Newsgroups: sci.environment


Paul Burnett asks:

>AM>Numerous "poor underdeveloped countries" have postcolonial economies depende
>  >on a few cash crops established during colonial times. The cash crops -- e.g
>  >sugar, coffee, tea, tobacco, pineapples, etc -- have taken a tremendous toll
>  >on native ecosystems -- replacing them entirely over vast tracts.
>
>Somehow, this reminds me that the same holds true for the majority of
>crops grown in _this_ country - tobacco, cotton, corn, sugar, etc.  Do
>you worry about the "vast tracts" of native ecosystems replaced here?

In one word -- yes.

The temperate zone is less biodense than the tropics and has much lower rates
of endemism (geographical restriction of a species' range, usually to region of
origin) so the loss in *species* diversity is higher in the tropics per unit
area destroyed than in the temperate zone. However, there has certainly been 
considerable loss in the temperate zone, especially at the *population* level
of diversity. 

The impact of temperate zone habitat loss is also ongoing:

*) Range restriction in the temperate zone resulting from agricultural 
conversion has pushed many species well below the total numbers needed to 
maintain polymorphism (of *currently* selectively neutral variation) against
genetic drift, reducing their future adaptation potential and possibly their
intrinsic speciation potential. 

*) It has pushed many species and geographical populations down to numbers low 
enough that their chances of persistence against extinction per thousand 
years -- or even per century -- are poor to dismal. 

*) By eliminating many local populations, it has changed the metapopulation 
structure of huge numbers of species, with consequences that conservation 
biologists have only recently begun to study -- but which often seem to induce
augmentation of the rate of loss of overall genetic diversity and further 
reduction of persistance probabilities. Reduction of gene flow and of the
probability of recolonization after a local extinction often occur as a
result of these metapopulation reductions. Some species that were formerly
very widespread are now entirely dependent upon a single geographical 
population for all their future prospects. 

*) Homogenization of temperate zone environments -- reduced spatial diversity --
means reduced gradients of biotic selection factors from locale to locale.
This, especially when coupled with reduced genetic diversity, almost certainly
means a reduction of overall speciation rates. 

	*	*	*	*	*	*	*	*

Habitat loss in the temperate zone is by no means a thing of the past: it is
continuing apace and indeed accelerating. Anyone who has spent a few decades
outside an urban core can remember woods and swamps, meadows and abandoned
fields that are no more, and can perhaps recall animals that are seen no more
in those parts. All that has been replaced by shopping centers, golf courses,
airports, industrial parks...

But then of course -- *we* don't have a population problem in *this* country.
Just ask the many technological supremecists who hang out in this group, 
chatting about their souped-up reactors, occasionally flashing an arithmetic
switchblade for their peers' admiration.

------------
Alan McGowen

"Models are tools for thinkers, not crutches for the thoughtless." -- Michael
Soule.

Xref: sparky sci.environment:14370 sci.energy:6741