The effect of the change in vegetation structure from closed rain forest to tall open forest on the small mammal assemblage was studied by live trapping at three sites where the ecotone was very narrow (<20 m) near the southern end of the Wet Tropics World Heritage Area of Australia. Habitat heterogeneity was significantly higher in the mixed open forest/ecotone area than in the adjacent rain forest. There was a large change in the structure of the small mammal assemblage coincident with the

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... etation discontinuity. Although the species richness of small mammals was relatively constant across the gradient, the evenness and diversity of the assemblage declined across the transition from open forest into rain forest and biomass increased, largely due to the high abundance of Rattus fuscipes in the rain forest. The results suggest that the species richness of the small mammal assemblage was not determined by the spatial heterogeneity of the vegetation structure. The species composition of the rain forest is probably related to the historical biogeography of the area whereas the species richness of the wet sclerophyll forest is probably due to a mass-area effect from the adjacent large areas of rain forest and dry sclerophyll forest. However, the evenness, and therefore the diversity of the assemblage, was strongly affected by habitat heterogeneity. I NT R OD U C TI O N Many areas of rain forest in north Queensland are bounded on their western edge by a narrow band (less than 5 km wide) of tall open forest, with mixed casuarina and rain forest elements (Winter 1988) . This band of tall open forest, often referred to as wet sclerophyll forest, forms a transitional zone between the rain forest and the extensive xeric woodland to the west of the main dividing range. Often the transition from closed forest to the wet sclerophyll forest