More than 70% of the earth is covered by oceans and major seas and there are more than 1.6 million kilometres of coastline. People depend on the resources provided by oceans and coasts for survival and well-being in many ways. More than a billion people rely on fish as their main or only source of animal protein.
Yet our marine world is under threat: over-exploitation of its resources, habitat destruction, pollution and climate change are all driving biodiversity loss. Arguably the most insidious threat however, is the one posed by marine invasive species.
Marine habitats are populated by different species of animals, plants and microorganisms that have evolved in isolation, separated by natural barriers. But humans have overcome these barriers with shipping, air travel and other transport means. As a result, species are now moving far beyond their natural ranges into new areas. Species that have been moved, intentionally or unintentionally, as a result of human activity, into areas where they do not occur naturally are called ‘introduced species’ or ‘alien species.’ Many of them perish in their new environment but some thrive and start to take over native biodiversity and affect human livelihoods—these are known as invasive species.
When a species establishes in a new environment, it is unlikely to be subjected to the natural controls that kept its population numbers in balance within its natural range. Without such control by predators, parasites or disease, such species tend to increase rapidly, to the point where they can take over their new environment. Marine invasive species have had an enormous impact on biodiversity, ecosystems, fisheries and mariculture (breeding and farming marine organisms for human consumption), human health, industrial development and infrastructure.
It is difficult to predict which species will become invasive. Sometimes a species can be present for a considerable length of time at low numbers, lulling human observers into a false sense of security until conditions change and become suitable for populations to expand. One example of this is the Chinese mitten crab (Eriocheir sinensis) which was present off the shores of the UK for around 60 years without noticeable signs of being invasive. Then a series of very dry summers in the 1990s reduced the flow of rivers in the south of the country allowing the crabs to settle, reproduce and boom in number. The crabs travel long distances upstream, feeding on native species. They also burrow into stream and river banks leading to bank collapse. British zoologists fear that the Chinese mitten crab could both eat and out-compete vulnerable freshwater species and that native crayfish (which are already in decline) could be affected.
Did you know?
An estimated 7,000 species are carried around the world in ballast water every day. Archaeological records show that the Vikings brought home with them a species of large North American clam, probably for food. The comb jellyfish was introduced into the Black Sea through ship ballast water in the early 1980s and by 1994, the area’s anchovy fishery had almost disappeared. The estimated cost of dealing with the freshwater zebra mussel introduction in the US for the period 1989-2000 is between US$ 750 million and US $1 billion. An invasion of black striped mussels in a Northern Australian marina was discovered in time to successfully eradicate it. The operation involved using sharpshooters to protect divers from crocodiles!
For more information read "Marine Menace: alien invasive species in the marine envorinment".