According to Wikipedia, "Marsupial success over placental mammals in Australia has been attributed to their comparatively low metabolic rate, a trait which would prove helpful given Australia's characteristic low fertility and aridity."

If this is the case, then why have introduced placentals like cats and rabbits caused such havok in Australia? Or have they?

I am going to demonstrate my ignorance here! I thought (probably wrongly) that marsupials developed in isolation in Australia in parallel to (but obviously less advanced than) mammals. If that is the case then surely it is not that marsupials had success over placental mammals, rather they evolved to best fit the niche within Australia at that time.

The wikipedia article cites Richard Dawkins's The Ancestor's Tale for the statement 'in Australia, marsupials displaced any placental mammals entirely, and have since dominated the Australian ecosystem', which I think is a strange source for this. Dawkins actually writes that other mammals in Australia died out early (with specific reference to a couple of teeth attributed to an extinct group of placentals found in Australia, but nothing younger than 55 million years). There is a difference between 'placentals died out' and 'placentals were displaced'. And there is no source cited by the Wikipedia article for the metabolic reasons.

Australia has been isolated for a long time, in which marsupials and monotremes had practically no competition from placentals. Bats and some rodents did get into Australia more recently but the great majority of placental mammals came with the first human settlers; after which placental mammals have been wreaking havoc. South America was also a haven for marsupials until it joined up with North America and placental mammals swooped in from the north, the so called Great American Interchange. During this exchange, northern placentals moving south was more prominent than southern marsupials moving north (the few exceptions being the N. American opossum). And in the south, placentals pretty much took over the ecosystem.

As much as us Aussies would like to claim marsupials as our own (like pavlovas, vegemite, the refrigerator, rotary clothes line, man’s first powered flight (I kid you not, but he was an Englishman!), etc, etc, etc) as Manabu says marsupials are found in other places such as Indonesia, New Guinea and the Americas. It would be interesting to know whether they have evolved in parallel in these places (or if they have evolved much at all)? And to further Manabu’s comment about placentals reeking havoc after their introduction, kangaroos at least have been more than holding their own over the last 20 years or so - and they have only been on the ‘menu’ fairly recently for most Aussies! My feeling was that many of Australian ‘rare’ marsupials have been more subject to man-made environmental pressures than from ‘feral’ or ‘natural’ placentals. What I didn’t know was that according to Wikipedia marsupials (and monotremes) lack a ‘gross communication (corpus callosum)’ between the right and left brain hemispheres - that’s got to mean something in an evolutionary sense?

Last edited by Steve Lolait (29th Nov 2010 23:57:39)

Regarding the introduced cats etc, you can probably put the damage they cause down to more general principles of invasive species, rather then a specific adavantage over marsupials. Where a new animal (like a cat or rabbit in Australia and New Zealand) is introduced into an ecosystem and proves to be "invasive", that ecosystem is pushed away from its previous balance point. For example, in the absense of ground-based predators in New Zealand a whole slew of flightless birds have evolved. With the introduction of rats, cats and mustelids, those niches have been basically wiped clean of birds on the mainland. That isn't because the mammal predators are in some way better or more efficient then birds in general, it just means that the ecosystem has been altered and bird-occupied niches have contracted or disppeared entirely.