Loss of Biodiversity and Invasive Species

The loss of biodiversity usually evokes the demise of such charismatic mega-fauna as Indian tigers, Chinese pandas, and Kenyan cheetahs. All these species are greatly endangered, but in terms of irreplaceable ecosystemic services their loss would not even remotely compare with the loss of economically important invertebrates. Both Europe and North America have seen a gradual decline of pollinators, including domesticated honeybees and wild insects. Pollination by bees is an irreplaceable ecosystemic service responsible for as much as one-third of all food consumed in North America, from almonds and apples to pears and pumpkins (fig. 4.11) (NRC 2006a).

Pollinators are also needed to produce a full yield of seed for such important feed crops as alfalfa and red clover (Proctor et al. 1996). Spreading infestations of varroa mites and tracheal mites (they either kill bees outright or introduce lethal viruses into their bodies) are very difficult to control (NRC 2006a). Introduced African bees, which began their northward expansion in Brazil in 1956, have been destroying native wild colonies in the Americas, and indiscriminate use of pesticides has been the most important human factor in roughly halving the North American honeybee count during the second half of the twentieth century (Watanabe 1994; Kremen et al. 2002). This worrisome trend took a dramatic turn during the winter of 2006-2007, when beekeepers across North America reported widespread collapses of entire bee colonies. The usual suspects included a variety of pathogens, pesticides, and the high-fructose sugar syrup diets used to feed the hives in winter (and their contaminants). The most likely cause has been a virus from Australia (Stokstad 2007).

Loss of biodiversity takes place mostly because of the destruction or substantial alteration of natural habitats (Millennium Ecosystem Assessment 2005). Agricultural land is now the largest category of completely transformed, much less biodiverse land. In 2005 its extent, including permanent tree crops, was about 15 million km2, and the three largest grain crops—cultivars of wheat (originally from the Middle East), rice (from the Southeast Asia), and corn (from Mesoamerica)—are now grown on every continent and occupy a combined area of about 5 million km2, more than all remaining tropical forests in Africa. Land under settlement and

European honeybee, Apis mellifera, a pollinator without whose toil we would not have many of our fruit and nut crops. Photo from National Human Genome Research Institute, courtesy U.S. Department of Agriculture.

European honeybee, Apis mellifera, a pollinator without whose toil we would not have many of our fruit and nut crops. Photo from National Human Genome Research Institute, courtesy U.S. Department of Agriculture.

bearing industrial and transportation infrastructures adds up to about 5 million km2, and water reservoirs occupy about 500,000 km2. Human activities have thus entirely erased natural plant cover on least 20 million km2, or 15% of all ice-free land surface.

Areas that still resemble natural ecosystems to some degree but that have been significantly modified by human actions are much larger. Permanent pastures total about 34 million km2, and at least one-quarter of this area is burned annually in order to prevent the growth of trees and shrubs. A very conservative estimate of the global extent of degraded forests is at least 5 million km2, and the real extent may be twice as large. Total area strongly or partially imprinted by human activities is thus about 70 million km2, no less than 55% of nonglaciated land. Another way to appraise the significance of this transformation is to estimate the share of global photosynthesis that is consumed or otherwise affected by human actions. Vitousek et al. (1986) calculated that during the early 1980s humanity appropriated 32%-40% of terrestrial photosynthesis through its field and forest harvests, animal grazing, land clearing, and forest and grassland fires. A more recent recalculation (Rojstaczer, Sterling, and Moore 2001) closely matched the older estimate.

The remaining wilderness has been pushed to subboreal and polar regions. During the late 1980s an inventory of its large (>400,000 ha) contiguous blocks found that more than one-third of the global land surface (nearly 48 million km2) is still wilderness, but 40% of the total was in Arctic or Antarctic (McCloskey and Spalding 1989). Territorial shares of remaining wilderness ranged from 100% for Antarctica and 65% for Canada to less than 2% for Mexico and Nigeria and, except for Sweden (about 5%), to zero for even the largest European countries. There were also no undisturbed large tracts of land in such previously biodiverse ecosystems as the tropical rain forests of the Guinean Highlands, Madagascar, Java, and Sumatra, and the temperate broadleaf forests of eastern North America, China, and California.

This enormous transformation and degradation will continue (in Asia largely unabated, in Africa much accelerated) during the first half of the twenty-first century, with the tropical deforestation and conversion of wetlands causing the greatest losses of biodiversity (fig. 4.12). Quantifying this demise has been difficult. Nearly 850 species were lost worldwide between 1500 and 2000 (Baillie, Hilton-Taylor, and Stuart 2004), an average rate of one to two per year that is roughly 1 OM higher than the natural rate of extinction. However, for the past century the rate may have been about 100 times faster than indicated by the fossil record, and depending on the rate of habitat destruction, it may become 1,000 times faster during the next 50 years (Millennium Ecosystem Assessment 2005). The best compilation suggests that 12% of known bird species, 23% of mammals, 25% of coniferous trees, and 32% of amphibians are threatened with extinction (IUCN 2001), and unlike recent extinctions, which took place mostly on oceanic islands, continental extinctions will be the norm.

