Because ecosystems extend across borders, Mexico is included as part of North America in this section. North America's biodiversity increases along a latitudinal gradient from north to south. Some 7 807 plants, 233 mammals and 160 birds are endemic or unique to North America (Commission for Environmental Cooperation 1999). Endemism is particularly high in Mexico, where as many as 40-50 per cent of flowering plants, more than half of the reptiles and amphibians, nearly 50 per cent of the fish, and 33 per cent of the mammals are considered endemic (World Bank 1995). In contrast, Canada has relatively low endemism, with unique species mainly limited to islands and areas that escaped glaciation (Government of Canada 1996). Endemism in the United States is highest on islands, especially the Hawaiian Islands, where 44 per cent of the higher plants, 95 per cent of the molluscs, 43 per cent of the birds and 30 per cent of inshore fishes are unique to those islands (Allison and others 1995, Hourigan and Reese 1987). Much less is known about species richness and endemism in marine habitats.
Over the past century, habitat destruction, over-zealous hunting or harvesting, and competition from introduced species has led to the decline and extinction of many North American species (Langner and Flather 1994). The past century also witnessed a decrease in the genetic diversity of agricultural crops and livestock (Government of Canada 1996). In more recent decades, threats attributed to hunting and over-harvesting have diminished in comparison to those from habitat destruction, degradation and fragmentation. Indeed, habitat loss and alteration have become a major threat to the continued diversity of wildlife in North America.
Wetland habitats, which are essential to many forms of wildlife, are particularly threatened. Since the 17th century, many of Canada's wetlands have been lost or severely degraded. Drainage for agriculture accounted for about 85 per cent of these losses, and for 80 per cent of the estuary marches in the Fraser River Delta, 70 per cent of prairie potholes, 68 per cent of southern Ontario wetlands, and 65 per cent of Atlantic coastal salt marshes (Rubec 1994). Similarly, in the United States, more than half of the wetlands have been drained, dredged or modified in some way. Most of this wetland loss (48 million hectares) has occurred in the 48 conterminous states, with Alaska losing only a fraction of its original 68 million hectares (OECD 1996). Overall, though, wetlands still comprise some 1.27 million km2, 13 per cent of Canada's area; this amounts to one-quarter of the world's wetlands (Government of Canada 1996).
There are 50 wetlands of international importance in North America (the Ramsar sites that parties to the Convention on Wetlands of International Importance especially as Waterfowl Habitat agree to establish as wetland reserves). They cover 14.2 million hectares, of which 13 million hectares are in Canada (Ramsar 1997). Wetland loss has contributed, among other things, to the long-term declines of some duck populations (Caithamer and Smith 1995). Over the past decade, awareness of wetland loss has been growing and efforts to protect these habitats have increased. But losses continue to outpace the gains made through wetland restoration projects. Coastal marshes and other aquatic ecosystems are particularly prone to degradation because the concentration of settlements in coastal areas and around rivers and lakes continues to grow (Langner and Flather 1994).
The Migratory Bird Treaty Act, enacted by the United States in 1918, helped in the recovery of many North American birds suffering from population declines due to excessive hunting (Harrington 1995). More recent cooperative efforts, including the North American Breeding Bird Survey (begun in 1966 by both the United States and Canada), have led to better information on status and trends in North American migratory species (LaRoe and others 1995). Problems with migratory species remain, however, with habitat loss one of the biggest threats to migratory populations in North America. Many birds, bats, butterflies and sea mammals summer in the northern reaches of North America and winter in its southern climes, whereas others migrate exclusively within the tropics. Mexico hosts 51 per cent of all migratory bird species from its northern neighbours, and the loss of critical over-wintering sites, due to deforestation and other land-use changes, may threaten the survival of these populations (Robinson 1997, Greenberg 1990). The transformation or degradation of breeding habitat in the north, along with deprivation of vital stopover areas along developed coastal areas, is also contributing to a decline in migratory songbirds (Temple 1998, Robinson 1997, Terborgh 1989). Other migrants are under multiple pressures. The extraordinary migration of the Monarch butterfly, for example, may be threatened by coastal development in California, the deforestation of its habitat in Mexico's cloud forests, and threats to milkweed habitat in Canada (Malcolm 1993, Schappert 1996).
