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.
Toby
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