Geographical patterns of species turnover in aquatic plant communities

1. A classic theory in biogeography predicts that high latitude communities are unstable. This may be because of decreased species richness or decreas ed environmental predictability and productivity towards the poles. 2. We studied latitudinal patterns in long-term community persistence of aq uatic vascular plants in 112 Finnish lakes, situated within a 1000-km range from the northernmost to the southernmost lake. 3. Contrary to theoretical predictions, we found that the turnover rate of plant species in 45 years was inversely related to latitude. That is, plant communities in northern lakes were more persistent than communities in sou thern lakes. When we used multiple regression to find the best predictors o f species turnover rate (TR), latitude was the only variable that was highl y significantly related to species turnover rate. Area, species number, wat er transparency, pH and change in transparency did not notably explain the gradient observed. 4. The latitudinal trend was mainly because of lower species immigration ra tes at higher latitudes, whereas extinction rate did not so strongly decrea se with increasing latitude. Immigrations and extinctions in the lakes were not in balance: the species numbers between the 1930s and 1980s increased more strongly in the southern than northern lakes. 5. We suggest that the inverse relationship between latitude and plant spec ies TR in Finland is most probably caused by human influence on lakes, espe cially eutrophication and immigration of new species in southern latitudes. In addition, although species richness per lake did not decrease towards t he north, the total species pool probably does, which means that in the nor th there are fewer species that can actually immigrate.