untitled

the juveniles leave voluntarily, their behavior being innately determined by their

genes (e.g. in Belding’s ground squirrels (Spermophilus beldingi); Holekamp 1986);

or that adults forcibly exclude juveniles. Saturationdispersal is seen in many large

mammals (Sinclair 1992). In this case dispersal occurs when a population reaches a

threshold density determined by food limitation. Dispersal then is density dependent

(see Chapter 8 for an explanation of this mechanism) so that population density remains

the same on the initial area. Examples of this have been described for Himalayan

tahr (Hemitragus jemlahicus) as they spread through the Southern Alps of New

Zealand (Parkes and Tustin 1985), and for wood bison (Bison bison) as their popu-

lation increased through their former range in the boreal forest of Canada (Larter

et al. 2000) (see Section 7.6 for modeling range expansion).

The likelihood of dispersal differs markedly between individuals of a population.

Figure 7.1 shows a sample of distances dispersed by juvenile kangaroo rats

(Dipodomys spectabilis) ( Jones 1987), a solitary, nocturnal, grain-eating desert

rodent. Females averaged 29 m and males 66 m, but the majority of individuals did

not disperse at all. Jones (1987) reported that adults of this species do not disperse

much: 70% of adult males and 61% of adult females remained in one mound for the

rest of their lives. Juvenile females of red deer (Cervus elaphus) seldom disperse but

adopt home ranges that overlap those of their mothers. In contrast, males leave the

natal home range between the ages of 2 and 3 years, mostly joining stag groups in

the vicinity (Clutton-Brock et al. 1982).

Patterns of dispersal are related to the type of mating system (Greenwood 1980,

1983; Dobson 1982; Greenwood and Harvey 1982). Thus, in mammals, females are

concerned with obtaining resources while males compete for mates. In general, males

disperse in promiscuous and polygynous species because they are more likely to find

new mates by doing so, while females are philopatric (i.e. remain at their birth site)

because they are more likely to find food in areas they know well. Both sexes dis-

perse in monogamous species. Amongst higher vertebrates, one sex is more prone

to dispersal than the other. Thus, in mammals males are the dispersers whereas in

birds it is the females which disperse, although there are exceptions for both groups.

For example, in mammals females are the dispersers in wild dogs (Lycaon pictus) and

zebra (Equus burchelli). In fishers (Martes pennanti) and wolves, both sexes disperse

equally (Arthur et al. 1993; Boyd and Pletscher 1999).

The causes of dispersal fall into three broad categories: competition for mates, avoid-

ance of inbreeding, and competition for resources ( Johnson and Gaines 1990). In

DISPERSAL, DISPERSION, AND DISTRIBUTION 91

50

40

30

20

10

0

0 1 50 100 150 200 250 300

Dispersal distance (m)

Number of individuals

Female (n = 70) Male (n = 72)

50

40

30

20

10

0

0 1 50 100 150 200 250 300

Fig. 7.1The frequency
distributions of
distances dispersed by
juvenile rat kangaroos.
(Data from Jones 1987.)