Details of the geographic replacement of the Red Squirrel (Sciurus vulgaris) by the Grey Squirrel (Sciurus carolinensis) in eastern England.

Abstract

1. Details of the replacement are revealed by a series of twenty-two annual distribution maps (1960-81).
2. Colonization of the region by invading grey squirrels was patchy, and pioneer populations were often detached from the main distribution of the species. Similarly, the red squirrel in decline was reduced to scattered 'island' populations.
3. Interaction between the two species could at the most provide a partial explanation of red squirrel decline: in many cases red squirrels became locally extinct before establishment of grey squirrels in the same area. In other cases, coexistence occurred for up to 16 years, or continued in 1982. Grey squirrel presence per se did not enhance the probability of red squirrel extinction. However, the more plausible possibilities of density-dependent interaction could not be tested in this extensive approach.
4. The loss of local red squirrel populations showed no temporal or spatial grouping.
5. A viral disease caused high death-rates locally in red squirrel populations, sometimes resulting in local extinction, but was not recorded in grey squirrels. Outbreaks frequently arose outside the current geographical range of the grey squirrel, weakening the hypothesis of the latter species as an immune vector. Outbreaks showed no temporal grouping.