(Sherman 1976; Salsbury and Armitage 1994; Bowman
et al. 1999; Maier 2002), raising the possibility that peri-
odic long-range dispersal is important in maintaining ge-
netic relatedness among distantly separated populations.
Telfer et al. (2003) observed that populations of water voles
(Arvicola terrestris) living on islands separated by distances
large enough to prohibit regular long-distance dispersal
had substantially lower levels of genetic diversity than did
large, continuous populations on the mainland.
Conservation of species
Dispersal also has important implications for the conser-
vation of species. Dispersal facilitates gene flow between
populations, and can be important in maintaining genetic
diversity within populations. Maintaining gene flow is espe-
cially important in small populations, which are more likely
than large populations to become inbred, because they pro-
vide fewer options for choosing mating partners. Dispersal
may in fact counteract the stochastic tendency for alleles to
be lost in small populations (Aars and Ims 2000).
Barriers to dispersal can hinder gene flow, and thus de-
crease genetic variability within a population and increase
genetic differentiation among populations (e.g., Santos et al.
1995; Landry and Lapointe 2001). In the long term, changes
in the genetic structure of populations can decrease their
viability and lead to local population extinctions (e.g., Aars
et al. 2001; Roach et al. 2001). Habitat destruction associ-
ated with human activity can cause continuous populations
to become fragmented. Moreover, inhospitable terrain be-
tween fragments of undisturbed habitat can impede dis-
persal. For example, root voles in experimentally subdi-
vided populations suffered increased predation when they
attempted to disperse across inhospitable area separating
habitat patches (Aars et al. 1999). Introduction of exotic
predatory species can also reduce large populations to dis-
connected fragments and hinder dispersal, as was observed
by Aars et al. (2001) among water vole populations in Scot-
land after introduction of the American mink (Mustela vi-
son). Thus maintaining dispersal routes in fragmented hab-
itat can be vital to the continued existence of populations in
the area.
Use of dispersal corridors — narrow areas continuous
with and connecting larger habitat areas — for dispersal
has been observed in undisturbed populations of Colum-
bian ground squirrels transferring between distant colonies
(Wiggett and Boag 1989). Preserving dispersal corridors in
fragmented habitat has been suggested to beneficially aug-
ment gene flow in a variety of rodent species, including root
voles (Andreassen et al. 1998; Aars and Ims 1999; Ims and
Andreassen 1999; Andreassen and Ims 2001), gray-tailed
voles (Microtus canicaudus;Davis-Born and Wolff 2000),
and Piute ground squirrels (Spermophilus mollis;Antolin
et al. 2001). Dispersal corridors can also enhance gene flow
by allowing individuals to make excursions to distant habi-
tat to mate with unfamiliar individuals. Such mating excur-
sions have been suggested to occur in root voles and Japa-
nese wood mice (Apodemus argenteus;Ohnishi et al. 2000).Breeding DispersalBreeding dispersal occurs after successful reproduction,
when an adult animal departs its breeding area and moves
to a new one. Breeding dispersal has not been studied as ex-
tensively as has natal dispersal. However, three general hy-
potheses have been proposed to explain adaptive functions
of breeding dispersal.
First, adult animals may leave their home areas to avoid
mating with their offspring in the future. For example, in
Belding’s ground squirrels, mating is dominated by a small
number of males during the breeding season (Sherman
1976). These males typically disperse from the breeding area
despite their high reproductive success. Males who domi-
nate mating are typically the largest and most successful at
winning fights with other males, and presumably are good
competitors. Thus these males are most likely not dispers-
ing in response to competition for mates or environmen-
tal resources. Rather, Sherman (1976) proposed that males
who have mated with several females will be fathers of a
large proportion of females in the next generation, and dis-
perse so that their reproductive tenure in the area will not
overlap with that of their daughters.
Second, adult animals may disperse so that their off-
spring can inherit their territories. Territorial bequeathal
has been observed in red squirrels (Tamiasciurus hudsoni-
cus;Berteaux and Boutin 2000), Columbian ground squir-
rels (Harris and Murie 1984), and white-footed mice (Wolff
and Lundy 1985). Berteaux and Boutin (2000) suggested
that breeding dispersal by adult female red squirrels is a
form of parental investment. Adult females are better able
than young to establish themselves in vacant territories.
They observed that by leaving their territories, mothers
allowed some of their young to remain on the natal site,
which increased survival of the young. Female red squirrels
who engaged in breeding dispersal tended to be older and
more experienced than nondispersing females, and emi-
grated when food resources were abundant, thus minimiz-
ing the potential costs of dispersal.
However, Lambin (1997) suggested that territorial be-
queathal might be adaptive only when young inherit a crit-
ical resource such as a midden, as has been observed in red
squirrels (Berteaux and Boutin 2000), or a burrow system,156 Chapter Thirteen