Distribution patterns of vertebrates in relation to an extensive rainfall gradient and variation in soil texture in the tropical savannas of the Northern Territory, Australia

The abundance, richness and species composition of frog, reptile, bird and mammal faunas varied along an extensive gradient from 470 to 1406 mm annual rainfall, and between three contrasting soil types (clay, loam and sand) i n northern Australia. Patterns varied between and within vertebrate classes . In general, the most fertile soils in the highest rainfall sites supporte d the greatest species richness and abundance, but this association with fe rtility broke down at lower rainfall sites. Frogs were richest and most abu ndant at high rainfall sites, especially on clay soils, presumably because these had greatest water availability. Clay soils supported few reptile spe cies, but these were often at relatively high abundance. High rainfall site s supported the richest reptile faunas. On sand and loam soils, bird specie s richness varied little along the rainfall gradient, but richness declined very substantially on clay soils. This was probably largely due to the far more marked vegetation structural change on clay soils than on other subst rates. Few mammals were reported, and no clear trends were associated with either rainfall gradient or soil texture. Turnover in species composition a long the rainfall gradient was gradual and limited on sand and loam soils, but far more marked on clay soils. There were few cases of replacements of ecologically comparable species along the gradient. These patterns reflect the disparate history, fragmentation and landscape positioning of clay soil environments, relative to the far more homogeneous eucalypt-dominated vege tation on sand and loam soils. Although comparable studies are lacking on o ther continents, patterns revealed here may be idiosyncratic due to the vir tual monopolisation by eucalypts of the environment across the very extensi ve rainfall gradient.