Extinction, Colonization, and Population Structure: A Study of a Milkweed Beetle

Abstract

The turnover of local populations that results from a continuing pattern of extinction and colonization can be an important determinant of population genetic structure because it can enhance opportunities for both genetic drift and gene flow. This can either enhance or diminish the genetic differentiation of local populations depending on the specifics of the process. Thus, its general significance can be determined only by detailed studies of a diversity of species. In this study, extinction rates and the composition of colonizing groups were investigated for a milkweed beetle, Tetraopes tetraophthalmus. Four of 33 natural populations of T. tetraophthalmus became extinct over 6 yr of observation, yielding an extinction probability of about 2% per generation, sufficient to influence population structure. A 3-yr study of migration into eight experimental patches of milkweed showed that an average of three beetles invade an unoccupied site each year, demonstrating that significant genetic drift could be associated with the founding of populations. Variation about this average was significant from site to site and from year to year, tending to reduce the average effective number of colonists. The number of colonists arriving at sites ranged from 1 female to 21 individuals of both sexes. Although the sex ratio of colonists, pooled across sites and years, was close to 50:50, significant but opposite deviations from this ratio were observed in two of three years, when analyzed separately. Individuals migrating into these experimental patches persisted for 6 days, on the average, about as long as adults found in small, natural milkweed patches. The relative isolation of some of the experimental patches from potential sources of the migrants demonstrated that at least occasional dispersal events on the order of several kilometers occur in this species, raising the possibility that considerable gene flow could also be associated with the founding events.