Are invasive species good or bad?

Are invasive species good or bad? This is the question which search engines have directed to my web log over the last few weeks.

The answer depended a little on what is meant by good or bad. If by bad we mean do invasive species cost us money which could be used somehow somewhere for something else, then invasive species are bad. From clogged canals such as water hyacinth (Eichhornia crassipes) to ravenous insects such as Formosan termites (Coptotermes formosanus), from toxic weeds like giant hogweed (Heracleum mantegazzianum), to disease carrying insects like the Asian tiger mosquito, (Aedes albopictus), from plants such as Canada thistle (Cirsium arvense) which reduces harvest, to birds such as the mute swan (Cygnus olor) which reduces habitat, the list is long and impressive.

The danger of attaching a monetary value without careful consideration is that some invasive species may potentially have a monetary value as in kudzu (Invasive species, BARC, Kudzu and Bio-fuel) ) for ethanol, and thus be good becasue they produce money (wealth). Or for that matter, English ivy (Hedera helix) for landscapes as an evergreen ground cover. The two examples have a positive short term market preference value which is not easily compared to long term public interest or good valuation, as well as difficult to compare off-setting costs for destruction of ecosystem services.

Buried in all of this is the native versus exotic definition challenge which places poison ivy and white tailed deer outside the official definition of an invasive species, and thus contributes to confusion for the out-side-not-full-time participant to the discourse and question. .
I wager I did not answer the question, and that stakeholders on all sides of the issue will find absolutes which I missed. Welcome to the wickedly inconvenient world of invasive species. .