, 90% of quaking aspen (Populus tremuloides Michx.) stands burned recently; the other valleys had no fire. We created predictive models of elk pellet density that incorporated bottom-up and top-down variables. All valleys had a high elk pellet density (≥10 per 100 m2). Wolf scat density was similar where there was no fire, but one order of magnitude greater in burned areas. Elk pellet density was lower in the North Fork, a predation-related response. In all valleys, site-specific elk density declined as impediments to detecting or escaping wolves increased, and elk avoided aspen, except for North Fork unburned areas. Models that best predicted elk density contained bottom-up and top-down effects. At local scales, high predation risk negatively influence elk occurrence, suggesting that even with minimal wolf exposure elk avoid risky sites.
Eisenberg et al. (2013): Wolf, elk, and aspen food web relationships: Context and complexity. DOI: 10.1016/j.foreco.2013.01.014.
To assess the relationship between predation risk perceived by elk (Cervus elaphus L., 1758) as evidenced by vigilance, we conducted focal animal observations in elk winter range. We stratified our observations in Glacier National Park, Montana, USA, and Waterton Lakes National Park, Alberta, Canada, in valleys with three wolf (Canis lupus L., 1758) population levels (Saint Mary Valley: no wolf; Waterton Valley: moderate wolf; North Fork Valley: high wolf). Although the lowest elk vigilance occurred in Saint Mary and the highest in the North Fork, our analysis revealed a complex picture. Our model included distance to forest edge, group size, distance to road, social class, and impediments to detecting and escaping wolves. In Saint Mary, none of the variables were significant. In Waterton, vigilance decreased as elk group size increased (p < 0.00001) and increased as impediments increased (p = 0.0005). In the North Fork, vigilance increased as group size increased (p = 0.03), bulls were more vigilant (p = 0.02), and the interaction between group size and impediments was significant (p = 0.03). Where a high wolf population existed, elk did not exhibit uniform or expected response to predation risk factors. High wolf presence may necessitate adaptive elk behaviour that differs from response to moderate wolf presence.
Gervasi et al. (2013): Decomposing risk: Landscape structure and wolf behavior generate different predation patterns in two sympatric ungulates. DOI: 10.1890/12-1615.1.
Like most ecological communities, aspen (Populus tremuloides) forests are influenced by a synergy of bottom-up (resources-driven) and top-down (predator-driven) processes. Since the 1920s, ecologists have observed the decline of many aspen communities throughout the Intermountain West. The extent and possible drivers of this decline are topics of much recent scientific study. In addition to bottom-up effects, which include drought, fire suppression, and disease, ungulate herbivory is a contributing factor. Trophic cascades are ecological relationships in which an apex predator produces strong top-down, direct effects on its prey and indirect changes in faunal and floral communities at lower trophic levels. Apex predators, such as the gray wolf (Canis lupus), have been linked to aspen vigor and recruitment, via trophic cascades mechanisms. Scientists have hypothesized that returning wolves to the landscape enables aspen to recruit into the forest overstory, via the density-mediated and behaviorally-mediated effects of wolves on their ungulate prey, primarily elk (Cervus elaphus). We present a synthesis of scientific findings on this topic, identify trends in the ecological impacts of wolves in aspen communities in a variety of ecosystems, and suggest areas for further investigation. Knowledge gaps include the interaction of top-down (e.g., predators) and bottom-up (e.g., drought, fire, hydrology, logging) effects, and how the ecological context of the interaction affects the outcome. Future horizons involve exploring these food web relationships as a complex of inter-level interactions in a more integrated, empirical manner. We suggest adopting a new standard for the aspen/wolf ecology literature by shifting its emphasis and lexicon from trophic cascades to food web studies. Such an integrated approach can help managers create more resilient aspen communities.
Miller et al. (2012): Trophic cascades linking wolves ( Canis lupus ), coyotes ( Canis latrans ), and small mammals. DOI: 10.1139/z11-115.
