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Invasion of the Asian Shore Crab: Its Effect on the Local Ecosystem
Transcript of Invasion of the Asian Shore Crab: Its Effect on the Local Ecosystem
By: Renee Colbath Change in creature interactions within the intertidal community: The introduction of a new aggressive species means competition for food, space and shelter with local species.
While some local species can hold their ground against this small predator, others, especially juvenile creatures are at risk losing the battle for territory with this aggressive little crab.
"Recent trends show numbers of shore crabs are steadily increasing while native crab populations are declining" (Richerson) The Asian shore crab is becoming the dominant species in many intertidal ecosystems on the northeastern coast of the U.S. Due to its ability to thrive in its new environment, and its competitiveness with other crabs, the Asian shore crab has the potential to "bring other crabs to extinction" (Westgate 135). The Asian Shore Crab is an invasive species that is causing drastic changes and wrecking havoc on the natural ecosystem of the northeastern United States coastline. The Asian shore crab, Hemigrapsus sanguineus, commonly called the Asian shore crab is a very small species of crab, growing to adulthood at only '" 35mm to 42 mm wide'" (ISSG Database: Ecology quotes Benson). It is fascinating how much of an impact that such a small animal can have on an ecosystem. Invasive species are animals and plants that are transported away from their natural habitats to other areas of the planet where they would not have otherwise been able to migrate to on their own. As they are not naturally occurring, many invasive species cause problems to the ecosystem of the affected area where they have been moved. The Asian shore crab is native to Japan shoreline and its surrounding areas in the '"western Pacific Ocean, from 20 to 50 north latitude...'" (ISSG Database: Ecology quotes McDermott). It was first transported to the northeastern coast of the United States in 1988 where it was discovered in New Jersey. Since then it has been found in large numbers down the coastline from Maine to North Carolina (Are Invasive Species Bad...). Asian shore crabs have adapted so well to their new environment that their populations have become "denser in North America than in their native range in Asia (Are Invasive Species Bad...). Many microorganisms and larvae are taken into ships in the ballast water tanks (Richerson). These vessels then travel all around the world. The Asian shore crab was most likely introduced to its new environment in the United States when ballast water was dumped out of ships hailing from the Western Pacific. Hemigrapsus sanguineus is dependent on rock cover" (ISSG Database: Impact) and other types of shelter for survival. The ISSG Database quotes Fukui (1988) that in Japan, the Asian shore crab inhabits "crevices among boulders on rocky intertidal shores. The Asian Shore Crab able to thrive in the conditions of the northern Atlantic coastline, because there are many rocky places to hide, and because it can adapt well to many different environments due to its "broad temperature and salinity tolerance" (Westgate quotes Main (2007) 134). Change to the food web: Hemigrapsus sanguineus is an "opportunistic carnivore" (Brousseau). Diet "macroalgae, salt marsh grasses, and small invertebrates such as amphipods, gastropods, bivalves, barnacles and polychaetes" (Brousseau). This diet allows the Asian shore crab to survive in various environments. This crab also has "seasonal diet shifts, a diet that changes as the organism grows, individual diet specialization", and is more carnivorous while producing eggs (Griffen 2546 - 2549). There is an interesting effect that the introduction of the Asian shore crab has caused to the European green crab population. The green crab is another invasive species that occupy’s the same environment as the Asian shore crab in the U.S. (ISSG Database: Impact). In the 1990's there was a large decrease in green crabs and a sharp increase in Asian shore crabs (Westgate quotes Lohrer (2002) 131-132). It's population in the U.S. declined by about 90% (Westgate quotes Main (2007) 134). The two crabs act aggressively towards one another, and the Asian shore crab is the "stronger competitor" against young green crabs (Westgate quotes Nelson (2005) 131). The green crab caused more harm to the American clamming industry than the Asian shore crab. Since the introduction of the Asian shore crab, there have been less green crabs and the green crabs have been "eating less shellfish and are growing slower, leading to higher mortality from predation" (Kaplan). The introduction of the Asian shore crab might eliminate the problem of the invasive European green crab, but in turn, as the Asian shore crab population continues to grow, it may just cause a new problem for the U.S. coastline. There is nothing controlling the spread of the Asian shore crab. In its natural habitat, its population is controlled by natural predators and "parasites which are not present along the US Atlantic coast" (Richerson). Since each female can produce up to 200,000 eggs per year at "50,000 per clutch" (Richerson) and each crab lives "around eight years" (ISSG Database Ecology), than each female has the potential to spawn 1,600,000 offspring in her lifetime.
During the crab's one month period as a larvae, an Asian shore crab can be transported very far, spreading the species great distances with each generation (Richerson). Predators will not keep the population down. The population will most likely continue to grow and spread until the species reaches an ecosystem with intolerable conditions.
Hemigrapsus sanguineus has invaded not only the United States, but also "Croatia, France, the Mediterranian, and the Netherlands" (ISSG Database: Distribution).
As any active attack on the Asian shore crab by humans would harm local species as well, the only thing that humans can do about this problem is to stop new species from being moved to new ecosystems. "Ballast water management is being researched to reduce or eradicate new introductions" (ISSG Database: Management). There are many possible effects that the invasion of the Asian shore crab may have in the future. It could possibly cause the extinction of some of the natural inhabitants of the Northeastern U.S. intertidal zones. Possible effect on humans: due to its consumption of and competition with commercially important local species. Hemigrapsus sanguis readily consumes three species of commercial shellfish, blue mussels (Mytilus edulis), soft shelled clams (Mya arenaria), and oysters (Crassostrea virginica)" (Brousseau). It also competes with larger species such as the blue crab and lobster" (Richerson) for food and shelter. They "prey upon juvenile lobsters" (Are Invasive Species Bad...) which could decrease the lobster population in the future. THE END Have a Great Summer !!!