Species encroaching on alien territories
Causes of the dispersal of marine organismsSince humans began to sail the seas, other species have been travelling around the globe with them. These are not limited to useful plants and animals, nor to pests such as pathogenic agents or rats, but also include significant numbers of marine organisms. Historical records and archaeological finds show that the sailing ships of the early explorers were colonized by up to 150 different marine organisms that lived on or in the wooden hulls, or used the metal parts such as anchor chains as a substrate . If the growth became a nuisance, the organisms were scraped off while at sea. In other cases the organisms remained on the rotting hull of a ship when it was scrapped and could no longer be repaired. It is hardly surprising then that many wood-boring species such as the shipworm Teredo navalis are found around the world today. However, it is no longer possible to determine whether these species were already cosmopolitan before the European voyages of discovery. It is common knowledge, however, that humans have contributed to the dissemination of many species. Increasing numbers of marine organisms are now transported over the oceans as a result of globalization, trade and tourism. It is estimated that the water in ballast tanks used to stabilize freighters is alone responsible for transporting tens of thousands of different species between geographically-distant regions. Most of these exotics die during the trip or at the destination, while only a small fraction are able to successfully reproduce and form a new population . But a study of six harbours in North America, Australia and New Zealand has shown that, in spite of all obstacles, one to two species were able to successfully establish themselves per year at each of the sites investigated. Geographical barriers can also be overcome by canals. Over 300 species have already migrated through the Suez Canal from the Indian Ocean into the Mediterranean Sea. In addition, rivers and other waterways are responsible for species exchange, such as between the Baltic Sea and the Black Sea. Another important cause for the dispersal of marine organisms is the trade of living marine organisms for aquaculture, aquaria and the food industry.
Specialists divide the coastal waters of the world into a total of 232 ecoregions which are either separated from each other by geographical barriers such as land bridges, or are clearly different from each other with respect to certain environmental characteristics such as salinity. According to a report issued in 2008, new species have already been introduced by humans to at least 84 per cent of these 232 ecoregions. Investigations in the North and Baltic Seas show that at least 80 to 100 exotic species have been able to establish themselves in each of these areas. In San Francisco Bay, 212 foreign species have already been identified, and for the Hawaiian Islands it is assumed that about a quarter of the marine organisms that can be seen without a microscope have been imported. Relatively little is known about the distribution of microorganisms or other plants and animals that are hard to identify. Species records are also sketchy for many marine regions where access is difficult. Experts assume that in future exotic organisms will have better chances to establish themselves in some regions due to climate warming. Organisms from South-East Asia, for example, which prefer a warm climate, could take root in regions that were previously too cold for them.
- 5.9 > Invasive species thrive particularly well in certain coastal ecoregions of the Earth. Most affected are the temperate latitudes. Regions where immigrants do not encroach on or displace native species are shown in green.
New species alter biodiversityMany exotic species infiltrate the native flora and fauna without dominating them, thus increasing the diversity of the species association. Natural catastrophes can completely destroy habitats and be fatal to entire species communities. In these cases a completely different species assemblage develops in the affected regions through the influx of new species. An example of this is the Baltic Sea which was formed after the last Ice Age – that is, in the relatively recent geological past – and is only around 7000 years old in its present form as a brackish sea. One indigenous species alone evolved there, the alga Fucus radicans. All other species native to this area today migrated from habitats such as the North Sea or the White Sea. The immigration of species from other regions is thus not always problematic nor caused by human activity.
Since Christopher Columbus travelled to America in 1492, exchange between distant parts of the Earth has steadily increased. It has thus become more likely that species will encroach on ecoregions that are far removed from their natural areas of origin. Sometimes the new species create problems. They may displace a number of native species and thereby lead to a decrease in biodiversity. This is especially likely to occur if they have no natural enemies in the new location. For example, within only 15 years of its initial discovery in Monaco the Australian green alga Caulerpa taxifolia had overgrown 97 per cent of all the suitable ground between Toulon and Genoa, and had spread into the northern Adriatic and as far as Sicily. The alga produces a repellent substance that makes it unpalatable to most herbivores. There are organisms that feed on Caulerpa and have adapted to the repellent, but these species are not present in the Mediterranean Sea.
The Asian algae Sargassum muticum and Gracilaria vermiculophylla also formed practically monospecific stands in some coastal areas after their introduction to Europe. The northern Pacific seastar Asterias amurensis, on the other hand, established itself in south-eastern Australian waters in the mid-1980s. Only 2 years after it was first detected in Port Philipp Bay, a large bay off Melbourne, more than 100 million specimens were estimated. This starfish too found practically no natural enemies in its new habitat, enabling it to decimate stocks of native starfish, mussels, crabs and snails. The biomass of the starfish eventually exceeded the total amount of all commercially fished marine animals in the region.
