EXTINCTION-RECOLONIZATION DYNAMICS IN THE MYCOPHAGOUS BEETLE PHALACRUS-SUBSTRIATUS

The population structure of the mycophagous beetle Phalacrus substriat us is characterized by many small, local populations interconnected by migration over a small spatial scale (10 X 75 m(2)). Each local P. su bstriatus population has a relatively short expected persistence time, but persistence of the species occurs due to a balance between freque nt local extinctions and recolonizations. This nonequilibrium populati on structure can have profound effects on how the genetic variation is structured between and within populations. Theoretical models have st ated that the genetic differentiation among local populations will be enhanced relative to an island model at equilibrium if the number of c olonizers is less than approximately twice the number of migrants amon g local populations. To study these effects, a set of 50 local P. subs triatus populations were surveyed over a four-year period to record an y naturally occurring extinctions and recolonizations. The per populat ion colonization and extinction rate were 0.237 and 0.275, respectivel y. Mark-recapture techniques were used to estimate a number of demogra phic parameters: local population size (N = 11.1), migration rate ((m) over cap = 0.366), number of colonizers (k = 4.0), and the probabilit y of common origin of colonizers (phi = 0.5). The theoretically predic ted level of differentiation among local populations (measured as Wrig ht's F-ST) was 0.070. Genetic data obtained from an electrophoretic su rvey of seven polymorphic loci gave an estimated degree of differentia tion of 0.077. There was thus a good agreement between the empirical r esults and the theoretical predictions. Young populations (<(theta)ove r cap>(young) = 0.090) had significantly higher levels of differentiat ion than old, more established populations (<(theta)over cap>(old) = 0 .059). The extinction-recolonization dynamics resulted in an overall i ncrease in the genetic differentiation among local populations by c. 4 0%. The global effective population size was also reduced by c. 55%. T he results give clear evidence to how nonequilibrium processes shape t he genetic structure of populations.