Ocean School

Invasive Species in Marine Ecosystems

by Susan Kynast, Ocean School Marine Biologist

 

Invasive marine species are one of the four greatest threats to the world’s oceans! Unlike other forms of marine pollution, such as oil spills, where ameliorative action can be taken and from which the environment will eventually recover, the impacts of invasive marine species are most often irreversible!

 

Plants, protozoans, and even bacteria and viruses can also be invasive species. For marine species we have made life especially easy. Besides being able to live in whole ecosystems on the hull of ships, ships also use ballast water to stabilize vessels. Ballast water is pumped into the ship in one port and emptied in another, providing a moving aquarium for any aquatic invader which happens to be in the water column when the water is pumped into the tanks – up to and including fish. And since almost all marine organisms have planktonic life stages, virtually any marine life form can travel by ballast water. A total of 3 to 5 billion tons of ballast water are carried by ships between countries every year! It is estimated that at least 7,000 species are in transit in ballast water every day. If the conditions in the port of arrival are similar to the ones in the environment where the ship came from, the species will survive – and reproduce. Scientists think that approximately once every 1 to 7 days an invasive species establishes itself in a marine ecosystem somewhere in the world. The European zebra mussel in the Great Lakes is one of the most notorious examples of marine invasions, and even in Maine where marine ecosystems are still relatively healthy, 33 invasive species have been identified including such familiar creatures as the common periwinkle and the green crab which have especially in the case of the periwinkle dramatically modified Maine’s intertidal zone. Our own species can become dangerous invaders in other ecosystems. The comb jelly Mnemiopsis leidyi for exampleis implicated in the destruction of the entire Black Sea ecosystem.

Besides hitching rides on or in ships, species are also transported on the increasing amounts of marine debris floating in our oceans. Since ocean current systems are global, and plastic marine debris floats for decades or even centuries, sessile species can reach new environments anywhere in the world by this method.

 

Saccorhiza polyschides, an alga which does not normally occur on the Belgian coast, found on a stranded plastic drum at beach at Oostende, Belgium. The algal growth also harbours small crabs.
Image and caption courtesy of Francis Kerckhof, Management Unit of the North Sea Mathematical Models, Belgium. http://www.mumm.ac.be

 

 Aquaculture is another means of travel for invasive marine species. The aquaculture species themselves can escape, as was the case with the aquaculture salmon in Maine which ended up interbreeding with the native salmon, possibly contributing to the decline of wild Atlantic Salmon through a process of genetic mixing called outbreeding depression. Aquaculture species also carry bacteria, viruses, parasites, and various algae on their bodies since there is no way to disinfect a living organism, and those ‘hitchhikers can then ‘infect’ native species. Infectious Salmon Anemia which spread to the Maine aquaculture industry from Norway and could easily spread to Maine’s wild fish stocks is one example.

 

Invasive species can also travel with seafood and its packaging such as seaweed designed to keep certain commercial species (and unfortunately also undesired invaders) alive. One survey found ‘extra’ species in 83 percent of seafood shipments!

And finally the pet trade is a source of exotic species as people dump the contents of their aquarium into the next body of water. The live food markets of places like Chinatown also provide a source of invasive species which people buy and release out of humanitarian concerns. This is not to say that humanitarian animal purchases are wrong! Countless turtles and tortoises for example have been saved in this manner by concerned individuals. The problem is not in the purchase, but in the release. Any rescued animal should be given to an appropriate rescue and rehabilitation organization which will be glad to have it, and the contents of an aquarium should be returned to a reputable pet shop. The reason is not only the possibility of the animal itself becoming invasive and destroying the ecosystem (such as the northern snakehead), but also the chance that the animal might be carrying a pathogen which could be transferred to a wild population. This can happen even if a native animal which had contact with an exotic species while in captivity is released. The most tragic example is the spread of Mycoplasma into the California Desert Tortoise population which killed off a large percentage of the wild population. Exposure to pathogens from land turtles might have also caused the herpes virus to jump into sea turtle population, with its spread possibly facilitated through the aquaculture (turtle farming) operations on Grand Cayman. Currently almost all sea turtle populations in the world are being decimated by herpes-caused fibropapillomatosis.

 

Invasive species management came close to home last week as scientists presented a study which found 18 invasive species in Casco Bay at Maine’s Marine Invasion, a forum hosted by the Casco Bay Estuary Project and Maine Sea Grant.

International Maritime Organization (IMO)

Invasive species are a global conservation problem. A species in its native habitat is constrained by predator-prey and competitive relationships. In short, its numbers are controlled by others which eat it or eat its food. If the organism can somehow hitch a ride to a new habitat, it leaves its predators and competitors behind. Organisms reaching new habitats has always happened throughout evolution – this is how isolated islands and continents get colonized – but in the past it has happened on a time frame of tens of thousands of years. Hitching a ride on a whale, a branch, a coconut, or a seagull is not as easy as hitching a ride in or on a ship or even aircraft (or we would all swim…). Some introductions have even been intentional as humans thought they could ‘improve’ the ecosystem. Stories of ecological disaster include the brown tree snake on Guam, rats, cats, and goats on tropical islands, and rabbits in Australia to name just a few.

 

More Information:
Maine’s Marine Invasion Forum Website (under construction) http://www.cascobay.usm.maine.edu/invasives.html
Invasivespecies.gov http://www.invasivespecies.gov/
IMO Global Ballast Water Management Programme http://globallast.imo.org/