Chapter 20 Elk Foraging Behavior Gower, Claire N.; Garrott, Robert A.; White, P.J.
Terrestrial Ecology,
2008, Letnik:
3
Book Chapter
Large herbivores that inhabit seasonal environments face considerable challenges when trying to balance the conflicting demands of satisfying physiological requirements and avoiding predation. The ...harsh environment imposes strong nutritional constraints which influence activity patterns and foraging strategies. In addition, the need to evade predators strongly determines the way in which prey behave (Lima and Dill 1990). The traditional view is that prey manage food acquisition and predation risk by trading one behavior at the expense of the other. However, if a range of anti‐predator behaviors can collectively be adopted at appropriate temporal scales, then this trade‐off may be somewhat reduced (Lind and Cresswell 2005). Plasticity in prey behaviors may balance the need to forage and minimize predation risk, thereby allowing prey to remain in an environment that both provides adequate forage resources but also poses a high threat of predation. We observed the behavior of elk (Cervus elaphus) in the Madison headwaters area of Yellowstone National Park during 15 consecutive winter field seasons, accruing approximately 3100 h of behavioral data before and after wolf (Canis lupus) reintroduction to Yellowstone, when elk were exposed to varying degrees of wolf predation risk. We hypothesized that environmental and temporal factors would play a fundamental role governing the likelihood of winter foraging by elk. We proposed competing hypotheses regarding the effect of wolves. Based on the traditional view of the foraging‐vigilance trade‐off, we suspected that the addition of wolves to the system would manifest a higher degree of behavioral alertness in elk and reduce the likelihood of foraging. Alternatively, increasing behavioral alertness when wolves were actively hunting at night would manifest increased diurnal foraging. Furthermore, we incorporated our knowledge of the range of other behavioral responses exhibited by elk to determine if they behaviorally compensated to mediate any fitness costs associated with behaviors that reduced predation risk.
While the elk herd that winters on the northern range of Yellowstone National Park has been the subject of almost continuous investigations since the inception of the park, the elk herd that occupies ...the Madison headwaters area in the central portion of Yellowstone has received much less scientific attention. This is a particularly interesting population for the study of regulatory processes because the elk remain within the confines of the park year‐round and, thus, are not subjected to harvest by human hunters. Historic records also suggest that the herd was not targeted by market hunters during the 1800s nor managed by the National Park Service through intensive culling, as was common for ungulates occupying the northern range until 1968 when the Park Service adopted the natural regulation policy (Cole 1971, 1983). Hence, the dynamics of this herd have not been influenced to any appreciable extent by human manipulations other than the extirpation of wolves from the Park in the early 1900s. This portion of Yellowstone is also subjected to harsh winter conditions with periods of intense cold temperatures and deep snow pack in most years. These conditions provide an excellent opportunity to study the influence of density‐dependent and density‐independent factors and their interactions in regulating population processes in an ungulate herd. In this chapter, we present the results of an intensive demographic study of this population that was conducted just prior to the reestablishment of wolves.
What a predator eats when given choices, and the subsequent effects of this behavior on ecosystem stability, has long been a topic of interest for ecologists. Prey selection is influenced by the ...absolute and relative abundances of prey types, the life history characteristics of predators and prey, and the attributes of the environment in which these interactions occur. Strong preference by a predator for a particular prey type can lead to ecosystem instability, while prey switching can lessen predation effects on the less abundant prey and enhance system stability. Evaluating prey selection in large mammal systems is difficult due to the broad spatial and temporal scales at which these predatory interactions occur, and investigations, particularly with wolf‐ungulate systems, typically involve only the primary prey. Multiple prey species characterize most large mammal predator‐prey systems, therefore research into predator‐multiple prey dynamics has the potential to yield important ecological insights. We studied winter prey selection during 1996–1997 through 2006–2007 in a newly established wolf‐elk‐bison system where prey differed substantially in their vulnerability to wolf (Canis lupus) predation and wolves preyed primarily on elk (Cervus elaphus) but also used bison (Bison bison) to varying degrees within and among winters and packs. We analyzed the relative influences of prey abundance, predator abundance, and environmental variables on the selection of prey species and age classes and evaluated whether wolves exhibited prey switching from elk to bison.
The nonmigratory elk population in the Madison headwaters area of Yellowstone National Park appeared to be regulated near ecological carrying capacity by food limitation for at least three decades ...prior to the reestablishment of wolves. Eight years of post‐wolf data indicated a substantial proportion of wolf predation was additive and overwhelmed any potential for the elk population to demographically compensate. Thus, wolf predation resulted in a 60–70% decrease in elk abundance and the system transitioned from being bottom‐up regulated in the absence of a significant predator to strong top‐down limitation due to wolf predation. However, it is uncertain if predation will ultimately regulate the elk population at a lower, alternate state or if predation and other factors influencing elk vulnerability will interact to result in further decreases in elk abundance. Fundamental to this question is the role of bison as an alternative prey for wolves. We discuss regulatory processes in predator–prey systems and present conceptual models that provide contrasting predictions for wolf‐ungulate dynamics. We also characterize various aspects of prey vulnerability that may influence the effects of alternative prey on predation of the preferred prey and, in turn, the stability or instability of wolf multiple‐prey systems. We conclude by evaluating support for these processes in data we have collected on Madison headwaters elk and predicting the future trajectory of the herd.
