suppression, logging, and grazing (Crane and Fischer 1986;
Steele et al. 1986; Sherman and Runge 2002), and by inva-
sion of exotic grasses that lack adequate seed heads to pro-
vide overwinter food (Sherman and Runge 2002). Southern
Idaho ground squirrels have a small range in west-central
Idaho. The native shrub and bunchgrass habitat favored
by this subspecies has been mostly converted to a fire-
dependent grassland dominated by exotic annuals, or to in-
tensive agriculture; rodent control negatively affects the re-
maining small populations (Whisenant 1990, Eric Yensen,
pers. comm). The Washington ground squirrel has been
affected by agricultural conversion of grasslands, invasion
of exotic plants, intensive grazing, and control measures
(Betts 1999, Rickart and Yensen 1991). Habitat conversion
to agricultural, mining, and urban uses, as well as desertifi-
cation associated with climate change affect the Mojave
ground squirrel near the Los Angeles urban area (Best 1995).
The imperiled Mohave, Idaho, and Washington ground
squirrels are below the median body length of North Amer-
ican ground squirrels and are found in relatively moist habi-
tat islands within the drier areas of ground squirrel ranges
in North America (fig. 39.1). These ground squirrels may
have reached an adaptive limit for tolerating arid environ-
ments and have therefore declined as their relatitvely mesic
habitat islands have shrunk following the most recent Pleis-
tocene glacial period. They are then vulnerable to further
climatic drying, increasing irregularlity of rainfall associ-
ated with climate change, invasion by exotic annual species
that make quick use of surface moisture before drying out
(as compared to the deeper-rooted native bunchgrasses),
and shrinkage and fragmentation of remaining habitat.
Where populations are disjunct, population genetics
demonstrate spatial structuring (c.f. Antolin et al. 2001).
Maintaining the genetic variability represented by such
structuring may be important in both localized adaptation
and longer-term evolution, especially in response to cli-
matic change. At a minimum, conservation of recognized
subspecies should be important to maintaining a portion
of this genetic variability. There are nineteen subspecies
of ground squirrels and prairie dogs recognized in North
America; sixteen of these are of particular conservation
concern or of unknown status (table 39.2). Most of these
subspecies are associated with globally common species:
the northern and southern Idaho ground squirrels are an
exception, however, as subspecies of an imperiled species.
The Coachella round-tailed ground squirrel is considered
“critically imperiled” and is a candidate for US federal list-
ing. The northern and southern subspecies of the Idaho
ground squirrel are considered “imperiled” (NatureServe
2004); the northern subspecies is federally listed in the
US as “threatened,” while the southern is a candidate for
listing. Seven subspecies are “vulnerable.” Three subspecies
are “apparently secure.” The remaining five are too poorly
known to rank. All of the imperiled subspecies have his-466 Chapter Thirty-Nine
Figure 39.5 Ranges of North American prairie dogs (Patterson et al. 2003). Black-tailed, Mexican, and Utah species are of conser-
vation concern. These include species designated “vulnerable,” “critically imperiled,” or “imperiled” by NatureServe. Range sizes are
distorted by map projections.