Immigrants to habitats occupied by conspecific organisms are usually expected to be competitively inferior, because residents may be locally adapted. If residents are inbred, however, mating between immigrants and residents results in offspring that may enjoy a fitness advantage from hybrid vigor. We demonstrate this effect experimentally in a natural Daphnia metapopulation in which genetic bottlenecks and local inbreeding are common. We estimate that in this metapopulation, hybrid vigor amplifies the rate of gene flow several times more than would be predicted from the nominal migration rate. This can affect the persistence of local populations and the entire metapopulation.

If colonization of empty habitat patches causes genetic bottlenecks, freshly founded, young populations should be genetically less diverse than older ones that may have experienced successive rounds of immigration. This can be studied in metapopulations with subpopulations of known age. We studied allozyme variation in metapopulations of two species of water fleas (Daphnia) in the skerry archipelago of southern Finland. These populations have been monitored since 1982. Screening 49 populations of D. longispina and 77 populations of D. magna, separated by distances of 1.5-2180 m, we found that local genetic diversity increased with population age whereas pairwise differentiation among pools decreased with population age. These patterns persisted even after controlling for several potentially confounding ecological variables, indicating that extinction and recolonization dynamics decrease local genetic diversity and increase genetic differentiation in these metapopulations by causing genetic bottlenecks during colonization. We suggest that the effect of these bottlenecks may be twofold, namely decreasing genetic diversity by random sampling and leading to population-wide inbreeding. Subsequent immigration then may not only introduce new genetic material, but also lead to the production of noninbred hybrids, selection for which may cause immigrant alleles to increase in frequency, thus leading to increased genetic diversity in older populations. M ANY populations exist as metapopulations, that dynamics affect genetic diversity has received less attenis, as populations structured into interconnected tion. Theory predicts that turnover dynamics mostly demes with local "turnover" dynamics of extinction and (but not invariably) lead to increased genetic differentirecolonization (Andrewartha and Birch 1954; Hanation and decreased local genetic diversity as compared ski 1999). Evolutionary processes in metapopulations to similarly structured populations without extinction differ in many aspects from those in large, uniform popand recolonization dynamics (Slatkin 1977; Wade and ulations because gene flow among demes is restricted, McCauley 1988; Whitlock and McCauley 1990; local demes may be small, and turnover dynamics lead Austerlitz et al. 1997Austerlitz et al. , 2000; Le Corre and Kremer to genetic bottlenecks during recolonization (Andrew-1998;Pannell and Charlesworth 1999). artha and Birch 1954; Hanski and Gilpin 1997; HanThis can be studied empirically in metapopulations ski 1999). Moreover, metapopulation structure may be in which the age of local demes is known. If turnover important for evolutionary processes even in populadynamics increase genetic differentiation, young demes tions that do not exist as ecological metapopulations should be more strongly differentiated than old demes, (i.e., with turnover dynamics too weak to influence debecause age structure in metapopulations is a direct mography). This is because many migrants are needed consequence of turnover. Indeed, a number of empirito homogenize...

Several models have been suggested to explain variation in parasite richness among populations. Most of these models are based on epidemiological factors (population size, number of host species), biological factors (patch quality, interspecific competition) and the spatial and temporal structure of the host metapopulation. We studied the parasites of 137 rock pool populations of the planktonic crustacean Daphnia magna to determine the factors that account for total parasite richness, richness of endoparasites and epibionts, and the presence/absence patterns of individual parasite species. The rock pools of 86 of these populations have been studied since 1982, and it is known how long these pools have been continuously inhabited by Daphnia. By far the best predictor of total parasite richness was host population age, which explained ϳ50% of the variance. While endoparasite richness increased linearly with age over 16 yr, epibiont richness saturated ϳ3 yr after pool colonization, which may be explained by the higher dispersal rate of epibionts. After we corrected for host population age, endoparasite richness was positively correlated with the water volume of the rock pool (an estimator for host population size), and epibiont richness was correlated with water conductivity. Pools with lower water conductivity (less influenced by the brackish water of the Baltic Sea) had more epibiont species. The local network size of the host metapopulation (local pool density and number of pools per island) hardly influenced parasite richness. There was also no strong indication of spatial effects (isolation by distance and island effects) on the parasite community. The factors that were correlated with species richness were, however, not the same as those related to the presence of single parasite species. At least for certain epibionts, it appears that presence/absence patterns were influenced by interspecific competition. In conclusion, our analysis shows that predictions derived from epidemiological and temporal models, but not from spatial models, can explain parasite richness patterns, despite apparent conflicting patterns found for individual parasite species. Our analysis extends the scope of these models, which were previously supported mainly with helminths, to bacteria and protozoa.

The genetic structure of metapopulations offers insights into the genetic consequences of local extinction and recolonization. We studied allozyme variation in rock pool metapopulations of two species of waterfleas (Daphnia) with the aim to understand how these dynamics influence genetic differentiation. We screened 138 populations of D. magna and 65 populations of D. longispina from an area in the archipelago of southern Finland. The pools from which they were sampled are separated by distances between 1.5 and 4710 m and located on a total of 38 islands. The genetic population structure of the two species was strikingly similar, consistent with their similar metapopulation ecology. The mean F PT value (differentiation among pools with respect to the total metapopulation) was 0.55 and a hierarchical analysis showed that genetic differentiation was strong (40.25) among pools within islands as well as among whole islands. Within islands, pairwise genetic differentiation increased with geographic distance, indicating isolation by distance due to spatially limited dispersal. Previous studies have shown strong founder events occurring during colonization in our metapopulation. We suggest that the genetic population structure in the studied metapopulations is largely explained by three consequences of these founder events: (i) strong drift during colonization, (ii) local inbreeding, which results in hybrid vigour and increased effective migration rates after subsequent immigration, and (iii) effects of selection through hitchhiking of neutral genes with linked loci under selection.

Climate change is expected to alter the range and abundance of many species by influencing habitat qualities. For species living in fragmented populations, not only the quality of the present patches but also access to new habitat patches may be affected. Here, we show that colonization in a metacommunity can be directly influenced by weather changes, and that these observed weather changes are consistent with global climate change models. Using a long-term dataset from a rock pool metacommunity of the three species Daphnia magna, Daphnia longispina and Daphnia pulex with 507 monitored habitat patches, we correlated a four-fold increase in colonization rate with warmer, drier weather for the period from 1982 to 2006. The higher colonization rate after warm and dry summers led to an increase in metacommunity dynamics over time. A mechanistic explanation for the increased colonization rate is that the resting stages have a higher exposure to animal and wind dispersal in desiccated rock pools. Although colonization rates reacted in the same direction in all three species, there were significant species-specific effects that resulted in an overall change in the metacommunity composition. Increased local instability and colonization dynamics may even lead to higher global stability of the metacommunity. Thus, whereas climate change has been reported to cause a unidirectional change in species range for many other species, it changes the dynamics and composition of an entire community in this metacommunity, with winners and losers difficult to predict.