Calf Recruitment Research Articles

HUMAN ACTIVITY MEDIATES A TROPHIC CASCADE CAUSED BY WOLVES

Experimental evidence of trophic cascades initiated by large vertebrate predators is rare in terrestrial ecosystems. A serendipitous natural experiment provided an opportunity to test the trophic cascade hypothesis for wolves (Canis lupus) in Banff National Park, Canada. The first wolf pack recolonized the Bow Valley of Banff National Park in 1986. High human activity partially excluded wolves from one area of the Bow Valley (low-wolf area), whereas wolves made full use of an adjacent area (high-wolf area). We investigated the effects of differential wolf predation between these two areas on elk (Cervus elaphus) population density, adult female survival, and calf recruitment; aspen (Populus tremuloides) recruitment and browse intensity; willow (Salix spp.) production, browsing intensity, and net growth; beaver (Castor canadensis) density; and riparian songbird diversity, evenness, and abundance. We compared effects of recolonizing wolves on these response variables using the log response ratio between the low-wolf and high-wolf treatments. Elk population density diverged over time in the two treatments, such that elk were an order of magnitude more numerous in the low-wolf area compared to the high-wolf area at the end of the study. Annual survival of adult female elk was 62% in the high-wolf area vs. 89% in the low-wolf area. Annual recruitment of calves was 15% in the high-wolf area vs. 27% without wolves. Wolf exclusion decreased aspen recruitment, willow production, and increased willow and aspen browsing intensity. Beaver lodge density was negatively correlated to elk density, and elk herbivory had an indirect negative effect on riparian songbird diversity and abundance. These alternating patterns across trophic levels support the wolf-caused trophic cascade hypothesis. Human activity strongly mediated these cascade effects, through a depressing effect on habitat use by wolves. Thus, conservation strategies based on the trophic importance of large carnivores have increased support in terrestrial ecosystems.

Modeling the decline of the Porcupine Caribou Herd, 1989-1998: the importance of survival vs. recruitment

The Porcupine caribou (Rangifer tarandus granti) herd increased from approximately 100 000 animals during the 1970s to 178 000 in 1989, then declined to 129 000 by 1998. Our objective was to model the dynamics of this herd and investigate the potential that lower calf recruitment, as was observed during 1991-1993, produced the observed population changes. A deterministic model was prepared using estimates of birth and survival rates that reproduced the pattern of population growth from 1971-1989. Then, parameters were changed to simulate effects of lower calf recruitment and adult survival. Reducing recruitment for 3 years caused an immediate reduction in population size, but the population began to recover in 5-6 years. Even a dramatic temporary reduction in recruitment did not explain the continuing decline after 1995. In contrast, a slight but persistent reduction in adult survival caused a decline that closely followed the observed pattern. This suggests that survival of adults, and perhaps calves, has declined since the late 1980s.

Habitat selection and demography of a nonmigratory woodland caribou population in Newfoundland

Several studies have shown that habitat selection and population limitation of woodland caribou (Rangifer tarandus caribou) is typically associated with the interaction among human-induced habitat alteration, wolf predation (Canis lupus), and the availability of alternative prey such as moose (Alces alces) and white-tailed deer (Odocoileus virginianus). Although woodland caribou inhabiting Newfoundland have been exposed to moderate levels of timber harvesting, black bears (Ursus americanus), not wolves, are the principal large predator in this system, and moose represent the alternative large prey species. During a 4-year period of timber harvesting, we monitored the location and status of 24 adults and 46 calves to determine coarse-scale habitat selection and survival rates of a nonmigratory and demographically isolated woodland caribou herd in Newfoundland. We also estimated pregnancy and recruitment rates and determined potential causes of mortality. Habitat selection significantly varied among seasons but was consistent with the hypothesis that caribou avoid habitats where the likelihood of contact with a predator or alternative prey was high. Population growth rate (rs = –0.01 ± 0.06 (mean ± 1SD)), calf recruitment (percentage of 10-month old calves = 13%), and adult survival rate (0.87) indicated that the population was stable. Mean pregnancy rate (96%), parturition date, and the lack of evidence for malnutrition in 22 carcasses suggested that the availability of quality forage is not a key factor limiting population growth. Although predation could not be linked to the death of adult caribou, black bears were responsible for 5 of 15 calf fatalities. This study suggests that black bears need to be considered more seriously as a factor influencing coarse-scale habitat selection and demography in woodland caribou herds.

