Caribou survival, fate, and cause of mortality in Newfoundland
Caribou Rangifer tarandus populations are declining globally, and all woodland caribou in Canada are designated as "At-Risk" except for the Newfoundland population. However, Newfoundland’s caribou population has declined from nearly 94,000 animals in the late 1990s to just under 34,000 in 2012 and a change in the "At-Risk" status from the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) is very possible.
The Caribou Data Synthesis and Calf Mortality Study showed that low calf survival rates were the main demographic cause of this decline. These studies form the foundations of the Newfoundland Caribou Strategy, a 5-year study that sought, in part, to determine factors underlying low calf survival rates and is the subject of this report.
Survival rates can be influenced by many different factors or combinations of these factors, such as the size or density of the population. A higher population density often attracts more predators, increases rates of disease, and decreases the amount of food per individual. When a demographic rate (e.g., survival) changes with population density, it is termed density dependent. Conversely, climatic factors such as a harsh winter can influence survival independent of density. Both density-dependent and density-independent factors can differentially influence a population depending on its phase, i.e., whether the population is increasing or decreasing, and are termed phase dependent.
This study expands upon previous work and is one of the largest of its kind in the world in terms of the numbers of animals collared, reflecting the long-term commitment of the Government of Newfoundland and Labrador to caribou management and conservation. The following conclusions are based on data gathered from 2003 to 2012 in five different study areas in Newfoundland as part of the Calf Mortality Study and the Newfoundland Caribou Strategy:
- Caribou survival was relatively constant from 1980 to 1997 during the population increase (ca. 66%). After a gap in telemetry studies from 1998 to 2002, calf survival rates were extremely low (ca. 7%). From 2003 to 2012, calf survival generally increased, but the survival rate never reached the levels of the 1979 to 1997 period and the population continues to decline, albeit at a considerably lessened rate (hereafter, results are for 2003–2012 unless noted otherwise).
- Calf survival decreased as population size (density) increased, i.e., a density-dependent relationship. Given the relatively constant survival during the population increase, this suggests a phase-dependent influence on survival rates, i.e., that population size must exceed some threshold before it influences calf survival. Further, most calves collared as neonates die within the first 3 months, and especially the first 5–6 weeks. Heavier calves had a higher probability of surviving than lighter calves.
- Climate had little influence on caribou calf survival. This is perhaps not surprising given that Newfoundland is a relatively mild climate for caribou and there are no over-winter predators of significance, i.e., caribou are not likely to die from extreme winter events and there are no wolves (Canis lupis) or other predators that hunt more efficiently as snow depth increases.
- Survival was high and relatively constant for over-winter calves (calves approximately 6 months to 1 year old), as well as yearlings (12–24 months), 2-year-olds (24–36 months), and adults (> 36 months). Changes in the survival rates of these older cohorts must be carefully monitored as they can have more profound population-level influences than changes in calf survival rates.
- Predation was the main cause of calf mortality (ca. 90%). A greater percentage of calves suffered predation during the population decrease than during the increase.
- Predation was less important as a cause of mortality for older cohorts. Of the few animals that died, roughly half of the older calves died because of predation and one-quarter of the adults did.
- Black bear (Ursus americanus) and coyote (Canis latrans) were the dominant predators. Predation by lynx (Lynx canadensis) and Bald Eagles (Haliaeetus leucocephalus) was much less common. Low calf predation by lynx is a departure from earlier findings.
- Although coyote are a common predator, it is difficult to determine whether they are simply killing calves that other predators might have taken anyway. Therefore, it is difficult to determine their role in the population decline. However, it is clear that there is little evidence to support anecdotal reports of high levels of predation on older caribou, especially, during the winter.
- Caribou calf survival has not yet reached the level where the caribou population will stabilize. This may mean a change to "At-Risk" under COSEWIC and have implications for development in the province.
- The conclusions of this study could not have been realized without the Caribou Data Synthesis, the Calf Mortality Study, and the Newfoundland Caribou Strategy and underscores the importance of long-term data (1979–2012) to research and management.