Why did the ringtail cross the road?

April 2002

Because there’s now a simple and safeway to do so - thanks to the efforts ofRainforest CRC student Nigel Weston.Road kills are an obvious and tragicresult of vehicle-wildlife interactionswhere animal habitats have beenfragmented by roads. However, thereis also concern about the less obviouseffects of roads on rare species that donot readily move at ground level. Thisled Nigel to base his Masters study onthe value of canopy bridges acrossroads in the Wet Tropics.

The rainforest ringtailsCanopy bridges are not a new concept,and have been used to assist themobility of squirrels in Britain, andmonkeys in Kenya, Brazil and Belize.The arboreal animals with most to gainfrom such structures in the WetTropics are definitely the possums, andthe ringtails in particular. Rainforestringtail possums are endemic to theWet Tropics region. Species include thebeautiful green ringtail (Pseudochiropsarcheri), the imposing Herbert Riverringtail (Pseudochirulus herbertensis), itsclose relative the Daintree Riverringtail (P. cinereus), and the acrobaticlemuroid ringtail (Hemibelideuslemuroides). The first three arerestricted to areas generally above 400metres altitude, with the lemuroidfurther restricted to areas above 800metres. Rainforest ringtails are listedas rare under state nature conservationlegislation, and as near threatened by theWorld Conservation Union.

Causes for concernThe effects of fragmentation on ringtailpossums were first documented by DrBill Laurance in a former CRC land-mark study on the Atherton Table-lands in the mid 1990s. This studyfound that in fragmented forest, thenaturally abundant lemuroid ringtaildeclined by more than 97 percentwithin 35-60 years in 40-80 hectarefragments, and within 9 years insmaller fragments.

Fragmentation of habitat poses a veryreal threat to species survival. Clearedland around forest fragments canconstitute a barrier to the introductionof new genes which then leads toinbreeding and eventual extinction ofisolated populations. Dr Laurancefound a patch of over 600 hectares,cleared relatively recently, which was

totally devoid of lemuroid ringtails.The fact that the lemuroid rarelyventures onto the ground, means thateven roads and powerline clearingscan pose this kind of threat to thespecies.

The first bridgeIn 1995, Rupert Russell, a well knownlocal conservationist, installed the firstcanopy bridge in the Wet Tropics overa disused logging track in rainforestnear Lake Tinaroo, with the aid ofFNQEB and WTMA. The bridgeremains in place today, but wasunmonitored until early 2000 whenNigel began work on his study. Usingthis well-established bridge as his firststudy site, he set up infrared camerasand scat nets to determine whichanimals were using the bridge.

Right: The original canopy bridge installednear lake Tinaroo by Rupert Russell in 1995

By mid 2000, he had evidence oflemuroid, Herbert River and greenringtails, melomys and long-tailedpygmy possums having been on thebridge. Spotlighting was then used toconfirm that these animals were actu-ally using the bridge to cross the road.A data logger was also installed on thebridge to count numbers of animalscrossing.

Time to modifyThe design of the initial bridge was likea rope tunnel, to offer some protectionfrom aerial predators. However, in allthe photographic evidence and spot-lighting observations, animals pre-ferred using the top of the structure totraverse the road. Only one animal wasobserved inside the tunnel, but thisseemed mostly an attempt to avoid thedata logger. Armed with this newknowledge, and the support of bothMain Roads Department and Austral-ian Geographic, Nigel set up modifiedbridges at two further sites on theAtherton Tablelands. These newbridges were simplified to resemblesimple rope ladders, a design thatgreatly decreased the costs of bothconstruction and maintenance.

Gaps that need bridging mostThe two new sites were very different.The first re-designed bridge wasinstalled in December 2000 in awell-forested area near Millaa Millaa.This site was surrounded by manynatural connections between treesacross the road and there was alreadya good deal of animal movementthrough this canopy before the bridgewent in. Little activity was observedon this bridge over the followingmonths.

The second site was over a gazettedroad - the Old Palmerston Highway,where no natural connections occurredresulting in little or no animal move-ment across the road via the canopy.At 15 metres, this bridge was twice thelength of the original bridge installedby Rupert. It took a mere five monthsto find conclusive evidence that thenew bridge was being regularly usedby lemuroid and Herbert Riverringtails, as well as the copperybrushtail possum.

Implications for ConservationClearly there is no need for canopybridges over roads which retain goodcanopy coverage. There is, however,good cause for their installation wherethere is no natural connection betweentrees for arboreal animals to traverse aroad or other linear clearing. In Brit-ain, their installation and specificationfor the protection of squirrels is firmlyentrenched and they are now both astandard feature of road constructionand a common sight in the landscapein the north and south where the na-tive red squirrel is still abundant. Theremay be an equally strong case for theirinclusion into roads constructionpolicy in the Wet Tropics to protect andfoster the free movement of our rain-forest ringtail possums and other ar-boreal species.

For further information contact:Nigel Weston, Rainforest CRCPO Box 6811, Cairns, QLD 4870Phone: (07)4042 1223Fax: (07)4042 1247Email: Nigel.Weston@jcu.edu.au

Above: Infrared camera shots of animals crossing the canopy bridge. Centre: Melomys,

Left: an adult Herbert River ringtail and right: a juvenile of the same species

(Camera: Nigel Weston)