Invertebrate communities and turnover in wetland ponds affected by drought

1. Changes to the macroinvertebrate fauna found in small ponds on a freshwater marsh (Aberlady Bay, Scotland) in 1986, 1987 and 1992 were used to assess the impact of the 1988–92 drought on taxon turnover in pond communities. 2. Permanent ponds accumulated taxa over the study period. Ponds that were wet throughout 1986–87 but dried in 1992 lost some of the many taxa associated with permanent water but acquired a reduced fauna typical of temporary ponds. Ponds that were temporary in 1986–87 were dry during most of 1992 and lost almost all aquatic taxa. 3. Faunal turnover was considerable even in permanent ponds. Extinction rates were high for taxa typical of permanent or temporary ponds. Colonization rates were poor for the taxa from permanent water, but high for the taxa from temporary ponds. 4. The importance of colonization and extinction rates as main predictors of the distribution of species between the ponds was looked at using metapopulation incidence functions, where observed incidence of a taxon can be used to predict colonization and extinction rates. Predicted rates were then compared with observed rates. Incidence functions gave reasonable predictions of observed colonization rates but were poor predictors of extinction, even for taxa that appeared likely to be true metapopulations. 5. For the pond fauna, including fugitive species adapted to temporary ponds, whilst colonization may well depend on environmental stochasricity (how long a pond holds water), subsequent survival depends on other, demographic, processes (e.g. finding a mate, predation) rather than the pond drying out.