An Analysis of a Method for Comparing Residents and Colonists in a Natural Population of Microtus ochrogaster

Dueser et al. (1981) described a method for identifying individuals as dispersers or residents in a natural population of Microtus pennsylvanicus. They operationally defined dispersers to be those individuals which, when first captured, weighed at least 30 g. All animals that weighed less than 30 g were defined as residents. They then used standard demographic variables to compare the dispersers and the residents once the residents had grown to at least 30 g. They reasoned that for a very trappable species, trapped very frequently, and with closely spaced traps, the probability of missing an individual until it was at least 30 g was very small and most of these individuals would have been captured had they been born on the grid. This method allowed them to test for differences between these two groups which may relate to different life history strategies involving the costs and benefits of dispersing or remaining in the population (Gaines and McClenaghan, 1980; Johnson, 1984; Johnson and Gaines, 1985). In fact, Dueser et al. (1981) found the disperser group exhibited relatively greater proportions of reproductively active individuals and was predominantly composed of males. The high reproductive activity of dispersers would be necessary for dispersal to be an evolutionarily stable strategy (Johnson, 1984; Johnson and Gaines, 1985), since dispersal has been shown to increase the risk of mortality (Ambrose, 1972; Metzgar, 1967). Recently, the Dueser et al. (1981) technique has been the subject of some controversy as to its usefulness and justifiability. Tamarin (1984) argues the error rates of misclassifying dispersers are high. He cites studies which indicate that a large percentage of dispersing individuals are under 30 g. These animals would be mistakenly classified as residents. In rebuttal, Dueser et al. (1984, p. 728) state the 30 g criterion is valid since "... the relatively low body weight of 30 g is most likely to err in the direction of identifying residents as 'dispersers', rather than vice versa . . , " real differences between dispersers and residents will be reduced by misclassification. Thus, their technique is conservative. The disagreement about misclassification may, in part, stem from Dueser et al.'s use of the term disperser. Their disperser group is really the post-dispersal subset of the larger set of animals which actually dispersed from a population. Tamarin (1984) cites studies which identify dispersers as animals appearing in vacant habitat. While an individual of any size or physical condition may be able to colonize empty habitat, there is no reason to assume those which successfully colonize in an established population should be similar to those which immigrate into the rather uncommon condition of unoccupied but suitable habitat. There are several reasons to believe that animals colonizing in an existing population may differ substantially. Larger animals have lower energetic costs of transport per unit distance, may be less susceptible to death by exposure and predation, and may have a better chance of establishing a home range in a population of strange and potentially aggressive animals. To avoid misinterpretation, we have relabeled Dueser et al.'s dispersers as colonists and emphasize that the Dueser et al. (1981) technique attempts to monitor dispersers after they have entered an established population. In this paper, we test the appropriateness of Dueser et al.'s method, keeping in mind that we are comparing colonists (successful dispersers) and residents (nondispersers), not dispersers and residents. Though we cannot quantitatively estimate the accuracy of the resident classification, we can determine if it is correct at least more often than not. The accuracy of the colonist classification can be quantitatively estimated. Using M. ochrogaster, we also compare demographic characteristics of colonists and residents in an attempt to replicate Dueser et al.'s results. Our data collecting protocol was the same as Dueser et al. (1981). The data were collected as part of another study (Johnson 1984; Johnson and Gaines, 1985) from December 1979 to November 1981. We utilized two 0.8 ha live-trapping grids, one open (A) and one fenced (Y), which were about 500 m apart. Voles could immigrate and emigrate to and from the open grid at all times. Only emigration was allowed from the enclosed grid. Voles dispersed from the enclosure by crossing a 1-m wide mowed strip next to the