Islands have interesting biology thanks to their relative isolation from other land masses, and though the British flora is a little impoverished after the last ice age it contains some real gems. Just as in exotic islands though, our local ecology is threatened when non-native species invade and run rampant in the absence of disease, pests, or invertebrates adapted to using them as food. Keeping the foreigners out might sound like prejudice, but is based on experience of the choking effect species such as Himalayan Balsam exert, pushing out our more self-effacing natives.
Like other good riparian keepers the fishing clubs of The Camel are aware of this threat and try to address it, though attempting to eradicate these invaders must feel like starting the undercoat on the Forth Bridge, a Whac-a-mole or a modern labour of Hercules.