Being high is better: effects of elevation and habitat on arctic ground squirrel demography

Abstract

We investigated the effect of local environment on the demography and population dynamics of arctic ground squirrels (Spermophilus parryii plesius ) by comparing reproduction, survival, and population trends of squirrels living in low elevation boreal forest and high elevation alpine tundra sites in southwestern Yukon Territory, Canada. Contrary to the trend for most birds and mammals, reproduction was significantly lower at the lower elevation and females living at higher elevation did not delay the age at which they first reproduced. Even though survival in the boreal forest was lower in summer than in the alpine, it was higher over winter so annual adult female survival was similar between sites. Sensitivity analysis of model parameters revealed that in the forest, population growth rate (l) was most sensitive to small changes in adult active season survival whereas for the alpine population, l was most sensitive to changes in juvenile winter survival. In their respective habitats, these parameters also showed high year to year variation and thus contributed greatly to the population trends observed. Even though ground squirrels persisted in the boreal forest, the measured demographic rates indicate the forest was sink habitat (lB/1) and may have relied on nearby grassy meadows for immigrants. In contrast, the alpine habitat maintained a ground squirrel population in the absence of immigration (l0/1).The variation in demographic rates between ground squirrels living at high and low elevation may arise from phenotypic responses of squirrels to different habitat structure. Arctic ground squirrels rely on sight to detect predators from a safe distance, and the boreal forest, with its lower visibility and higher predator density, appears to be suboptimal habitat.

Evidence for Selective Caching by Arctic Ground Squirrels Living in Alpine Meadows in the Yukon

“…We collected cheek-pouch contents from ground squirrels live-trapped as part of a study examining arctic ground squirrel demography and dispersal (Gillis, 2003;Gillis et al, 2005 Lacey, 1991). We ear-tagged each squirrel with a unique number the first time it was trapped, and on every subsequent recapture we recorded identity, age (juvenile or adult), and gender.…”

Factors Governing the Distribution and Abundance of Arctic Ground Squirrels

“…3; mean (± SE) residual of mass over structural size = −11.0 ± 10.2 vs. 20.5 ± 6.1 g; Werner et al, 2015a). Females that are in poor condition often exhibit lower reproductive success, a fact that has been reported for ground squirrels in the boreal forest (Gillis et al, 2005). However, I also found that forest squirrels gained mass at an accelerated rate, and by autumn, they had attained parity with squirrels in more favorable habitats (both groups weighed 520 ± 15 g; Werner et al, 2015a).…”

“…These "sink" habitats are net importers, and without sufficient immigration from source habitats, extinction would be inevitable (Dias, 1996). Previous studies have identified forests as potential sink habitats that require a constant influx of squirrels from surrounding source habitats to sustain them (Gillis et al, 2005;Donker and Krebs, 2012). To better understand the consequences of this source-sink relationship, I compared body mass and condition of adult females in boreal forest and adjacent meadows using data accumulated between 2000 and 2013.…”

How will the greening of the Arctic affect an important prey species and disturbance agent? Vegetation effects on arctic ground squirrels

“…Firstly, some prey species rely on high visibility for predator detection (Blumstein et al 2006) and open escape routes (Schooley et al 1996). Secondly vegetation stature can affect the type of predator, predator foraging strategy (Gillis et al 2005a), or predator density. Tall vegetation may, however, also have positive effects by providing cover for foraging herbivores (Hannon et al 2006).…”

“…Changes to their density may result in feedbacks to the ecosystem state, due to the ecosystem effects they impose via extensive burrowing and as prey to many avian and terrestrial predators (Wheeler and Hik 2013). Comparative survival and density trends at boreal, shrub-dominated and more open alpine tundra sites show characteristics of source-sink dynamics, and elevated predation risk in sites with taller vegetation may underlie these site differences (Gillis et al 2005a;Krebs 2011, 2012;Wheeler 2012). However, experimental food enrichment and subsequent population trends after cessation of enrichment suggest that forage may also play an important role in population dynamics .…”