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Home , Habitat destruction

Biodiversity – Threats Introductory article

Weston W Sechrest, University of Virginia, Charlottesville, Virginia, USA Article Contents Thomas M Brooks, Center for Applied Science – Conservation International, . Introduction Washington DC, USA . . Genetic and Behavioural Degradation of Taxa . Any direct or indirect human activity that threatens the planet’s biological diversity in the Destruction . form of genes, , species, , or other levels of biological organization Habitat Degradation and Fragmentation . is considered a threat to survival. . Global Change . Introduction . Disease . Exploitation Biological diversity or biodiversity is defined as the variety . Summary of the planet’s living organisms and their interactions. The . Acknowledgements term biodiversity encompasses all of life’s variation, expressed in genes, individuals, populations, species, communities and ecosystems. Quantitative measures of The transition that began around 10 000 years ago from biodiversity most often focus on a taxonomic unit, gathering and cultures to sedentary agricultural typically the species, although aspects of ecological societies spurred human growth from several diversity can also be measured. Biodiversity is a dynamic million to over 6 billion in the year 2000. This population entity, and has changed throughout the history of life on growth, coupled with the proliferation of new technologies Earth. and accompanying resource consumption, has created a The mechanisms responsible for biodiversity change are biodiversity threat rivalling all past natural threats. evolutionary processes of and extinction, along Modern human actions threaten biological diversity on a with ecological processes over shorter time periods. worldwide scale, over an extremely short geological time Extinction and speciation are well catalogued in the period. The sources of danger to biodiversity include palaeontological record as instrumental to biodiversity taxonomically specific threats such as exploitation, intro- fluctuations. Species interactions, environmental change, duced species, and genetic or behavioural degradation, all and even cosmic disturbances (meteors, tidal interactions, of which can interact and ultimately result in extinction. and solar processes) have played key roles in shaping past These threats combine with the community and and present biodiversity. Throughout the course of life on level threats of habitat degradation, fragmentation, and this planet, average background extinction rates have been destruction, pollution and global , causing punctuated by extinction episodes, the five most devastat- disruption and alteration of community and ecosystem ing of which are termed mass . Over 99% of all structure and function. Humans have triggered a sixth species that have existed on this planet are now extinct. mass extinction. Present biodiversity is nonetheless impressive, with over 1.75 million species described and at least an order of Extinction magnitude more species still unknown (Figure 1). A sixth new mass extinction is now underway. This The most obvious loss of biodiversity is the extinction of recent threat to biodiversity arose in the early , unique taxa. Extinction occurs when no more individuals marking the beginning of hominid activities affecting of a taxonomic group survive, either within a specified part biodiversity on increasingly larger scales, aided by tool and of their range or forever lost across their entire range. The fire use. Prehistoric extinctions triggered by humans taxonomic unit of extinction is usually measured as a include loss of in North America, Australia, species, though extinction can be assessed at subspecific or Asia, and to a lesser extent and Africa. For population levels. A species, by definition, is evolutionarily example, Australia lost 23 of 24 large terrestrial vertebrate unique; each species has distinct genetic, evolutionary, genera (with body size greater than 45 kg) within a short behavioural and ecological attributes that once lost cannot time period around 46 000 BP. This extinction trend be replaced. continued into the late Holocene with oceanic The process of extinction and speciation has been extinctions in the Pacific and other regions, including continual; as new species have arisen others have dwindled and , decimating several thou- and become extinct. The one constant of evolutionary sand species along with losses of other taxa, change has provided varying amounts of diversity over transforming many island ecosystems. geological time. Throughout the history of life on this

ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Publishing Group / www.els.net 1 Biodiversity – Threats

Viruses Prokaryotes Algae Protozoa Fungi Lower plants Higher plants Nematodes Annelids Arthropods (incl. insects) Vertebrates Soil macrofauna Marine macrofauna Terrestrial macrofauna Terrestrial flora All species 1 × 103 1 × 104 1 × 105 1 × 106 1 × 107 1 × 108 Number of species Described up to 1995 Expert opinion Extrapolation