Recolonizing carnivores can have a large impact on the status of wild ungulates, which have often modified their behavior in the absence of predation. Therefore, understanding the dynamics of reestablished predator–prey systems is crucial to predict their potential ecosystem effects. We decomposed the spatial structure of predation by recolonizing wolves (Canis lupus) on two sympatric ungulates, moose (Alces alces) and roe deer (Capreolus capreolus), in Scandinavia during a 10‐year study. We monitored 18 wolves with GPS collars, distributed over 12 territories, and collected records from predation events. By using conditional logistic regression, we assessed the contributions of three main factors, the utilization patterns of each wolf territory, the spatial distribution of both prey species, and fine‐scale landscape structure, in determining the spatial structure of moose and roe deer predation risk. The eestablished predator–prey system showed a remarkable spatial variation in kill occurrence at the intra‐territorial level, with kill probabilities varying by several orders of magnitude inside the same territory. Variation in predation risk was evident also when a spatially homogeneous probability for a wolf to encounter a prey was simulated. Even inside the same territory, with the same landscape structure, and when exposed to predation by the same wolves, the two prey species experienced an opposite spatial distribution of predation risk. In particular, increased predation risk for moose was associated with open areas, especially clearcuts and young forest stands, whereas risk was lowered for roe deer in the same habitat types. Thus, fine‐scale landscape structure can generate contrasting predation risk patterns in sympatric ungulates, so that they can experience large differences in the spatial distribution of risk and refuge areas when exposed to predation by a recolonizing predator. Territories with an earlier recolonization were not associated with a lower hunting success for wolves. Such constant efficiency in wolf predation during the colonization process is in line with previous findings about the naïve nature of Scandinavian moose to wolf predation. This, together with the human‐dominated nature of the Scandinavian ecosystem, seems to limit the possibility for wolves to have large ecosystem effects and to establish a behaviorally mediated trophic cascade in Scandinavia.
Painter et al. (2015): Recovering aspen follow changing elk dynamics in Yellowstone: evidence of a trophic cascade? DOI: 10.1890/14-0712.1.
When large carnivores are extirpated from ecosystems that evolved with apex predators, these systems can change at the herbivore and plant trophic levels. Such changes across trophic levels are called cascading effects and they are very important to conservation. Studies on the effects of reintroduced wolves in Yellowstone National Park have examined the interaction pathway of wolves (Canis lupus L., 1758) to ungulates to plants. This study examines the interaction effects of wolves to coyotes to rodents (reversing mesopredator release in the absence of wolves). Coyotes (Canis latrans Say, 1823) generally avoided areas near a wolf den. However, when in the proximity of a den, they used woody habitats (pine or sage) compared with herbaceous habitats (grass or forb or sedge)– when they were away from the wolf den. Our data suggested a significant increase in rodent numbers, particularly voles (genus Microtus Schrank, 1798), during the 3-year study on plots that were within 3 km of the wolf den, but we did not detect a significant change in rodent numbers over time for more distant plots. Predation by coyotes may have depressed numbers of small mammals in areas away from the wolf den. These factors indicate a top–down effect by wolves on coyotes and subsequently on the rodents of the area. Restoration of wolves could be a powerful tool for regulating predation at lower trophic levels.
Ripple et al. (2001): Trophic cascades among wolves, elk and aspen on Yellowstone National Park’s northern range. DOI: 10.1016/S0006-3207(01)00107-0.
To investigate the extent and causes of recent quaking aspen (Populus tremuloides) recruitment in northern Yellowstone National Park, we measured browsing intensity and height of young aspen in 87 randomly selected aspen stands in 2012, and compared our results to similar data collected in 1997–1998. We also examined the relationship between aspen recovery and the distribution of Rocky Mountain elk (Cervus elaphus) and bison (Bison bison) on the Yellowstone northern ungulate winter range, using ungulate fecal pile densities and annual elk count data. In 1998, 90% of young aspen were browsed and none were taller than 200 cm, the height at which aspen begin to escape from elk browsing. In 2012, only 37% in the east and 63% in the west portions of the winter range were browsed, and 65% of stands in the east had young aspen taller than 200 cm. Heights of young aspen were inversely related to browsing intensity, with the least browsing and greatest heights in the eastern portion of the range, corresponding with recent changes in elk density and distribution. In contrast with historical elk distribution (1930s–1990s), the greatest densities of elk recently (2006–2012) have been north of the park boundary (~5 elk/km2), and in the western part of the range (2–4 elk/km2), with relatively few elk in the eastern portion of the range (<2 elk/km2), even in mild winters. This redistribution of elk and decrease in density inside the park, and overall reduction in elk numbers, explain why many aspen stands have begun to recover. Increased predation pressure following the reintroduction of gray wolves (Canis lupus) in 1995–1996 played a role in these changing elk population dynamics, interacting with other influences including increased predation by bears (Ursus spp.), competition with an expanding bison population, and shifting patterns of human land use and hunting outside the park. The resulting new aspen recruitment is evidence of a landscape‐scale trophic cascade in which a resurgent large carnivore community, combined with other ecological changes, has benefited aspen through effects on ungulate prey.