Cases of newly imported species displacing native species have been documented in 78 per cent of the 232 coastal ecoregions of the world. Many cases have been reported from the temperate latitudes in particular, those regions of the Earth where it is neither extremely hot nor extremely cold. With the exception of Hawaii and Florida, the 20 coastal ecoregions most strongly afflicted by invasive marine organisms are located exclusively in the temperate North Atlantic and North Pacific or in southern Australia, and nine of these regions are in Europe. Some places, like San Francisco Bay, are now dominated by non-native species. There, the encroaching species are often considered to be a threat to marine biodiversity, although so far not a single case is known in which a species introduced from outside has caused the extinction of native organisms.
- 5.10 > The number of ecologically or economically problematic imported species in the most affected marine regions of Europe.
Economic impacts of alien speciesIntroduced marine organisms can inflict economic losses on fisheries. The warty comb jelly Mnemiopsis leidyi, native to America, brought about a collapse of the coastal fisheries in the Black Sea 25 years ago, an area already greatly weakened ecologically at that time due to overfishing and eutrophication. Specimens that were probably introduced with ballast water were first sighted there in 1982. The jellyfish spread rapidly and ravaged native species, especially fish, by feeding on their eggs and larvae. The commercial fishery harvest decreased by around 90 per cent. In 1989 counts of 240 specimens per cubic metre of water were made, the greatest concentration of M. leidyi in the world. Only the unintentional introduction of another comb jelly, Beroe ovata – a predator – was effective in repelling the population and allowing a comeback of the fish population. Invasive species are also causing problems on the eastern coast of North America. There the European common shore crab Carcinus maenas caused a decline in the clam fishery harvest. In some cases invasive marine organisms can even present a hazard to human health. One example of this is illustrated by microalgae of the genus Alexandrium, which produce a nerve toxin. Species of Alexandrium have recently been discovered in many coastal areas where they probably did not exist just a few decades ago. Such phenomena can obviously have extremely negative effects on tourism.
Introduced species are not only transported unintentionally in the ballast water of ships. Entrepreneurs often import marine organisms from other countries to nonindigenous habitats for aquaculture breeding. This may provide short-term commercial profits, but also poses the risk of imported species displacing native organisms, leading to intermediate- or long-term economic or ecological damage. Studies have shown that at least 34 per cent of the 269 introduced marine organisms investigated were deliberately imported for aquaculture breeding. One example is the Japanese giant oyster Crassostrea gigas, which has taken up residence and established itself in at least 45 ecoregions. Between 1964 and 1980 in particular, large amounts of young oysters, called spat, were imported into Europe. In many cases the ecological impact was devastating. In North America and Australia the giant oyster forms dense colonies that displace native species. Furthermore, they frequently cause eutrophication of the coastal waters because they excrete indigestible particles agglutinated with mucus, which cause the additional organic contamination of the water. The presence of giant oysters in France has also led to pollution of the waters. In addition, a decline in the zooplankton as well as larger animals has been observed. In the Netherlands and Germany the giant oysters tend to settle on blue mussel banks. This is threatening an important species of traditional fishery.
It is assumed that besides the giant oyster at least 32 additional species have been unintentionally introduced into the North Sea, including the common Atlantic slippersnail Crepidula fornicata and the alga Gracilaria vermiculophylla, both of which have proven to be ecologically problematic. In order to avoid this kind of hazard in the future, a standard assessment system would be helpful. This could be used to estimate the potential of a species displacing other organisms. In addition, it could be used to weigh up the advantages and disadvantages of introducing an external species to a certain habitat.
By comparing problematic and harmless imported species, experts have been trying for some time to identify characteristics that indicate a high potential for displacement of the native species. For example, some algal species float while others sink. Whether the species drifts and can thus easily disperse depends essentially on this factor. But so far it has proven difficult to draw conclusions about the displacement potential of a species based on individual traits. Perhaps it will never be possible to make confident predictions about the behaviour of a species in a new location, because numerous critical factors are at play. This prediction is further complicated as a species establishes itself in a new habitat over an extended time period, living through numerous phases. After an initial expansion phase, during which a species thrives, there usually follows a decline before the species has completely adapted to the new habitat. Before the displacement potential of a species can be confidently estimated, it has to be known which phase the species is in at a given time. But that is very difficult to determine.
Can future introductions be avoided?Caution is necessary when dealing with foreign marine organisms because species introduction is largely irreversible. Any kind of mechanical removal of established species is virtually impossible. Many species go through microscopic dormant or larval stages during which they are free-floating. During such phases the organisms defy all efforts to control them. It may be possible to introduce natural enemies to the new habitat, but then these organisms could later become a threat themselves. Government policy and environmental management will therefore have to take a stronger stance to control the primary causes of species introduction. It is important that this includes uninterrupted monitoring of aquaculture and ballast water, for example. Unilateral efforts at the national or local levels, however, will hardly be effective. International strategies practiced by all states bordering an ecoregion have greater chances of success.