Density compensation in allopatric populations of the pygmy shrewSorex minutus on Gotland and the outer hebrides: evidence for the effect of interspecific competition.

Pygmy shrewsSorex minutus occur allopatrically in Ireland, the Outer Hebrides and Gotland in Sweden, and sympatrically with common shrewsS. minutus on the European mainland and in England. Competition theory redicts higher population density in allopatric populations as a consequence of relaxed competition. Here this prediction is tested by comparing allopatric populations of pygmy shrews in the Outer Hebrides and in Gotland with sympatric populations on the Swedish mainland. Population densities were higher in allopatry than in sympatry. Lower summed densities in allopatry, which is predicted by niche theory, was found on the Outer Hebrides, but not on Gotland. Size distributions of carabids, which are important food items, could not explain the differences in population densities. Since both avian predators and snakes are present in all areas, population density of pygmy shrews during autumn in concluded to be regulated primarly by competition with common shrews in areas of sympatry.