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Forecasting caribou population trajectories using the DG-Sim SyncroSim package

DG-Sim Fortymile herd population size forecasts
DG-Sim forecasts of the Fortymile caribou herd population size (2017-2022). Photo census estimates are shown for 2017 and 2022.

In collaboration with ApexRMS, population models and forecasts for the Fortymile caribou herd were recently presented at the 2023 North American Caribou Workshop and Arctic Ungulate Conference by Mike Suitor, a caribou biologist of the Yukon Government’s Department of Environment.

Over the past century, the range and population size of the Fortymile caribou herd has fluctuated dramatically, with the most recent peak in population size in 2017, according to photo census data. Following this peak, indicators of the herd’s deteriorating health suggested a declining population size, but the severity of decline was unknown due to a lack data. While photo census data are considered the gold standard of herd population size estimates, weather conditions are rarely favorable for census flights, posing the question of whether accurate estimates of caribou population size could be forecast using a model.

Together with Yukon and Alaska government biologists, we generated successive forecasts for the Fortymile herd population trajectory from 2017 through 2022 using DG-Sim, a population modeling package for SyncroSim. The accuracy of the DG-Sim model, built using historic data sets of population variables such as birth and death rates, was confirmed via the 2022 photo census, conducted for the first time in 5 years. DG-Sim models also confirmed that using radio and satellite collar data on births and deaths yields much more accurate forecasts of population size compared to fall composition surveys alone. This collaborative project has supported the stewardship of the Fortymile caribou herd and will influence future policies surrounding herd management.