In recent decades, the deliberate or inadvertent introduction of exotic species has emerged as a growing threat to native biodiversity that has both economic and biological costs. In the United States, for example, at least 4 500 species of foreign origin have established populations, with approximately 15 per cent of these species causing harm (US Congress 1993). High-impact species, such as the gypsy moth, imported fire ants (Solenopsis invicta), purple loosestrife (Lythrum salicaria) and the zebra mussel (see box), can affect many national interests including agriculture, industry, human health and the protection of natural areas.
In November 1990, the Non-indigenous Aquatic Species Act was passed by the US Congress, and in May 1993, the first ballast water law in the world was adopted. The ballast water law requires ships entering the Great Lakes with ballast water to exchange that water on the high seas. While ballast water exchange reduces the risk of introducing invasive species, it does not eliminate it. Recommendations for future strategies to eliminate undesirable introductions include alternatives in ship design and treatment of ballast water with, for example, heat, ultraviolet light or ozone to remove foreign organisms (Mills and others 1998).
Overall, there are an estimated 430 threatened species of mammals, birds, reptiles, amphibians and fishes in North America (see bar chart above). One-third of the region's freshwater fish stocks are threatened or rare. The United States had the greatest diversity of freshwater mussels in the world but 55 per cent of the species are now either extinct or threatened with extinction (Williams and Neves 1995). Concern with vanishing plant species has also become a regional and global concern, particularly in relation to genetic stock. A 1997 report assessing the condition of approximately 20 500 species of native US plants and animals gives about two-thirds of the species satisfactory marks while about one-third are of conservation concern (Stein and Flack 1997). Organisms with especially poor marks include animals depending on freshwater habitats such as mussels, crayfish, fishes and amphibians. Flowering plants also receive low scores, with one-third of their species - a total of some 5 144 species - in trouble.
Limited data on the status of marine species are available from the US National Marine Fisheries Service within the National Oceanic and Atmospheric Administration (National Marine Fisheries Service 1997). At least 85 species of marine mammals are found in US marine waters, including the Atlantic Coast, Gulf of Mexico and Pacific Coast. Eighteen of these species are listed as threatened or endangered under the US Endangered Species Act. Increases in populations have resulted from the prohibition of commercial whaling but decreases have resulted from the by-catch associated with commercial fishing, illegal killings, strandings, entanglement, disease and exposure to contaminants (Kinsinger 1995).
Throughout North America, some 2.5 million km2 of land, freshwater and marine areas have been set aside as national parks and other types of protected areas (see illustration right) that help to sustain and preserve rare, threatened or vulnerable ecosystems, and the species and genetic resources that they harbour (Commission for Environmental Cooperation 1999). This amounts to approximately 9 per cent of North America's total land area, and the number and amount of protected lands are growing. In Mexico, more than 10 new biosphere reserves were decreed in the past decade (Secretaría de Medio Ambiente Recursos Naturales y Pesca 1996), while in Canada protected areas have increased by 15 per cent since 1990 (Government of Canada 1996) and will continue to increase with the implementation of Marine Protected Areas and National Marine Conservation Areas. The US system of protected areas doubled in size in 1980, with the creation of Tongass National Park in Alaska.
An encouraging trend is the growing recognition of the need to protect representative areas of all the region's diverse ecosystems. Eco-regional assessment, however, has not turned up good news. In an examination of eco-regions of North America, the World Wildlife Fund found that eco-regions in the United States and southern Canada are under severe threat (Ricketts and others 1997). The most critical eco-regions include temperate broadleaf and mixed forests, temperate grasslands, savannahs and shrublands; 60 per cent of the eco-regions classified as critical or endangered are part of these habitat types.
Over the next 10 years, with the implementation of new legislation such as the Marine Protected Areas and the National Marine Conservation Areas, the number of protected areas may increase. Despite efforts to protect biodiversity, however, human activities are likely to continue to encroach on ecosystems and to jeopardize the habitats of threatened species in North America. The expansion of regional and international trade may increase the introduction of exotic species into North America. On a longer time scale, climate change may also force rapid adaptation, alter communities of plants and animals, and their migration patterns, and cause extinctions.
The need to conserve biodiversity requires the development of an analytical framework for monitoring its status and for establishing priorities. One North American example is the Canadian Biodiversity Strategy developed by the Canadian Government (Environment Canada 1994). This strategy sets out national goals and strategic directions for the conservation and sustainable use of biodiversity, and acts as a planning framework for federal, provincial and territorial governments. There are also many initiatives in the area of environmental indicator development that lend support to the implementation of the strategy.