[permaculture] Are we a weed?
hemenway at jeffnet.org
Wed Mar 26 12:42:24 EST 2003
I was interested in the possibility that one-way weed travel is just a
myth, since people have cited a few counter-examples (though the
examples are mostly of animals, which have very different dispersal
systems than plants). I pulled down my copy of Ecological Imperialism:
The Biological Expansion of Europe, by Alfred Crosby, and browsed
around. He cites data showing that of 500 farmland weeds (plants) in
the US, over half are from Europe, and in Canada, over 60% are
European. Roughly the same numbers are found in Australia and S.
America. But in Europe, less than 10% of the weeds are not from Europe
or Asia. Not a single Australian or NZ weed was found in Europe until
after 1920. The only major weeds in Europe from North America are two
fleabanes, Canadian waterweed, and a handful of others. Most of the
most aggressive US weeds, like ragweed, goldenrod, and milkweed,
haven't gained a foothold in Europe. Yet roughly the same amount of
shipping goes to Europe as from it, so you'd think just as many species
have been imported both ways.
Thus the (mostly) one-way weed establishment is a fact. One reason for
this, in addition to those I cited in my last post, is that
counter-directional species usually travel alone, separate from their
guilds, while European weeds travel with their guilds: We bring cattle,
sheep, and all the other animals that have adapted with Euro-weeds
(which have burrs adapted for our livestock, pass through their guts
unharmed, etc); we bring pollinators by the millions (European
honey-bees love Euro-weeds), and we bring many of the other insect and
plant associates that are allies of the weeds. That broad-band traffic
doesn't go the other way, and thus non-Euro-weeds don't thrive. You
could say they die of loneliness.
For a look at my book on ecological gardening,
Gaia's Garden: A Guide to Home-Scale Permaculture, visit
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