Swallowwort invasion, Spencerville, Ont. August 2004 painting by A. Karstad

Each organism is wonderful in its own way. You just have to become familiar with it, and it becomes a part of you – shared experience. You don’t like to have judgements made of you as an individual, or even as a group. Individuals can be useful in their places. But too much of anything upsets the balance of nature. Too much bare, disturbed soil invites opportunistic plants, which in turn can alter an ecosystem, reducing its diversity and stability.

Opportunist (sometimes called “weedy”) species are adaptable and robust, fast growing and prolific – admirable in a spartan sort of way. However, usually they don’t have an opportunity to show their stuff unless a space is made in an ecosystem for them. That space can be a simple mechanical event, such as exposing mineral soil, which offers initial freedom from competition – or it can be introduction into an entirely new part of the world, which offers freedom from being eaten – or global warming, which offers freedom from frost damage. Some species are even rare or scattered in their native places (like Pale Swallowwort and Japanese Knotweed) where they face their nativer herbivores, but become opportunistic and even invasive when introduced where they have an advantage.

Opportunistic species become invasive when they go beyond just finding free space and safety, and begin to exclude native species and become themselves the agent of change in ecosystems. We think it’s most instructive to regard a species as invasive when habitats it’s moved into have lower biodiversity than the places it hasn’t gotten to yet.