7 June 1999

Beetles shed new light on rainforests

The odd-shaped genitals of dung beetles are giving scientists a remarkable new insight into the deep history of the Australian rainforest - and, potentially, a new way to care for it in future.

Researchers at Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) have discovered the rich variety of dung beetles that inhabit the rainforest offers a new picture of the ebb and flow of the forests over the last 5 million years.

"Scientists and explorers have been collecting dung beetles from the rainforest for over 150 years. Together, these collections give us a remarkable insight into the way the forest has changed over millions of years," explains Dr Peter Cranston.

Dung beetles may evoke less public admiration than some rainforest species, but their diversity is proving a great boon to scientists trying to piece together the forest's behaviour over time - and predict its future under climate change and human impacts.

Studying different species of dung beetles and where they occur, enables researchers to spot breaks in continuity that show how the forest came and went over millions of years.

Dung beetles play a vital role as recyclers, processing not only dung from birds and marsupials but also fruit, fungi and rotting vegetation back into fertile soil.

They are also among Australia's most ancient inhabitants, with their ancestral tree reaching back 100 million years to the time when it was joined to the other southern continents as Gondwana, and the Cretaceous period (145-65 million years ago).

"They are exceptionaly sensitive indicators of the forest's ebb and flow. Flightless dung beetles can linger on in patches of rainforest too small to sustain larger animals like marsupials.

"Conversely, when the forest returns, flighted species of beetles readily recolonise regrowth areas," Dr Cranston explains.

But telling one species of dung beetle from over 150 others is not easy - and that is where the genitalia come in. A dung beetle's genitals are as useful as a fingerprint for identification purposes, says Dr Chris Reid of the Australian Museum.

"The shape of the male beetle's penis is quite distinctive - but so too are the female genitalia. In some species she has a unique locking mechanism that keeps the male in place. It's also a way of making sure she can identify the right mate.

"I can tell one species from another by examining the intricate shape of this mechanism," he explains.

"In all, we've got about 40,000 dung beetle specimens from Australian rainforests in various collections, which is unrivalled in the world - a tremendous resource on which to construct a picture of the ancient and recent rainforests."

Dung beetles are also easily acquired, so if there is a gap in the picture it can easily be filled. To trap the beetles, scientists use the most basic form of bait - the researchers' own droppings.

From the evidence accumulated particularly by keen collectors over the last 30 years, researchers can see how dung beetles divided into new species as their habitats changed and geographic obstacles like river valleys separated the populations.

"There is so much data, so many species, and so many specimens we can be very confident of matching the patterns of speciation to the geography and interpreting what has gone on," Dr Reid says.

"We use the beetles as a very fine-scale tool to explore the significance of particular pockets of the rainforest to the landscape.

"We can use them to understand the forest's relative age and historical value in the landscape, assess its biodiversity, monitor forest health, and see whether restoration work is succeeding or failing."