Ripple & Beschta (2004): Wolves, elk, willows, and trophic cascades in the upper Gallatin Range of Southwestern Montana, USA. DOI: 10.1016/j.foreco.2004.06.017.
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.
Ripple & Beschta (2007): Restoring Yellowstone’s aspen with wolves. DOI: 10.1016/j.biocon.2007.05.006.
We summarized the status of wolves (Canis lupus), elk (Cervis elaphus), and woody browse conditions during the 20th century for the upper Gallatin elk winter range in southwestern Montana, USA. During this period, wolves were present until about the mid-1920s, absent for seven decades, and then returned to the basin in 1996. A chronosequence of photographs, historical reports, and studies indicated willows (Salix spp.) along streams became heavily browsed and eventually suppressed following the removal of wolves, apparently due to unimpeded browsing by elk. However, after wolf establishment in 1996, browsing intensity on willows lessened in some areas and we hypothesized that, at both a landscape and fine scale, browsing pressure reflects terrain configurations influencing predation risk (nonlethal effects), in conjunction with lower elk densities (lethal effects). We measured browsing intensity and heights of Booth willow (S. boothii) along 3000 m reaches of the Gallatin River and a tributary to examine the potential influence of wolf/elk interactions upon willow growth. Where the Gallatin Valley is relatively narrow (high predation risk), willows began releasing in 1999 and by 2002 were relatively tall (150–250 cm). In contrast, willow heights along a wider portion of the Gallatin Valley, along the open landscape of the tributary, and an upland site (all low predation risk) generally remained low (<80 cm). We identified terrain and other features that may contribute to the perceived risk of wolf predation, by elk for a given site. Although alternative mechanisms are discussed, changes in willow communities over time following wolf removal and their subsequent reintroduction were consistent with a top-down trophic cascade model involving nonlethal and possibly lethal effects. If similar top-down effects upon vegetation hold true in other regions of North America and other parts of the world where wolves have been extirpated, wolf recovery may represent a management option for helping to restore riparian plant communities and conserve biodiversity.
Ripple et al. (2011): Can restoring wolves aid in lynx recovery? DOI: 10.1002/wsb.59.
Wolves (Canis lupus) were reintroduced to Yellowstone National Park in 1995–1996. We present data on a recent trophic cascade involving wolves, elk (Cervus elaphus), and aspen (Populus tremuloides) in Yellowstone’s northern winter range that documents the first significant growth of aspen in over half a century. Results indicate reduced browsing and increased heights of young aspen during the last 4–5 years, particularly at high predation risk sites (riparian areas with downed logs). In contrast, young aspen in upland settings generally showed continued suppression with only a slight decrease in browsing levels and only a slight increase in height. Our findings are consistent with the combined effects of a behaviorally-mediated and density-mediated trophic cascade. Results provide an improved perspective for understanding trophic dynamics and spatially variable plant community growth patterns in this recovering ecosystem.
Ripple & Beschta (2012a): Trophic cascades in Yellowstone: The first 15 years after wolf reintroduction. DOI: 10.1016/j.biocon.2011.11.005.
Herein, we examine the hypothesis that relatively low densities of snowshoe hares (Lepus americanus) and the imperiled status of lynx (Lynx canadensis) may be partially due to an ecological cascade caused by the extirpation of gray wolves (Canis lupus) in most of the conterminous United States decades ago. This hypothesis focuses on 2 plausible mechanisms, one involving “mesopredator release” of the coyote (C. latrans), which expanded its distribution and abundance continentally following the ecological extinction of wolves over the temperate portion of their geographic range. In the absence of wolves, coyotes may have affected lynx via increased predation on snowshoe hares, on which the lynx specializes, and/or by direct killing of lynx. The second mechanism involves increased browsing pressure by native and domestic ungulates following the declines in wolves. A recovery of long‐absent wolf populations could potentially set off a chain of events triggering a long‐term decrease in coyotes and ungulates, improved plant communities, and eventually an increase in hares and lynx. This prediction, and others that we make, are testable. Ecological implications for
the lynx may be dependent upon whether wolves are allowed to achieve ecologically effective populations where they recolonize or are reintroduced in lynx habitat. We emphasize the importance of little‐considered trophic and competitive interactions when attempting to recover an endangered carnivore such as the lynx.