Figure 1 Number of described species and estimates of species numbers, including expert opinions of taxonomic specialists and various extrapolations (Pimm et al., 1995) planet, losses of biodiversity have been common. There is Extinction, by reducing overall diversity, creates a more palaeontological evidence for five mass extinctions, during biologically and ecologically uniform biosphere. In addi- which many taxonomic groups lost a majority of species. tion to the loss of ecological diversity, the phylogenetic The current extinction crisis has seen species lost at a rate history of each species or taxonomic group is lost for ever. perhaps 1000 to 10 000 times the average background rate The ultimate tool for increasing diversity, evolutionary identified by the fossil record. Indeed, the present change, requires extremely long geological time periods. extinction episode may eventually rival in rate and Ecosystem recovery from drastic biodiversity losses in magnitude all previous episodes on this planet. The cause previous mass extinctions required millions of years. of virtually every present extinction lies ultimately in anthropogenic actions. Current taxonomic extinction risk has been system- atically assessed by several organizations, most notably the Genetic and Behavioural Degradation International Union for the Conservation of Nature and of Taxa Natural Resources (IUCN). Detailed information is available for well-known groups of organisms, including Biological diversity encompasses the diversity of popula- most vertebrates and flowering plants, and to a much lesser tions within species, as well as genetic and behavioural extent for invertebrates, other plants and fungi. Since 1600, diversity within populations. Species differences are the at least 1.84% of and 1.20% of bird species have most easily recognizable form of diversity, although become extinct. Present calculations estimate 25% of differences at the population and genetic levels are mammals and 12% of at risk of extinction with a necessary components for species survival. As the line probability of at least 10% over the next 100 years. Species between species is sometimes vague – for example, due to at risk are mainly those that have small range or population hybridization or asexual reproduction – the distinction sizes, especially species that have become rare due to between populations within species is likewise often human activity. Delayed extinction following habitat loss difficult to establish. There are two main mechanisms of or other mechanisms of and range reduction genetic and behavioural degradation, the outright loss of may take tens to thousands of years, even if all present populations and alteration of populations as a result of anthropogenic threats cease. Invertebrates and marine human activity. Extreme examples of both can be seen in taxa have had comparatively little scientific attention paid captive populations, of relevance to biodiversity when to them, though by all calculations they contain a most or all surviving individuals of a species are in significant amount of species at risk of extinction in the captivity, such as Spix’s macaw (Cyanopsitta spixi). foreseeable future. Certain terrestrial and marine geo- However, the main threat from such degradation is to graphic regions, especially in the , contain high wild populations. proportions of endemic species. These areas often con- Many behavioural differences exist among species’ currently face disproportionately greater threat from populations; for example, separate chimpanzee ( habitat loss and other perils. troglodytes) populations utilize different tools, such as sticks to extract from their mounds or rocks to

