Sometimes it has been difficult to decide which is the greatest threat to our countryside, alien invaders or our home-bred pests Cameron and Clegg and their Defra ministers who — despite a promise to be our greenest ever government — seem hell-bent on allowing their rich mates and political sponsors to ruin our countryside.

This week let’s concentrate on alien species and nowhere is their pernicious influence felt so greatly than in and on our ponds, lakes, canals, rivers and other watercourses.

I’ve written about the red signal crayfish and its exotic Asian cousins that have almost driven our native crays to extinction and about the notorious killer shrimp (Dikerogammarus villosus) and the epic battle the Broads National Park has waged to keep it under control. 

That killer shrimp came all the way from the waterways around the Caspian Sea and now it seems some other species from that same region are arriving in British waters.

Chief threat among the newest invaders is a foreign mussel recently found in the river Wraysbury — a tributary of the Thames. 

The Quagga mussel (Dreissena rostriformis bugensis) turned up there on October 1 and immediately brought a cold chill to the hearts of river scientists.

It has already caused huge destruction all over the waterways of Europe causing chaos by blocking water pipes, sluices, irrigation systems and killing native water plants and animals. 

Quagga filters out blue-green algae, leading to more sunlight reaching the riverbed which can also change the nature of sediment, hitting snail populations, mayfly larvae and native freshwater shrimps. 

This dangerous and destructive bivalve is just one of a group of freshwater species that has been spreading westward from the Caspian region of eastern Europe.

Now a new study by a University of Cambridge team suggests that the Quagga and the killer shrimp are just the advance guard in what will be an invasion of as many as two dozen equally destructive species.

Five species are already in Britain — including the demon shrimp (Dikerogammarus haemobaphes) and killer shrimp. All are likely to increase in number and distribution. Computer modelling suggests that four species are likely to already be in Britain undetected. 

Two of those, Echinogammarus ischnus and Limnomysis benedeni, are omnivorous shrimp that pose a threat to British species. There is evidence that they might predate our native shrimps, our insect larvae and fish eggs. 

The south-east of England, especially the lower reaches of the Thames, Great Ouse, Severn and the Broads, are most at risk from these invaders.

The study warns that the first wave of invaders, including the Quagga, are likely to make it easier for further arrivals from the Caspian region. Many of these organisms have — over long evolution — developed a cosy relationship with each other. They tend to help each other thrive.

For example there is a close relationship between the killer shrimp and the zebra mussel (Dreissena polymorpha). 

The shrimp feeds on waste produced by the mussel, hides between the mussel shells and has even evolved to have a zebra camouflage pattern.

So how do these invading mussels get here? Chief suspect is in the bilges of trading vessels from Europe. They would be killed by the salt seawater if they were simply attached to the ship’s hull. 

Many of these alien species are already well-established in the Netherlands — Britain’s biggest European maritime trading partner. 

Damp boots, waders and fishing tackle of European anglers coming to Britain on fishing holidays are another route.

The small river Wraysbury was home to a little endangered British bivalve called the depressed river mussel (Pseudanodonta complanata). 

With the arrival of the Quagga that little mussel, along with all those concerned about waterway biodiversity, are likely to get even more depressed.