Do Wild Reindeer Exhibit Grazing Compensation during Insect Harassment?

We studied wild reindeer (Rangifer tarandus tarandus) to determine whether a wild ungulate can sufficiently increase its grazing time to compensate for loss of ingestion and rumination caused by insect harassment. We also tested the effect that insect harassment may have on autumn carcass weights. During summer 1997 and 1998, we recorded 24-hr activity budgets and frequency of change in activity bouts for wild reindeer. Following diurnal periods' insect harassment and decreased grazing, animals did not compensate by increasing grazing time or intensity during the night when insect harassment was low. Adult females and calves weighed significantly less in autumn 1997 following a summer of diurnal insect harassment, compared to autumn 1998 following a summer of little insect harassment. This indicates that the effect of daily insect harassment on activity budgets and grazing intensity had a negative effect on the body weights of adult females and calves. For reindeer, as for most wild ungulates, reduced body weight indicates poor condition and can jeopardize survival, reproduction, and calf recruitment.

Dynamics of Interacting Elk Populations within and Adjacent to Rocky Mountain National Park

We studied population subdivision and density-dependent and independent factors influencing population processes between 1965 and 2001 for elk (Cervus elaphus) inhabiting Rocky Mountain National Park (park) and the adjacent Estes Valley (town), Colorado, USA. Elk numbers within the park were held relatively constant by management controls until 1967, after which time they were allowed to increase without human interference. Radiotelemetry of 73 elk indicated limited exchange between the subpopulations; combined with clear distinctions in population dynamics, this suggests that these subpopulations are relatively independent despite the absence of physical barriers between them. The elk subpopulation within the park initially increased at 6.5%/year between 1968 and 1970, then growth gradually slowed-exhibiting density-dependent reductions both in calf survival and recruitment with increasing population size-and approached an estimated carrying capacity of 1,069 ± 55 (?±SE). Since 1991, this subpopulation has remained within ±5% of this equilibrium. The adjacent Estes Valley subpopulation grew at an estimated maximum 5-year average rate of 11.0% from 1979 to 1983 and is still increasing at 5.2%/year (1991-2001 average). Estimated town population currently is about 70% of our projected carrying capacity of 2,869 ±415 elk based on projection of observed calf recruitment decline with increasing population. Both carrying-capacity estimates are consistent with independent estimates based on forage biomass and energy considerations. Adult cow survival rate was not found to differ between park and town, and we estimated a constant rate of 0.913 [95% CI = 0.911, 0.915]. Bull survival rates increased in the park from 0.52 to 0.79 between 1965 and 2001, but remained constant at 0.42 [0.35, 0.47] in the Estes Valley. Colder winter temperatures were correlated with reduced calf recruitment (calves:cow at age 0.5 yr) and with reduced calf survival (between age 0.5 and 1.5 yr) in town. Recruitment of town elk also increased with warmer summer temperatures and greater summer precipitation. No weather covariates were significantly correlated with calf recruitment or survival in the park. Declining calf recruitment has been nearly linear and similar in both the park and town. In the park, calf survival responded little to density when the population was well below carrying capacity, but responded at an increasing rate as the population neared carrying capacity. This pattern may explain why calf survival response to density has not yet been detected in town. We estimated current combined population size of 3,049 [2759, 3369] elk in 2001. Elk in the town sector currently outnumber elk in the adjacent national park by almost 2:1 and are projected to increase by 46% before being nutritionally limited, suggesting that human-elk conflicts likely will increase in the absence of active management intervention.

