The studies that have looked carefully at dispersal have tended to bear out its importance. In a long-term and intensive investigation of a population of great tits, Parus major, near Oxford, UK, it was observed that 57% of breeding birds were immigrants rather than born in the population (Greenwood et al., 1978). In a population of the Colorado potato beetle, Leptinotarsa decemlineata, in Canada, the average emigration rate of newly emerged adults was 97% (Harcourt, 1971). This makes the rapid spread of the beetle in Europe in the middle of the last century easy to understand (Figure 6.13).
A profound effect of dispersal on the dynamics of a population was seen in a study of Cakile edentula, a summer annual plant growing on the sand dunes of Martinique Bay, Nova Scotia. The population was concentrated in the middle of the dunes, and declined towards both the sea and the land. Only in the area towards the sea, however, was seed production high enough and mortality sufficiently low for the population to maintain itself year after year. At the middle and landward sites, mortality exceeded seed production. Hence, one might have expected the population
to become extinct (Figure 6.14). But the distribution of Cakile did not change over time. Instead, large numbers of seeds from the seaward zone dispersed to the middle and landward zones. Indeed, more seeds were dispersed into and germinated in these two zones than were produced by the residents. The distribution and abundance of Cakile were directly due to the dispersal of seeds in the wind and the waves.
Probably the most fundamental consequence of dispersal for the dynamics of single populations, though, is the regulatory effect of density-dependent emigration (see Section 6.3.3). Locally, all that was said in Chapter 5 regarding density-dependent mortality applies equally to density-dependent emigration. Globally, of course, the consequences of the two may be quite different. Those that die are lost forever and from everywhere. With emigration, one population's loss may be another's gain.
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