2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net Biodiversity – Threats break open nuts. Behaviours are not reserved to verte- Habitat Destruction brates; invertebrates can also alter or lose behaviours as a result of human influence. For example, the monarch One of the most devastating threats to biodiversity is the butterfly (Danaus plexippus), though not endangered as a outright loss of habitat due to human activity. Habitat loss species, has populations that undergo remarkable migra- typically involves conversion of for other uses, tions along the west and east of North America, including urban and agricultural areas. Once removed, a involving several generations, from California and Mex- natural habitat is often permanently lost, although natural ico, respectively, northward and back to small over- or artificial restoration of some is possible over wintering areas where they are threatened with habitat time. loss. The loss of unique behaviours such as this will take a Terrestrial ecosystems suffer habitat destruction in a toll on overall diversity, and indeed can have long-term variety of ways, such as , desertification, cascading effects on other populations and species. and artificial burning. Many terrestrial Alteration of behaviours in response to human activity ecosystems have been converted to urban and agricultural also lessens natural diversity, as many species, such as areas. Temperate regions in and North America house sparrows (Passer domesticus) and Northern racoons have lost almost all primary vegetation over the last few (Procyon lotor), have become acclimated to humans and centuries, often for the inefficient purpose of raising crops have lost many of their natural behaviours. to feed animals for human consumption – although some Populations within species often show many genetic areas have since regenerated secondary . The differences that are translated into different biological and situation is different in many tropical regions, where most ecological roles in local ecosystems. Over the range of a primary vegetation losses have occurred over the last species, populations may have different life histories, such century. In the tropics, where most necessary nutrients are as onset of breeding. Populations of a species in separate contained in the living biomass, loss of vegetation over geographic areas may be part of different communities and large areas can result in permanent land transformation, as associated food webs or competitive interactions. Loss of a soils are relatively poor in nutrients. For example, population can have consequences for the biodiversity of historically Madagascar was largely forested, but over local ecosystems, as well as the overall genetic diversity the course of the last thousand years or so, humans have within the species. cleared the majority of forests, typically using fire. This has Genetic diversity is critical for survival over evolution- resulted in the island’s interior being mostly converted to ary time, as loss of populations within species lowers grassland, agricultural fields and denuded land, with genetic diversity, making them less equipped to adapt to virtually no regeneration. environmental or ecological changes. Conservation genet- Habitat destruction is also a major threat to biodiversity ics is a field that focuses on the genetic consequences of in aquatic regions. This type of habitat loss occurs from population and species fragmentation and decline. A large dam construction, filling, water flow diversion, oil effective population size, between 5000 and 10 000 drilling, pollution and bottom trawling in addition to other individuals, is theorized to be sufficient to ensure long- human activities. The majority of freshwater ecosystems term survival. Theoretical and empirical studies of small have been altered, and many vital wetland and aquatic populations have demonstrated the potential negative habitats have been destroyed. Wetland loss particularly effects of inbreeding depression and loss of genetic affects diversity, as are often important centres of adaptability, demographic and environmental stochasti- regional and local diversity. In coastal environments, city, and susceptibility to disease and catastrophes such as habitat has been destroyed in many river deltas, where hurricanes. Often, intraspecific competition and mortality large quantities of , pesticides, and industrial decrease in species at low density and/or small total pollutants empty into gulfs and bays, creating zones population size, leading to an increase in density and without viable habitats. Runoff from the Mississippi River numbers. However, some species at a low density and/or has created a ‘dead zone’ of low marine productivity total population size exhibit a direct relationship between stretching over thousands of square kilometres of the Gulf rate and density, which is called the of Mexico. Harvesting of marine life using bottom Allee effect. This can be a result of lower recruitment or trawling is equivalent to deforestation on land, and has higher mortality, but ultimately this process can prevent transformed large areas of biologically diverse sea bottom population recovery and lead to further decline and into barren landscape. extinction. Reduction in population size can precipitate The impact of habitat loss on biodiversity can be problems such as appearance of deleterious alleles and loss described using a simple empirical model, the species–area of overall genetic diversity that can potentially lead to relationship. This states that the number of species found population or species extinctions. These problems are in an area is consistently related to the size of that area exacerbated by other threats to biodiversity, including (specifically, by a power function with an exponent of a and climatic change, such that small quarter). Thus, if an area is reduced in size by half, population size is often a brief prelude to extinction. approximately a sixth of the species found within it will

ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 3 Biodiversity – Threats

Preisolation Postisolation Extraction of resources, such as removing certain plant species, can affect ecosystem structure and function. Another form of degradation includes activities or management techniques that disrupt natural cycles or regimes. These include crop irrigation, which disrupts water flow, and managed natural fire suppression in areas where fire-adapted species occur. Freshwater ecosystems have been degraded; over 60% of the world’s ABCtotal water flow has been altered, mostly for power generation by dams, , flood control and drinking water. The impacts on biodiversity are clear; in Stotal Asia, the Aral Sea has lost two-thirds of its volume and over half of its surface area due mainly to irrigation, and Soriginal Sfragment salinity levels have tripled. None of the 24 native fish species remain. Unfortunately, habitat fragmentation has also increased in ecosystems as a result of human alteration and destruction of habitat. Natural habitats always have some degree of heterogeneity, but increased disturbance due to

Number of species human causes has created a new problem. Isolation of habitats to fragments of their original size leads to ‘edge effects’, habitat , decreased organismal dispersal

