Populations of rodents isolated on islands often show systematic differences in demography, reproduction, behavior, and morphology when compared to mainland populations. These differences, termed the island syndrome, include higher and more stable densities, better survival, increased body mass, and reduced aggressiveness, reproductive output, and dispersal. We synthesize information in the literature on island rodent populations and construct a conceptual model to explain the island syndrome. Population density and other manifestations of the island syndrome are predicted to increase with island isolation and to decrease with island area. The effect of isolation is direct by limiting dispersal, while the effect of area is less direct. As area increases, predators, 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 morphologically and trophically divergent species) is primarily responsible for the area effect and serves as a principal factor differentiating island from mainland populations of rodents. Other characteristics associated with the island syndrome may be the result of both short-term and long-term processes or responses. Short-term responses include reproductive, body size, and behavioral changes that may be phenotypic reaction norms in response to higher island densities. Thus, higher densities lead to reduced reproductive output, which then leads to greater body size. Initial behavioral differences may be owing to better survival and reduced dispersal, which result in less population turnover, greater neighbor familiarity, and less aggression. Long-term 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 reduced mortality schedules, and reduced aggressiveness. While the model is specific to rodent populations, the conclusions may be applicable to other systems if characteristics such as body size, vagility, and community composition are considered.

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A population of white-footed mice (Peromyscus leucopus) in southeastern Massachusetts was lived-trapped monthly in five habitat types for 5 yr in order to study the demography of a habitat generalist. We identified three demographic groups (two of low density and one of high density), which differed primarily in density, adult survival, proportion of males breeding, and variability in the proportion of males. The low-density segments of this population were at times able to achieve demographic performance equivalent to the high-density segments. Several demographic variables were related linearly to microhabitat gradients derived from a principal components analysis of 24 habitat variables, but similar demographic structure was found in different habitat types. A simple model is presented that relates demography to environmental suitability in a habitat generalist. According to this model, population density, productivity, and survival increase, and numerical and demographic variability decrease, along a gradient of increasing environmental suitability. Intensity of intrinsic regulation also may increase with environmental suitability, which would contribute to population stability. We hypothesize that in habitat generalists such as Peromyscus leucopus, population numbers are regulated along a stable trajectory in areas of high suitability, but may show little or no intrinsic regulation in poorer habitats. Furthermore, demographic structure may shift as environmental conditions improve or decline in quality. This highly flexible demographic structure may enable populations of generalists to persist in poor habitats.

A seed-removal experiment was conducted to assess the role of Proechimys semispinosus (Central American spiny rat) as potential predator and disperser of Astrocaryum standleyanum (black palm) seeds. One hundred fresh ripe A. standleyanum fruits were placed in semipermeable cages on each of 14 small islands in Panama, and seed disappearance rates were calculated for each island. Spiny rat density and biomass were determined by live-trapping on each island for 5 consecutive days and 4 nights. Censuses of fruiting trees were conducted on each island to control for effects of food availability on removal of palm fruits. Disappearance rates were related positively to total spiny rat density, density of adult and subadult spiny rats, and spiny rat biomass, but the density of fruiting trees accounted for very little variation. Spiny rats evidently prey heavily on A. standleyanum seeds, based on the high removal rates and on feeding observations of captive individuals. Since spiny rats may scatterhoard A. standleyanum seeds, they may also function as effective seed dispersers if seeds are removed to favourable germination sites unavailable to other seed predators. Results indicate that spiny rats, because of their abundance and wide distribution, may be important but overlooked predators and dispersers of A. standleyanum seeds and of other large-seeded tree species.

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