CHANGES IN SMALL MAMMAL ASSEMBLAGE STRUCTURE ACROSS A RAIN-FOREST OPEN FOREST ECOTONE

The effect of the change in vegetation structure from closed rain fore st to tall open forest on the small mammal assemblage was studied by l ive trapping at three sites where the ecotone was very narrow (<20 IT I) near the southern end of the Wet Tropics World Heritage Area of Aus tralia. Habitat heterogeneity was significantly higher in the mixed op en forest/ecotone area than in the adjacent rain forest. There was a l arge change in the structure of the small mammal assemblage coincident with the vegetation discontinuity. Although the species richness of s mall mammals was relatively constant across the gradient, the evenness and diversity of the assemblage declined across the transition from o pen forest into rain forest and biomass increased, largely due ro the high abundance of Rattus fuscipes in the rain forest. The results sugg est that the species richness of the small mammal assemblage was not d etermined by the spatial heterogeneity of the vegetation structure. Th e species composition of the rain forest is probably related to the hi storical 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. Howeve r, the evenness, and therefore the diversity of the assemblage, was st rongly affected by habitat heterogeneity.