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There are fully documented cases of woody plants and grasses that chemically
inhibit or prevent the growth of competing species. The most common one is
Juglands, a toxin secreted by Black Walnut trees, which kills or seriously
injures many woody plants. In some cases, grasses inhibit their own species,
thereby limiting the population. This is most noticeable with certain bunch
grasses in very dry conditions. 

Whether this occurs in aquatic species is not proven, __so far as I know.__ If
there IS such an effect, the toxin will be clearly identifiable by chemical
analysis, and the affect can then be demonstrated by adding the chemical
substance to water containing the competing plants. The competing plants will
then exhibit unmistakable symptoms of poisoning. 

Anybody have LOTS of money, to subsidize a Graduate Student and provide the
necessary lab equipment, etc?  Darn. That is what I thought.  :-(

Until then, the best we can do is to use proper Scientific analysis, e.g.
planting tanks identically, with single species, lighting identically, etc.
Then, add the suspected competing plants, and record the results. No fair
changing fish load, lighting, water changing, etc. Until then, all we will
have is "anecdotal" evidence, which is not very useful. :-(

If you study George and Karen's posts, you will notice that they DO make very
careful measurements of growth rates, growth conditions, etc., and still
cannot prove some effects. Scientific studies are NOT easy!!


Jean Olson
JOlson8590 at AOL_com
Out in the Boonies, near
Cambridge, Iowa