Despite abundant evidence that the species composition of a given ecosystem can change considerably over time, until recently there have been no attempts to identify the main causes of species turnover by distilling general trends from multiple ecological studies. Biologists inFinlandandGermanydrew on information extracted from 99 papers on aquatic ecosystems, defining species turnover as the rate at which an index of assemblage similarity declined with time. Estimates of species turnover showed a strong negative correlation with the duration of sampling. This was explained by the fact that because new species tend to be found less often as the length of a sampling episode increases, the rate of change per unit time tends to be higher for shorter studies than for longer ones. Species turnover was also affected by latitude, being fastest when based on within-year data from low-latitudes or on between-year data from high latitudes. The reasons for this pattern are not entirely clear, but the authors speculate that it may be linked to differences in longevity and generation times associated with higher energy inputs and faster life cycles in the tropics. A third factor affecting species composition was body size, with smaller organisms showing higher rates of turnover. Possible explanations for this result include the tendency for small species to have rapid life histories and efficient modes of dispersal. Finally, species turnover was also faster in larger ecosystems, and more rapid in lakes than in stream or marine assemblages. In summary, species turnover seems to be driven by a wide range of ecological, geographical and physical effects.
Reference: Korhonen, J.J., Soininen, J. & Hillebrand, H. 2010. A quantitative analysis of temporal turnover in aquatic species assemblages across ecosystems. Ecology 91(2), 508–517.
Freshwater Research News 
Posted on March 4, 2011