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Population Ecology of Rattus Rattus Across a Desert‐Montane Forest Gradient in the Galapagos Islands

Clark, David B.
Ecology 1980 v.61 no.6 pp. 1422-1433
Rattus rattus, arid zones, biomass, breeding, correlation, drought, fur, males, microhabitats, montane forests, population ecology, rats, savannas, seasonal variation, sex ratio, shrublands, trapping, vegetation index, woodlands, Galapagos Islands
Populations of introduced black rats (Rattus rattus) were studied by removal trapping in five nonagricultural habitats in the Galapagos Islands; montane forest, montane scrub, tropical deciduous woodland, thorn scrub, and savannah. Density of rats ranged from 0.4 to 18.9/ha, and both density and biomass of rats were significantly correlated with an index of vegetation biomass. The estimated quantity of vegetation per individual and per unit biomass decreased with increasing rat density. These results suggest that rat poplations were food—limited. A 2—mo drought significantly lowered condition (mass/length) of montane forest and montane scrub rats. Analysis of size and reproductive classification of resident and immigrant rats showed significant heterogeneity among subgroups bases on sex and period of capture. Rats from the two habitats of lowest rat density appeared to have more specific microhabitat requirements than rats from denser populations. Breeding was confined to the hot and early cool seasons (January through July) in all habitats. Mean and minimum body lengths in all populations were positively correlated with time since arrival of arid zone rains, while the coefficients of variation of these quantities were negatively correlated. Sex ratio varied seasonally; the percentage of males increased significantly during the nonbreeding season. Intraisland pelage differentiation was observed. Compared to other tropical rodents, Galapagos Rattus rattus have been very successful in range of habitats occupied and in biomass maintained.