Distributional relations of two species of Psednura (Orthoptera : Pyrgomorphidae) in the Evans Head area of New South Wales

The distribution of Psednura pedestris and the northern race of P. musgvavei was studied in relation to habitat in the heathy vegetation of the sandy coastal plain near Evans Head, N.S.W. The density of both species was in general very low, corresponding to an average of only one specimen captured per man-hr; in a few patches it rose to several times that figure. The distribution of the two species was in the main mutually exclusive in a mosaic pattern reflecting the mosaic distribution of the respective preferred habitats, pedestvis occurring on the margins of swamps and musgravei on somewhat better drained sites. However, at three locations mixed populations were found in the ecotone between the two habitats. At one of these, which was studied in detail, the zone of overlap was never wider than 12 m, and the highest density of each species occurred within a 2-m strip on either side of a median line related to a prominent habitat feature. The situation is compared and contrasted with that in parapatric species and races of morabine grasshoppers.