March, 2003: Scouts visiting Crom Estate found themselves battling aliens – and winning.

It wasn't the small greys they encountered, or the jelly pseudopods, or even the Vermicious Knids. These aliens were of the leafy green vegetative variety.

Boys from First Lisnaskea and First Rossorry troops gathered at Crom one weekend to wage war on the invaders.

The nature conservation warden explained to them that dogwood, or Cornus sp to give it its Latin name, had been planted years ago as an ornamental shrub.

Now it had spread into reed beds and wetlands at the estate where it was causing damage to native habitats, pushing out the plants and animals they support.

Staff had been clearing patches of dogwood from the wetlands throughout the winter, but were beginning to find themselves overwhelmed.

So when the scouts stepped in, the National Trust was glad of the help.

Over two days, the boys laboured to return the wetlands to their natural condition, some cutting down the dogwood, others carrying it up the lake bank and piling it where it could be collected by tractor.

Conservation staff explained how invasive species – plants and animals including mink – were damaging the wetlands of the British Isles.

"Some of the 'foreigners' we love to grow in our gardens remain where we planted them, but unfortunately a lot of them do not.

"Invasive species have a tendency to spread uncontrollably, cover everything and prevent anything else from growing.

"As they do so, native species are gradually losing their battle against these stronger newcomers.

"The knock-on effect is a reduction in the variety of plant species left and consequently a reduction in the variety of animals species that depend on them," the National Trust spokesman explained.

Alien plants are one of the biggest threats to native flora today, she pointed out.

"To fight them off and prevent further spreading is going to be one of the biggest challenges to nature conservation this century," she said.