Dispersal and density

Density-dependent emigration was identified in Section 6.3.3 as a frequent response to overcrowding. We turn now to the more general issue of the density dependence of dispersal and also to the evolutionary forces that may have led to any density dependences that are apparent. In doing so, it is important to bear in mind the point made earlier (see Section 6.1.1): that 'effective' dispersal (from one place to another) requires emigration, transfer and immigration. The density dependences of the three need not be the same.

6.6.1 Inbreeding and outbreeding

Much of this chapter is devoted to the demographic or ecological consequences of dispersal, but there are also important genetic and evolutionary consequences. Any evolutionary 'consequence' is, of course, then a potentially important selective force favoring particular patterns of dispersal or indeed the tendency to disperse at all. In particular, when closely related individuals breed, their offspring are likely to suffer an 'inbreeding depression' in fitness (Charlesworth & Charlesworth, 1987), especially as a result of the expression in the phenotype of recessive deleterious alleles. With limited dispersal, inbreeding becomes more likely, and inbreeding avoidance is thus a force favoring dispersal. On the other hand, many species show local adaptation to their immediate environment (see Section 1.2). Longer distance dispersal may therefore bring together genotypes adapted to different local environments, which on mating give rise to low-fitness offspring adapted to neither habitat. This is called 'outbreeding depression', resulting from the break-up of coadapted combinations of genes - a force acting against dispersal. The situation is complicated by the fact that inbreeding depression is most likely amongst populations that normally outbreed, since inbreeding itself will purge populations of their deleterious recessives. None the less, natural selection can be expected to favor a pattern of dispersal that is in some sense intermediate - maximizing fitness by avoiding both inbreeding and outbreeding depression, though these will clearly be by no means the only selective forces acting on dispersal.

Certainly, there are several examples in plants of inbreeding and outbreeding depression when pollen is transferred from either close or distant donors, and in some cases both effects can be demonstrated in a single experiment. For example, when larkspur (Delphinium nelsonii) offspring were generated by hand pollinating with pollen brought from 1, 3, 10 and 30 m to the receptor flowers (Figure 6.9), both inbreeding and outbreeding depression in fitness were apparent.

6.6.2 Avoiding kin competition

In fact, inbreeding avoidance is not the only force likely to favor natal dispersal of offspring away from their close relatives. Such

Figure 6.9 Inbreeding and outbreeding depression in Delphinium nelsonii: (a) progeny size in the third year of life, (b) progeny lifespan and (c) the overall fitness of progeny cohorts were all lower when progeny were the result of crosses with pollen taken close to (1 m) or far from (30 m) the receptor plant. Bars show standard errors. (After Waser & Price, 1994.)

Figure 6.9 Inbreeding and outbreeding depression in Delphinium nelsonii: (a) progeny size in the third year of life, (b) progeny lifespan and (c) the overall fitness of progeny cohorts were all lower when progeny were the result of crosses with pollen taken close to (1 m) or far from (30 m) the receptor plant. Bars show standard errors. (After Waser & Price, 1994.)

dispersal will also be favored because it decreases the likelihood of competitive effects being directed at close kin. This was explained in a classic modeling paper by Hamilton and May (1977; see also Gandon & Michalakis, 2001), who demonstrated that even in very stable habitats, all organisms will be under selective pressure to disperse some of their progeny. Imagine a population in which the majority of organisms have a stay-at-home, nondispersive genotype O, but in which a rare mutant genotype, X, keeps some offspring at home but commits others to dispersal. The disperser X will suffer no competition in its own patch from O-type individuals but will compete against O-type individuals in their home patches. Disperser X will direct much of its competitive effects at non-kin (with genotype O), while O directs all of its competition at kin (also with genotype O). X will therefore increase in frequency in the population. On the other hand, if the majority of the population are type X, whilst O is the rare mutant, O will still do worse than X, since O can never displace any of the Xs from their patches but has itself to contend with several or many dispersers in its own patch. Dispersal is therefore said to be an evolutionarily stable strategy (ESS) (Maynard Smith, 1972; Parker, 1984). A population of nondispersers will evolve towards the ubiquitous possession of a dispersive tendency; but a population of dispersers will be under no selective pressure to lose that tendency. Hence, the avoidance of both inbreeding and kin competition seem likely to give rise to higher emigration rates at higher densities, when these forces are most intense.

There is indeed evidence for kin competition playing a role in driving offspring away from their natal habitat (Lambin et al., 2001), but much of it is indirect. For example, in the California mouse, Peromyscus californicus, mean dispersal distance increased with increasing litter size in males and, in females, with increasing numbers of sisters in the litter (Ribble, 1992). The more kin a young individual was surrounded by, the further it dispersed.

Lambin et al. (2001) concluded in their review, though, that whereas there is plentiful evidence for density-dependent emigration (see Section 6.3.3), there is little evidence for density-dependent 'effective' dispersal (emigration, transfer and immigration), in part at least because immigration (and perhaps transfer) may be inhibited at high densities. For example, in a study of kangaroo rats, Dipodomys spectabilis, over several years during which density varied, dispersal was monitored first after juveniles had become independent of their parents, but then again after they had survived to breed themselves. The kangaroo rats occupy complex burrow systems containing food reserves, and these remain more or less constant in number: high densities therefore mean a saturated environment and more intense competition (Jones et al., 1988). At the time ofjuvenile independence, density had no effect on dispersal (i.e. on emigration); but by first breeding, dispersal rates (i.e. effective dispersal rates) were lower at higher densities (inverse density dependence) (Figure 6.10). In males, this was mainly because they moved less between juvenile independence and breeding. In females, it occurred mainly because

Low density

High density

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Dispersal distance at first breeding (m)

Figure 6.10 Inverse density-dependent effective dispersal in the kangaroo rat, Dipodomys spectabilis: (a) males, (b) females. Natal dispersal distances were greater at low than at high densities. (After Jones, 1988.)

their survival rate in new patches was lower at high densities (Jones, 1988).

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