Conserving local frog diversity
Croydon, like most suburban areas of Melbourne is faced with the conundrum: people or wildlife. The two are often portrayed as mutually exclusive and to much of our wildlife this is often the case. Frogs however can be an exception; with public support and careful planning there is no reason why species diversity can not be maintained even in the suburbs of a large city like Melbourne.
Enter the friends and committee of Warrien Reserve located on Warrien Road, Croydon, Victoria. With support from local council, friends of the reserve have constructed new breeding habitat especially designed to support locally rare frog species. Historically Warrien Reserve supported a relatively diverse frog community comprising at least 5 of the 10 species listed for the region (see below) and including two locally rare species the Southern Brood Frog and Victorian Smooth Froglet. (Picture right © Warrien Management Committee)
Widely distributed and abundant in the region
Crinia signifera (Common Froglet)
Litoria ewingii (Southern Brown Tree Frog)
Limnodynastes dumerilii (Pobblebonk Frog)
Limnodynastes tasmaniensis (Spotted Marsh Frog)
Restricted distribution and locally rare in the region
Limnodynastes peronii (Striped Marsh Frog)
Geocrinia victoriana (Victorian Smooth Froglet)
Litoria verreauxii (Whistling Tree Frog)
Restricted distribution but locally abundant in the region
Litoria peronii (Peron's Tree Frog)
Locally extinct in the region
Pseudophryne semimarmorata (Southern Brood Frog)
Litoria raniformis (Growling Grass Frog)
Over a ten year period, pollution, weed invasion and alteration to drainage systems within Warrien Reserve has seen the demise of Pseudophryne semimarmorata and Litoria raniformis within the reserve. Pollution flowing down the storm water drains from near by residences enters the reserve system and eventually ends up in the frog breeding ponds.
(Picture right - Sediment trap constructed to capture polluted run-off water © Craig Cleeland)
June of 2000 was the last time calling individuals of the Southern Brood Frog where noted in the reserve. This was also the last known population of this species in the City of Maroondah.
(Picture right - A male Pseudophryne semimarmorata (Southern Brood Frog) on an egg nest © Craig Cleeland)
The brightly coloured ventral surface of the frog warns predators about the toxins produced by this species. Females deposit 100 to 200 eggs in nests excavated by the males. These nest sites are usually depressions or seepage lines that will fill up with winter rains allowing the tadpoles to hatch and continue their development. Unfortunately habitat loss and the movement of storm water along seepage lines in urban areas have contributed greatly to the demise of this species in built-up areas. (Picture right © Craig Cleeland)
Similarly, Geocrinia victoriana ( Victorian Smooth Froglet) had suffered considerable losses and in 2000 and 2001 and were represented by less than 5 calling males. Like the Southern Brood Frog, eggs are terrestrial, and spawned in moist leaf litter or in the bases of grass tussocks in areas that will later be flooded.
(Picture right - Victorian Smooth Froglet sits on top of a fresh clump of spawn deposited in the base of a sedge. Breeding occurs in autumn and clumps of spawn numbering somewhere between 90 and 160 eggs are deposited © Craig Cleeland)
The Mission Begins
Keen not to let the Victorian Smooth Froglet become locally extinct like the Southern Brood Frog the committee and friends of Warrien Reserve joined forces with the Maroondah City Council to build the species a brand new breeding habitat specially designed to meet the needs of this species. In particular the sited need to: only hold water over winter / spring then dry up over summer;be shallow enough in places to allow the pond water to warm up in the winter sun; have a northerly aspect free from shading trees to allow maximum winter light levels;have logs and sedges suitable for spawning sites;have a water supply free from polluted run-off.
Construction
A natural depression within the reserve system was selected that was not part of the main drainage system and hence free from contamination from storm water entering the reserve. Earth works were then carried out to excavate the depression and clay was then transported into the site and compacted to give the pond its seasonal water holding characteristics. (Picture right © Craig Cleeland)
About 4 species of locally indigenous species of plants were recommended for the project and over 100 individual plants were used in the reconstruction. Planting days spanned over several years to achieve the desired effect. (Picture right © Warrien Management Committee)
Large logs and rocks were brought into the site and positioned so that when the pond is dry they can become useful spawning sites.
(Picture right © Craig Cleeland)
Results
The first rains confirmed the integrity of the structure and numerous other frog species moved into the site to spawn. In fact this pond has become the major breeding pond for the Southern Brown Tree Frog in this area.
(Picture right © Craig Cleeland)
However the Victorian Smooth Froglets remained elusive and in 2002 & 2003 only one individual could be found and it seemed that we may have been to late to save this species. However 2004 brought much better news with two males calling from the pond and an egg mass uncovered. There is still a long way to go and issues such as inbreeding and the influence of random events are still major hurdles to overcome.