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Dégradation des Ecosystèmes

4. What are the most critical factors causing ecosystem changes?

  • 4.1 What is a "driver" and how does it affect ecosystems?
  • 4.2 What are the indirect drivers and how are they changing?
    • 4.2.1 Indirect drivers
    • 4.2.2 Changes in the impacts of indirect drivers
  • 4.3 What are the direct drivers of changes in ecosystem services?
    • 4.3.1 Direct drivers in specific ecosystems
    • 4.3.2 Most important direct drivers

4.1 What is a "driver" and how does it affect ecosystems?

The source document for this Digest states:

Natural or human-induced factors that directly or indirectly cause a change in an ecosystem are referred to as "drivers." A direct driver unequivocally influences ecosystem processes. An indirect driver operates more diffusely, by altering one or more direct drivers.

Drivers affect ecosystem services and human well-being at different spatial and temporal scales, which makes both their assessment and their management complex (SG7). Climate change may operate on a global or a large regional spatial scale; political change may operate at the scale of a nation or a municipal district. Sociocultural change typically occurs slowly, on a time scale of decades (although abrupt changes can sometimes occur, as in the case of wars or political regime changes), while economic changes tend to occur more rapidly. As a result of this spatial and temporal dependence of drivers, the forces that appear to be most significant at a particular location and time may not be the most significant over larger (or smaller) regions or time scales.

Source & ©: MA  Millennium Ecosystem Assessment Synthesis Report (2005),
Chapter 4, p.64

4.2 What are the indirect drivers and how are they changing?

    • 4.2.1 Indirect drivers
    • 4.2.2 Changes in the impacts of indirect drivers

4.2.1 Indirect drivers

The source document for this Digest states:

Indirect drivers

In the aggregate and at a global scale, there are five indirect drivers of changes in ecosystems and their services: population change, change in economic activity, sociopolitical factors, cultural factors, and technological change. Collectively these factors influence the level of production and consumption of ecosystem services and the sustainability of production. Both economic growth and population growth lead to increased consumption of ecosystem services, although the harmful environmental impacts of any particular level of consumption depend on the efficiency of the technologies used in the production of the service. These factors interact in complex ways in different locations to change pressures on ecosystems and uses of ecosystem services. Driving forces are almost always multiple and interactive, so that a one-to-one linkage between particular driving forces and particular changes in ecosystems rarely exists. Even so, changes in any one of these indirect drivers generally result in changes in ecosystems. The causal linkage is almost always highly mediated by other factors, thereby complicating statements of causality or attempts to establish the proportionality of various contributors to changes. There are five major indirect drivers:

  • Demographic Drivers: Global population doubled in the past 40 years and increased by 2 billion people in the last 25 years, reaching 6 billion in 2000 (S7.2.1). Developing countries have accounted for most recent population growth in the past quarter-century, but there is now an unprecedented diversity of demographic patterns across regions and countries. Some high-income countries such as the United States are still experiencing high rates of population growth, while some developing countries such as China, Thailand, and North and South Korea have very low rates. In the United States, high population growth is due primarily to high levels of immigration. About half the people in the world now live in urban areas (although urban areas cover less than 3% of the terrestrial surface), up from less than 15% at the start of the twentieth century (C27.1). High-income countries typically have populations that are 70–80% urban. Some developing-country regions, such as parts of Asia, are still largely rural, while Latin America, at 75% urban, is indistinguishable from high-income countries in this regard (S7.2.1).
  • Economic Drivers: Global economic activity increased nearly sevenfold between 1950 and 2000 (S7.SDM). With rising per capita income, the demand for many ecosystem services grows. At the same time, the structure of consumption changes. In the case of food, for example, as income grows the share of additional income spent on food declines, the importance of starchy staples (such as rice, wheat, and potatoes) declines, diets include more fat, meat and fish, and fruits and vegetables, and the proportionate consumption of industrial goods and services rises (S7.2.2).

In the late twentieth century, income was distributed unevenly, both within countries and around the world. The level of per capita income was highest in North America, Western Europe, Australasia, and Northeast Asia, but both GDP and per capita growth rates were highest in South Asia, China, and parts of South America (S7.2.2). (See Figure 4.1 and 4.2.) Growth in international trade flows has exceeded growth in global production for many years, and the differential may be growing. In 2001, international trade in goods was equal to 40% of gross world product. (S7.2.2).

Taxes and subsidies are important indirect drivers of ecosystem change. Fertilizer taxes or taxes on excess nutrients, for example, provide an incentive to increase the efficiency of the use of fertilizer applied to crops and thereby reduce negative externalities. Currently, many subsidies substantially increase rates of resource consumption and increase negative externalities.