Reindeer (Rangifer tarandus) and climate change: Importance of winter forage

As a consequence of increasing greenhouse gas concentrations, climate change is predicted to be particularly pronounced, although regionally variable, in the vast arctic, sub-arctic and alpine tundra areas of the northern hemisphere. Here, we review winter foraging conditions for reindeer and caribou (Rangifer tarandus) living in these areas, and consider diet, forage quality and distribution, accessibility due to snow variation, and effects of snow condition on reindeer and caribou populations. Finally, we hypothesise how global warming may affect wild mountain reindeer herds in South Norway. Energy-rich lichens often dominate reindeer and caribou diets. The animals also prefer lichens, and their productivity has been shown to be higher on lichen-rich than on lichen-poor ranges. Nevertheless, this energy source appears to be neither sufficient as winter diet for reindeer or caribou (at least for pregnant females) nor necessary. Some reindeer and caribou populations seem to be better adapted to a non-lichen winter diet, e.g. by a larger alimentary tract. Shrubs appear to be the most common alternative winter forage, while some grasses appear to represent a good, nutritionally-balanced winter diet. Reindeer/caribou make good use of a wide variety of plants in winter, including dead and dry parts that are digested more than expected based on their fibre content. The diversity of winter forage is probably important for the mineral content of the diet. A lichen-dominated winter diet may be deficient in essential dietary elements, e.g. minerals. Sodium in particular may be marginal in inland winter ranges. Our review indicates that most Rangifer populations with lichen-dominated winter diets are either periodically or continuously heavily harvested by humans or predators. However, when population size is mainly limited by food, accessible lichen resources are often depleted. Plant studies simulating climatic change indicate that a warmer, wetter climate may cause an altitudinal upward shift in the production of mat-forming lichens in alpine, sub-arctic regions. This is due to an increased potential for lichen growth at high altitudes, combined with increased competition from taller-growing vascular plants at lower altitudes, where the biomass of Betula nana in particular will increase. Matforming lichens dominant on dry, windblown ridges are easily overgrazed at high reindeer densities. This has longterm effects due to lichens’ slow regeneration rate, but may also reduce competition from vascular plants in a long time perspective. Fires may act in a similar way in some forested areas. Accessibility of winter forage depends on plant biomass, snow depth and hardness; ice crusts or exceptionally deep snow may result in starvation and increased animal mortality. Calf recruitment appears to be low and/or highly variable where winter ranges are overgrazed and hard or deep snow is common. Population decline in several Rangifer tarandus spp. has been associated with snow-rich winters. Effects tend to be delayed and cumulative, particularly on calves. This is mainly ascribed to feeding conditions for young animals which later affect age at maturation. Global warming may increase the frequency of deep or hard snow on reindeer ranges in Norway, due to increased precipitation and more frequent mild periods in winter. We hypothesise that potential benefits from increased plant productivity due to global warming will be counteracted by shifts in the distribution of preferred lichen forage, reduction of the areas of suitable winter ranges, and generally reduced forage accessibility in winter.

Dynamics of woodland caribou populations at the southern limit of their range in Saskatchewan

Woodland caribou (Rangifer tarandus caribou) in central Saskatchewan are aggregated into several spatially disjunct populations. Historical data indicate that some local populations were eliminated and that the regional metapopulation declined during the 1970s. Proximate causes of decline may have included hunting, severe winter weather, and the end of predator control, though the ultimate causes may relate to road construction and logging activity, which expanded after 1966. There has been no sport hunting since 1987, yet the caribou populations in the region appear to be declining. Based on serum progesterone levels, first conception by females occurred at 16 months and the overall pregnancy rate was 94%, and in addition the minimum parturition rate was 86%, all of which indicate adequate nutrition. The annual survival rate of adults was 84%, which is comparatively low, though similar to results from other studies on woodland caribou. We observed a low rate of calf recruitment (28 calves : 100 cows), which we suspect was due to predation. Wolves (Canis lupus) have long been thought to limit woodland caribou populations while subsisting on other ungulate prey, but we speculate that black bear (Ursus americanus) predation may be an important cause of poor calf survival.

