EXTINCTION-RECOLONIZATION DYNAMICS IN THE MYCOPHAGOUS BEETLEPHALACRUS SUBSTRIATUS

Abstract

The population structure of the mycophagous beetle Phalacrus substriatus is characterized by many small, local populations interconnected by migration over a small spatial scale (10 × 75 m ). Each local P. substriatus population has a relatively short expected persistence time, but persistence of the species occurs due to a balance between frequent local extinctions and recolonizations. This nonequilibrium population structure can have profound effects on how the genetic variation is structured between and within populations. Theoretical models have stated that the genetic differentiation among local populations will be enhanced relative to an island model at equilibrium if the number of colonizers is less than approximately twice the number of migrants among local populations. To study these effects, a set of 50 local P. substriatus populations were surveyed over a four-year period to record any naturally occurring extinctions and recolonizations. The per population colonization and extinction rate were 0.237 and 0.275, respectively. Mark-recapture techniques were used to estimate a number of demographic parameters: local population size (N = 11.1), migration rate (m∘=0.366), number of colonizers (k = 4.0), and the probability of common origin of colonizers (φ = 0.5). The theoretically predicted level of differentiation among local populations (measured as Wright's F ) was 0.070. Genetic data obtained from an electrophoretic survey of seven polymorphic loci gave an estimated degree of differentiation of 0.077. There was thus a good agreement between the empirical results and the theoretical predictions. Young populations (θ∘young=0.090) had significantly higher levels of differentiation than old, more established populations (θ∘old=0.059). The extinction-recolonization dynamics resulted in an overall increase in the genetic differentiation among local populations by c. 40%. The global effective population size was also reduced by c. 55%. The results give clear evidence to how nonequilibrium processes shape the genetic structure of populations.

Hierarchical genetic structure and effective population sizes in Phalacrus substriatus

“…An earlier study focused on the population structure of P substriatus over a small spatial scale (<1000 m2), corresponding roughly to a single population of the host plant Carex nigra (Ingvarsson et al, 1997). In that study we found evidence for a strong degree of differentiation among local I?…”

“…The increase in genetic differentiation could be further enhanced if recurrent extinctions and recolonizations of local populations are frequent and especially if founders of new populations come from a limited number of sources (founders have a high probability of common origin, sensu Whitlock & McCauley, 1990). In an earlier study (Ingvarsson et a!., 1997) we have shown that local population turnover is common in P substriatus and that founders of new populations indeed have a high probability of common origin because they come from only one or two source populations. We also showed that local extinctions and recolonizations result in an 40 per cent increase in the degree of differentiation among populations on island 3.…”

“…This, in turn, influences the estimate of Ne(j) and will consequently lead to a rather imprecise estimate of Ne(i). Secondly, eqn (4) does not explicitly take population turnover into account, which the method used by Ingvarsson et aL (1997) does. As frequent extinctions and recolonizations can greatly reduce the global effective population size and because turnover of local populations is a frequently occurring phenomenon in P substriatus (at least on island 3; Ingvarsson et at., 1997), this will compromise the use of eqn (4) in determining Ne().…”

“…Secondly, eqn (4) does not explicitly take population turnover into account, which the method used by Ingvarsson et aL (1997) does. As frequent extinctions and recolonizations can greatly reduce the global effective population size and because turnover of local populations is a frequently occurring phenomenon in P substriatus (at least on island 3; Ingvarsson et at., 1997), this will compromise the use of eqn (4) in determining Ne(). However, as we lack detailed demographic data from islands 1 and 2, the use of eqn (4) provides the only means by which we can obtain at least a crude estimate of the global effective population size for these two islands.…”

“…How gene flow among populations influences the estimation of effective population size depends both on average gene frequency in migrants and in individuals within the population receiving the migrants. The migration rate among local P substriatus populations is far from negligible (m = 0.366; Ingvarsson et al, 1997), so the estimate of the effective population size is clearly going to be affected by migration from surrounding populations. Omitting the effects of migration when estimating the effective population size ignores an important source of gene frequency change.…”