Annual subsidies to conventional energy, which encourage greater use of fossil fuels and consequently emissions of greenhouse gases, are estimated to have been $250–300 billion in the mid-1990s (S7.ES). The 2001–03 average subsidies paid to the agricultural sectors of OECD countries were over $324 billion annually (S7.ES), encouraging greater food production and associated water consumption and nutrient and pesticide release. At the same time, many developing countries also have significant agricultural production subsidies.

  • Sociopolitical Drivers: Sociopolitical drivers encompass the forces influencing decision-making and include the quantity of public participation in decision-making, the groups participating in public decision-making, the mechanisms of dispute resolution, the role of the state relative to the private sector, and levels of education and knowledge (S7.2.3). These factors in turn influence the institutional arrangements for ecosystem management, as well as property rights over ecosystem services. Over the past 50 years there have been significant changes in sociopolitical drivers. There is a declining trend in centralized authoritarian governments and a rise in elected democracies. The role of women is changing in many countries, average levels of formal education are increasing, and there has been a rise in civil society (such as increased involvement of NGOs and grassroots organizations in decision-making processes). The trend toward democratic institutions has helped give power to local communities, especially women and resource-poor households (S7.2.3). There has been an increase in multilateral environmental agreements. The importance of the state relative to the private sector—as a supplier of goods and services, as a source of employment, and as a source of innovation—is declining.
  • Cultural and Religious Drivers: To understand culture as a driver of ecosystem change, it is most useful to think of it as the values, beliefs, and norms that a group of people share. In this sense, culture conditions individuals’ perceptions of the world, influences what they consider important, and suggests what courses of action are appropriate and inappropriate (S7.2.4). Broad comparisons of whole cultures have not proved useful because they ignore vast variations in values, beliefs, and norms within cultures. Nevertheless, cultural differences clearly have important impacts on direct drivers. Cultural factors, for example, can influence consumption behavior (what and how much people consume) and values related to environmental stewardship, and they may be particularly important drivers of environmental change.
  • Science and Technology: The development and diffusion of scientific knowledge and technologies that exploit that knowledge has profound implications for ecological systems and human well-being. The twentieth century saw tremendous advances in understanding how the world works physically, chemically, biologically, and socially and in the applications of that knowledge to human endeavors. Science and technology are estimated to have accounted for more than one third of total GDP growth in the United States from 1929 to the early 1980s, and for 16–47% of GDP growth in selected OECD countries in 1960–95 (S7.2.5). The impact of science and technology on ecosystem services is most evident in the case of food production. Much of the increase in agricultural output over the past 40 years has come from an increase in yields per hectare rather than an expansion of area under cultivation. For instance, wheat yields rose 208%, rice yields rose 109%, and maize yields rose 157% in the past 40 years in developing countries (S7.2.5). At the same time, technological advances can also lead to the degradation of ecosystem services. Advances in fishing technologies, for example, have contributed significantly to the depletion of marine fish stocks.

Source & ©: MA  Millennium Ecosystem Assessment Synthesis Report (2005),
Chapter 4, pp.64-66

4.2.2 Changes in the impacts of indirect drivers

The source document for this Digest states:

Consumption of ecosystem services is slowly being decoupled from economic growth. Growth in the use of ecosystem services over the past five decades was generally much less than the growth in GDP. This change reflects structural changes in economies, but it also results from new technologies and new management practices and policies that have increased the efficiency with which ecosystem services are used and provided substitutes for some services. Even with this progress, though, the absolute level of consumption of ecosystem services continues to grow, which is consistent with the pattern for the consumption of energy and materials such as metals: in the 200 years for which reliable data are available, growth of consumption of energy and materials has outpaced increases in materials and energy efficiency, leading to absolute increases of materials and energy use (S7.ES).

Global trade magnifies the effect of governance, regulations, and management practices on ecosystems and their services, enhancing good practices but worsening the damage caused by poor practices (R8, S7). Increased trade can accelerate degradation of ecosystem services in exporting countries if their policy, regulatory, and management systems are inadequate. At the same time, international trade enables comparative advantages to be exploited and accelerates the diffusion of more-efficient technologies and practices. For example, the increased demand for forest products in many countries stimulated by growth in forest products trade can lead to more rapid degradation of forests in countries with poor systems of regulation and management, but can also stimulate a “virtuous cycle” if the regulatory framework is sufficiently robust to prevent resource degradation while trade, and profits, increase. While historically most trade related to ecosystems has involved provisioning services such as food, timber, fiber, genetic resources, and biochemicals, one regulating service—climate regulation, or more specifically carbon sequestration—is now also traded internationally.

