AN organism often multiplies explosively when carried to a new environment. Rabbits in Australia are a familiar example, and now we have water hyacinth (Eichornia crassipes) in south-east Asia, the Nile, and the Congo1–3. In time, enemies of the invader will probably evolve in the new environment, or be introduced into it, and restore a balance, but that may take many years. In the meantime, there is disruption of old patterns of life and effort is therefore put into attempts to eradicate the invader and restore the status quo. This is obviously wise, but it is by no means certain that eradication will be successful, so that it may be prudent to try to find how best to live in the new circumstances. The invader may often be useful.