Southern Brown Tree Frog, Litoria Ewingi
Frogs play an important part in the food web as both prey and predator and are sentinels of waterway health.The strategy will consider opportunities to improve habitat conditions for our native frog communities in the catchment, through on-ground works and other initiatives that improve habitat for frogs or reduce threats to frog populations e.g.
- Enabling wetting and drying periods for floodplain wetlands to support frog breeding cycles
The community plays a key role in helping build our local knowledge on frogs through Melbourne Water's Frog Census app, a community frog monitoring program.
Frogs are a key value of rivers, streams and wetlands, and are an important part in the food web as both prey and predator, and are sentinels of waterway health.
While many people are unlikely to know what species of frog occur in their local areas, or to recognise frog calls over those of some insects and birds, they consider them to be important. During the recent extended drought, some people – particularly those in rural areas – noted that they missed the sound of frogs calling which led to feelings of depression or anxiety about the future.
Our frog ‘value’ is based on the presence, abundance and variety of frogs (abundance and species richness), which incorporates underlying factors such as the availability of suitable breeding habitat and adequate ongoing breeding success for persistent populations that are resilient and selfsustaining.
Currently we take the position that south-eastern mainland Australian species of frog that arrive through natural expansion (for example, in response to climate change) or even through ‘unintentional’ human assistance might add to the frog species richness of the region (such as the Eastern Dwarf Tree Frog). But other species would not add to our value, especially when they have
the potential to become over-abundant and/or pest animals (for example, the Cane Toad and the Asian Black-spined Toad).
Regional frog data comprise records from the Victorian Biodiversity Atlas, Melbourne Water’s Frog Census, and records of consultants and academics.
The status score is measured by considering the variety of frog species present, compared to the species we would expect to find in an undisturbed area.
Frog species recorded include threatened species such as the Growling Grass Frog and the Southern Toadlet. Frogs across much of the catchment have been impacted by spreading urbanisation, land use intensification, introduced predators, and deteriorating water quality. The condition of frogs in the Lower Dandenong Creek sub-catchment has been assessed as low, with only seven of the expected 14 species being present. However the condition of frogs in the Middle Dandenong Creek sub-catchment is assessed as moderate with nine of the expected 14 species being present.
Scores are likely to improve in the higher parts of the catchment; however, a lack of survey data means that an assessment cannot be made in some of these management units.
Frog species recorded include threatened species such as the Growling Grass Frog, Bibron’s Toadlet and the Southern Toadlet. Deep Creek Lower sub-catchment offers some of the best habitat for frogs in the regions, with 9 of the 13 expected species observed between 2012 and 2017. Overall, the status score for the Maribyrnong catchment is low, which means that across much of the catchment only half of the expected species are to be found today. Despite the success of a new Frog Census app, frog data are sparse and too few to generate frog condition scores for 6 of the 9 management units in the Maribyrnong catchment.
Frog species occurring include threatened species such as the Growling Grass Frog, Bibron’s Toadlet and the Southern Toadlet; although neither of the two toadlet species have been recorded in the catchment since the Millennium Drought.
Our frog condition metric is derived from the proportion of expected species actually recorded over the assessment period (2012 to 2017). The expected species list varies between sub-catchments, from 11 species for the Werribee lowlands to 13 species expected in the upper reaches of the Werribee and Lerderderg. However, frog data are sparse and too few to generate frog conditions scores for 7 of the 12 sub-catchments in the Werribee catchment. Of the five sub-catchments for which we can estimate condition, two have ‘high’ frog condition scores (the Lerderderg and Middle Werribee) and three are ranked as ‘moderate’.
There are up to 14 species of frogs expected to occur across the catchment. This includes threatened species such as the Growling Grass Frog and the Southern Toadlet, with more common species such as Southern brown tree frog, Striped marsh frog, Eastern Banjo Frog, Spotted Marsh Frog and Common Froglet.
The Mornington Peninsula Western Creeks and Bunyip River Lower sub-catchments have high scores for frogs, and the Cardinia, Toomuc, Deep and Ararat Creeks sub-catchment has a moderate score. However, there are insufficient observations in the other eight sub-catchments to generate assessment scores.
Frog species recorded include threatened species such as the Growling Grass Frog and the Brown Toadlet. Fifteen species of frogs are expected to occur in the catchment.
However, and despite the success of the new Frog Census ‘app’, data are sparse and too few to generate frog conditions scores for almost three quarters of the sub-catchments. Of the sub-catchments for which we can estimate condition, one has a low frog score (Darebin Rural), two a moderate score (Woori Yallock and Yarra Middle and Lower) and three a high score (Diamond Rural, Plenty Source, Plenty Rural and Lower). Two threatened species frog, Bibron’s Toadlet (Endangered in Victoria) and Southern Toadlet (Vulnerable in Victoria) have seemingly disappeared from several areas in the catchment since the Millennium Drought.
- Lack of native vegetation along side streams and around wetlands
Native vegetation is vital for providing habitat, and for supporting different stages of lifecycles.
- Altered water regimes in waterways
High flow rates can wash tadpoles out of wetlands, and negatively change the physical wetland environment that acts as protection and habitat.
- Human disturbance
Noise and artificial light can disrupt breeding behaviour of frogs.
- Introduced pests
Species such as introduced fish, foxes and cats can prey on eggs, larvae or adult frogs.
- Water quality
Frogs are sensitive to poor water quality and pollution.
- Loss of connectivity
Floodplains and wetlands that have wet and dry periods are critical for breeding of some frog species. When these areas are disconnected from waterways, frog populations tend to decline. Proximity and access to alternative wetlands is critical to healthy population and life cycle dynamics.
A major threat to frog populations is the disease Chytridiomycosis. This infectious disease is caused by a fungus and is now found in frogs throughout much of the world, including Australia. The fungus attacks the skin of the frog and also damages the nervous system, resulting in death.
Potential management actions
- Protect, improve and maintain habitat quality
Undertake works in and around waterways and establish new vegetation zones, place rocks and/or woody debris around waterways, vary water levels to facilitate diversity and complexity of vegetation structure.
- Wetland improvement and creation
Improve the design of existing constructed wetlands and create new wetlands that are positioned in clusters or are large enough to increase habitat for frogs.
- Reduce populations of introduced pests
Protect frogs from predation by introduced fish.
- Improve water quality in floodplain wetlands
Manage inputs into waterways such as sediment and stormwater.
- Reduce the impacts of human disturbance
Install road underpasses along new roads proposed for construction near wetlands and waterways.
- Improve connectivity
Link breeding habitat by creating vegetation corridors along drainage lines.