Wednesday, December 21, 2011
Predator-prey relationships between timber wolves (Canis lupus) and moose (Alces alces) were studied from 1970 to 1974 in Isle Royale National Park, a 544-km2 island in Lake Superior, as part of a continuing research program begun in 1958.
Initial studies in the late 1950s and early 1960s showed that a single large pack of wolves hunted the entire island, preying in midwinter on old adult moose and calves. In nonwinter months wolves relied heavily on moose calves; beaver were a minor food supplement. Moose productivity was very high and was attributed to intensive predation by wolves.
By 1970, moose productivity had declined markedly and there was increased evidence of winter nutritional stress. Young adult moose and calves became especially vulnerable to wolves in winter due to the effects of malnutrition early in life and to the unusually deep snow. Also, wolves relied heavily on an increased beaver population. The food resources for wolves thus were expanded significantly. Concurrently, the size of the original pack's territory was reduced, allowing a second pack to establish itself in the summer of 1971. As a result, the winter wolf population increased from 20 in 1971 to 31 in 1974.
The two principal wolf packs were tracked by plane in midwinter for 234 "pack-days." The packs traveled an average of 11 km/day and 33 km/kill. Travel for both packs was least when, because of snow conditions, moose were easier to kill.
Food availability for pack members ranged from 4.4 kg to 10.0 kg/wolf/day and declined for both packs between 1971 and 1974. Concurrently, the amount of spatial overlap between the two packs increased, leading in 1974 to direct conflict and the death of one wolf.
A social hierarchy existed in both packs, with dominant, or alpha, males and females leading the pack in daily activities. Alpha wolves did most of the mating and discouraged courtship behavior among subordinate wolves. Leadership in both packs exhibited pronounced year-to-year stability.
Wolf predation accounts for most of the adult moose mortality, which occurs primarily in winter. Normally, losses between the ages of 1 and 7 years are relatively light but moose mortality increases steadily thereafter. Average annual adult mortality was estimated by a life-table to be 13%. Male moose die slightly sooner than females and exhibit a higher incidence of arthritis and malnutrition. The oldest males and females recorded were 15.5 and 19.5 years, respectively.
The incidence of moose aged 1-6 years among wolf kills increased from 13% prior to 1970 to 53% since 1970. This reflected a major increase in vulnerability of young moose which appears to have resulted from increased malnutrition early in life and resultant adverse effects on growth and development. Bone measurements revealed that calf size at birth was correlated with the severity of the previous winter, and generations of calves born after winters of nutritional distress account for almost all of the young adult moose killed by wolves.
Increased nutritional stress among Isle Royale moose in the early 1970s apparently resulted from a combination of plant successional trends which reduced browse supply, increased winter severity because of deep snow, and an increase in the moose population during the 1960s.
Relations with Nonprey Species
In winter, wolves encounter scavengers for which moose carcasses are a principal source of food. Besides the red fox, many birds also utilize wolf-killed moose—primarily the raven, gray jay, black-capped chickadee, and an occasional eagle.
While wolves were seen chasing foxes six times in winter 1972-74, none was caught. Foxes can often run on light snow crusts where wolves break through, and they invariably outrun wolves when chased overland in snow. In the only chase seen on ice, the fox had such a long head start that it reached the shore with no trouble. In 1972, the East Pack was observed just leaving a fox it had killed on the open ice of Malone Bay. The area was matted with wolf tracks, and much hair had been pulled from the fox, though it was not eaten.
The fox's ability to outrun wolves in most snow conditions may be an important reason for its continued coexistence with wolves on Isle Royale. Coyotes, however, disappeared from the island around 1957, less than a decade after the arrival of the wolf. Foxes have thrived recently on Isle Royale, and perhaps even increased after the disappearance of coyotes. While foxes have been observed on Isle Royale since the mid-1920s, long-time island residents report that foxes were uncommon, at least relative to coyotes, before wolves became established.
Moreover, less competition for food resources exists between wolves and foxes than between wolves and coyotes. Johnson (1969) reported that snowshoe hares were the most important year-round food for Isle Royale foxes, and that at certain seasons they made extensive use of insects and fruit. Coyotes relied heavily on moose carcasses. Wolves apparently eliminated coyotes on Isle Royale (Mech 1966; Krefting 1969; Wolfe and Allen 1973), probably through direct killing and competition for food.
Wolves occasionally were indifferent to the presence of foxes. In 1973, the West Pack bedded down on the ice after feeding on a moose carcass. Soon a fox approached, cautiously staying out of sight of the wolves when possible. At the carcass, the fox chased away several ravens and woke the wolves in the process, but they merely raised their heads for a brief look.
