Understanding Caribou Population Cycles

The complex population dynamics of caribou (Rangifer tarandus) were studied to determine the patterns of their population cycles and the processes driving them. It is well established, via previous archaeological research and Indigenous knowledge, that large migrating caribou herds found in and around the tundra at northern latitudes experience population boom and busts roughly every several decades. However, the processes driving the dynamics of these cycles are relatively unknown, which makes managing caribou herds for recreational and subsistence harvests difficult. It has been hypothesized that a combination of intrinsic and extrinsic factors shape these cycles, with density-dependence, predation, harvest, climate, and others likely all playing a role. I aimed to determine whether caribou herds experience population cycling and, if so, estimate the period and amplitude of their cycles and determine which factors drive them. I collected population data on 43 caribou herds throughout the world, and in doing so, assembled the largest caribou population database to date. I used statistical interpolation to fill in the gaps between available data due to low sampling frequency. I quantified whether herds were cycling by fitting populations to sine waves and using periodograms to distinguish cycling tendencies from white-noise stochasticity. I collected additional information on other factors hypothesized to affect caribou cycles, including predator presence data, climate oscillation data, subspecies and ecotype data, and the latitudes of each herd. I used the interpolated data for each herd to determine the variables influencing the periods and amplitudes of caribou population cycles. The median period length was 40.5 years and the amplitude, standardized about the mean population size, was .871; period length and amplitude were also positively correlated. In addition, cycle amplitude was best predicted by period length, subspecies, biome, and average winter minimum temperature. Period length was best predicted by amplitude, latitude, subspecies, biome, NDVI, and average winter minimums. A better understanding of caribou population dynamics could help wildlife professionals and policymakers adapt their caribou management strategies. Climate appears to be a strong driver of these cycles, and with climate change becoming an increasingly apparent reality in the Arctic, cyclic tendencies may prove to disappear, or become amplified and spell disaster for caribou populations. Caribou management strategies will need to adapt to an ever-changing world if we want to preserve natural caribou population cycles—but what that entails remains to be seen