THE ISLAND SYNDROME IN RODENT POPULATIONS

Populations of rodents isolated on islands often show systematic diffe rences in demography, reproduction, behavior, and morphology when comp ared to mainland populations. These differences, termed the island syn drome, include higher and more stable densities, better survival, incr eased body mass, and reduced aggressiveness, reproductive output, and dispersal. We synthesize information in the literature on island roden t populations and construct a conceptual model to explain the island s yndrome. Population density and other manifestations of the the island syndrome are predicted to increase with island isolation and to decre ase with island area. The effect of isolation is direct by limiting di spersal, while the effect of area is less direct. As area increases, p redators, competitors, and habitat structure increase in diversity. We suggest that the intensity or absence of density-depressing factors ( primarily predation in communities comprised of only a few morphologic ally and trophically divergent species) is primarily responsible for t he area effect and serves as a principal factor differentiating island from mainland populations of rodents. Other characteristics associate d with the island syndrome may be the result of both short-ten and lon g-ten processes or responses. Short-ten responses include reproductive , body size, and behavioral changes that may be phenotypic reaction no rms in response to higher island densities. Thus, higher densities lea d to reduced reproductive output, which then leads to greater body sit e. Initial behavioral differences may be owing to better survival and reduced dispersal, which result in less population turnover, greater n eighbor familiarity, and less aggression. Long-ten changes may be due to directional selection for reduced dispersal, increased body size in response to increased intraspecific competition, reduced reproductive output (smaller litters and delayed maturation) in response to reduce d mortality schedules, and reduced aggressiveness. While the model is specific to rodent populations, the conclusions may be applicable to o ther systems if characteristics such as body size, vagility, and commu nity composition are considered.