Collapse of Arctic Ground Squirrel Populations in the Yukon Boreal Forest Charles J. Krebs, Jeffery R. Werner, and Rudy Boonstra University of British Columbia and University of Toronto, Canada

Outline Distribution and natural history Population dynamics Explanations for recent population collapse Predictions Experiments in progress

Arctic Ground Squirrel (Urocitellus parryii) Burrowing herbivore (500-800 g) Hibernate 8 months of winter Occupy alpine, forest, grassland habitats One litter per year (4-7) Live in groups of related females Density 1-3 per ha

Geographical Distribution Most of geographical range is in arctic and alpine tundra

Altitudinal Distribution

Altitudinal Distribution ALPINE Population collapse Limited or no recovery BOREAL FOREST High density Stable populations 500 meters 1000 2000 2001 - 2014

Population Fluctuations - Boreal Forest Declines driven by predation

Local Dynamics – The Problem # 1 Ground squirrel population changes have been driven by predation as secondary prey Low snowshoe hare abundance in the past 14 years along with climatic warming have allowed increased shrub growth Ground squirrels depend on warning calls to alert the colony to predator presence

Local Dynamics – The Problem # 2 Hypothesis: increased shrub growth has reduced predator detection and ground squirrels have been driven to local extinction in the boreal forest Prediction: forest habitats are sinks, and recolonization must come from source areas Source areas could be alpine populations or small meadows within the forest

Landscape Distribution - 2013 A regional effect, not a local effect

Decision Tree

Dispersal Limitation - Observations Continuous dispersal within forests and low grasslands of males shown by radio-collaring No movements of radio-collared squirrels from the alpine down into the boreal forest - alpine population are not a source When we radio-collar juveniles in boreal forest areas in 2013 and 2014, they either move and die or stay and die by predation

Decision Tree

Decision Tree continued from above Experimental re-introductions ?

Possible Allee Effect in Arctic Ground Squirrels Declines driven by predation

Further Questions Global warming is increasing winter temperatures and causing freeze-thawing and rainfall during winter This could produce ice in hibernation burrows, resulting in overwinter mortality If this is correct, the icing effect should affect both alpine and forest populations

Summary Arctic ground squirrels have disappeared from boreal forest sites where they were formerly very abundant They appear to be in a predator pit caused by: - habitat changes in the forest caused by reduced herbivory and global warming - an Allee effect associated with predator warning calls within a colony

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