0 capability, and genetic isolation. Habitat edges experience Atotal Afragment 0 a different microclimate than that experienced within the Area interior of a habitat. Changes in the local microclimate can Figure 2 Typical species–area relationships. Larger areas (A) have more result from air currents, increased sunlight, and . species than smaller ones (B, C), and areas that have long been isolated – Many species, including mammalian carnivores such as such as islands – have proportionally fewer species (C) than do equal sized areas that are nested within continuous habitat (B) (Pimm and Brooks, grey wolves (Canis lupus), require large home ranges in 1999). relatively continuous tracts of land. Fragmentation upsets the ecosystem by decreasing the ability of species with large home ranges or specialist habitats to survive, while at the eventually become extinct (Figure 2). Clearly, all of these same time providing opportunities for species that extinctions will not happen immediately (although some proliferate on the edges of habitat, such as the nest- will), but will occur over a time-lag, the length of which is parasitic Brown-headed Cowbird (Molothrus ater) in the dependent on the absolute size of the area remaining. For forest/grassland ecotone. example, the length of the time-lag for the loss of New genetic research has begun to uncover the species from the Indonesian Greater Sunda Islands importance of continuous habitat in the maintenance of following the rise of sea levels at the end of the Pleistocene populations. Fragmentation of habitat can result in was on the order of millennia. In contrast, the extinction of decreased populations and range size for many species. bird species from forest fragments in Kenya has been Some species cannot or will not disperse across fragmented shown to have occurred over less than a century. In habitats, especially when the intervening landscape con- summary, maintenance of viable habitats is crucial for tains barriers such as . Planned corridors are maintaining biodiversity. necessary for continued connection between populations and maintenance of genetic diversity. The genetic con- sequences of are exacerbated by Habitat Degradation and disruption of dispersal patterns in fragmented habitats. Fragmentation Less devastating but more insidious than outright destruc- Pollution tion of habitats is their fragmentation and degradation. Degradation of habitats occurs when some aspect of the Pollution is defined as contamination of the natural natural environment is removed or altered. Alteration can environment. Pollution can be in the form of liquids, include addition of pollutants, which make habitats less solids, gases, or even forms of electromagnetic radiation suitable for some organisms. Other activity such as heavy input into air, water, or land. Since the industrial human or usage can also degrade habitat quality. revolution, the input of organic and inorganic substances