Population growth and status of the Nushagak Peninsula caribou herd in southwest Alaska following reintroduction, 1988 - 1993

Caribou were reintroduced to the Nushagak Peninsula, Alaska in February 1988, after an absence of over 100 years. The purpose was to reestablish caribou in the area and once again provide hunting to local residents. The Nushagak Peninsula caribou herd (NPCH) has grown rapidly from 146 reintroduced caribou to over 1000 in 6 years at an exponential rate of increase of r = 0.317 or about 38%. The dramatic growth of the herd was attributed to the initial high percentage of females in the herd, high calf production and survival, pristine range, few predators and no hunting. Abundant high quality forage on the Nushagak Peninsula is the probable reason for the enhanced body condition and high natality even among 2-year-olds, and it has most likely contributed to the high calf survival and recruitment. Lack of predators and hunting has allowed calf and adult mortality to remain low. Although the size of the NPCH has grown steadily over the past 6 years, no significant dispersal from the peninsula has occurred. The population density of the NPCH was estimated to be 1.0/km2 in 1993. We believe the herd will continue to grow, and could reach a density of 2.3/km2 by 1998, even with a 10% harvest beginning in 1995. While the current growth of the NPCH makes the réintroduction a success, the increasing density, lack of dispersal and potential for over-grazing, presents managers with hard decisions.

Demography of the George River Caribou Herd: Evidence of Population Regulation by Forage Exploitation and Range Expansion

The George River caribou herd in northern Quebec/Labrador increased from about 5000 animals in 1954 to 472,200 (or 1.1 caribou/km) prior to the 1984 calving season. The range used by the herd expanded from 160,000 to 442,000 sq km for the period 1971-84. The exponential rate of increase (r) was estimated at 0.11 in the 1970s. Calf:female ratio in autumn was relatively constant (x=0.52) from 1973 to 1983, but decreased to about 0.39 in 1984-86. The harvest rate was relatively low in the 1970s (about 3%/y), but seemingly increased in the mid-1980s to 5-7% as a result of more liberal regulations and a greater impetus to exploit caribou for subsistence. The cumulative impact of lower calf recruitment and more intensive hunting may have appreciably depressed the growth rate of the herd in 1984-86. A greater year-round competition for food resources and a greater energy expenditure associated with range expansion are presented as probable regulatory factors for the George River herd. It is argued that the nature of caribou-habitat interactions in continental regions generate long-term fluctuations in caribou numbers if human exploitation remains low. At present, wolf predation does not appear to be an important mortality factor capable of regulating the George River herd.Key words: caribou, population regulation, food limitation, northem Quebec, Labrador, wolf predation

Dynamics of caribou and wolves in northern British Columbia

Major fluctuations in caribou (Rangifer tarandus) numbers have occurred since the settlement of European man in British Columbia. Caribou declined in the late 1930's and 1940's after moose (Alces alces) expanded their range and wolves (Canis lupus) increased. Following a wolf reduction program from 1949 to 1962, the caribou expanded until a high was reached in the late 1960's, after which the herds again diminished as wolf numbers increased and calf recruitment decreased. Three caribou populations were censused between 1976 and 1982, and an experiment was conducted on one of these areas (with the others as controls) to test the effect of wolf numbers on herd growth. The Spatsizi–Lawyers and Level–Kawdy populations inhabited ranges where wolf densities were near 9–10wolves/1000 km2;Spatsizi–Lawyers declined between 1977 and 1982(r = −0.13) and Level–Kawdy declined between 1977 and 1982(r = −0.12). We reduced wolves at the experimental Horseranch population from 10 to 1–4/1000 km2 and these caibou increased from 1977 to 1982 at an average exponential rate of 6% per year. None of the populations were heavily hunted; however, both declining populations had low calf survival with a mean of 55% of the calves dying within the first 2 weeks of life. Calf survival increased significantly at the Horseranch in the 3 years that wolves were reduced. In conclusion, we believe that the Spatsizi–Lawyers and Level–Kawdy populations declined as a result of predation on calves and adults. Yet, if wolves are managed to provide stabilizing recruitments for caribou, it should be possible to dampen the natural fluctuations of caribou and wolves that are frequent in unperturbed systems and maintain higher numbers of both species.

Causes of Neonatal Moose Calf Mortality in South Central Alaska

Causes of Neonatal Moose Calf Mortality in South Central Alaska