Biogeography and assemblage structure of small mammals across a transition zone in southern Chile

Includes bibliographical references (pages 161-182)The structure and correlates of small terrestrial mammal assemblages were investigated across a major ecological transition in southern Chile. Habitats ranged from wet primary growth Valdivian temperate rainforest to deciduous southern beech (Nothofagus') woods, shrub (matorral) , and finally pre-Patagonian pampa/steppe. In five visits over two years, 1750 small mammals of 13 species were collected from 36 traplines with a total effort of 9169 trapnights. Traplines yielded 1 to 7 species each. Species were heterogeneously distributed across the transition whereas small mammal carrying capacities and assemblage parameters such as richness, evenness, diversity, and biomass demonstrated no obvious patterns. Assemblages contained more species on average than expected by chance. This may have been related to patterns of trophic and microhabitat diversification. There were nonetheless only two recurrent species groupings of 32 observed, suggesting that assemblage composition may be unpredictable. Competitive interactions may determine the distributions of some species here but structural vegetative characteristics and climate appear to be the proximate and ultimate factors, respectively, influencing the presence of most species here. Tree and litter cover especially emerge as parameters to which these mammals respond. Litter depth, shrub cover, and number of shrub species also appear influential. These data agree with conclusions obtained from work with heteromyid rodent assemblages in the North American Southwest, including: (1) mammalian assemblages are composed of species individualistically distributed according to unique requirements for food and other resources; (2) local biotas may be quite variable; (3) local assemblages respond to environmental changes on relatively small spatial scales; and (4) local assemblage composition may not be representative of communities of larger regions. The fact that assemblages as ecologically and historically distinct as heteromyids from the North American Southwest and southern South American sigmodontines share such fundamental patterns of structure suggests these explanations may be more general in applicability.M.S. (Master of Science