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GEO-6 Regional Assessment for North America


Jersey to Cape Cod. Later the crab began moving north, and its arrival in Maine coincided with dramatic declines in the soft clam fishery. Green crabs are a major predator of bivalves, can crack open clams and mussels faster than other crabs, and can out-compete native crabs for food. A second major invasion occurred in 1989 in San Francisco Bay, where the crabs probably arrived as larvae in ballast water or in seagrass or kelp used in shipments of lobsters and bait worms. Grosholz and Ruiz (1995) documented their spread and effects, including losses of up to half of the Manila clams (Venerupis philippinarum) in California. As the species continues to move north it is a concern for Dungeness crab (Metacarcinus magister), oyster and clam fisheries in the Pacific Northwest.


The Chinese mitten crab (Erocheir sinensis), native to Korea and China, is believed to have been accidentally released in the early 1900s in Germany and spread into many European rivers and estuaries, where it feeds on native species and burrows into river banks, often leading to their collapse. It became established on the North American west coast in the 1990s, and is considered a threat to native invertebrates, to the structure of freshwater and estuarine communities, and to some commercial fisheries.


Tunicates (sea squirts) attach to hard substrates, including artificial structures like floating docks and pilings. Some (Botryllus schlosseri and Botrylloides violaceus) settle on eel grass blades and reduce light penetration, thereby reducing growth and survival of the grasses, which are important habitats for numerous animals. The carpet sea squirt (Didemnum vexillum), which reproduces rapidly and spreads easily, has invaded and overgrown marine communities in New England and mid-Atlantic coasts, as summarized by Lambert (2009).


While the common reed, Phragmites australis, is native to North America, a new genetic strain from Europe is a major invader in east coast brackish and freshwater marshes. It out-competes native plants, replacing diverse plant assemblages in freshwater and brackish wetlands. Detritus from Phragmites enters food webs the same way as the


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detritus from the native Spartina, so it has a similar trophic function. The Atlantic cordgrass (Spartina alterniflora), native to the east coast of the US, has been planted elsewhere for coastal protection, but when moved to the west coast it became invasive (Daehler and Strong 1996). It is changing tidal mudflats into dense vegetation, affecting migratory waterfowl, shorebirds and wading birds that forage in the open mudflats. In San Francisco Bay, it has hybridized with native Spartina species. Trends in invasive aquatic species are upwards in the Great Lakes (National Academy of Sciences 2008), Gulf of Maine (Pappal 2010), in freshwater habitats in Rhode Island (Watershed Counts 2014) and elsewhere.


Urban encroachment


Habitat fragmentation was discussed in the previous section on land-use change. The encroachment of urban space into ecosystems remains a key stressor on biota in North America. Radeloff et al. (2010) found that in the United States, between 1940 and 2000, 28 million housing units were built within 50 km of protected areas, and 940 000 were built within national forests. Housing growth rates during the 1990s within 1 km of protected areas (20 per cent per decade) outpaced the national average (13 per cent). If long- term trends continue, another 17 million housing units will be built within 50 km of protected areas by 2030 (1 million within 1 km), greatly diminishing their conservation value. US protected areas are increasingly isolated, and housing development in their surroundings is decreasing their effective size; some national forests are even threatened by habitat loss within their administrative boundaries.


Recent work by Flather (2015) looked at songbird species richness and abundance at the interface between protected areas and housing developments. He found that housing developments immediately outside the perimeter of a protected area had an impact on species richness and abundance within the protected area, especially in the perimeter zone. His findings suggest that conservation policy and decision makers seeking to create protected areas to preserve species richness and diversity need to create protected areas large enough so that the edge effects of


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