Patterns of Island Occupancy Explained by Colonization and Extinction Rates in Shrews

Abstract

This is the first study reporting patterns of island occupancy in a guild of mammals with detailed information on the processes affecting these patterns: dispersal, colonization, and extinction. We surveyed the presence of three species of shrews, Sorex araneus, S. caecutiens, and S. minutus, on 108 islands in three lakes in Finland. A significant compensatory effect between island area and isolation was detected in one species, but most islands were so little isolated (< 1 km) that isolation played no great role in this study. The position and the slope of the incidence function varied greatly among the species. Fitting the model J = 1/(1 / y/Ax) to the observed incidence functions, and assuming that the annual extinction probability is 1 on islands of A = 0.5 ha, yielded quantitative predictions of the rates of colonization and extinction in the three species. Observed rates agreed remarkably well with the predicted ones (median difference 20%). Overwater dispersal rate and/or ability was lowest in the smallest species (S. minutus), probably because of its short starvation time and slow swimming rate. Unexpectedly, this species had the highest colonization ability (rate of successful colonization conditional on arriving at an island). There were no significant differences between the species in colonization rate, but interspecific differences in extinction rate were highly significant and negatively correlated with body size. We suggest that these differences are due to increasing importance of environmental stochasticity with decreasing body size in shrews. This hypothesis is supported by larger coefficients of variation of abundance in small than in large species on the mainland.