northern boreal forest demonstrated a two-fold increase in
population density associated with removal of predators,
but a four-fold increase associated with food supplementa-
tion. Predation may have been particularly intense in their
system, as the experiment was conducted during a decline
and low phase in the snowshoe hare cycle, when predators
were high and alternative prey low in density. The added
food resulted in improved body condition, earlier emer-
gence of juveniles, and increased reproductive rates (Karels
et al. 2000). Although more than 50% of juvenile Colum-
bian ground squirrels were lost to badgers one year (Murie
1992), higher survivorship of remaining juveniles may have
compensated.
Given that production of food plants is critically impor-
tant to maintaining viable populations of ground squirrels,
competition with invasive plants may be the greatest cause
of decline (aside from outright loss of habitat to agricultural
or other development.) Exotic annuals compete with native
perennials for scarce water in dry environments, but provide
only an ephemeral food supply to herbivores. Fire, over-
grazing, and other anthropogenic disturbances can encour-
age these invasives and the invasives, in turn, encourage the
spread of fire. Reduced or more irregular rainfall associated
with climate change will exacerbate this problem.
Habitat Fragmentation
Habitat fragmentation for ground squirrels is produced
by conversion of land for agriculture, roads, and urban de-
velopment. Prairie dogs live in dense colonies, and so are
already unevenly distributed across the landscape. Perhaps
their extreme sociality makes prairie dogs more susceptible
to both control efforts and plague. Indeed, sylvatic plague
(Yersinia pestis;Barnes 1982) has caused local extinction of
many colonies. Disease is not restricted to prairie dogs; a
widespread decline, possibly resulting from plague, led to
apparent extinction across much of the range for Townsend
ground squirrels in Utah beginning in 1936 and persisting
at least 18 years (Hansen 1956).
For prairie dogs, areas that appear on the basis of vege-
tation to be uniform habitat may contain soils unsuitable
for colonization, adding to population fragmentation. Vari-
ation in topography, soils, and land-use history contribute
to fragmentation (Koford 1958). The need to be able to dig
burrows that will remain sufficiently dry without excessive
energy expenditure restricts these species to reasonably uni-
form, colloidial, and arid to somewhat mesic soils that are
lacking in aggregate rock. Sandy soils are particularly un-
suitable for animals with larger body sizes, so that we may
find kangaroo rats (Dipodomysspp.), for instance, in soils
too sandy for burrows of larger ground squirrels or prairie
dogs. Prairie dog colonies in the Pawnee National Grass-
lands of Colorado were located less often than expected on
Olney fine, sandy loam, based on its availability, possibly
because of avoidance of soils with deep, sandy horizons.
In a fragmented population, animals that disperse to ge-
netically dissimilar colonies may produce more successful
and robust offspring than those that remain and breed with
more genetically similar animals. Long-distance dispersal
has been more closely associated with inbreeding avoidance
(Dobson et al. 1997) than with lack of food resources or
other factors. Prairie dogs occasionally disperse long dis-
tances, sometimes more than 5 km (Garret et al. 1982; Gar-
rett and Franklin 1988). Dispersal is risky, however, expos-
ing animals to predators in unfamiliar territory. Dispersing
juvenile arctic ground squirrels suffered higher mortality
than philopatric squirrels, and this effect increased with
dispersal distance (Byrom and Krebs 1999).
Dependence on burrows for hibernation, thermoregu-
lation, escape from predators, and reproduction raises the
costs of dispersal away from the home burrow system. Con-
structing burrow systems de novois challenging and risky,
as digging animals are exposed to predators and weather, so
that animals most often settle in areas with existing burrow
systems (Boag and Murie 1981; Murie and Harris 1984;
Knowles 1985; Weddell 1989), even if the entrances to such
systems have been effectively covered over with loose soil.
Finding existing burrows can be difficult for highly social
species that must cover wide areas of unoccupied habitat
to disperse between social groups. Hence, ground squirrels
show pronounced genetic population structuring, in which
similarity decreases directly with distance or with distance
along dispersal routes such as drainages (Gavin et al. 1999;
Roach et al. 2001; Dobson, chap. 14 this volume). Because
of the energetic costs and risks of burrow construction, and
the corresponding energetic savings and safety they offer,
burrow systems are valuable resources (King 1984).Conservation and Management ImplicationsGround squirrels can strongly affect ecosystem processes as
a rich diurnal prey item (Marti et al. 1993) by foraging and
defecating and through burrow construction and soil mix-
ing (Yensen and Sherman 2003). In nature, ground squir-
rels are largely regulated by environmental “crunches” in
bottom-up nutrient availability. There is abundant evidence
of the sensitivity of these species to reduction in nutrient
availability through prolonged winter, periodic drought,
and invasion of exotic annuals. Climate change, especially
where it affects cycles of precipitation and their predicta-
bility, could push extensive areas of the ranges of these spe-
cies below a critical production threshold, particularly in470 Chapter Thirty-Nine