The local paper reported an Osprey pair nesting on an oyster lease. First clutch unsuccessful - flood drowned them. 2nd clutch successful.

Think I recall the Eastern Barred Bandicoot was thought extinct till a colony was found living in old cars in a junkyard in SA.

As for importance - its a complex argument. In essence for fauna, I guess if something artificial is supplementing a natural scarcity or simply lack of a key habitat component (eg cavities under bridges where micro-bats roost), then its significant. Several studies have shown where placement of specific replacement habitat components have enhanced biodiversity from laying fence posts in a grassy woodland to replace fallen logs, to everyone's favorite - nest boxes where hollows don't exist. However, where the artificial component is suppressing say development of key habitat components, like lantana, depending on the species, its obviously going to be a positive or negative.

Finally, I have read the Green-Thighed Frog and Common Planigale have been recorded sheltering in the dense leaf litter of lantana.

I have followed some similar studies to the one Bob Moffat has posted. During the late 1990's we headed into mouse plagues in several places in NSW. One landholder in the Southern Tablelands asked me how to encourage raptors back into bare cereal paddocks. One paddock,about 300ha, was his main concern because of a late cereal crop just emerging. I suggested that as Telstra were replacing line poles in the area, he might try to obtain some, and, if the paddock was rectangular, to place six poles, with crosstrees, at each corner and two at the centre edge of the longer boundary. The owner phoned in great excitement, a couple of months later, that two pairs of Ospreys had actually built nests on two poles situated diagonally across the longer axis of the paddock. I mentioned that many raptors like a "base station" to begin quatering an area and if this absent, they may not return to a previous hunting ground.  The owner passed this information on to two other interested landholders and they also followed the same practice with old poles.

>10% of nests (n118) of Ospreys Pandion haliaetus in NSW (a listed vulnerable species in TSC Act 1995) were located on artificial structures in 2003.  Artificial structures incl transmission towers, power poles, bridges and purpose-built  poles with nesting cradles.  Nests on artificial structures generally produce more fledged young per nest cf natural nests (usually in dead trees) as the former are more structurally more robust thus better survive storm events. It is not surprising that the state Osprey population is increasing (from ~ 10 nests in c1980) with a major contribution from nests on artificial (incl purpose-built) structures.

I think the general rule is that weed infestation (e.g. lantana) and dumped rubbish do provide additional habitat for a small number of native species. However, there are many more species that can't adapt to these degraded habitat conditions.Tim Low's book The New Nature: Winners and Losers in Wild Australia (2002, Penguin Books Australia, Camberwell) provides many examples of species that have adapted or not adapted to living in modified urban and rural environments. It is well worth a read.

One of my favourite examples from the book related to a critically endangered plant species in Tasmania (can't remember the species name). It was thought to be extinct, but several individual plants were subsequently found in a paddock containing grazing horses. The immediate conservation response was to remove the horses so that they did not graze on the endangered plants. The longer-term effect of removing the horses was the extinction of the critically endangered plant population because it was unable to compete with weeds and introduced grasses for space and resources.

The Red-browed Firetail (Neochmia temporalis) appears to have extended its range considerably in the last few decades, partly as a result of the invasion of exotic grasses into woodlands and roadside reserves in south-eastern Australia. This species is able to feed on the seeds of both exotic and native grasses and herbs. In contrast, the Diamond Firetail (Staganopleura guttata) is more dependent on the seeds of native grasses and herbs as sources of food. Therefore, this latter species is declining, partly because exotic grasses are invading its foraging habitat, as well as increased competition from Red-browed Firetails who move into Diamond Firetail habitat along with the exotic grasses. So here we have two closely related native finch species - one a winner (Red-browed Firetail), the other a loser (Diamond Firetail) partly because of the spread of exotic grasses. I'm currently involved in some long-term roadside woodland bird surveys along the Hume Highway (between Albury and the Wagga Wagga turnoff) and the different fortunes of these two finch species are quite obvious in that area.  

It depends on the critter I think.

Without styrofoam cups and building rubble I doubt there would be a Cumberland Plain Snail remaining in western Sydney. Just cleaning up a site can have a big impact on this species.

But it is hard to recommend the retention of garbage. Almost as hard as explaining to a client the importance of a snail.

And in the plant world there are the cases of those opportunists that come in on fill (some orchids and Acacia pubescens for example). What do people do with them?

I recently attended the Royal Zoological Society annual conference. A short discussion about the use of artificial / non-natural habitat took place. For example: old dumped cars and other rubbish such as currogated iron creating suitable habitat for reptiles, Lantana giving small birds shelter from predators and the like. I would be interested to hear some other peoples point of view about how much weight such 'habitat' should be given during the assessment process.