James Crants wrote on 11-May-10 13:05:
There's a difference between saying that two species are not ecologically equivalent and saying that two categories of species are not ecologically equivalent.
But, ecological "equivalents" are not really "equal" in such a way that they are substitutable in a community. I mean, you can't just say, take a Clay-colored Robin from Panama and replace the American Robin (even though they might be considered ecological equivalents) and then expect their roles to just fit right in in their new places.
If exotic species (as a category) were ecologically equivalent to native ones, you would still find that every species would differ from every other species by at least a few measures. I'm saying that, as a category, exotic species are ecologically different from native ones.
Now do you mean "until they are naturalized"? After all, take the House Sparrow, that has now crossed the continent and invaded many places in the Americas. Is it still ecologically different from natives?
I would suggest that if you took both native and introduced species, and did a blind study, in which you looked at survival, interactions and so on, you would not get a clear cut difference in ecological characters that would identify (say, through a discriminant function analysis) introduced and native species. Take the persimmons I have here in my yard here in southern Brazil. Clearly introduced from Japan (I will eliminate them once I have a native fruit tree to replace them with), but they attract leaf-cutter ants to consume leaves, bees and other insects visit the flowers, all kinds of animals eat the fruits, and they seeds are quite viable and the plant could easily become invasive and probably is in many places. If you took a native plant here, like the Scheflera (Didymopanax) and checked it out, you would find that, as a sapling, it cannot handle our cold winters (frost burns every year), it gets hit by aphids so badly that it is often worse than the frost, and the leaf cutter ants also nail it. In the same time my one native sapling has remained at the same size (short, < 1 m tall), a persimmon has grown from a seed and is now producing fruit and is about 3 m tall. The Scheflera is at least 9 years old, while the persimmon is about 3. I would suggest that through any objective measurements by a naive observer, they would think that the Scheflera was NOT native and that the Persimmon was.
So, my point is, that using objective measurements, I think we would not find that there are clear distrinctions between native and introduced organisms. We may find certain kinds of trends, but the errors associated with using those trends as guides to recognize native or introduced organisms will be large and so not very useful.
Cheers, Jim
