November 2007

Tonight I conducted a Melbourne Water Frog Census at the small wetland at the bottom of Mulla Mulla Grasslands (Bush’s Paddock).

A frog census consists simply of recording the calls made by frogs on a small cassette recorder. On the previous Sunday during a BOCA quarterly bird survey it was noticed that the wetland had a significant amount of water in it, with a quantity of emergent aquatic vegetation. It was decided to do a frog census here while there was still water in the dam.

During the week there was significant rain for about a day, & by Thursday there was considerably more water in the dam. As the evening grew darker a couple of Spotted Marsh Frogs (Limnodynastes tasmaniensis) began to call. A few frogs continued to call sporadically for a while.

It was decided to simulate the sound of rainfall to see if this would cause a reaction. Shortly after scooping up water then letting it fall back into the dam, numerous marsh frogs began to call. Before long there was a loud chorus of frogs. Whether this was because of the sound of the falling water, or because they became used to the presence of a visitor, or perhaps it just became dark enough to cause them to call, is not known.

This trick may be tested on another occasion to see if it works again. The only frogs heard here were Spotted Marsh Frogs, but in the distance a few Pobblebonks (Limnodynastes dumerilli) could be heard calling from the Western Water treatment pond, with their loud distinctive “bonk” calls. Unfortunately there were swarms of blood-sucking mosquitoes. There were also masses of tadpoles in the water, of different sizes. Hopefully the water will remain in the dam long enough for most of these to survive to become adult frogs.

A large proportion of the marsh frogs had a different call from other marsh frogs heard in & around Melton. Melton marsh frogs have a single distinctive “tic” call, as do Spotted Marsh Frogs in the rest of southern Victoria. Some marsh frogs in Bush’s Paddock have this expected single “tic” call. But a significant proportion has a “rapid fire” call, a series of 4-5 “tics”, as heard in northern & central Victoria.

The line of demarcation between the two populations may not be as strictly defined as existing records may suggest. These different calls can be heard on Frogs of Australia.

We already have some special frogs in Melton South. These are the Spadefoot Toads found only in the small wetland in Rees Road. To date they have been recorded on the frog census nowhere else in the greater Melbourne area.

It now appears we may have some special frogs in Mulla Mulla Grasslands (Bush’s Paddock) also.

Daryl Akers