More information on the Common Starling which I forgot to mention in my previous comment...

The arid and semi-arid zones of southern Australia appear to have been a relatively effective barrier in preventing the movement of Common Starlings from South Australia into south-western WA. However, cage traps containing decoy birds at Eyre Bird Observatory (about 250 km west of the WA/SA border) trap progessively more Common Starlings each year. Moreover, at least two Common Starlings have been observed in Alice Springs in recent times (one as recently as September 2009), suggesting that this species is becoming more adapted to surviving Australian semi-arid and arid conditions.  What will happen once the deserts of central and southern Australia no longer act as a significant barrier to the westward movement of Common Starlings?

Brian, I think one needs to be careful about distinguishing between introduced species that are "settled in their new ecosystems" and those wreaking havoc. There are numerous examples globally of introduced species being "dormant" in their new environments, only to become pests at a later stage.

A good example is the Common Starling in eastern Australia. It was released deliberately in eastern Australia in the 1850s & 1860s, primarily to control insect pests in agricultural regions, but it wasn't until the mid- to late-1910s (in NSW & Victoria) and the 1920s/30s (in Qld) did signiifcantly population explosions occur.

In my earlier example, it is important to establish whether Aldrovanda vesiculosa is native to Australia or introduced. If the latter, is there a danger that a future environmental trigger could lead it to choking aquatic environments much like Salvinia or Alligator Weed?

Interestingly, I heard a couple of years ago that Cotton had "escaped" plantations in northern Australia and had significantly infested some waterways and wetlands in Kakadu NP. Who would have thought this would happen?

In my last post I was echoing Derek’s suggestion that, at least from the perspective of conservation biology, the most useful distinction is not between “native” and “non-native” species, but between species that are “settled” in their ecosystems and those that are wreaking havoc.

That is a good point Elizabeth, though it relates more to whether a species is in the early stages of invasion (or colonisation) or not, rather than whether a species is “native” as currently defined in Australia.  I do think the definition of “native” as existing here prior to 1750 (or whenever) is ecologically meaningless.  Perhaps “old” or “new” would be better terms.           

Actually they have very important ecological implications. An introduced species can behave quite differently in its new home compared with its native state. Studies of such differences can shed light on ecological processes for the species and the ecosystems as well as on the drivers of population dynamics - essential information for conservation management.

Stephen's example is also interesting, because knowing whether it is a relic species or one hitching a ride on migrating birds it may tell us which direction the species is heading - expanding or contracting - and so help in assigning scarce conservation dollars.

In my opinion, the words "introduced" and "native" are ecologically meaningless.  Who cares when a species arrived here?  Australia's history of northwards drift is a history of successive biological "invasions", and I would guess that most of the "native" species here today are invaders or the descendants of invaders.  The trouble is, people often use "native" to mean "good", and "introduced" to mean "bad".  The underlying supposition is that "nature" is unchanging; that the situation in Australia in, say, 1750, is how Australia was "meant" to be; that divergences from this situation are bad and that, as conservationists, our goal is to try and return the land to how it was back then.  But I don't think there's any scientific rationale for this way of thinking.

Another conundrum arises when you are not sure if a species is native to Australia and has been introduced to overseas countries or vice versa, or is a cosmopolitan species.

A case in point is the carnivorous freshwater aquatic plant, Aldovandra vesiculosa (a type of aquatic trigger plant), a species that is rare and is globally threatened.  It is found only in a few wetlands in Australia, wetlands along the east coast, one wetland in Armidale NSW, a few wetlands in northern Australia and another in Esperance (south-western Australia). The same species also occurs in isolated pockets in southern and eastern Europe, northern Africa, east Asia (including Japan, China, south-east Asia, Indonesia) and New Guinea.

So have shorebirds (waders, freshwater terns, etc), gallinules (swamphens, moorhens, coots, etc), herons, egrets ibses or passerines bathing/drinking in wetlands been responsible for introducing this plant to different continents that are along their migratory pathways, or did the plant species once have a more widespread global distribution, but now occurs as isolated global remnants? If birds have been vectors involved in the dispersal of the plant species across continents, where did the plant originate and where was it introduced?

If Aldovandra vesiculosa was introduced in to Australia, and was introduced naturally by migrating birds or by other means, it would have to be regarded as a species that is native to Australia. After all, Cattle Egrets and Sacred Ibis dispersed naturally into Australia in the late 1970s as a result of global expansions of their ranges, and both are now considered species that are native to Australia.

Incidentally, a genetic study of global populations of Aldovandra vesiculosa will be the subject of an honours project at the University of WA next year. The results of the study may shed some light onto the evolutionary biology of this species and perhaps pathways of dispersal between continents.

quite a paradox Deryk. I would argue that the best approach is battlefield triage, ie address these species where they are a problem ie a threatening process, and leave where they are "unnatural" but not a problem (eg filling a vacant niche such as the House Mouse in the Kite's diet) for the time where we have the luxury of resources and time (ie 30% of our biodiversity is NOT likely to be extinct by 2050).

If you look up Dingo on the DECCW website you get "The dingo is Australia's wild dog. As the largest native carnivorous mammal in the country, it is a magnificent animal in its natural habitat and plays a vital role in maintaining the balance within ecosystems. The dingo's origin is uncertain, though scientists now believe that it is related to Asian and Middle Eastern wolves that probably arrived in Australia between 3,500 and 4,000 years ago". Here is recognition that an introduced animal (one that arrived in Australia) "plays a vital role in maintaining ecosystem balance". So for this species, to go from introduced to "native" the line in the sand is drawn at 3500 years. But what about clumps of blackberry or lantana that occur in a cleared paddocks? These, in the absence of native plants, provide sheltering and breeding sites for native species such as wrens and finches. Removal of these introduced plants (as required by the Noxious Weeds Act) would cause the local extinction of these native species. Therefore, does and introduced species become "native" once, again like the Dingo, it provides a role in maintaining the ecosystem balance? Some introduced plants are all that’s left in some ecosystems, native animals becoming adaptable to the resources these provide. The root systems of these plants also provide a function is soil stability. Native animals are adaptable to the presence of introduced species (particularly in the absence of native “equivalents”), for example select species of birds that have learnt to turn cane toads over (and feed on their stomachs) thereby avoiding their poison sacks. Also, for example, a study looking at the diet of Black-shouldered Kites in the Liverpool Local Government Area (NSW) and identified that the introduced Mouse made up around 98% of this species dietary intake.  I just throw it out there for discussion. We channel vast sums of money into pest control and weed eradication but are our actions actually having a negative effect on maintaining the balance of ecosystems, by removing resources native species have adjusted their life cycle patterns to utilise? Could that money be better spent on other ecological issues such as the purchase of land and inclusion of that into the conservation reserve network, or research into issues such as the treatment of Tasmanian Devil mouth ulcers. Maybe, just for argument sake, we should say "okay X,Y and Z species have been around long enough and are fulfilling an ecological role, therefore these animals/plants etc are now considered native and were not going to waste any more money on them". Maybe that’s the definition/yard stick we should be using, if a species is fulfilling an ecological role and maintaining/adding to, the ecosystem balance, then its native. Thoughts?