Ripple & Beschta (2012b): Large predators limit herbivore densities in northern forest ecosystems. DOI: 10.1007/s10344-012-0623-5.
The 1995/1996 reintroduction of gray wolves (Canis lupus) into Yellowstone National Park after a 70 year absence has allowed for studies of tri-trophic cascades involving wolves, elk (Cervus elaphus), and plant species such as aspen (Populus tremuloides), cottonwoods (Populus spp.), and willows (Salix spp.). To investigate the status of this cascade, in September of 2010 we repeated an earlier survey of aspen and measured browsing and heights of young aspen in 97 stands along four streams in the Lamar River catchment of the park’s northern winter range. We found that browsing on the five tallest young aspen in each stand decreased from 100% of all measured leaders in 1998 to means of <25% in the uplands and <20% in riparian areas by 2010. Correspondingly, aspen recruitment (i.e., growth of seedlings/sprouts above the browse level of ungulates) increased as browsing decreased over time in these same stands. We repeated earlier inventories of cottonwoods and found that recruitment had also increased in recent years. We also synthesized studies on trophic cascades published during the first 15 years after wolf reintroduction. Synthesis results generally indicate that the reintroduction of wolves restored a trophic cascade with woody browse species growing taller and canopy cover increasing in some, but not all places. After wolf reintroduction, elk populations decreased, but both beaver (Caster canadensis) and bison (Bison bison) numbers increased, possibly due to the increase in available woody plants and herbaceous forage resulting from less competition with elk. Trophic cascades research during the first 15 years after wolf reintroduction indicated substantial initial effects on both plants and animals, but northern Yellowstone still appears to be in the early stages of ecosystem recovery. In ecosystems where wolves have been displaced or locally extirpated, their reintroduction may represent a particularly effective approach for passive restoration.
Ripple et al. (2013): Widespread mesopredator effects after wolf extirpation. DOI: 10.1016/j.biocon.2012.12.033.
There is a lack of scientific consensus about how top-down and bottom-up forces interact to structure terrestrial ecosystems. This is especially true for systems with large carnivore and herbivore species where the effects of predation versus food limitation on herbivores are controversial. Uncertainty exists whether top-down forces driven by large carnivores are common, and if so, how their influences vary with predator guild composition and primary productivity. Based on data and information in 42 published studies from over a 50-year time span, we analyzed the composition of large predator guilds and prey densities across a productivity gradient in boreal and temperate forests of North America and Eurasia. We found that predation by large mammalian carnivores, especially sympatric gray wolves (Canis lupus) and bears (Ursus spp.), apparently limits densities of large mammalian herbivores. We found that cervid densities, measured in deer equivalents, averaged nearly six times greater in areas without wolves compared to areas with wolves. In areas with wolves, herbivore density increased only slightly with increasing productivity. These predator effects
are consistent with the exploitation ecosystems hypothesis and appear to occur across a broad range of net primary productivities. Results are also consistent with theory on trophic cascades, suggesting widespread and top-down forcing by large carnivores on large herbivores in forest biomes across the northern hemisphere. These findings have important conservation implications involving not only the management of large carnivores but also that of large herbivores and plant communities.
Ripple et al. (2014): Status and Ecological Effects of the World’s Largest Carnivores. DOI: 10.1126/science.1241484.
Herein, we posit a link between the ecological extinction of wolves in the American West and the expansion in distribution, increased abundance, and inflated ecological influence of coyotes. We investigate the hypothesis that the release of this mesopredator from wolf suppression across much of the American West is affecting, via predation and competition, a wide range of faunal elements including mammals, birds, and reptiles. We document various cases of coyote predation on or killing of threatened and endangered species or species of conservation concern with the potential to alter community structure. The apparent long-term decline of leporids in the American West, for instance, might be linked to increased coyote predation. The coyote effects we discuss could be context dependent and may also be influenced by varying bottom-up factors in systems without wolves. We make recommendations for ecological research in light of ongoing wolf recovery in parts of the West. Strong ecological effects of wolf repatriation may not occur outside of large reserves where wolves are prevented from achieving ecologically effective densities because of wolf hunting or wolf
control programs. Finally, we advocate for more studies relating to the management of coyotes that compare exploited and unexploited populations and evaluate the influence of anthropogenic food subsidies on coyote densities.
