Last year the invasive Quagga Mussel (Dreissena bugensis) arrived in UK waters amid the typical flurry of media scare stories. Billed as the single biggest threat to British wildlife it appeared the Quagga was going to take on its close relation the Zebra Mussel (Dreissena polymorpha) for notoriety amongst the UK’s worst invasive species, who is rarely omitted from top 10 countdowns of the most dangerous invasive species.
Arriving in the UK over 200 years ago the Zebra Mussel has now colonized most of the inner water systems of central and western Europe,as well as spreading to the Great Lakes of the USA from it’s native home of the Caspian Sea. Throughout their steady colonization along our waterways they have grown themselves a reputation for playing havoc with infrastructure. Almost undoubtedly arriving as larvae in the ballast of commercial ships, the mussel will then settle upon any hard surface in the area such as pipes that deliver water to cities. A recent estimate of the economic cost of the Zebra Mussel to the USA amounts to a hefty $2 billion dollars annually.
Economic damages are usually more than enough to condemn an invasive species, yet the zebra mussel can also be implicated in a variety of environmental problems such as having a detrimental affect upon North American unionids. As previously mentioned Zebra Mussels have a tendency to settle upon any hard surfaces, and exposed clam shells provide a perfect place, thus reducing the clams ability to burrow and move through sediment. An attached Zebra Mussel can also make it harder for clams to feed by obstructing the openings of their shells, as well as simply outcompete clams for food. Nobody is sure whether their ability to outcompete clams for food or their trespassing on clam shells is the main cause that Zebra Mussels have nearly eradicated native clams from their new, invaded homes.
It is easy to point the finger of blame solely at the alien invasion of Zebra Mussels for unionids decline, however unionids have been in trouble since before the introduction of Zebra Mussels to North America nearly 30 years ago. Habitat destruction caused by building dams, pollution and overharvesting are all key players in the decline of unionids, as well as the loss of the host fish that parasitic larvae unionids need to complete their life cycles. It is possible that Zebra Mussels are merely filing in a gap left by the loss of native clams and as a result is now the subject of a persecution campaign, however it is just as likely that the arrival of the Zebra Mussel is actually responsible for accelerating already under pressure species into local extinctions.
So does the Zebra Mussel have any redeeming features other than its extraordinary ability to colonise new habitats and as a prolific breeder? Their ability to filter water has a mixture of impacts upon their surroundings, most notably it improves water clarity, which is often used in defence of the Zebra Mussel. The clearer water is actually a result of the mussels removing phytoplankton and reducing non-toxic algae and thus reducing competition between algae strains. This can result in algae blooms, which last year caused 400,000 people to be left without drinking water when Lake Eerie suffered from a major algae bloom.
Potentially the Zebra Mussels most redeeming feature is their value as a food source. They can quickly become a staple of the diet of molluscivorous fish after they enter a new habitat as well as for waterfowl. The consumption of Zebra Mussel by migratory birds is well documented and is regarded as the cause for dramatic increases in flock sizes amongst various species.
The Zebra Mussel is a well documented invasive species, hence why we know so much regarding its positive and negative impacts upon invaded habitats. It must be admired in its own right as a prolific colonizer however it is also a drain upon the economy as its impacts cost billions of pounds worldwide annually. Does the Zebra Mussel truly deserve its place amongst the worst of the invasive species or is it actually deeply misunderstood and merely taking advantage of the damaged waterways we have provided for it? Well that’s up to you to decide.
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