Migration and re-colonization enable organisms to persist in metapopulations. Re-colonization success may be limited by the number of arriving migrants or by patch quality. In a well-studied rock pool Daphnia metapopulation, it is frequently assumed that re-colonization is limited by the number of arriving migrants, and that all patches are equally suitable for colonization. This assumption strongly influences how observations about dynamics, epidemiology and population genetics for the entire metapopulation are interpreted. Here we test this assumption.
In 627 rock pools, we found that high pH, high Ca++ and high water conductivity were positively correlated with the presence of D. magna. The experimental release of D. magna into randomly chosen natural pools revealed the highest colonization success in pools with high pH. Next, we elevated pH and Ca++ concentrations in natural pools by adding a system-specific natural source of calcium carbonate (either from crushed oyster shells or from eider duck droppings, which contain blue mussel shells). These treatments led to a rapid increase in pH and Ca++ and strongly raised the likelihood that introduced D. magna would establish persistent populations. Therefore, we conclude that low pH and Ca++ result in unsuitable colonization conditions in two-thirds of the untreated pools.
A further experiment revealed that natural colonization rates were about five times higher in calcium-treated pools than in untreated pools. Finally, we observed that eider droppings are more frequently found in the catchment area of occupied pools, than they are in those of unoccupied pools, suggesting that the blue mussel shells contained in the eider droppings play an important role in making pools suitable for colonization and in enabling D. magna to persist. Thus, eider ducks are ecosystem engineers in this system.
We recalculate typical metapopulation parameters to account for the unsuitable pools, resulting in a much more dynamic picture of this metapopulation than previously believed, with colonization rates and gene flow three to five times higher. These results have strong implications for metapopulation persistence, local and global genetic diversity, genetic rescue, gene flow and local adaptation. Our results emphasize that without verifying patch suitability, estimated rates of metapopulation dynamics can severely underestimate the true rates.