Seager et al. (2013): Patterns and consequences of ungulate herbivory on aspen in western North America. DOI: 10.1016/j.foreco.2013.02.017.
Large carnivores face serious threats and are experiencing massive declines in their populations and geographic ranges around the world. We highlight how these threats have affected the conservation status and ecological functioning of the 31 largest mammalian carnivores on Earth. Consistent with theory, empirical studies increasingly show that large carnivores have substantial effects on the structure and function of diverse ecosystems. Significant cascading trophic interactions, mediated by their prey or sympatric mesopredators, arise when some of these carnivores are extirpated from or repatriated to ecosystems. Unexpected effects of trophic cascades on various taxa and processes include changes to bird, mammal, invertebrate, and herpetofauna abundance or richness; subsidies to scavengers; altered disease dynamics; carbon sequestration; modified stream morphology; and crop damage. Promoting tolerance and coexistence with large carnivores is a crucial societal challenge that will ultimately determine the fate of Earth’s largest carnivores and all that depends upon them, including humans.
Winnie (2012): Predation risk, elk, and aspen: tests of a behaviorally mediated trophic cascade in the Greater Yellowstone Ecosystem. DOI: 10.1890/11-1990.1.
Quaking aspen (Populus tremuloides) forests develop complex, multi-story structure and speciose plant communities, which provide habitat for ungulates and diverse wildlife species. Successfully recruiting aspen sprouts and seedlings provide important sources of structural, functional and genetic diversity vital to resilient aspen forests. Chronic ungulate browsing of regenerating aspen can degrade aspen community structure and diversity. This simplifies food webs and can have negative implications for ecosystem resilience. This paper explores how patterns of ungulate herbivory in aspen forests are influenced by and affect bottom–up and top–down forces in aspen ecosystems. We outline management strategies aimed at decreasing ungulate and livestock impacts on aspen and increasing sprout survival and recruitment. The body of aspen research indicates that herbivory is more heterogeneous in areas that contain human hunters, predators, or fire on the landscape. The complexities of ungulate herbivory and fire on aspen ecosystems, especially in relation to scale, are imperfectly understood. Wildlife agencies responsible for elk (Cervus elaphus) and deer
(Odocoileus spp.) populations should consider management strategies that use ungulate herbivory impacts on ecosystems such as aspen as indicators of sustainable herd densities. To increase aspen resilience in the face of current and future environmental change, we recommend a multi-faceted approach that involves enhancing bottom–up forces while decreasing top–down impacts from ungulates.
Winnie & Creel (2017): The many effects of carnivores on their prey and their implications for trophic cascades, and ecosystem structure and function. DOI: 10.1016/j.fooweb.2016.09.002.
Aspen in the Greater Yellowstone Ecosystem are hypothesized to be recovering from decades of heavy browsing by elk due to a behaviorally mediated trophic cascade (BMTC). Several authors have suggested that wolves interact with certain terrain features, creating places of high predation risk at fine spatial scales, and that elk avoid these places, which creates refugia for plants. This hypothesized BMTC could release aspen from elk browsing pressure, leading to a patchy recovery in places of high risk. I tested whether four specific, hypothesized fine‐scale risk factors are correlated with changes in current elk browsing pressure on aspen, or with aspen recruitment since wolf reintroduction, in the Daly Creek drainage in Yellowstone National Park, and near two aspen enclosures outside of the park boundary. Aspen were not responding to hypothesized fine‐scale risk factors in ways consistent with the current BMTC hypothesis.
Despite some controversy, a wide range of research across multiple taxa have established that carnivores strongly influence prey population dynamics both through direct offtake and indirect risk effects. Because of these powerful top-down effects carnivores can influence ecosystems across multiple trophic levels. Here we discuss research addressing carnivore direct- and indirect effects on prey, and how these effects can influence overall ecosystem structure and function.