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Twelve Western Canadian Caribou Subpopulations Had Estimated Effective Population Size Below 50

Woodland caribou in Jasper National Park, Alberta, Canada
Research · Genetics & genomics

Genome-wide data from 759 woodland caribou (Rangifer tarandus caribou) in British Columbia, Canada and Alberta showed very small estimated effective population sizes in several subpopulations. Among the 32 individual subpopulations for which contemporary effective population size could be estimated, 12 had point estimates below 50. In seven of those, even the uncertainty around the estimate remained entirely below that threshold. The study was first published on 7 August 2026.

Effective population size, written Ne, is not the number of caribou present. It is a genetic measure: roughly, the size of an ideal breeding population that would experience the same rate of genetic drift or inbreeding as the real population. The study uses the widely applied 50/500 framework, in which Ne below 50 is associated with short-term inbreeding risk and values below 500 with reduced long-term adaptive potential. The authors also note that these thresholds are debated and are best treated as conservation benchmarks rather than absolute biological cut-offs.

Most estimable subpopulations were below the 500 benchmark

Twenty-eight of the 32 subpopulations with contemporary Ne estimates had point estimates below 500, and in 23 the estimate still remained entirely below 500 even when statistical uncertainty was taken into account. When the researchers accounted for population structure at the broader metapopulation level, three of six metapopulations — Jasper-Banff, South-Eastern and Itcha-Ilgachuz — also remained below 500.

Census size and effective population size did not tell exactly the same story. The number of mature animals in a subpopulation was related to the level of inbreeding. The contemporary Ne estimate was not. The authors therefore treat census size, effective size and direct genomic measures of inbreeding as complementary rather than interchangeable indicators.

DNA carried signs of both older and recent inbreeding

The researchers analysed 33,346 single-nucleotide polymorphisms and measured runs of homozygosity — stretches of the genome where the two inherited chromosome copies are identical. Shorter runs can reflect older shared ancestry, while long runs are more consistent with recent mating among relatives. Most subpopulations contained a mixture of both, and small endangered subpopulations, particularly in the south, generally showed the highest levels of inbreeding. Some larger populations also had elevated inbreeding, so small census size was not the only warning signal.

Historical reconstructions also showed a broad north-to-south shift in the timing of population bottlenecks. Northern groups declined around 1700–1780, central groups around 1780–1860 and southern groups around 1860–1940. The authors say this timing broadly coincides with regional climatic shifts and increasing human disturbance, while cautioning that reconstructing the most recent demographic changes in structured populations carries uncertainty.

Sources and image credit

  1. Bourbon, C., Deakin, S., Michalak, A., Hughes, M. M., Cavedon, M., Neufeld, L., Pelletier, A., Polfus, J., Schwantje, H., Thacker, C., Musiani, M., & Poissant, J. (2026). Inbreeding and Demographic History of Caribou (Rangifer tarandus) in Western Canada Inferred From Genome-Wide SNP Data. Evolutionary Applications, 19, e70311. https://doi.org/10.1111/eva.70311Primary open-access research article first published 7 August 2026. The study analysed 33,346 SNPs from 759 woodland caribou representing 45 subpopulations in British Columbia and Alberta. Twelve subpopulations had contemporary effective population size Ne<50 and 28 had Ne<500; three of six metapopulations were below Ne<500 after accounting for population structure. Small southern subpopulations generally showed high inbreeding, and historical Ne reconstructions showed a broad north-to-south gradient in bottleneck timing.

Image: ThartmannWiki / Wikimedia Commons — mountain-type woodland caribou (Rangifer tarandus caribou) in Jasper National Park, Alberta, Canada, 4 September 2014; CC BY-SA 4.0. Representative image from the study region, not an identified study animal. · CC BY-SA 4.0 · Source

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