Global biodiversity outlook 31
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Genetic diversity Genetic diversity is being lost in natural ecosystems and in systems of crop and livestock production. Important progress is being made to conserve plant genetic diversity, especially using ex situ seed banks. The decline in species populations, combined with the fragmentation of landscapes, inland water bodies and marine habitats, have necessarily led to an overall significant decline in the genetic diversity of life on Earth. While this decline is of concern for many reasons, there is particular anxiety about the loss of diversity in the varieties and breeds of plants and animals used to sustain human livelihoods. A general homogenization of landscapes and agricultural varieties can make rural populations vulnerable to future changes, if genetic traits kept over thousands of years are allowed to disappear. An example of the reduction in crop diversity can be found in China, where the number of local rice varieties being cultivated has declined from 46,000 in the 1950s to slightly more than 1,000 in 2006. In some 60 to 70 per cent of the areas where wild relatives of rice used to grow, it is either no longer found or the area devoted to its cultivation has been greatly reduced148,149. Significant progress has been made in ex situ conservation of crops, that is the collection of seeds from different genetic varieties for cataloguing and storage for possible future use. For some 200 to 300 crops, it is estimated that over 70% of genetic diversity is already conserved in gene banks, meeting the target set under the Global Strategy for Plant Conservation150. The UN Food and Agriculture Organization (FAO) has also recognized the leading role played by plant and animal breeders, as well as the curators of ex situ collections, in conservation and sustainable use of genetic resources151,152,153. However, major efforts are still needed to conserve genetic diversity on farms, to allow continued adaptation to climate change and other pressures. Additional measures are also required to protect the genetic diversity of other species of social and economic importance, including medicinal plants, non-timber forest products, local landraces (varieties adapted over time to particular conditions) and the wild relatives of crops. Standardized and high-output systems of animal husbandry have led to an erosion of the genetic diversity of livestock. At least one-fifth of livestock breeds are at risk of extinction. The availability of genetic resources better able to support future livelihoods from livestock may be compromised. Twenty-one per cent of the world’s 7,000 livestock breeds (amongst 35 domesticated species of birds and mammal) are classified as being at risk, and the true figure is likely to be much higher as a further 36 per cent are of unknown risk status [See Figure 13]. More than 60 breeds are reported to have become extinct during the first six years of this century alone154. The reduction in the diversity of breeds has so far been greatest in developed countries, as widely-used, high-output varieties such as Holstein-Friesian cattle come to dominate. In many developing countries, changing market demands, urbanization and other factors are leading to a rapid growth of more intensive animal production systems. This has led to the increased use of non-local breeds, largely from developed countries, and it is often at the expense of local genetic resources155. Government policies and development programmes can make matters worse, if poorly planned. A variety of direct and indirect subsidies tend to favour large-scale production at the expense of small-scale livestock-keeping, and the promotion of “superior” breeds will further reduce genetic diversity. Traditional livestock keeping, especially in drylands, is also threatened by degradation of pastures, and by the loss of traditional knowledge through pressures such as migration, armed conflict and the effects of HIV/AIDS156. The loss of genetic diversity in agricultural systems is of particular concern as rural communities face ever-greater challenges in adapting to future climate conditions. In drylands, where production is often operating at the limit of heat and drought tolerances, this challenge is particularly stark157,158. Genetic resources are critically important for the development of farming systems that capture more carbon and emit lower quantities of greenhouse gases, and for underpinning the breeding of new varieties. A breed or variety of little significance now may prove to be very valuable in the future. If it is allowed to become extinct, options for future survival and adaptation are being closed down forever. Current pressures on biodiversity and responses The persistence and in some cases intensification of the five principal pressures on biodiversity provide more evidence that the rate of biodiversity loss is not being significantly reduced. The overwhelming majority of governments reporting to the CBD cite these pressures or direct drivers as affecting biodiversity in their countries. They are:
Habitat Loss and Degradation Habitat loss and degradation create the biggest single source of pressure on biodiversity worldwide. For terrestrial ecosystems, habitat loss is largely accounted for by conversion of wild lands to agriculture, which now accounts for some 30% of land globally. In some areas, it has recently been partly driven by the demand for biofuels159,160. The IUCN Red List assessments show habitat loss driven by agriculture and unsustainable forest management to be the greatest cause of species moving closer towards extinction161. The sharp decline of tropical species populations shown in the Living Planet Index mirrors widespread loss of habitat in those regions162,163. For example, in one recent study the conversion of forest to oil palm plantations was shown to lead to the loss of 73-83% of the bird and butterfly species of the ecosystem164. As noted above, birds face an especially high risk of extinction in South-east Asia, the region that has seen the most extensive development of oil palm plantations, driven in part by the growing demand for biofuel165. Infrastructure developments, such as housing, industrial developments, mines and transport networks, are also an important contributor to conversion of terrestrial habitats, as is afforestation of non-forested lands. With more than half of the world’s population now living in urban areas166, urban sprawl has also led to the disappearance of many habitats, although the higher population density of cities can also reduce the negative impacts on biodiversity by requiring the direct conversion of less land for human habitation than more dispersed settlements167. Even though there are no signs at the global level that habitat loss is declining significantly as a driver of biodiversity loss, some countries have shown that, with determined action, historically persistent negative trends can be reversed. An example of global significance is the recent reduction in the rate of deforestation in the Brazilian Amazon, mentioned above. For inland water ecosystems, habitat loss and degradation is largely accounted for by unsustainable water use and drainage for conversion to other land uses, such as agriculture and settlements. The major pressure on water availability is abstraction of water for irrigated agriculture, which uses approximately 70 per cent of the world’s withdrawals of fresh water, but water demands for cities, energy and industry are rapidly growing168. The construction of dams and flood levees on rivers also causes habitat loss and fragmentation, by converting free-flowing rivers to reservoirs, reducing connectivity between different parts of river basins, and cutting off rivers from their floodplains169. In coastal ecosystems, habitat loss is driven by a range of factors including some forms of mariculture, especially shrimp farms in the tropics where they have often replaced mangroves. Coastal developments, for housing, recreation, industry and transportation have had important impacts on marine ecosystems, through dredging, landfilling and disruption of currents, sediment flow and discharge through construction of jetties and other physical barriers170. As noted above, use of bottom-trawling fishing gear can cause significant loss of seabed habitat. Climate Change Climate change is already having an impact on biodiversity, and is projected to become a progressively more significant threat in the coming decades. Loss of Arctic sea ice threatens biodiversity across an entire biome and beyond. The related pressure of ocean acidification, resulting from higher concentrations of carbon dioxide in the atmosphere, is also already being observed. Ecosystems are already showing negative impacts under current levels of climate change (an increase of 0.74ºC in global mean surface temperature relative to pre-industrial levels), which is modest compared to future projected changes (2.4-6.4 ºC by 2100 without aggressive mitigation actions)171.). In addition to warming temperatures, more frequent extreme weather events and changing patterns of rainfall and drought can be expected to have significant impacts on biodiversity. Impacts of climate change on biodiversity vary widely in different regions of the world. For example, the highest rates of warming have been observed in high latitudes, around the Antarctic peninsula and in the Arctic, and this trend is projected to continue172. The rapid reduction in the extent, age and thickness of Arctic sea ice, exceeding even recent scientific forecasts, has major biodiversity implications [See Box 15 and Figure 14].
Already, changes to the timing of flowering and migration patterns as well as to the distribution of species have been observed worldwide179,180,181,182, 183,184,185. In Europe, over the last forty years, the beginning of the growing season has advanced by 10 days on average186. These types of changes can alter food chains and create mismatches within ecosystems where different species have evolved synchronized inter-dependence, for example between nesting and food availability, pollinators and fertilization. Climate change is also projected to shift the ranges of disease-carrying organisms, bringing them into contact with potential hosts that have not developed immunity. Freshwater habitats and wetlands, mangroves, coral reefs, Arctic and alpine ecosystems, dry and sub-humid lands and cloud forests are particularly vulnerable to the impacts of climate change. Some species will benefit from climate change. However, an assessment looking at European birds found that of 122 widespread species assessed, about three times as many were losing population as a result of climate change as those that were gaining numbers187. The specific impacts of climate change on biodiversity will largely depend on the ability of species to migrate and cope with more extreme climatic conditions. Ecosystems have adjusted to relatively stable climate conditions, and when those conditions are disrupted, the only options for species are to adapt, move or die188,189. It is expected that many species will be unable to keep up with the pace and scale of projected climate change, and as a result will be at an increased risk of extinction, both locally and globally190. In general climate change will test the resilience of ecosystems, and their capacity for adaptation will be