Climate change could become a major cause of extinction in addition to excessive exploitation and extensive habitat loss (to which climate change directly

Deforestation in the Amazon Basin, Bolivia, from logging, ranching, and settlement. Remaining forest shows as dark patches. Satellite image, January 8, 2000, from USGS EROS Data Center Satellite Systems Branch, <http://209.15.138.224/bolivia_maps/s_deforestation_bolivia.htm>.

Deforestation in the Amazon Basin, Bolivia, from logging, ranching, and settlement. Remaining forest shows as dark patches. Satellite image, January 8, 2000, from USGS EROS Data Center Satellite Systems Branch, <http://209.15.138.224/bolivia_maps/s_deforestation_bolivia.htm>.

contributes). The theoretical potential for extinctions driven by climate change is substantial (Lovejoy and Hannah 2005). Thomas et al. (2004) used projections of species distributions (including hundreds of plants, mammals, birds, frogs, reptiles, and invertebrates) to asses extinction risks in a sample region covering about 20% of the continental surface and found that by the year 2050 minimal, medium, and maximum climate change scenarios would condemn, respectively, 18%, 24%, and 35% of species to extinction. Given the many uncertainties, these rates are not to be seen as predictions but as worrisome indicators of the extent of possible change. By 2050 the biosphere is likely to lose thousands of species ranging from Indian tigers to Central American tree frogs to cycad plants.

Further degradation of biodiversity will take place because of the continuing deliberate introductions and unintended invasions of alien plants, animals, and invertebrates that have already changed ecosystems on all continents (Mooney and Hobbs 2000; Baskin 2002; Cox 2004; Sax et al. 2005). Croplands are obviously anthropogenic ecosystems, but many seemingly natural landscapes are as well. For example, 99% of all biomass in parts of the San Francisco Bay is not native (Enserink 1999), and bioinvasions have had particularly devastating effects on islands. Oahu's native vegetation now occupies less than 5% of the island, and even the Galápagos Islands, a national park where 97% of land is protected, have been heavily invaded by rats, pigs, goats, cats, ants, and quinine trees (Kaiser 2001).

The recent rush to cultivate energy crops should be seen "as adding biofuels to the invasive species fire" (Raghu et al. 2006, 1742) because such high-yielding grass species as giant reed, reed canary grass, and switchgrass have been shown to be actually or potentially invasive in U.S. ecosystems. Finally, there are also the largely unexplored consequences of the global spread of microbes in the ballast water of commercial ships (Ruiz et al. 2000). New bioinvasions are largely unpredictable, but their progress is often very rapid, and once they are under way, they are nearly impossible to stop. The rising volume of global trade and travel will only multiply the opportunities for unintended species introductions.

The consequences of bioinvasions often include a profound transformation of affected ecosystems and a high economic cost. Zebra mussels and water hyacinth are two notable examples. The mussels were carried by ships in European ballast water to North America for generations, but they took hold in Ohio and Michigan in 1988, and by the end of 2000 they had penetrated all the Great Lakes and the Mississippi basin. Their massive colonies cloak and clog underwater structure and pipe inlets, reduce the presence of native mussel species, and cause economic damage

European zebra mussels, Dreissena polymorpha, and their distribution in North America. From USGS (2006).

European zebra mussels, Dreissena polymorpha, and their distribution in North America. From USGS (2006).

in billions of dollars per decade (fig. 4.13) (USGS 2006). Water hyacinth (Eichhor-nia crassipes, an Amazonian native) has taken over many river, lake, and reservoir surfaces on five continents, with major local and regional economic impacts. It asphyxiates native biota; interferes with fishing, electricity generation, and recreation; increases evaporation from infested surfaces; and serves as a breeding ground for disease vectors (McNeely 1996).

Why does all this matter? It is clear that overexploitation, loss of natural habitats, and species introductions and invasions lead to ecosystemic impoverishment and homogenization. These change previously unique and species-rich communities into homogenized assemblages dominated by a few generalists, be they pests or weeds. Sparrows, crows, pigeons, rats, mice, and feral dogs are the inheritors of the biosphere molded by humans. But there is much more than this esthetic impoverishment and lamentable loss of genetic information that took so many generations to select and perfect. The loss of biodiversity and bioinvasions have major economic consequences, and if severe enough, they can reduce the stability and resilience of ecosystems and seriously compromise the delivery of irreplaceable ecosystemic services.

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