High population persistence in a system with high turnover

The concept of turnover—the repeated extinction and immigration of populations in ecological communities—has been prominent for many years1. Here we show that a system of island orb spiders has consistently high turnover during a five-year period. This might seem to imply that most populations are ephemeral, but ˜50% of the populations present in the system at any one time are persistent over at least the moderately long term. Curves of persistence through time for ensembles of populations are more concave than exponential curves, indicating highest extinction rates in young populations; species vary greatly in the slope of this population 'persistence curve'. In any one-year period, small populations become extinct much more frequently than larger populations. Over longer periods, however, larger populations do not diminish and become extinct; instead, smaller populations re-immigrate and become extinct repeatedly. The total number of individuals belonging to populations becoming extinct during intervals of up to five years is small compared to those persisting. Thus, a layer of small, ephemeral populations is superimposed against a backdrop of large, persistent populations.