Rocky substrate as a water basin
Rocky substrate is important for species living in xeric re-
gions of the world because depressions in rocks form small
water catchments during periods of rain (Streilein 1982).
These rock pools can increase the humidity of the region
around the rocks, leading to a more mesic habitat and pro-
viding the opportunity for species to live where they oth-
erwise could not (Streilein 1982; Morris 2000). Red-back
voles (Clethrionomys gapperi), for example, live in forests
in the northern part of their range, but in the southern part
of their range they are confined to living in rock outcrops,
presumably because outcrops are the only habitat capable of
providing a sufficiently high humidity to avoid dehydration
(Wolff and Dueser 1986). The Australian pebble-mound
mouse (Pseudomys chapmani) uses resourcefulness in com-
bination with rocky substrate to create its own mesic mi-
crohabitat. The pebble-mound mouse scatters a mound of
pebbles around its burrows. Each morning, the air temper-
ature around the pebbles warms up faster than the pebbles
themselves, causing small droplets of dew to form by con-
densation. The dew collected on pebbles within each 1-
meter-diameter mound can then be ingested by the pebble-
mound mouse (Nowak 1991). Through the use of ingenuity
or merely a strategic choice of habitat, rodents ensure their
survival in what would otherwise be harsh, inhospitable en-
vironments by using rocky substrate to obtain water.
Unique microenvironments and floral diversity
The complex topography of rocky habitat, including the
presence of catchment sites for water, can lead to the exis-
tence of unique microenvironments throughout rocky sub-
strate and increased plant species richness (Freeland et al.
1988; Trainor et al. 2000; Fredericksen et al. 2003). A sur-
vey of plants surrounding a rock escarpment in western
Arnhemland, Australia, revealed a larger number of plant
species, a larger number of tree species, and greater tree spe-
cies diversity with increasing proximity to the escarpment
(Freeland et al. 1988). Higher levels of floral diversity near
the escarpment are thought to result from increased water
retention by the rock surface during seasonal rains (either
due to water absorption by the rock, water catchment sites
within the rock face, or water runoff from the escarpment).
This particular escarpment in Arnhemland is home to six
species of mammal that are restricted to rock outcrops. Be-
cause of the high diversity of rock-specialists on this out-
crop and the extensive floral diversity surrounding the out-
crop, Freeland et al. (1988) proposed that the uniqueness of
microflora environments surrounding rocky habitat might
lead to speciation within rock-dwelling taxa. If correct, this
supposition could help to explain the large diversity of
mammals that associate with rocky habitat throughout the
world. Further studies are needed to test this hypothesis,
however, against the more traditional explanation of speci-
ation through geographic isolation (see Braun and Mares
2002, for example).Effect of fire and over-grazing
Two of the major factors threatening faunal diversity within
habitats of Australian rock-dwelling rodents are fire and the
overgrazing of food plants by large herbivores (Begg 1981;
Begg et al. 1981; Morris 2000; Trainor et al. 2000; Brook
et al. 2002). Populations of both the critically endangered
Carpentarian rock-rat (Zyzomys palatalis) and the Arnhem
Land rock-rat (Zyzomys maini) are thought to be highly
susceptible to these concerns (Begg et al. 1981; Churchill
1996; Trainor et al. 2000; Brook et al. 2002). Both fire and
overgrazing, however, appear to be less of a problem the
rockier the habitat (Trainor et al. 2000). For the Arnhem
Land rock-rat, a fire greatly reduced population numbers
and led to decreased reproduction the following breeding
season (Begg et al. 1981). Although this species was initially
most abundant in closed forest, that habitat was severely
impacted by the fire, so the Arnhem Land rock-rat relo-
cated after the fire to less disturbed scree habitat (Begg et al.
1981). This study suggests that many Australian mammals
may choose to live in rocky habitat so as to reduce their
exposure to fire and problems of a similar nature caused by
overgrazing.More stable microclimate
Because rocky substrate can provide abundant refuges, en-
hance thermoregulation, lead to increased floral diversity,
improve water availability, and reduce the extent of dam-
age from fires and overgrazing, a more stable microclimate
often exists in and around rocky habitat compared to sur-
rounding regions (Streilein 1982; Trainor et al. 2000). In
some cases, such favorable conditions can lead to the avail-
ability of year-round resources, and may result in smaller
home range sizes for rock-dwelling mammals in compari-
son to related taxa that do not live in rocks (Pavey et al.
2003). The overall stability of rocky habitat also makes it
suitable for use as a temporary refuge by species that nor-
mally reside in other habitats (Withers and Edward 1997).
For example, a large-scale temporal analysis of the distri-
bution of mammals in Australia indicates that rocky habi-
tat is increasingly occupied during times of reduced ground-
water levels (Braithwaite and Muller 1997). Since rocky
habitat provides a stable microclimate that is exploitable by
rock-dwelling species all year round and non-rock-dwelling
taxa during critical times, it is not surprising that over one-420 Chapter Thirty-Five