Er.. I don't think it works like that. You talk as if there was some sort of inevitability in evolution. I think its all about chance and probability-and some organisms just get lucky:).

Speaking as one member of a herd of about 6,000,000,000 large mammals that evolved in Africa and are a fair way down the path to devastating their entire planet, this one is quite close to home.

Some introduced species just die out. Others seem to have a huge advantage. This might be because the new conditions remove some constraint on them - rainfall, soil quality, predation etc. Moves to control incomers by introducing controlling species are usually failures, remeniscent of the old lady who swallowed a fly.

The Cane Toad is a well-studied example. It was introduced into Puerto Rica (successfully) to control the Cane Beetle which destroyed sugar cane plantations. But in Australia, it seems to have poisooned many of the local species. Further, its population density in Oz is 50-100 times the natural density in its home territory.

Aggressive expansion is probably not a good long-term strategy for a species (sorry: there is no sense of purpose in evolution, but it's a useful short-cut for talking about it). In the long term, any over-successful species will result in an equally widely-spread antidote. And in fact, explosive population growth seems to invoke instability in population through some sort of chaos-theory phenomenon.

I'm a little doubtful about the randomness of evolution filling niches. I heard recently there are 400 distinct species of fig-tree. And, wildly unexpectedly, every one has a unique species of fig wasp that has evolved with it, and fertilization of the fruit can only happen with that specific tree/wasp pairing. That speaks loudly for niches being extremely densely packed.

Sorry - one click too many.

I actually was avoiding this thread because it is far too large for a balanced treatment.  It is closely involved with a wide range of very general evolutionary topics. What is worse, many of the concepts are as much quantitative as qualitative, so it is very difficult to make good predictions, and even more difficult to make long-term predictions "especially about the future".

Consider the NZ ground nesters; they should have generated predators and parasites from opportunistic species should they?  Probably they did to some extent, but it does not follow that such creatures would have  found the opportunities particularly attractive. Remember that at the time of separation NZ did not simply house a job lot of random organisms, but a running ecology. None of the niches for attacking ground nesters was a major long-term winner and ground-nesting birds were probably a humourless lot with big feet and small toleration of visitors. Probably within a matter of centuries rather than millions of years, ground nesting became the most attractive option for most of the large birds. Any organism that tried to exploit the fact would first have a lot of evolving to do in the face of unfavourable selection. The mammals that were imported later had had some 100- 200 million years of evolving in continental sized areas, and you want NZ to match that from under a longish white cloud in a few million? Try again!

PP says:"I'm a little doubtful about the randomness of evolution filling niches. I heard recently there are 400 distinct species of fig-tree. And, wildly unexpectedly, every one has a unique species of fig wasp that has evolved with it, and fertilization of the fruit can only happen with that specific tree/wasp pairing. That speaks loudly for niches being extremely densely packed."

Well, in this case the niches certainly densely packed, in direct negation of the NZ situation I  just mentioned. But think about how it happened. The fig species generally evolved notionally paraphyletically, meaning that they were in some ways prevented from interbreeding, typically by geographical factors. Now, each population, sometimes a mere handful fo trees, was viably interbreeding, or they would not be of interest here because they otherwise would have died out anyway, and this implies that they had resident populations of fig wasps suited to their needs, and those wasps would have evolved and stuck to the trees or they in turn would have died out. After just a few millennia at most, unrelenting selection would have fitted the wasps to their respective figs, not only selective, but effective. 

In this example each niche would have brought its incumbent with it, so the niches would have had 100% density of packing in anticipation, so to speak.

Actually, I am not well informed about fig wasp biology, but I have an idea that some figs have more than one pollinater (but don't quote me on that one) and I am pretty sure that I read somewhere that some of the wasps have parasitoids that exploit them without doing much pollination.

Often invasive species can survive because new niches have been created by humans, who alter habitats and reduce numbers of competitors. Also, the local wildlife has not had time to adapt to the new introductions and the predators and diseases limit their numbers in their native habitat do not exist in their new habitats.