[PCA] Disturbance and invasion

[Steve Erickson]

I's somewhat disturbed by the apparent assumption by some 
posters here that all invasions are traceable to anthropogenic 
disturbance. There are ecosystems where disturbance is a key 
factor in their creation and persistence, whether very frequent 
(e.g., salt march - daily tidal), less frequent (e.g. prairie - 
annual fire; forest - blow down). These disturbances create and 
maintain the structure, function, and composition of these 
systems, with no additional anthropogenic disturbance necessary 
for invasion by introduced species. Examples in my part of the 
planet (western Washington state) include:

Spartina spp - low salt marsh.
Epilobium hirsutum - high salt marsh, freshwater wetlands, pond fringe.
Cytisus scoparius - prairie.
Hedera helix; Ilex aquifolium - forest understory.

In all of the above examples these invasive species change 
fundamental processes and attributes of the invaded systems 
through mechanisms including direct exclusion of the native 
species, structural change, and changing the systems' nutrient 
levels and pathways.

And there really is no obvious commercial use that would cause 
them to be hunted to extinction or even kept in check.

Ultimately, this is a philosophical question. I guess I'm a Leopoldian:

"A thing is right when it tends to preserve the integrity, 
stability, and beauty of the biotic community. It is wrong when 
it tends otherwise."

I take "stability" in the cyclic sense, not as in "static" or 
"unchanging." And when a system is or will be fundamentally 
changed by an invader, I come down on the side of the existing 
system and its integrity.

-Steve
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Whidbey Environmental Action Network
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Box 53, Langley, WA   98260
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