greatly affected by the intensity of other pressures that continue to be imposed. Those ecosystems that are already at, or close to, the extremes of temperature and precipitation tolerances are at particularly high risk. Over the past 200 years, the oceans have absorbed approximately a quarter of the carbon dioxide produced from human activities, which would otherwise have accumulated in the atmosphere191. This has caused the oceans (which on average are slightly alkaline) to become more acidic, lowering the average pH value of surface seawater by 0.1 units192193. Because pH values are on a logarithmic scale, this means that water is 30 per cent more acidic. The impact on biodiversity is that the greater acidity depletes the carbonate ions, positively-charged molecules in seawater, which are the building blocks needed by many marine organisms, such as corals, shellfish and many planktonic organisms, to build their outer skeletons194,195196,197,198199. Concentrations of carbonate ions are now lower than at any time during the last 800,000 years200. The impacts on ocean biological diversity and ecosystem functioning will likely be severe, though the precise timing and distribution of these impacts are uncertain. Pollution and Nutrient Load Pollution from nutrients (nitrogen and phosphorous) and other sources is a continuing and growing threat to biodiversity in terrestrial, inland water and coastal ecosystems. Modern industrial processes such as the burning of fossil fuels and agricultural practices, in particular the use of fertilizers, have more than doubled the quantity of reactive nitrogen - nitrogen in the form that is available to stimulate plant growth - in the environment compared with pre-industrial times. Put another way, humans now add more reactive nitrogen to the environment than all natural processes, such as nitrogen-fixing plants, fires and lightning201,202,203. In terrestrial ecosystems, the largest impact is in nutrient-poor environments, where some plants that benefit from the added nutrients out-compete many other species and cause significant changes in plant composition. Typically, plants such as grasses and sedges will benefit at the expense of species such as dwarf shrubs, mosses and lichens. Nitrogen deposition is already observed to be the major driver of species change in a range of temperate ecosystems, especially grasslands across Europe and North America, and high levels of nitrogen have also been recorded in southern China and parts of South and South-east Asia. Biodiversity loss from this source may be more serious than first thought in other ecosystems including high-latitude boreal forests, Mediterranean systems, some tropical savannas and montane forests204. Nitrogen has also been observed to be building up at significant levels in biodiversity hotspots, with potentially serious future impacts on a wide variety of plant species205. Large parts of Latin America and Africa, as well as Asia, are projected to experience elevated levels of nitrogen deposition in the next two decades. Although the impacts have mainly been studied in plants, nitrogen deposition may also affect animal biodiversity by changing the composition of available food. 206 In inland water and coastal ecosystems, the buildup of phosphorous and nitrogen, mainly through run-off from cropland and sewage pollution, stimulates the growth of algae and some forms of bacteria, threatening valuable ecosystem services in systems such as lakes and coral reefs, and affecting water quality207,208,209,210. It also creates “dead zones” in oceans, generally where major rivers reach the sea. In these zones, decomposing algae use up oxygen in the water and leave large areas virtually devoid of marine life. The number of reported dead zones has been roughly doubling every ten years since the 1960s, and by 2010 had reached around 500211 [See Figure 15]. While the increase in nutrient load is among the most significant changes humans are making to ecosystems, policies in some regions are showing that this pressure can be controlled and, in time, reversed. Among the most comprehensive measures to combat nutrient pollution is the European Union’s Nitrates Directive212 [See Box 16 and Figure 16].
Overexploitation and Unsustainable Use Overexploitation and destructive harvesting practices are at the heart of the threats being imposed on the world’s biodiversity and ecosystems, and there has not been significant reduction in this pressure. Changes to fisheries management in some areas are leading to more sustainable practices, but most stocks still require reduced pressure in order to rebuild. Bushmeat hunting, which provides a significant proportion of protein for many rural households, appears to be taking place at unsustainable levels. Overexploitation is the major pressure being exerted on marine ecosystems, with marine capture fisheries having quadrupled in size from the early 1950s to the mid 1990s213. Total catches have fallen since then despite increased fishing effort, an indication that many stocks have been pushed beyond their capacity to replenish. The FAO estimates that more than a quarter of marine fish stocks are overexploited (19%), depleted (8%) or recovering from depletion (1%) while more than half are fully exploited214. Although there have been some recent signs that fishing authorities are imposing more realistic expectations on the size of catches that can safely be taken out of the oceans, some 63% of assessed fish stocks worldwide require rebuilding215. Innovative approaches to the management of fisheries, such as those that give fishermen a stake in maintaining healthy stocks, are proving to be effective where they are applied. [See Box 17].
Invasive Alien Species |