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  1. allelopathy
    by Stephen Pushak <teban/> (Mon, 08 Dec 1997)


by Stephen Pushak <teban/>
Date: Mon, 08 Dec 1997

Sorry to dash cold water on you guys plans to gather anecdotal data on
allelopathic competition in plants however I figured you'd want to be
aware of the scientific work which has been done in this area.

Check out the articles by Diana Walstad in TAG issues 8:4, 8:5 and 8:6.
There is an extensive list of reference material in those TAG articles
for those with an interest.

1) allelopathic compounds and interactions are much more common in
terrestrial plants. Water dilutes the chemicals too much for them to
have significant inter species competitive effects in aquaria. 

2) allelopathic effects in water plants are extremely difficult to
measure. Competitive effects and incidental effects due to nutrient
conditions and light are the dominant causal factors for plants not
growing well in aquaria. Plants compete for light and nutrients and
these effects are NOT allelopathic. Any anecdotal observations that
folks may have about growing aquatic plants under hobby conditions are
worthless for drawing conclusions about allelopathic effects.

3) if you still want to study allelopathic effects, you need to set up a
series of double blind experiments where the plants are grown in almost
perfectly identical conditions and then growth rates measured carefully
by weighing dried samples. You need to employ a large number of samples
to avoid drawing incorrect conclusions from statistical scatter caused
by variations in conditions between the tanks.

4) allelopathic chemicals are used primarily in aquatic plants to
prevent the plant from being eaten by animals more than as a measure to
compete with other plants. It is currently thought that some aquatic
plants can inhibit the growth of unicellular algae however scientists do
not all agree on this subject. Critics of the allelopathic theories
suggest that these are in fact, competitive phenomena.

5) there is an enormous diversity of organic chemical compounds which
accumulate in aquariums from the decomposition of plant material. You
can really increase the concentration of these chemicals if you use peat
in your substrate or filter. These DOC (dissolved organic compounds)
have a variety of effects: moderating pH, inhibiting the growth of
bacteria, algae, plants and animals. Mostly these effects are positive
for our desirable aquarium inhabitants. There are a few plants such as
Aponogetons (A madagascariensis in particular) which we theorize are not
able to tolerate high DOC.

Of far greater importance to aquarists are competition of plants. See
articles by David Sutton in TAG 8:4, Walstad in TAG 9:2 and Paul
Krombholz in TAG 5:4

Various plant species have evolved competitive advantages which enable
them to grow under specialized environments such as alkaline or acidic
conditions, low light and low redox mud. Plants grow at vastly differing
rates and fast growing plants easily out compete slow growing neighbours
for light. Slow growing plants capture and hold nutrients and are able
to withstand changes in environmental conditions. This is why you are
able to grow aquatic plants which out survive algae in your aquariums.
Blue green and green water algae cannot survive low light conditions and
do not reproduce quickly under low nutrient conditions. Filament algae
however, are similar to macrophytes (large plants) and are much better
adapted to survive changes in their environment. That is one reason why
it's a good idea to try to grow plants free of filament algae by
avoiding introducing them into your aquarium in the first place. Of
course, virtually all plants which come from the fish store are
contaminated by a variety of filament algae so the only way to prevent
contamination is by doing it from day one of setting up an aquarium.

It will be beneficial if we can steer the discussion away from
allelopathy and onto competitive effects which are MUCH more important.

By the way, one term for referring to plants growing cooperatively is


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