The granite gnammas (rock-holes) across the north of the Eyre Peninsula (EP), are a unique ephemeral freshwater habitat. A complex community of invertebrates is known to be associated with these sites, but macroinvertebrate predator occurrence is often sporadic both spatially and temporally. Due to the potential driving effect of predator occurrence/abundance on community structure, the aim of this project was to determine what predicts predator occurrence in the EP gnammas.
A total of 14 gnammas were sampled across five granite outcrops along the Eyre Highway in 2016. Sites were classified physically and ecologically and extensive dip-net sampling was undertaken to determine damselfly presence. Representative specimens were collected for identification and imaging.
Three species of damselfly were identified to be associated with the EP gnammas: Ischnura aurora, Xanthagrion erythroneurum, and Astrolestes annulosus. All three damselflies were observed co-occurring in a single gnamma at Pildappa Rock. This gnamma was notable as it had the greatest depth of all those surveyed, and the longest predicted hydroperiod. Additionally, this gnamma was the only one containing a floating-leaved, rooted aquatic macrophyte, resulting in a complex, three-dimensional vegetative structure across the whole water depth in the gnamma: swamp lily, Ottelia ovalifolia (Hydrocharitaceae).
Due to their exclusive co-occurrence, damselflies were hypothesised to be dependent on the presence of O. ovalifolia, possibly as a result of the plants providing a complex three-dimensional habitat for the nymphs. The results of this study contribute to the previously very limited ecological literature regarding the EP gnammas. Increased understanding of the factors that influence species diversity, is critical in maintaining these threatened habitats.