4 ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net Biodiversity – Threats into the environment by humans has become a growing In the case of acid deposition and other aerosol pollutants, threat to biodiversity. soil and water far from the site of pollution emission are Pollution can be acute, with a single incident, or chronic, often affected. The upper atmosphere’s ozone layer has with the addition of substances to the environment over a been reduced due to chlorofluorohydrocarbons and other continuous time period. Examples of acute environmental ozone-depleting chemicals released into the atmosphere. disasters include oil spills, refinery and shipping accidents, This has allowed more penetration of ultraviolet light, and nuclear accidents. Although the initial effects of these which can be harmful to biological organisms such as open disasters can result in massive , there are plankton communities. often longer lasting repercussions as well. An example of an acute disaster with long lasting effects is the prolonged ecological impact of radioactive material downwind of the Chernobyl nuclear power plant in Ukraine following an Global Climate Change explosion in 1986. Sources of chronic pollution from human activity include industrial emissions, aerosol Biological organisms interact with their environment and release from biomass burning, agricultural runoff, pesti- vice versa, so environmental change is a key determinant of cides, erosion, and automobile emissions. Although the which organisms speciate, which thrive and which become immediate effects of chronic pollution may be small, extinct. Perhaps the most crucial mechanism of environ- sustained rates and accumulation of chronic pollution can mental change is the global climate. Climate change is be more devastating than acute environmental disasters. reflected in alterations in atmospheric, hydrological and There are many examples of pollution and its impact on biogeochemical cycles. These changes are associated with biodiversity. Many released toxic elements and com- volcanic activity, changes in atmospheric chemistry, tidal pounds resist degradation and accumulate in the environ- changes, glaciation and melting of ice caps. Fluctuations ment. Organic chemicals with these properties are called such as slight changes in average daily temperatures, the persistent organic pollutants (POPs); organisms incorpo- duration of rainy seasons, night-time temperature, the rate toxins in their tissues and the pollutants are subse- , and solar radiation, among others, can affect quently biomagnified through food webs. Many species at biological organisms. Plants respond to critical climatic high trophic levels, especially marine mammals such as the variables such as daily high temperatures, extended beluga whale (Delphinapterus leucase), have been found , and other changes. Invertebrates have physio- with impaired reproductive or immunological function as a logically established tolerance levels, and cannot survive result of pollutants. outside of certain ranges. Vertebrates, such as mammals, Artificial lights can change predator–prey dynamics, are associated with certain habitat types that are dependent and underwater sonic pollution has been shown to on climate. Species that migrate, such as caribou (Rangifer adversely affect behaviour adversely. tarandus), are susceptible to local changes in climate, and Adaptation by organisms in response to pollution can as such could be the first to be affected. reduce diversity. For example, resistance to pesti- The average temperatures in the twentieth century were cides can create hardy taxa that have the potential to approximately 0.6 degrees Celsius higher than over recent devastate natural ecosystems. While the attention called to centuries. Recent research on ice cores and tree rings, along these problems by Rachel Carson’s Silent Spring has led to with other evidence, has established the scientific data the introduction of strict regulation of pesticide use in necessary to demonstrate this trend of increasing tempera- many countries, harmful chemicals including DDT are still ture. The worldwide international authority on climate used as pesticides in much of the tropics. change, the Intergovernmental Panel on Climate Change The biodiversity effects of pollution often hit higher (IPCC), has conclusively acknowledged that human levels of organization, altering community and ecosystem activity has played a role in the current climatic warming. structure and functions. Natural substances redistributed Models are being constantly revised to predict possible or manufactured by humans can also pollute the environ- consequences and the magnitude of global climate change ment. Examples include freshwater eutrophication, acid due to human activities, with an expected increase of 1.4 to deposition and . Eutrophication of fresh- 5.8 degrees Celsius in the twenty-first century. water bodies results from the chronic additions of nutrient Human activities that affect global climate change runoff from agriculture, such as nitrogen and phosphorus, include the production of from sources such as well as atmospheric nutrient deposition from industry. as fossil fuel combustion and burning of forests. Agricul- This affects hard-to-measure changes like reduction in ture also plays a large role by reducing available forests microorganism diversity and can have cascading effects on that serve as carbon sinks releasing from local plant diversity and the animals dependent on them. slash and burn agriculture and by the release of methane, a Acid deposition (either dry or wet) is caused by the potent from livestock. emission of sulfur dioxide and nitrous oxides, mainly from The message is grim for the planet’s biological diversity. industrial and automobile emissions, into the atmosphere. Slight changes in climatic patterns could have major

ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 5 Biodiversity – Threats effects, while any large local, regional or global change and by goat (Capra hircus) and sheep (Ovis aries) grazing could have cataclysmic effects. Already, delicate oceanic on islands lacking large herbivores. Such introductions ecosystems have declined recently as ocean have contributed to the extinction and endangerment of temperatures have increased. Coastal regions could be many bird and plant species (along with associated quickly inundated due to rapidly rising sea levels, already invertebrate species), because naive insular populations estimated to have increased 0.1 to 0.2 metres over the last are extremely vulnerable to predation or competition from century. This could prove catastrophic for some of the non-. species and diverse communities in this ecotone. The increased fragmentation, degradation, and destruc- Other particularly are those that tion of habitats, along with other threats, will certainly cannot easily track climatic changes: those with small open more niches for non-native species introductions. range size; island or mountaintop species; those with low in ecosystems is apparently important in reproductive capability; and those with little dispersal preventing introduced species from establishing perhaps ability such as many plant species or freshwater organisms due to increased stability. The anthropogenic transport confined to dispersal through waterways. Past climatic and relocation of species threaten biogeographic differ- changes have resulted in ecosystems with different species ences in fauna and flora. The result could drastically lessen compositions, due to species’ different abilities to track or biodiversity, resulting in a taxonomically and ecologically adapt to climatic changes. The rate of human-induced homogeneous planet. climate change promises to far exceed any known previous warming, so the biological effect cannot be easily predicted. Disease

