Direct methods

The colonization of new habitats, such as the invasion of new ponds by freshwater zooplankton (Jenkins, 1995), provides direct, incontrovertible evidence for dispersal, although such observations impart very little information on the extent or frequency of movements between established populations. A direct method that can provide more detailed estimates of dispersal is mark--recapture. Individuals are first marked in one of a number of different ways that include rings on birds' legs or paint markings on insect wings or carapaces (Figure 4.2). Direct evidence of dispersal is obtained when marked individuals are later recaptured some distance from their source population. A major drawback to this approach is that individual marking is a very time-consuming exercise and often yields limited returns because many of the marked individuals will never be recaptured.

Figure 4.2 A ruddy darter (Sympetrum sanguineum) that can be identified from the label on its wing as individual K15. This dragonfly was marked using indelible ink and subsequently recaptured as part of a study on the dispersal behaviour of odonates (Conrad et al., 1999). Photograph provided by Kelvin Conrad and reproduced with permission

Figure 4.2 A ruddy darter (Sympetrum sanguineum) that can be identified from the label on its wing as individual K15. This dragonfly was marked using indelible ink and subsequently recaptured as part of a study on the dispersal behaviour of odonates (Conrad et al., 1999). Photograph provided by Kelvin Conrad and reproduced with permission

Even more intensive, although potentially a greater source of information, is the use of radio tracking or, in some cases, radar tracking. Radio tags have been used to monitor migratory movements of a number of taxa, including birds and butterflies (Figure 4.3). Although the expense and laboriousness associated with radio tracking means that relatively few individuals can be monitored in this way, it does have the strong advantage of providing data on all movements, not just on the sites of recapture. This can be particularly useful if we are interested in individual foraging behaviour or wish to reconstruct a migratory route. Other studies have used stable isotopes to mark individuals, one example of this being the use of 15N to mark more than 1.5 million aquatic stonefly larvae (Leuctra inermis). Subsequent screening of individuals at adjacent streams revealed only a few isotopically enriched adults (Briers et al., 2004). In this case, traditional mark--recapture methods, which for logistical reasons would have targeted far fewer individuals, would probably not have provided any evidence for dispersal.

Radio tracking and mark--recapture studies provide estimates of dispersal but not gene flow, because they provide no information on whether or not immigrants will reproduce. Other direct measures of dispersal that accurately reflect gene flow are based on parentage studies and have been used most commonly in plants. One study used this approach to determine how far animal frugivores were dispersing seeds in a Spanish population of the cherry tree Prunus mahaleb (Godoy and Jordano, 2001). Endocarp tissue in this species is maternally derived and therefore

Figure 4.3 A female willow ptarmigan (Lagopus lagopus) in northwest British Columbia, Canada, with a radio collar around her neck. Ptarmigan chicks are precocial, which means that they are mobile almost as soon as they hatch, and without radio tracking it is almost impossible to monitor adults and chicks as they move around the tundra. Author's photograph

Figure 4.3 A female willow ptarmigan (Lagopus lagopus) in northwest British Columbia, Canada, with a radio collar around her neck. Ptarmigan chicks are precocial, which means that they are mobile almost as soon as they hatch, and without radio tracking it is almost impossible to monitor adults and chicks as they move around the tundra. Author's photograph is genetically identical to the source tree. By comparing the genotypes of 180 adult trees with the genotypes of endocarp tissue from 95 seeds that had been dispersed naturally, the authors were able to match seeds with maternal trees in 78 cases, and in each case the seed had originated from within the population. The remaining 17 seeds did not match the genotypes of any trees within the population and therefore must have been transported by frugivores from elsewhere. A similar approach has been used in other studies that have estimated pollen-mediated gene flow by comparing the genotypes of seedlings to the genotypes of potential pollen-donors (e.g. Dunphy, Hamrick and Schwagerl, 2004).

Estimating gene flow by comparing the genotypes of offspring and putative parents can provide a comprehensive picture of dispersal, but this approach is impractical or impossible in large populations or in species that often travel long distances. In fact, a common denominator to all direct studies of dispersal is that they are costly, time-consuming, and generally can be conducted only on a relatively small number of individuals. Furthermore, with the exception of parentage analysis, direct evidence for dispersal does not necessarily translate into evidence of gene flow. For these reasons researchers in recent years have turned to alternatives, and we shall look now at the most common of these: indirect estimates of gene flow based on genetic differentiation among populations.

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