Community Ecology of Aquatic Invertebrates in Gnammas (Rock-Holes) of North-Western Eyre Peninsula, South Australia

Abstract

Gnammas are small hollows, usually on granite outcrops, that contain water intermittently. They are of two basic types: pans are shallow (<20 cm deep) and form by weathering along horizontal laminations whereas pits are deeper (>50 cm) and form by weathering on a hemispherical front in solid granite, but often aided by vertical joints. Much is known about those in southern Western Australia, but similar gnammas on granite rocks of north-western Eyre Peninsula (EP), South Australia are unknown. This study shows such pools are less speciose than in the west, with fewer large branchiopods, cladocerans, ostracods, and copepods, no snails and overall hardly any endemics. This is largely explained by isolation effected by the large dry Nullarbor Plain between southern WA and EP; this isolation also affects the fauna within the EP region because highly isolated rocks lack some species. As in WA, EP pits have a largely eurytopic fauna of insects but with a few numerous and characteristic crustaceans whereas pans are dominated by crustaceans, though by different species than in WA. Community structure in pits on the EP resembles those in WA. However, EP pans are different, largely due to the influence of endemics, and consequently, the fauna of pits and pans on the EP differ less than in WA. Although the physical environment is similar in the two areas, biogeographical differences lead to distinctive communities in each. Almost none of these gnammas are in the conservation estate and many of the pit gnammas are severely sedimented.