Effects of competition on natal dispersal distance
Introduction
Many vertebrate species experience competition for breeding territories (Stamps, 1994). Several models (Murray, 1967, Waser, 1985, Tonkyn and Plissner, 1991) have indicated that increased competition for territorial vacancies will increase the distance moved by an individual from its natal site to the site of first breeding (natal dispersal distance). However, a recent model demonstrates that increased competition can increase or decrease natal dispersal distance, depending on the nature and magnitude of competition (McCarthy, 1997a). The model can be modified to represent searching for mates (McCarthy, 1997b). It was based on individuals moving from multiple nests in a straight line to the nearest available vacancy. Dispersers either moved sequentially, with a disperser leaving its nest only after the previous individual had settled, or simultaneously, with individuals searching for vacancies at the same time. Individuals did not suffer mortality during dispersal (McCarthy, 1997a).
Real species are unlikely to conform to the model of McCarthy (1997a) in at least three respects: (1) search paths followed by dispersers are unlikely to be perfectly straight lines; (2) individuals may experience considerable mortality during dispersal (Belichon et al., 1996), and this may have a large influence on dispersal success and dispersal distance; and (3) in the model of simultaneous dispersal it was assumed that individuals compete for a set of vacancies at exactly the same time, in contrast to the sequential model. Reality may be better approximated by an intermediate model. Individuals will tend to disperse at similar times, but not at exactly the same time. Additionally, territories will become available over a period of time, and dispersers will compete for these vacancies as they arise. These aspects are examined below by extending the model to consider any type of search path, mortality during dispersal, and a model that is intermediate between sequential and simultaneous dispersal.
Section snippets
Dispersal paths
McCarthy (1997a) modelled dispersal of individuals that each moved simultaneously from their own nest in a straight line to the first encountered vacancy. Dispersing individuals competed for vacancies by site pre-emption. In this model, the probability of finding a vacancy was the same for all dispersers when they left their nest, but their chances of success decreased with distance moved as other individuals settled and the density of vacancies declined. It was assumed that nests and vacancies
Mortality during dispersal
In most previous models of competition and dispersal, it is assumed that individuals do not die during dispersal (Murray, 1967, Waser, 1985, McCarthy, 1997a). This is unlikely to be true for many species, which may suffer substantial mortality during dispersal (Belichon et al., 1996). The simplest way of including dispersal mortality is to assume that the risk of mortality is constant over space. In this case, the chance of an individual being alive after moving a distance of x is given by the
Simultaneous and sequential dispersal
The models developed above are based on identical individuals competing simultaneously for a set of vacancies. However, some individuals may be more likely to obtain a territory than others (Stamps, 1994), making the effective number of dispersers less than the total pool of individuals without territories. Because of a dominance hierarchy, only a proportion of the individuals may have a chance of obtaining a vacancy at a given point in time. Additionally, with the exception of catastrophic
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