Bridget Flanagan from the Great Ouse Valley Trust describes three species that have invaded our river and upset the ecological balance

We don’t see them coming, but they can quickly create a huge problem and disrupt local ecosystems, she says.

It is believed that they are the biggest threat to global diversity after habitat loss. Who or what is this enemy? They are collectively known as ‘invasive non-native species’.

Throughout history people have moved animals and plants around the world – either intentionally or inadvertently.

But some in-coming species flourish far too well in their new environments when free from their native predators or competitors. They are not only a threat to biodiversity, but often a very expensive economic problem affecting industries such as forestry, fisheries and horticulture.

Here along the River Great Ouse we are all aware of several non-natives who have colonised the river. American Mink escaped or were intentionally freed from fur farms in the 1960s and are now widespread along the waterways.

They are carnivorous and vicious predators of ground-nesting birds. The decline in numbers of one of the UK’s most endangered mammals – the water vole - is directly attributed to mink.

However, there may be good news in that the resurgent otter populations are thought to be displacing the mink.

Floating Pennywort, a native of the Americas, was brought to the UK in the 1980s for ornamental use in garden ponds. But some plants found their way into rivers, flourished and spread at exponential speed.

Pennywort forms dense mats on the surface of the water; this cuts out light for photosynthesis for other aquatic plants, depletes oxygen levels in the water thus threatening fish and invertebrates, and chokes navigation channels preventing boating.

Since 2014 the plant has been banned from sale, but the clear-up in our rivers is ongoing. The Environment Agency and many volunteer groups continue to remove every bit of this pernicious plant – handpicking each fragment and, even then, continually checking for any re-growth.

And finally, the Chinese mitten crab is now establishing itself in the Great Ouse. So-called because of a brown furry-like growth on its claws, it is nowhere near as attractive as its name.

Mitten crabs have been recorded in the Thames estuary since 1935, probably introduced by clinging on to ships’ hulls. They are relatively large - their overall leg span is about 25 cm – and they burrow in riverbanks often causing extensive damage and erosion.

Overall, they upset the ecological balance of the river through competition and predation, and, so far, like many of the other aliens, they are proving difficult to control. They look set to stay.