polygynous species, females invest more in each offspring than do males, and so their
reproductive success is determined by resource competition. Male reproductive suc-
cess is limited by the number of mates they can find, so competition for mates is
important.
Inbreeding avoidance is often cited as a cause of dispersal on theoretical grounds
(reviewed in Thornhill (1993); see Section 17.3.5 for an explanation of the genetics
of inbreeding depression). Inbreeding depression was observed in a captive wolf (Canis
lupus) population (Laikre and Ryman 1991). In contrast, there was no evidence of
inbreeding depression or avoidance in a social carnivore, the dwarf mongoose
(Helogale parvula) (Keane et al. 1996). In general, the occurrence of inbreeding depres-
sion depends on the species (Waser 1996). There are some instances where inbreed-
ing avoidance has been found, as in some species of birds (Pusey 1987; Keller et al.
1994), primates (Pusey 1992), rodents (Hoogland 1982), and marsupials (Cockburn
et al. 1985). However, there are many instances where populations occur in small
numbers, inbreeding is not avoided, and there is no deleterious effect of inbreeding
(Keane et al. 1996). In other cases there are multiple causes of dispersal (Dobson
and Jones 1985).
Dispersers tend to have lower survival than those that remain in their natal area.
In arctic ground squirrels (Spermophilus parryii) survival of philopatric juveniles was
73%, whereas survival of dispersing squirrels was in the range 25– 40%. Also, sur-
vival declines with the distance of dispersal due to the increasing probability of being
caught by predators (Byrom and Krebs 1999). The survival of dispersing ferrets (Mustela
furo) in New Zealand was 100% where predators had been removed experimentally
compared with only 19–71% in areas where predators were present (Byrom 2002).
However, survival of dispersing male San Joachin kit foxes (Vulpes macrotis mutica)
was higher than that for philopatric males (Koopman et al. 2000), indicating excep-
tions to the rule.Dispersions may be random, clumped, or spaced. The most common is a clumped
dispersion(sometimes called a contagious dispersion). If the area is divided into
quadrats and the frequency distribution of animals per quadrat is recorded, the vari-
ance of that distribution will equal its mean if the animals are randomly distributed
(a Poisson distribution), the variance will be greater than the mean if the animals
are clumped at that scale, and the variance will be less than the mean if the animals
space themselves.
Scale is important when dispersions are considered because two or more orders
of dispersion may be imposed upon each other: randomly distributed clumps of
animals for example. In these circumstances a quadrat in a grid of small quadrats
will include either part of a group or it will miss a group: its count will be of many
animals or of no animals. When the grid comprises large quadrats, an average
quadrat will contain several groups of animals and the variation in counts between
quadrats will be less marked. The dispersion is the same whether the quadrats used
to sample it are large or small, but in this case the clumping as measured by the vari-
ance /mean ratio will appear to be more intense when quadrats are small.
An alternative to characterizing dispersion in terms of the frequency distribution
of quadrats containing 0, 1, 2, etc., animals per quadrat is instead to record the
frequency distribution of nearest-neighbor distances or of the distances between
randomly chosen points and the nearest animal to each. The problem of quadrat size92 Chapter 7