When predators and prey are in close proximity, prey may change how they aggregate to reduce their probability of being attacked and killed. However, the best strategy to reduce the risk of predation ...may not be the best strategy to acquire the resources needed to meet physiological demands for body maintenance and reproduction. How animals balance the competing risks of starvation and predation mortality is an active area of research in a wide variety of predator–prey systems. The non‐migratory elk (Cervus elaphus) population in the Madison headwaters area of Yellowstone National Park provided an excellent opportunity to evaluate prey grouping tendencies in the absence of predators and, following wolf (Canis lupus) reestablishment, when faced with trade‐offs between avoiding predation and acquiring adequate resources. We collected long‐term data on elk grouping behavior prior to wolf reintroduction, during the colonization stage, and after wolves became fully established in the system. Wolf presence varied spatially and temporally within and among years, and there was a wide range of snow conditions during the 16‐year study. Our objectives were to (1) evaluate competing hypotheses on whether elk formed larger or smaller aggregations when wolves were present, (2) determine if grouping strategies changed under different abiotic conditions, and (3) determine if variability in group size increased following wolf reintroduction as elk responded to fine‐scale temporal variation in predation risk.
Chapter 17 Wolf Kill Rates Becker, Matthew S.; Garrott, Robert A.; White, P.J. ...
Terrestrial Ecology,
2008, Letnik:
3
Book Chapter
The ability of predators to successfully capture and kill prey is affected by the abundance and diversity of the prey assemblage, and such variation is a fundamental driver of ecosystem dynamics ...because per capita consumption rate strongly influences the stability and strength of community interactions. Descriptions of predatory behavior in this context typically include the functional response, specifically the kill rate of a predator as a function of prey density. Thus, a major objective in studying predator–prey interactions is to evaluate the strength of the numerous factors related to the kill rate of a predator, and to subsequently determine the forms of its functional response in natural systems because different forms have different consequences for ecosystem dynamics. Recent controversies over the nature of predation focus on the respective roles of prey and predator abundance in affecting the functional response. However, resolution requires more direct measures of kill rates in natural systems. We estimated wolf (Canis lupus) kill rates in a tractable and newly established wolf–elk (Cervus elaphus)–bison (Bison bison) system in the Madison headwaters area of Yellowstone National Park during winters 1998–1999 to 2006–2007 to document the transition from over seven decades without wolves to a well‐established top predator population. Wolf abundance, distribution, and prey selection varied during the study, concurrent with variations in the demography, distribution, and behavior of elk and bison. These dynamics enabled us to evaluate factors influencing variations in wolf kill rates and the forms of their functional response.
In the absence of an effective predator, spatial patterns of large herbivores in northern temperate regions are largely influenced by food acquisition and energy conservation during winter, when ...resources are limited and the energetic cost of movement is high. In these circumstances animals would be expected to minimize movement to avoid unnecessary energy expenditures. With the addition of a top predator such a strategy may not be compatible with avoiding predation risk, therefore animals may increase their movement to avoid detection or escape capture. Such increased movement may occur on relatively fine scales within an animal's home range or may be manifested in broader scale responses such as dispersal or migration. With the reintroduction of wolves (Canis lupus) to Yellowstone National Park in 1995 and 1996, much attention has focused on the behavioral responses of large herbivores to wolf predation risk and the implications of these responses on ecosystem structure and function. Investigations are numerous but evaluations are complicated due partly to a paucity of data on elk (Cervus elaphus) spatial patterns prior to wolf reintroduction. We quantified winter movement patterns of a non‐migratory elk herd in the Madison headwaters prior to the reintroduction of wolves, when animals were constrained only by nutritional restrictions, and following wolf colonization and establishment, when elk experienced significant wolf predation. We evaluated changes in home range size and fidelity and described broader scale elk movement patterns such as dispersal and migration. We predicted elk would shift from a winter strategy of being relatively sedentary and occupying areas based on energetic considerations in the absence of wolves, to a more spatially dynamic strategy as elk responded to increased predation risk from wolves. Furthermore, we incorporated our knowledge of landscape characteristics of the system to provide possible explanations for any changes in elk spatial responses.
Chapter 27 Bison Winter Road Travel Bruggeman, Jason E.; Garrott, Robert A.; White, P.J. ...
Terrestrial Ecology,
2008, Letnik:
3
Book Chapter
The effects of road grooming on bison (Bison bison) distribution and movements in Yellowstone National Park have been debated since the early 1990s. Meagher (1993) expressed concern that energy saved ...by bison traveling on packed snow, in combination with better access to foraging habitat, resulted in enhanced population growth and increased movements to boundary areas. Thus, she recommended prohibiting road grooming to reduce the number and rate of bison leaving the park and induce them to revert to their traditional (i.e., pre‐road grooming) distributions (Meagher 2003). Conversely, Bjornlie and Garrott (2001) suggested that grooming of roads during winter did not have a major influence on bison ecology because of minimal use of roads for travel (19%) compared to off‐road areas, decreased use of roads during the grooming season, and short distances traveled on roads. In 2003, the U.S. District Court for the District of Columbia rebuked the National Park Service for not identifying which argument it found persuasive and instructed the agency to gather necessary data to make a reasoned choice (294 F. Supp. 2d 92, 115). To aid in this decision, we documented (1) spatial patterns in bison road travel, (2) landscape attributes affecting these trends, and (3) abiotic and biotic factors influencing temporal variability in travel on and off roads during November‐May from 1996–1997 through 2005–2006 to evaluate if trends in road travel were facilitated by road grooming or a manifestation of general bison travel patterns throughout the landscape.