A special case of the threats to biodiversity caused by species introductions is the expansion of pathogens or Introduced Species parasites resulting from human activity. Disease can result from genetic disorders, pathogens such as or Introduced species are those that are found in areas outside , or parasites. Coevolution of hosts and pathogens of their native range due to direct or indirect human over evolutionary time results in coexistence of both host activity. The anthropogenic mechanisms for introduction and pathogens. Imbalance resulting from human activity, are direct transport, removal of competitors, habitat such as reduction of populations to small size, opens alteration and human-induced climate change. Only some conditions for pathogens to spread. Diseases are often species introduced into new areas become established, and transmitted across different species, with the new host only some of the established species cause large changes in species often devastated by the new pathogen. For native biodiversity. Certain species have become human example, American chestnut (Castanea dentata) trees are commensals, colonizing areas where humans inhabit, all but wiped out due to the introduction of chestnut blight including the house mouse (Mus musculus), Norway rat (Cryphonectria parasitica) that had evolved in Asia (Rattus norvegicus), black rat (Rattus rattus), dog (Canis with the closely related Chinese chestnut (Castanea familiaris) and pig (Sus scrofa). mollissima). Species that are successful invaders into new areas are Organisms that are affected by environmental contami- generally ones that can tolerate novel environmental nants, such as exposure to organochlorines, may play a role conditions, competition, predation and other ecological in lowering immune response and resistance to disease. interactions, and have intrinsic biological characteristics Compromised immune function resulting from contami- including high reproductive capability, broad diet and high nants or stress can potentially push populations or species dispersal rates. Few species are successful invaders, because at risk over the edge. Canine distemper killed most of many cannot survive in new environments. For instance, the remaining wild black-footed ferrets (Mustela nigripes), though there are over two thousand rodent species, only a forcing capture of the rest, which have subsequently been handful are commonly spread by humans. Introduced plant used in a reintroduction programme. Introduced diseases species have wreaked havoc on many ecosystems; for are often more deadly, as host–pathogen dynamics are example, in North America, purple loosestrife (Lythrum usually the product of a long history of coevolution. salicaria) has taken over many wetland areas and kudzu vine (Pueraria lobata) is common along the edges of forests. In Africa, the water hyacinth (Eichhornia crassipes), introduced from the New World, is causing ecological and economical Exploitation devastation to lake systems. Many islands have been successfully colonized by In general, any changes to an ecosystem can potentially animals that have filled vacant ecological niches. This is result in biodiversity loss. One of the most direct demonstrated across the Pacific, for example by feral cats anthropogenic mechanisms for the loss of diversity is (Felis catus) when introduced into predator-free islands extraction of biological organisms from their natural

6 ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net Biodiversity – Threats environment. Globally, most ecosystems have only rela- other treaties banning the trade of many endangered tively recently experienced large-scale extraction by hu- species, target species including (Panthera tigris) and mans. (Loxodonta africana) are still decimated by Humans extract biological organisms from nature for poaching. Many cultures attach economic, spiritual or food, energy and other resources. Some of the most medicinal significance to certain species. The combined widespread exploitation is in the form of fishing, hunting demand for captive pets and products, such as and . The majority of human food is obtained from parts for traditional Asian medicines, crocodile skins and domestic plant crops and animals, though hunting and rhinoceros horns, threatens to bring many species to the gathering in natural habitats is still common. The killing of point of extinction. Such practises and beliefs can be wildlife species by humans constitutes a serious threat to equally devastating to plant populations. As an example, biodiversity. The bush meat trade in Africa and Asia wild ginseng (Panax quinquefolium) and many other comprises a culturally based demand for wild animals, species are now extinct across much of the USA as a result mostly ungulate and primate species, and has decimated of overharvesting for the illegal trade in ‘herbal’ medicines many populations and species. In both freshwater and which end up lining the shelves of drugstores. marine areas, food extraction is primarily on wild species. A special class of exploitation is the logging of forests, The direct threat of harvesting on native biodiversity has which presents probably the greatest single threat to been felt, as most marine fisheries have experienced drastic biodiversity worldwide. The of most logging collapses in target species. operations even in temperate regions is highly question- Vertebrate species are especially targeted for food and able, and it is probable that all logging in the tropical other economic benefits. Despite the Convention on the forests is unsustainable. Already, most of the world’s International Trade in (CITES) and temperate and tropical regions have lost significant

