Arctic ground squirrel population collapse in the boreal forests of the Southern Yukon

Abstract

Context The arctic ground squirrel (Urocitellus parryii) comprised 17% of the biomass of herbivores in the Yukon boreal forest during the summer months from 1987 to 1996 and was responsible for 23% of the energy flow at the herbivore level. By 2000, ground squirrel populations in this region collapsed to nearly zero and have remained there. Aims We summarise the population monitoring (since 1975) and recent experimental work that has been done on this key herbivore in the Kluane area of the southern Yukon to test one mechanistic hypothesis as the possible explanation for this population collapse and subsequent lack of recovery: predation. Methods Ground squirrels are the preferred summer prey of bird and mammal predators when snowshoe hare (Lepus americanus) populations are declining. We used translocations into formerly occupied habitat and radiotelemetry to determine movements and causes of death from 2009 to 2014. We surveyed 158 sites between 2008 and 2013 to measure the disappearance of colonies in alpine and forest habitats over 25 000 km2. Key results Ground squirrels from 2000 to 2013 comprised a small fraction of the herbivore biomass in the boreal forest zone, down from 17% earlier. Most forest populations (~95%) are currently extinct, whereas just over half (65%) of low-elevation meadow populations are locally extinct. One hypothesis is that ground squirrels in the forest have been driven into a predator pit from which they cannot recover. They remain abundant in alpine tundra (93% occupancy rate) and around airport runways and human habitations (97% occupancy), but there is no apparent dispersal from alpine areas down into the boreal forest. Conclusion The predator pit hypothesis is a likely explanation for the initial collapse and sustained decline in population size from 2000 to 2013. Recent attenuation of the hare cycle and milder winter climate have allowed shrubs to expand throughout the forest, thereby reducing visibility and increasing predation risk. This conclusion will be tested in further research using reintroductions to formerly occupied sites. Implication If the loss of this herbivore from the boreal forest is not reversed, predator pressure on the other major herbivores of the montane forest zone is likely to change significantly.

Factors Governing the Distribution and Abundance of Arctic Ground Squirrels

“…For almost three decades (1973 -99), ground squirrel populations in the boreal forests of the Kluane region (SW Yukon) cycled in a predictable manner (Werner et al, 2015b) in concert with the snowshoe hare (Lepus americanus; Boutin et al, 1995). This 9 -10 year cycle was stable up until 2000.…”

“…Inverse (i.e., negative) density dependence at low population size can also result from social dysfunction (Allee, 1931). Distinguishing between these two mechanisms would be difficult today because populations in the boreal forest are now largely extinct (Werner et al, 2015b). Note, however, that some extrinsic factor must first reduce populations to that critical threshold for these intrinsic Allee effects to take over.…”

“…These surveys measured colony characteristics considered to reflect levels of food availability (e.g., richness and abundance of food plants, soil composition) and predation risk (distance-specific visibilities, slope angle, distance to predator cover). To estimate population density, I used a powder tracking method developed by Boonstra et al (1992) for lemmings and voles, but adapted for ground squirrels (Hubbs et al, 2000;Donker and Krebs, 2011;Werner et al, 2015b). Small (5 × 7 cm) black plastic tiles, covered in mineral oil and unscented talcum powder, were placed in all burrows within a specified area (minimum size 0.5 ha) and left for six hours.…”

A meeting of mice and men: rodent impacts on food security, human diseases and wildlife conservation; ecosystem benefits; fascinating biological models

“…Further observations and experiments on this wide-scale collapse of rodent numbers in Europe are essential. The collapse of arctic ground squirrels in the boreal forests of the Yukon is discussed by Werner et al (2015) as a paradigm of the sudden collapse of a common rodent species and the detective work needed to determine the causes of the collapse, being likely due to predation and tying into the problem of a landscape of fear generated by climate change. The Serengeti is well known for its large-mammal populations and community dynamics, whereas data on the 40 species of rodents is much less abundant.…”

“…However, rodents should be viewed in a much broader context. They make up some 42% of mammalian species and less than 10% of species pose significant impacts on humans in agricultural or urban settings (Singleton et al 2007), and, indeed, many play important roles in ecosystems (Dickman 1999;Werner et al 2015). It is a common interest in the breadth of research questions on rodents that leads to a gathering of international biologists every 4 years.…”