Urban demographic and economic growth has been increasing pressures on ecosystems globally, but affluent rural and suburban living often places even more pressure on ecosystems (C27-ES). Dense urban settlement is considered to be less environmentally burdensome than urban and suburban sprawl. And the movement of people into urban areas has significantly lessened pressure on some ecosystems and, for example, has led to the reforestation of some parts of industrial countries that had been deforested in previous centuries. At the same time, urban centers facilitate human access to and management of ecosystem services through, for example, economies of scale related to the construction of piped water systems in areas of high population density.

Source & ©: MA  Millennium Ecosystem Assessment Synthesis Report (2005),
Chapter 4, pp.66-68

4.3 What are the direct drivers of changes in ecosystem services?

    • 4.3.1 Direct drivers in specific ecosystems
    • 4.3.2 Most important direct drivers

The source document for this Digest states:

Direct drivers

Most of the direct drivers of change in ecosystems and biodiversity services currently remain constant or are growing in intensity in most ecosystems. (See Figure 4.3.) The most important direct drivers of change in ecosystems are habitat change (land use change and physical modification of rivers or water withdrawal from rivers), overexploitation, invasive alien species, pollution, and climate change.

Source & ©: MA  Millennium Ecosystem Assessment Synthesis Report (2005),
Chapter 4, p.68

4.3.1 Direct drivers in specific ecosystems

The source document for this Digest states:

For terrestrial ecosystems, the most important direct drivers of change in ecosystem services in the past 50 years, in the aggregate, have been land cover change (in particular, conversion to cropland) and the application of new technologies (which have contributed significantly to the increased supply of services such as food, timber, and fiber) (CWG, S7.2.5, SG8.ES). In 9 of the 14 terrestrial biomes examined in the MA, between one half and one fifth of the area has been transformed, largely to croplands (C4.ES). Only biomes relatively unsuited to crop plants, such as deserts, boreal forests, and tundra, have remained largely untransformed by human action. Both land cover changes and the management practices and technologies used on lands may cause major changes in ecosystem services. New technologies have resulted in significant increases in the supply of some ecosystem services, such as through increases in agricultural yield. In the case of cereals, for example, from the mid-1980s to the late 1990s the global area under cereals fell by around 0.3% a year, while yields increased by about 1.2% a year (C26.4.1).

For marine ecosystems and their services, the most important direct driver of change in the past 50 years, in the aggregate, has been fishing (C18). At the beginning of the twenty-first century, the biological capability of commercially exploited fish stocks was probably at a historical low. FAO estimates that about half of the commercially exploited wild marine fish stocks for which information is available are fully exploited and offer no scope for increased catches (C8.2.2). As noted in Chapter 1, fishing pressure is so strong in some marine systems that the biomass of some targeted species, especially larger fishes, and those caught incidentally has been reduced to one tenth of levels prior to the onset of industrial fishing (C18.ES). Fishing has had a particularly significant impact in coastal areas but is now also affecting the open oceans.

For freshwater ecosystems and their services, depending on the region, the most important direct drivers of change in the past 50 years include modification of water regimes, invasive species, and pollution, particularly high levels of nutrient loading. It is speculated that 50% of inland water ecosystems (excluding large lakes and closed seas) were converted during the twentieth Century (C20.ES). Massive changes have been made in water regimes: in Asia, 78% of the total reservoir volume was constructed in the last decade, and in South America almost 60% of all reservoirs have been built since the 1980s (C20.4.2). The introduction of nonnative invasive species is one of the major causes of species extinction in freshwater systems. While the presence of nutrients such as phosphorus and nitrogen is necessary for biological systems, high levels of nutrient loading cause significant eutrophication of water bodies and contribute to high levels of nitrate in drinking water in some locations. (The nutrient load refers to the total amount of nitrogen or phosphorus entering the water during a given time.) Non-point pollution sources such as storm water runoff in urban areas, poor or nonexistent sanitation facilities in rural areas, and the flushing of livestock manure by rainfall and snowmelt are also cause of contamination (C20.4.5). Pollution from point sources such as mining has had devastating local and regional impacts on the biota of inland waters.

Coastal ecosystems are affected by multiple direct drivers. Fishing pressures in coastal ecosystems are compounded by a wide array of other drivers, including land-, river-, and ocean-based pollution, habitat loss, invasive species, and nutrient loading. Upstream freshwater diversion has meant a 30% decrease worldwide of water and sediment delivery to estuaries, which are key nursery areas and fishing grounds (C19.ES). Approximately 17% of the world lives within the boundaries of the MA coastal system (up to an elevation of 50 meters above sea level and no further than 100 kilometers from a coast), and approximately 40% live in the full area within 50 kilometers of a coast. And the absolute number is increasing through a combination of in-migration, high reproduction rates, and tourism (C.SDM). Demand on coastal space for shipping, waste disposal, military and security uses, recreation, and aquaculture is increasing.

