Rolf O. Peterson

Wolves, Moose, and Tree Rings on Isle Royale

Investigation of tree growth in Isle Royale National Park in Michigan revealed the influence of herbivores and carnivores on plants in an intimately linked food chain. Plant growth rates were regulated by cycles in animal density and responded to annual changes in primary productivity only when released from herbivory by wolf predation. Isle Royales dendrochronology complements a rich literature on food chain control in aquatic systems, which often supports a trophic cascade model. This study provides evidence of top-down control in a forested ecosystem.

Ecosystem consequences of wolf behavioural response to climate

Because apex predators exert considerable influence on the structure and function of top-down ecosystems, their responses to climate may shape responses at lower trophic levels. Previous reports of trophic cascades and ecosystem dynamics induced by predators have focused on changes in their abundance, whereas we investigated whether changes in predator behaviour could precipitate cascades of similar ecological scale. Here we report the ecological consequences of predator behavioural response to global climatic variation using 40 years of data on wolf predation from Isle Royale, USA, where wolves limit abundance of moose, which limit productivity of fir trees. In response to increases in winter snow related to the North Atlantic Oscillation, wolves hunted in larger packs and, consequently, tripled the number of moose killed per day compared with less snowy years when they hunted in smaller packs. Following increased predation rates, moose abundance declined, and, following release from heavy browsing, growth of understory fir increased. Hence, cascading behavioural responses of apex predators may be a substantial link in the pathway from climatic change to ecosystem function.

Yellowstone after Wolves

Abstract With gray wolves restored to Yellowstone National Park, this ecosystem once again supports the full native array of large ungulates and their attendant large carnivores. We consider the possible ecological implications of wolf restoration in the context of another national park, Isle Royale, where wolves restored themselves a half-century ago. At Isle Royale, where resident mammals are relatively few, wolves completely eliminated coyotes and went on to influence moose population dynamics, which had implications for forest growth and composition. At Yellowstone, we predict that wolf restoration will have similar effects to a degree, reducing elk and coyote density. As at Isle Royale, Yellowstone plant communities will be affected, as will mesocarnivores, but to what degree is as yet undetermined. At Yellowstone, ecosystem response to the arrival of the wolf will take decades to unfold, and we argue that comprehensive ecological research and monitoring should be an essential long-term component of the management of Yellowstone National Park.

THE EFFECT OF PREY AND PREDATOR DENSITIES ON WOLF PREDATION

Predator kills rate (i.e., kills per predator per time) is routinely presupposed to depend exclusively on prey density. However, per capita rates of killing may typically depend on the density of both prey and predator. Unfortunately, our perception of many ecological phenomena may be limited by the inappropriate assumption that kill rates do not depend on predator density. One of many ways to represent the influence of predator density is ratio-dependent predation, where kill rate depends on the ratio of prey to predator rather than the actual numbers of prey and predator. Determining the role of ratio dependency in predation theory has been contentious. Assessments of the influence of predator density on kill rate have been primarily limited to theoretical considerations, indirect evidence, and simplified laboratory demonstrations. We directly observed the influence of both prey and predator density on kill rates in an unmanipulated terrestrial system of large mammals— wolves ( Canis lupus) and moose (Alces alces). Predator density explained more variation in kill rate than did prey density (R 2 5 0.36 vs. R 2 5 0.17, respectively). Moreover, the ratio-dependent model greatly outperformed the prey-dependent model. Nevertheless, the ratio-dependent model failed to explain most of the variation in kill rate (i.e., R 2 5 0.34). The ratio-dependent-prey-dependent controversy may dissipate with greater appreciation and acknowledgment that both models may be overly simplistic, both have value, and neither deserves primacy.

Population limitation and the wolves of Isle Royale

Population regulation for gray wolves in Isle Royale National Park, Michigan, was examined in 1987–1995 when wolves were in chronic decline following a crash of the population in 1981–1982. Canine parvovirus (CPV-2) was probably influential during the crash, but it disappeared by the late 1980s. High mortality abruptly ceased after 1988, but low recruitment in the absence of disease and obvious shortage of food prevented recovery of the wolf population. In 1983–1995, with a comparable number of moose ≥10 years old as potential prey, wolves were only half as numerous as in 1959–1980. A simulation of annual fluctuations in effective population size ( N e ) for wolves on Isle Royale suggests that their genetic heterozygosity has declined ca. 13% with each generation and ca. 80% in the 50-year history of this population. Inbreeding depression and stochastic demographic variation both remain possible explanations for recent low recruitment.

