St Bees Island July 2007
A young male koala from St Bees Island.
A young male was pursued to the outer branches of a 20 metre gum tree by a larger, mature male koala.
We watched the young male clinging to the outer-most branches until it was dark.
The mature male established itself in a way that blocked any escape by the younger male. It adopted an erect posture and fed actively.
At one stage the young male attempted to crawl along the underside of the branch but was driven back. Next morning both animals were gone and not seen again during the trip.
July 2007
St Bees Island field visit
Eleven volunteers joined me on St Bees Island this winter. They came from Australia, England and the United States of America. We arrived by boat in calm weather – thank goodness. Our routine task was to spend 12 days monitoring koala movements and tree use across the island. This adds to the nine years of accumulated data on this population. Mid trip we were joined by my partner Rhonda Melzer (in her capacity as Queensland Parks and Wildlife Service officer) and a member of her team, John Clarke. Some of us were to work with them for a week monitoring vegetation and installing environmental monitoring equipment to track climatic influences on plant growth. This is part of our collaborative study of the impact of goat browsing and climate change on island ecosystems and koala habitat. With them came colleague Dr William Ellis and Dr Sean Fitzgibbon. With the assistance of the volunteers they worked to catch selected koalas and remove and replace radio tracking collars.
The weather was fine and clear for the first week but a fog descended on the island for the last week providing a constant cool cloud cover with occasional light showers.
The trip ran smoothly and all our objectives were met. The team sighted 55 individual koalas, collected 128 day records of tree use and 32 night records. They measured 360 trees and collected six skeletons of koalas that had died since our last visit. Six koalas were caught and either recollared or had their collars removed.
Our observations of tree use showed that the koalas used a variety of tree species during the day but at night used predominantly their principal fodder species – Forest Redgum (Eucalyptus tereticornis).
The radio tracking data will take sometime to analyse.
The skeletons will be cleaned and used to study aspects of koala longevity and aspects of disease syndromes influencing survivorship. We don’t know how long koalas on the island live or what diseases are impacting on their lifespan.
Climate change and goats
Despite us having an enjoyable and productive field trip the island was in a poor state. Since October 2006 the island ground cover has collapsed. This was most evident on the exposed hill slopes. When we first visited the island in 1999 these slopes supported a dense cover of native grasses (mainly Imperata cylindrica and Aristida spp.). Commonly the grass was over a metre high or formed dense swards that covered rocks and concealed hollows, vertical drop-offs and loose rocks. Now these hillside grasslands have been replaced by sparse to dense shrublands and sparse herblands with extensive areas of bare earth. Previously almost impenetrable native shrublands had disappeared to be replaced by fields of dead timber and sparse herbs. Even the robust Xanthorrhoea latifolia was reduced to well prune remnants in many areas.
This ecological collapse was after record summer and winter rains. Goats are the main suspects. There are now hundreds if not thousands of goats across the island and we know that they will eat many species on the island. Over the last few years we have documented goats eating everything from rainforest trees and ferns, eucalypt woodland species, grassland species and mangroves. In the seasonally dry spring they are commonly seen searching for freshwater soaks along the high tide line. In particularly severely dry seasons they are seen to drink seawater. The impact of the goats has been exacerbated by an ongoing drought. This drought has affected most of eastern Australia including St Bee Island. We believe that the combination of the drought and the goat browsing has pushed the island vegetation over a threshold. The Queensland Parks and Wildlife Service is planning to cull the goats this year (October 2007). Has the cull been left too late? What will be the nature and direction of the post goat succession in the ground cover? We know that the main koala fodder tree species is not regenerating. Is this due to browsing by goats? We’ll be working with the Queensland Parks and Wildlife Service to monitor the recovery process.
Some trees are dieing on the island. These include the koala fodder species (E. tereticornis) and the non-fodder species (Corymbia intermedia). Because the Corymbia is dieing along with the E. tereticornis we are confident that the dieback is probably associated with the influence of long-term drought – not over browsing by koalas.
Like much of eastern Australia, the St Bees Island climate is slowly changing – increasingly less humid with more extended and more intense dry seasons. Plant communities are changing. After 10 years research we think we have some sense of the koalas strategies for dealing with a drying environment. If the drying trend continues we expect that the koala will disappear from many localities across its range in Queensland.
Drama in the tree tops
When you live in close proximity to your study animals occasionally the researcher gets an opportunity to observe the private life of the study animal.
During this trip we had a young male koala calling from a large eucalypt behind one of the houses. This male territorial call is a deep throated staccato grunting and low frequency bellowing. Late one afternoon we were attracted to a high pitched chattering. On investigation we observed a large mature male in the same tree as the young male. The young male moved to the outermost branches and assumed what appeared to be a supplicant position clinging almost upside down to the fine branches and foliage. The older male initially followed the younger male to the outer canopy but then retreated to the point where the young male’s branch joined the trunk of the tree. At this point the older males sat upright and commenced feeding in a very overt manner. The next morning both males had departed. The older male had a large and obviously active sternal gland. The younger male had a sternal gland that was just discernable. We assume that we chanced on the exclusion of a young male from the range of a larger mature male. Based on our radio tracking records the young male will be driven across the island as it tries to find some empty habitat within which to establish a range and continue to develop.
Our next field trip is in October – late spring and the peak season for male territorial activity.