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Part of the problem with "invasive" and what window of time to use is 
how well a plant and/or animal do in an ecosystem is depending on a 
large number of variables. One of the most important variables is the 
climate. The diansours were naitve but are no longer and if reintroduced 
would not fare well except in maybe a very small area.

The reason for this is climate change. For a specific plant the location 
of favoriable growing conditions is not static by it is dynamic. Climate 
change is both man made and natural. There is significant evidence today 
that the warming and cooling cycle of the earth is correleated with the 
oscillation of the eccentricity of the earth's orbit around the sun. The 
change in eccentricity is very slow and very small but it does exist and 
it does have significant impact on the warming and cooling of the earth, 
which has an impact on the mean temp and rainfall, etc. in a specific 
location.  Part of this oscillation is caused by small changes on the 
gravational field around the earth caused by movement of other bodies. 
Part of it is brought on by changes in the earth itself. For example the 
recent 9 earthquake in the Indian Ocean not only produced a killer 
Tsunami but it cause a slight compaction in the earth which caused a 
minor - but meaurable - increase in the rotation rate. While this effect 
is very small it is causing a slight shift in the eccentricity of the 
earths orbit. This will have a long term effect on world wide climate.

Eccentricity perturbation most likely has a much greater effect than the 
impact of mankind on climate change - but when they reinforce one 
another the climate change can be catastrophic.

Along with physical changes in the earth and the earth's orbit, there 
are the issues of ultraviolent radiation caused by fluctuations in the 
ozone layer. Some of these fluctuations are natural and have been going 
on for a long period of time - some are man made. Varying ultraviolent 
amounts of impact climate.

To some extent the discussion of invasive species is being taken out of 
context. It needs to be taken in context of the dynamics of the 
egosystem which has to be taken in context to the dynamics of the earth 
and man's interaction with the earth. What was native 1000 years ago in 
a specific location may not fare well today. What fares well today may 
not have 1000 years ago. That's not to say we should not be vigilant 
when it comes to invasive species, but must put the discussion into a 
larger context and cannot just pick a window of time and say "this is 
the way it was so it is the way it must be." We can't go backwards in time.

Truman


Karen Sullivan wrote:

> *Thank goodness it's a popular cause....and what is different now is that
> there is a better understanding of
> the damages due to advances in science and better understanding of fragile
> ecosystems. Just becuase exotics were introduced several hundred years ago,
> it's no reason turn our backs on it now...
>
>  


--

"Only two things are infinite, the universe and human stupidity, and I'm 
not sure about the former."  Albert Einstein





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