Sunday, 13 August 2006

A water penny for your thoughts

Water has left its mark in the Centre. It carved the gorges and sculpted Kata Tjuta and Uluru and wore ancient mountains into plains of sand. That was in a wetter time. There's not much evidence of water in the heart of the continent now.

But it's still there.

While most zoologists concentrated on the biogeography of land animals in the Centre, Jenny Davis and colleagues from Western Australia studied the distribution of freshwater invertebrates. They focussed on the George Gill Range near Watarrka (Kings Canyon), which is the wettest place in the desiccated heart. It has also experienced the smallest amount of damage from human activity. Although the streams are ephemeral, running only during rare floods, water holes among the rocks provide a permanent habitat for freshwater animals. They act as refugia from which animals disperse when the creeks flow again.

The team found no great surprises in the number of species in these water holes—the usual suspects were there in the usual quantities. More ostracods, copepods and chironomids than you could poke a stick at. But there were some notable anomalies.

There were no stoneflies, isopods or amphipods. In fact, they found very few of the shredders that normally feed on fallen leaves. Maybe there weren't enough leaves to support them?

But they did find some animals that couldn't move around much—among them, the waterpenny (Sclerocyphon fuscus), a small psephid beetle. It was almost certainly a relict species, stranded in the central ranges when the continent started to dry out.

What were the biogeographical relationships of the George Gill Range fauna? Although many species were cosmopolitan, occurring in suitable habitat across the continent, the waterpennies and others were distinctly southern in distribution. The strongest biogeographical signal suggested a central and south-east relationship. But the dragonflies showed another pattern—albeit weaker than the others. Their affinities were to the north-west.

So what's the story? And why haven't I mentioned snails yet?

Thank you for reminding me. I can't answer the first question but I can draw comparisons with the land snails. With their distinct northern affinity, the Camaenidae show a similar pattern to the dragonflies. In contrast, Bothriembryon sides with the others and has a southern connection. What makes that genus unusual is that the southern link includes the south-west, something the aquatic animals don't show.

Read more
Davis, J.A., Harrington, S.A. & Friend, J.A. (1993). Invertebrate communities of relict streams in the arid zone: the George Gill Range, central Australia. Australian Journal of Marine and Freshwater Research 44: 483–505.

1 comment:

Anonymous said...

Refugia? No.