all theory thread DESIGN PRINCIPLES used in our course not

Bob Howard rmhoward at
Sun Sep 3 20:44:36 EDT 2000

Interesting discussion on weed/pioneer species. One point to bear in mind is the
difference between 'native' and 'altered' landscapes. In Australia many of the
landscapes are substantially 'native' - that is they are dominated by endemic species.
The AUstralian flora is substantially diffrenet from other flora's (and fauna's).
HOwever this is not true throughout the continent. Road verges are a good indicator - in
West Australia these are corridors of native species and substantially uninvaded by
exotic weeds. Main Roads and many local governments here have a policy of controlling
exotic species and replanting local species.

Contrast this with Westerrn Victoria - the only sites that retain the original plains
vegetation are found in a couple of railway crossings near Melbourne. Road verges in
Western Victoria are 100% dominated by exotic species.

The introduction of exotic( i.e. new) species to either of these landscapes requires
different assessments.

There are weed (vigorous, invasive) species in both these environments. The rate of
invasion depends also on the rate of disturbance in either landscape and upon the
availability of niches within the environment. Undisturbed roadside habitats will
persist with few to minimal weeds without being invaded by plants from surrounding
farmland or disturbed areas because the available niches are all occupied.

Therefore it is not so much the vigorousness (or exoticness) of the species per se that
matters but whether it can exploit a niche in space and time in the surrounding
landscape that will determine it's rate of spread.

Lastly, one should bear in mind the definition of the weed as "a plant for which no use
has been found. yet".Pc's prescription of 'weed' (i.e. pioneer) species has always been
posited upon the species being useful. It seems logical to suggest that the way to
assess a plant that may be 'invasive' is to ask what uis it's use and is it really going
to be used.

Bob Howard
Albany W.A.

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