Geographical Ecology of Small Mammals in the Northern Chilean Arid Zone

Small mammal communities were studied along a 1000 km latitudinal transect on the northern Chilean coast. Vegetation here is dominated by evergreen and drought-deciduous shrubs in the south, which are progressively replaced by succulents in the more arid northern regions. Nine localities were sampled twice during 1973-74 for small mammals; measurements were taken also of vegetation characteristics including cover, height, volume and foliage density as well as site slope, number of hiding holes and soil hardness. Precipitation was determined from weather station records. Using two measures of mammal species numbers, the total number of species known to be present (TNS) and the number of species actually caught (NSC), a stepwise multiple regression with all environmental variables indicated significant relationships to logarithmically-transformed measures of precipitation and herbaceous cover. Both latter variables demonstrate a northward decrease, and are significantly correlated with each other. Since available ecological information indicates that none of the small mammals are water-independent, and a large component (the caviomorphs) are primarily herbivorous, precipitation may act as an ultimate factor and herbaceous cover a proximate one in limiting the number of small mammals in the northern arid zone. Although some northern localities have insular characteristics, recent glacial history and potential avenues of dispersal down Andes flanks and river valleys suggest that isolation has not accounted for the present northward decrease in species numbers.