Mediterranean Caucasus South-Central California Floristic Province Indo- Caribbean Burma Eastern Arc & Polynesia/ Coastal Forests Micronesia Mesoamerica Brazil’s of Tanzania & Cerrado Kenya Western Chocó/ Tropical West Andes African Ghats/ Darien/ Sri Lanka Western Forests Ecuador Brazil’s Sundaland Wallacea Atlantic Polynesia/ New Forest Micronesia Caledonia Central Succulent Southwest Chile Karoo Madagascar & Australia Indian Ocean Islands New Zealand Cape Floristic Province

Figure 3 The 25 hotspots (Myers et al., 2000).

ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 7 Biodiversity – Threats proportions of forested areas. For example, 15 of the Acknowledgements world’s 25 biodiversity ‘hotspots’ are in the tropical forests, each holding the entire global range of more than 0.5% of The authors would like to acknowledge K. Billmark, J. the world’s plant species and now retaining less than a third Pilgrim and C. Sechrest for their helpful comments on this of its historical forest cover (Figure 3). Only five major article. forested wildernesses remain: three in the tropics – the Amazon and Congo Basins and the island of New Guinea; plus the boreal forests of Canada and Russia. Further Reading Recently, focus has been on sustainable use of biological resources, most often setting quotas on number of Baillie J and Groombridge B (eds) (1996) IUCN Red List of Threatened organisms killed, or restrictions of certain areas. For Animals. Gland, Switzerland: International Union for the Conserva- marine areas, research has shown that fish stocks are often tion of Nature and Natural Resources. tied into source populations, which disperse larva or adults Carson R (1962) Silent Spring. Boston, MA: Houghton Mifflin. Grenfell BT and Dobson AP (1995) Infectious Diseases of Wildlife. to other areas through ocean currents. Protection of source Cambridge: Cambridge University Press. populations could mitigate some of the pressure on a Laurance WF and Bierregaard RO Jr (eds) (1998) Tropical Forest species. The success of long-term sustainable exploitation Remnants: Ecology, Management, and Conservation of Fragmented has yet to be proved, and use of scientific research and Communities. Chicago, IL: University of Chicago Press. monitoring is increasingly necessary to prevent complete Myers NA, Mittermeier RA, Mittermeier CG, da Fonseca GAB and collapse of target species. Kent J (2000) Biodiversity hotspots for conservation priorities. Nature 403: 853–858. Peters RL and Lovejoy TE (eds) (1992) Global Warming and Biological Diversity. Newhaven, CT: Yale University Press. Pimm SL and Brooks TM (1999) The sixth extinction: How large, where and when? In: Raven PH and Williams T (eds) Nature and Human Summary Society: The Quest for a Sustainable World pp. 46–62. Committee for the Second Forum on Biodiversity, National Academy of Sciences and The current threats to biodiversity all have one element in National Research Council. Washington DC: National Academic common. They are directly or indirectly the result of one Press. dominant species, Homo sapiens. This differs from other Pimm SL, Russell GJ, Gittleman JL and Brooks TM (1995) The future of major episodes of biodiversity loss, when natural environ- biodiversity. Science 269: 347–350. mental and ecological changes were the cause. Habitat loss, Soule´ ME (ed.) (1987) Viable Populations for Conservation. Cambridge: degradation and fragmentation, pollution and climate Cambridge University Press. Walter KS and Gillett HJ (1998) 1997 IUCN Red List of Threatened change, introduced species and disease, and direct ex- Plants. Gland, Switzerland: International Union for the Conservation ploitation have together precipitated a global disaster for of Nature and Natural Resources. biological diversity. As a result much of the Earth’s genetic, Williamson M (1996) Biological Invasions. London: Chapman & Hall. species and ecological diversity is at grave risk. Wilson EO (1993) The Diversity of Life. Cambridge, MA: Belknap Press.

8 ENCYCLOPEDIA OF LIFE SCIENCES / & 2002 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net