Abstract

The exposed granites of the Stanthorpe Plateau are pitted with shallow rock hollows known as pan gnammas. Most are <7 cm deep, much shallower than gnammas studied elsewhere in Australia. With a total of 35 aquatic invertebrate taxa, averaging 5.6 taxa per gnamma, they also host a less diverse invertebrate fauna than other Australian gnamma groups. Their species richness per inselberg is only about one-quarter of that in the Wheatbelt (WA) and less than half that on the Eyre Peninsula (SA) and in Central Victoria. Gnamma depth, as a surrogate for hydroperiod, is likely to play a significant role in determining biotic diversity in pan gnammas, as shown by a strong correlation between species richness and gnamma depth. The relative shallowness of the Stanthorpe Plateau gnammas is due to a different mode of formation, with rock dissolution occurring mainly at the air–water–rock interface at the gnamma edges, resulting in vertical C-shaped edges and sideways spread rather than deepening. The invertebrate metacommunity of Stanthorpe Plateau gnammas consists mainly of dipterans and crustaceans, of which 14% are gnamma obligates. We documented the life cycle of the clam shrimp Paralimnadia urukhai, endemic to gnammas on the northern New England Tablelands. Sixteen months of monitoring populations in one of the deepest Stanthorpe Plateau gnammas (7.5 cm) illustrates the constraints imposed by short hydroperiods, with only one of eight hydroperiods resulting in significant reproduction and the others resulting in probable egg bank depletion, or non-hatching in one case, due to low temperatures. Rough calculations of egg production and hatching suggest the population is presently sustainable, but future climatic changes could restrict clam shrimps to deeper gnammas, which are scarce on the Stanthorpe Plateau.