Does inter-annual variability in population density increase with time?

Temporal variability in population density has been shown to increase with increasing length of a study. This has serious implications for ecological studies and suggests that obtaining an objective measure of temporal variability may not be possible. To test this hypothesis more generally, I calculated inter-annual variability, using the standard deviation of the logarithm of annual mean population density (STD(log D)), for 70 populations of phytoplankton, zooplankton and fish which have been sampled over 10 to 51 consecutive yr in lakes around the world. STD(log D) fluctuates most widely during the first 3-6 yr of a study, but these changes do not necessarily represent long-term trends in temporal variability. Median changes in STD(log D) over time are small (<< 0.l unit, even over a logarithmic time scale) and decrease as more years are included in the analysis. The inter-annual variability of most freshwater populations appears to reach an asymptotic value. Several populations, however, show an alternation of plateaux and of sudden increases in STD(log D), suggesting that the asymptotic values found in most populations may be temporary and may not extrapolate beyond the temporal scale of this study (10-50 yr). Overall, the increases in inter-annual variability are expected to be small(<0.l units, on average, per 25 yr). The pattern and the magnitude of these changes did not differ significantly between algae, zooplankton and fish, and were similar to those reported for birds and mammals. Increases in temporal variability may only be important for questions addressed over long periods of time.