Wolves, Moose, and the Allometry of Population Cycles

After a decade of dramatic population fluctuations, protected populations of wolves and moose in Isle Royale National Park in Lake Superior returned in 1983 to the levels observed in the 1950s. Inherent lags in this predator-prey system and the strong recovery of the moose population following a wolf population crash suggest that these populations may continue to cycle with a period length of about 38 (95 percent confidence interval, �13) years. Such a long-term cycle is consistent with the proposal that period length of herbivore population cycles will characteristically scale according to the fourth root of body mass, a basic allometric relation linking physiological cycles to population processes.

The Rise and Fall of Isle Royale Wolves, 1975–1986

The wolf ( Canis lupus ) population in Isle Royale National Park, Michigan, increased steadily in the 1970s, reaching a peak of 50 wolves (91 wolves/1,000 km2) in 1980. A population crash followed in 1980–1982, then wolf numbers stabilized at 20–24, the same level as during the 1960s. After a lag of 2–3 years, wolf density followed trends in moose ( Alces alces ) mortality rate, which was correlated with wolf-pack food supply. The increase in numbers of wolves resulted from proliferation of reproducing packs when food became more abundant in the early to mid-1970s. During the increase phase, pack size averaged 9.5 wolves, with annual survival of 84–87%, but during the crash, pack size dropped to 4.7 and survival was only 49%. Equilibrium was reached in 1983–1986, with average pack size of 6.5 and annual survival of 66– 67%, coupled with annual recruitment of two pups/pack. For 2 years before the 1980–1982 crash, dispersal of individuals from packs was high, resulting in smaller pack size. Known mortality during the decline took the form of starvation and intraspecific killing, direct and indirect manifestations of reduced food supply.

The influence of top-down, bottom-up and abiotic factors on the moose (Alces alces) population of Isle Royale.

Long–term, concurrent measurement of population dynamics and associated top–down and bottom–up processes are rare for unmanipulated, terrestrial systems. Here, we analyse populations of moose, their predators (wolves, Canis lupus), their primary winter forage (balsam fir, Abies balsamea) and several climatic variables that were monitored for 40 consecutive years in Isle Royale National Park (544 km2), Lake Superior, USA. We judged the relative importance of top–down, bottom–up and abiotic factors on moose population growth rate by constructing multiple linear regression models, and calculating the proportion of interannual variation in moose population growth rate explained by each factor. Our analysis indicates that more variation in population growth rate is explained by bottom–up than top–down processes, and abiotic factors explain more variation than do bottom–up processes. Surprisingly, winter precipitation did not explain any significant variation in population growth rate. Like that detected for two Norwegian ungulate populations, the relationship between population growth rate and the North Atlantic Oscillation was nonlinear. Although this analysis provides significant insight, much remains unknown: of the models examined, the most parsimonious explain little more than half the variation in moose population growth rate.

Effects of population density and pack size on the foraging ecology of gray wolves

A decline in the gray wolf ( Canis lupus ) population in Isle Royale National Park prompted an intensive study of radiocollared individuals in 1988–1991, complementing an ongoing study begun in 1958. During winter, 1959–1991, the proportion of lone wolves was higher when the population was at low levels, whereas average size of packs declined with density of wolves. Sizes of territories (1971–1991) were not related to size of packs. In winter, food availability (kg wolf−1 day−1) and kill interval (days/kill) varied inversely with size of packs. Social relationships of radiocollared lone wolves were flexible; they often formed temporary groups of two or three wolves. Small groups (less than four) and lone wolves roamed large areas of the island (300 km2 and 540 km2, respectively), frequently moving through territories of established packs. Solitary wolves and pairs readily killed adult moose, in contrast to a common belief that larger packs benefit from cooperative hunting.

Balsam Fir on Isle Royale: Effects of Moose Herbivory and Population Density

Balsam fir (Abies balsamea) population parameters were analyzed in Isle Royale National Park, Michigan, to assess moose (Alces alces andersoni) herbivory in relation to the population density of both fir and moose. Fir population parameters were determined at nine study sites, each representing a different combination of relative fir and moose density (high, medium, and low levels, respectively). In general, moose suppression of fir height growth and recruitment to the canopy increased with increasing moose density and decreasing fir density. Where fir densities were low, sapling heights were restricted to < 1 m regardless of moose density. In such sites, sapling growth suppression has been continuous since a peak in the moose population in the late 1920s. Canopy fir trees at most sites established prior to arrival of moose on Isle Royale around 1910. At high fir density release from growth inhibition was common following a period of low moose numbers from the mid- 1970s to early 1 980s. Here this release should facilitate recruitment of fir and effect replacement of canopy losses due to senescence. In other areas, however, continuing canopy losses and lack of recruitment due to intensive herbivory by moose will greatly reduce the fir component of the canopy, effectively preventing any return to the fir- dominated forests of the past.