During winter periods when foxes were unable to catch snowshoe hares because of deep snow they relied heavily on carcasses of wolf-killed moose. Foxes have difficulty penetrating the thick hide of a moose—they depend on wolves not only to kill the moose but also to open it up. In winters when utilization of kills by wolves is less than usual, moose carcasses may attract a large number of foxes—as many as 10 at one time in 1972
The foraging and feeding ecology of gray wolves is an essential component to understanding the role that top carnivores play in shaping the structure and function of terrestrial ecosystems. In Yellowstone National Park (YNP), predation studies on a highly visible, reintroduced population of wolves are increasing our understanding of this aspect of wolf ecology.
Wolves in YNP feed primarily on elk, despite the presence of other ungulate species. Patterns of prey selection and kill rates in winter have varied seasonally each year from 1995 to 2004 and changed in recent years as the wolf population has become established. Wolves select elk based on their vulnerability as a result of age, sex, and season and therefore kill primarily calves, old cows, and bulls that have been weakened by winter.
Summer scat analysis reveals an increased variety in diet compared with observed winter diets, including other ungulate species, rodents, and vegetation. Wolves in YNP hunt in packs and, upon a successful kill, share in the evisceration and consumption of highly nutritious organs first, followed by major muscle tissue, and eventually bone and hide. Wolves are adapted to a feast-or-famine foraging pattern, and YNP packs typically kill and consume an elk every 2-3 d. However, wolves in YNP have gone without fresh meat for several weeks by scavenging off old carcasses that consist mostly of bone and hide.
As patterns of wolf density, prey density, weather, and vulnerability of prey change, in comparison with the conditions of the study period described here, the authors predict that there will also be significant changes in wolf predation patterns and feeding behavior.
On the 15th anniversary of the return of wolves to Yellowstone National Park, a quiet but profound rebirth of life and ecosystem health is emerging, scientists conclude in a new report.
For the first time in 70 years, the over-browsing of young aspen and willow trees has diminished as elk populations in northern Yellowstone declined and their fear of wolf predation increased. Trees and shrubs have begun recovering along some streams, providing improved habitat for beaver and fish. Birds and bears also have more food.
“Yellowstone increasingly looks like a different place,” said William Ripple, a professor in the Department of Forest Ecosystems and Society at Oregon State University, and lead author of the study.
“These are still the early stages of recovery, and some of this may still take decades,” Ripple said. “But trees and shrubs are starting to come back and beaver numbers are increasing. The signs are very encouraging.”
The findings of this report, based on a recent analysis done by OSU researchers and a review of many other studies as well, were just published in Biological Conservation, a professional journal. They outline an ecosystem renaissance that has taken place since wolves were restored to Yellowstone after being extirpated in the 1920s.
Along four streams studied in the Lamar River basin, 100 percent of the tallest young aspen sprouts were being browsed in 1998, compared to less than 20 percent last year. Heavy browsing by elk on this favorite food had caused new aspen tree recruitment to essentially grind to a halt in the mid-to-late 1900s, when wolves were absent, but new trees are now growing again in places.
Among the observations in this report:
- Since their reintroduction in 1995-96, the wolf population generally increased until 2003, forcing changes in both elk numbers and behavior due to what researchers call the “ecology of fear.”
- The northern range elk populations decreased from more than 15,000 individuals in the early 1990s to about 6,000 last year, and remaining elk now have different patterns of movement, vigilance, and other traits.
- By 2006, some aspen trees had grown tall enough they were no longer susceptible to browsing by elk, and cottonwood and willow were also beginning to return in places.
- Improved willow growth is providing habitat that allows for a greater diversity and abundance of songbirds such as the common yellowthroat, warbling vireo and song sparrow.
- The number of beaver colonies in the same area increased from one in 1996 to 12 in 2009, with positive impacts on fish habitat.
- Increases in beaver populations have strong implications for riparian hydrology and biodiversity – Wyoming streams with beaver ponds have been found to have 75 times more abundant waterfowl than those without.
- The coyote population decreased with the increase in wolf numbers, potentially allowing more small mammals that provide food for other avian and mammalian predators, such as red foxes, ravens and bald eagles.
Evidence of improved ecosystem health following the return of wolves is “becoming increasingly persuasive,” the scientists said in their report, though they also note that an increasing population of bison is continuing to impact young woody plants in the Lamar Valley.
“The wolves have made a major difference in Yellowstone,” said Robert Beschta, a professor emeritus of forestry at OSU and co-author on the study.
“Whether similar recovery of plant communities can be expected in other areas, especially on public lands outside national parks, is less clear,” Beschta said. “It may be necessary for wolves not only to be present but to have an ecologically effective density, and mechanisms to deal with human and wolf conflicts also need to be improved.”
But at least in America’s first national park, the return of this large predator is having an impact.
“Predation and predation risk associated with large predators appear to represent powerful ecological forces,” the researchers concluded in their report, “capable of affecting the interactions of numerous animals and plants, as well as the structure and function of ecosystems.”