Listening to the lovely sound of the bell miner, who could have thought that it could devastate our eucalypt forests? By itself it can’t, of course. All it does is eat honey. But it is particularly fond of a sugary excretion produced by sap-sucking native insects called psyllids, which feed on gum leaves.
These insects sit on the vein of tree leaves, sucking their sap and excreting a sugary solution known as lerp. “The bell miners suck off the lerp, but leave the insect, which keeps working its way around the leaf,” says Paul Meek, executive officer of the bell miner associated dieback (BMAD) working group. In return for their sugary meal, the aggressive bell miners ward off any other bird or insects that would eat the psyllid itself.
The forest can go from looking like this (image from the New Scientist article):
to this (from a 2006 SMH article):
It seems that in New South Wales alone, up to 2.5 million hectares of forest could be affected, and the phenomenon is not limited to NSW.
However, according to the article:
Vic Jurskis of the Institute of Foresters of Australia says tree foliage can start looking sick before any sign of psyllids or bell miner outbreaks – or, indeed, any other obvious factor that might kill the trees. So it looks like the problem runs deeper than bell miners.
The article goes on to say:
The mystery has revived a radical old theory, proposed over 40 years ago by ecologist William Jackson of the University of Tasmania. According to one of his former students, David Bowman, an ecologist also at the University of Tasmania, Jackson presented his work to the Ecological Society of Australia in 1968. Bushfires had ripped through his home town of Hobart, Tasmania, in 1967, prompting questions over frequency of natural fires.
Jackson proposed that the number of fires sweeping through the bush dictate what type of vegetation grows there. Very frequent fires favour grassland, an intermediate number would support eucalyptus forest, while infrequent fires produce a forest of dense, shade-tolerant plants, an ecosystem dubbed the “dry rainforest” for its structural similarity to the classical tropical rainforest. So, according to Jackson’s theory, without regular bushfires, many eucalyptus forests will die out, leaving species-poor dry rainforests devoid of gums.
There are many reasons why gums rely on fire. In the absence of regular fires, dry rainforest species grow up beneath the eucalyptus canopy, competing with the gums for water – and winning. These species also contribute a thick litter layer that changes the soil chemistry, further stressing the gums.
The problem goes beyond eucalypts and beyond Australia. The need is to establish the optimum fire regime, not too much and not too little, for a particular species and climate. The article gives this map of Australia showing where fire regimes need to be managed to maintain forest type and biological diversity:
There is much to be done.