The greatest threat to coastal systems is the development-related conversion of coastal habitats such as forests, wetlands, and coral reefs through coastal urban sprawl, resort and port development, aquaculture, and industrialization. Dredging, reclamation and destructive fishing also account for widespread, effectively irreversible destruction. Shore protection structures and engineering works (beach armoring, causeways, bridges, and so on), by changing coastal dynamics, have impacts extending beyond their direct footprints. Nitrogen loading to the coastal zone has increased by about 80% worldwide and has driven coral reef community shifts (C.SDM).

Source & ©: MA  Millennium Ecosystem Assessment Synthesis Report (2005),
Chapter 4, pp.68-69

4.3.2 Most important direct drivers

The source document for this Digest states:

Over the past four decades, excessive nutrient loading has emerged as one of the most important direct drivers of ecosystem change in terrestrial, freshwater, and marine ecosystems. (See table 4.1.) While the introduction of nutrients into ecosystems can have both beneficial effects (such as increased crop productivity) and adverse effects (such as eutrophication of inland and coastal waters), the beneficial effects will eventually reach a plateau as more nutrients are added (that is, additional inputs will not lead to further increases in crop yield) while the harmful effects will continue to grow.

Synthetic production of nitrogen fertilizer has been an important driver for the remarkable increase in food production that has occurred during the past 50 years (S7.3.2). World consumption of nitrogenous fertilzers grew nearly eightfold between 1960 and 2003, from 10.8 million tons to 85.1 million tons. As much as 50% of the nitrogen fertilizer applied may be lost to the environment, depending on how well the application is managed. Since excessive nutrient loading is largely the result of applying more nutrients than crops can use, it harms both farm incomes and the environment (S7.3.2).

Excessive flows of nitrogen contribute to eutrophication of freshwater and coastal marine ecosystems and acidification of freshwater and terrestrial ecosystems (with implications for biodiversity in these ecosystems). To some degree, nitrogen also plays a role in the creation of ground-level ozone (which leads to loss of agricultural and forest productivity), destruction of ozone in the stratosphere (which leads to depletion of the ozone layer and increased UV-B radiation on Earth, causing increased incidence of skin cancer), and climate change. The resulting health effects include the consequences of ozone pollution on asthma and respiratory function, increased allergies and asthma due to increased pollen production, the risk of blue-baby syndrome, increased risk of cancer and other chronic diseases from nitrates in drinking water, and increased risk of a variety of pulmonary and cardiac diseases from production of fine particles in the atmosphere (R9.ES).

Phosphorus application has increased threefold since 1960, with a steady increase until 1990 followed by a leveling off at a level approximately equal to applications in the 1980s. While phosphorus use has increasingly concentrated on phosphorus-deficient soils, the growing phosphorus accumulation in soils contributes to high levels of phosphorus runoff. As with nitrogen loading, the potential consequences include eutrophication of coastal and freshwater ecosystems,which can lead to degraded habitat for fish and decreased quality of water for consumption by humans and livestock.

Many ecosystem services are reduced when inland water and coastal ecosystems become eutrophic. Water from lakes that experience algal blooms is more expensive to purify for drinking or other industrial uses. Eutrophication can reduce or eliminate fish populations. Possibly the most apparent loss in services is the loss of many of the cultural services provided by lakes. Foul odors of rotting algae, slime-covered lakes, and toxic chemicals produced by some blue-green algae during blooms keep people from swimming, boating, and otherwise enjoying the aesthetic value of lakes (S7.3.2).

Climate change in the past century has already had a measurable impact on ecosystems. Earth's climate system has changed since the preindustrial era, in part due to human activities, and it is projected to continue to change throughout the twenty-first century. During the last 100 years, the global mean surface temperature has increased by about 0.6° Celsius, precipitation patterns have changed spatially and temporally, and global average sea level rose by 0.1-0.2 meters (S7.ES). Observed changes in climate, especially warmer regional temperatures, have already affected biological systems in many parts of the world. There have been changes in species distributions, population sizes, and the timing of reproduction or migration events, as well as an increase in the frequency of pest and disease outbreaks, especially in forested systems. The growing season in Europe has lengthened over the last 30 years (R13.1.3). Although it is not possible to determine whether the extreme temperatures were a result of human-induced climate change, many coral reefs have undergone major, although often partially reversible, bleaching episodes when sea surface temperatures have increased during one month by 0.5-1° Celsius above the average of the hottest months. Extensive coral mortality has occurred with observed local increases in temperature of 3° Celsius (R13.1.3).

Source & ©: MA  Millennium Ecosystem Assessment Synthesis Report (2005),
Chapter 4, pp.69-70

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