Population ecologists have long been interested in two key topics: the first is the relative importance of intrinsic factors (such as the inhibition of reproduction at high population densities) and extrinsic environmental variations in determining population fluctuations; the second is nonlinearity in the processes that generate these fluctuations. On page 674 of this issue, Grenfell and co-workers discuss both of these issues. From their studies of fluctuations in two populations of Soay sheep living on islands 3.5 km apart in the St Kilda archipelago (in the Outer Hebrides), they provide a new insight into the synchronization of population fluctuations at separate, but not too distant, locations.
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