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The source document for this Digest states:
Biodiversity change is caused by a range of drivers. A driver is any natural or human-induced factor that directly or indirectly causes a change in an ecosystem. A direct driver unequivocally influences ecosystem processes. An indirect driver operates more diffusely, by altering one or more direct drivers. Important direct drivers affecting biodiversity are habitat change, climate change, invasive species, overexploitation, and pollution (CF4, C3, C4.3, S7).
No single measure or indicator represents the totality of the various drivers. Some direct drivers of change have relatively straightforward indicators, such as fertilizer usage, water consumption, irrigation, and harvests. Indicators for other drivers, including invasion by non-native species, climate change, land cover conversion, and landscape fragmentation, are not as well developed, and data to measure them are not as readily available (S7).
Changes in biodiversity and in ecosystems are almost always caused by multiple, interacting drivers. Changes are driven by combinations of drivers that work over time (such as population and income growth interacting with technological advances that lead to climate change) or level of organization (such as local zoning laws versus international environmental treaties) and that happen intermittently (such as droughts, wars, and economic crises). Reviews of case studies of deforestation and desertification reveal that the most common type of interaction is synergetic factor combinations: combined effects of multiple drivers that are amplified by reciprocal action and feedbacks (S7.4).
Drivers interact across spatial, temporal, and organizational scales, and any specific ecosystem change is driven by a network of interactions among different drivers. Though some of the elements of these networks are global, the actual set of interactions that brings about an ecosystem change is more or less specific to a particular place. For example, a link between increasing producer prices and the extension of production can be found in many places throughout the world. The strength of this effect, however, is determined by a range of location-specific factors including production conditions, the availability of resources and knowledge, and the economic situation of the farmer (S7.4). No single conceptual framework captures the broad range of case study evidence about the interactions among drivers. Based on the findings of the sub-global assessments of the MA and recent literature, some examples of causal linkages for ecosystem change can be given (SG-Portugal, SG-SAfMA). (See Figures 3.8 and 3.9 and Box Box 3.1)
Box 3.1. Direct Drivers: Example from Southern African Sub-global Assessment
Source & ©: Millennium Ecosystem Assessment
Chapter 3, p.47
The source document for this Digest states:
Biodiversity change is most clearly a consequence of the direct drivers. However, these reflect changes in indirect drivers—the root causes of changes in ecosystems. These can be classified into the following broad categories: change in economic activity, demographic change, sociopolitical factors, cultural and religious factors, and scientific and technological change.
- Global economic activity increased nearly sevenfold between 1950 and 2000 (S7.SDM), and in the MA scenarios it is projected to grow a further three- to sixfold by 2050. The many processes of globalization have amplified some driving forces of changes in ecosystem services and attenuated other forces by removing regional barriers, weakening national connections, and increasing the interdependence among people and between nations (S7.2.2).
- Global population doubled in the past 40 years, reaching 6 billion in 2000 (S7.2.1). It is projected to grow to 8.1–9.6 billion by 2050, depending on the scenario. Urbanization influences consumption, generally increasing the demand for food and energy and thereby increasing pressures on ecosystems globally.
- Over the past 50 years, there have been significant changes in sociopolitical drivers, including a declining trend in centralized authoritarian governments and a rise in elected democracies, which allows for new forms of management, in particular adaptive management, of environmental resources (S7.2.3).
Culture conditions individuals’ perceptions of the world, and by influencing what they consider important, it has implications for conservation and consumer preferences and suggests courses of action that are appropriate and inappropriate. The development and diffusion of scientific knowledge and technologies can on the one hand allow for increased efficiency in resource use and on the other hand can provide the means to increase exploitation of resources (S7.2.4, S7.2.5).
Source & ©: Millennium Ecosystem Assessment
Chapter 3, p.49
The source document for this Digest states:
Figure 3.10 Main direct drivers
Figure 3.11 Effect of land use intensity
Figure 3.12 Extend of cultivated systemsDirect drivers vary in their importance within and among systems and in the extent to which they are increasing their impact. Historically, habitat and land use change have had the biggest impact on biodiversity across biomes. Climate change is projected to increasingly affect all aspects of biodiversity, from individual organisms, through populations and species, to ecosystem composition and function. Pollution, especially the deposition of nitrogen and phosphorus, but also including the impact of other contaminants, is also expected to have an increasing impact, leading to declining biodiversity across biomes. Overexploitation and invasive species have been important as well and continue to be major drivers of changes in biodiversity (C4.3). (See Figure 3.10)
For terrestrial ecosystems, the most important direct driver of change in the past 50 years has been land cover change (C4.3, SG7). Only biomes relatively unsuited to crop plants, such as deserts, boreal forests, and tundra, are relatively intact (C4). Deforestation and forest degradation are currently more extensive in the tropics than in the rest of the world, although data on boreal forests are especially limited (C21). Approximately 10–20% of drylands are considered degraded (medium certainty), with the majority of these areas in Asia (C22). A study of the southern African biota shows how degradation of habitats led to loss of biodiversity across all taxa. (See Figure 3.11)
Cultivated systems (defined in the MA to be areas in which at least 30% of the landscape is in croplands, shifting cultivation, confined livestock production, or freshwater aquaculture in any particular year) cover 24% of Earth’s surface. (See Figure 3.12) In 1990, around 40% of the cropland is located in Asia; Europe accounts for 16%, and Africa, North America, and South America each account for 13% (S7).
For marine ecosystems, the most important direct driver of change in the past 50 years, in the aggregate, has been fishing. Fishing is the major direct anthropogenic force affecting the structure, function, and biodiversity of the oceans (C18). Fishing pressure is so strong in some marine systems that over much of the world the biomass of fish targeted in fisheries (including that of both the target species and those caught incidentally) has been reduced by 90% relative to levels prior to the onset of industrial fishing. In these areas a number of targeted stocks in all oceans have collapsed—having been overfished or fished above their maximum sustainable levels. Recent studies have demonstrated that global fisheries landings peaked in the late 1980s and are now declining despite increasing effort and fishing power, with little evidence of this trend reversing under current practices (C18.3). In addition to the landings, the average trophic level of global landings is declining, which implies that we are increasingly relying on fish that originate from the lower part of marine food webs (C18.3). (See Figures 3.13 and 3.14) Destructive fishing is also a factor in shallower waters; bottom trawling homogenizes three-dimensional benthic habitats and dramatically reduces biodiversity.
For freshwater ecosystems, depending on the region, the most important direct drivers of change in the past 50 years include physical changes, modification of water regimes, invasive species, and pollution. The loss of wetlands worldwide has been speculated to be 50% of those that existed in 1900. However, the accuracy of this figure has not been established due to an absence of reliable data (C20.3.1). 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 were built since the 1980s (C20.4.2). Water withdrawals from rivers and lakes for irrigation or urban or industrial use increased sixfold since 1900 (C7.2.2). Globally, humans now use roughly 10% of the available renewable freshwater supply, although in some regions, such as the Middle East and North Africa, humans use 120% of renewable supplies—the excess is obtained through mining groundwater (C7.2.3). The introduction of non-native invasive species is now a major cause of species extinction in freshwater systems. It is well established that the increased discharge of nutrients causes intensive eutrophication and potentially high levels of nitrate in drinking water and that pollution from point sources such as mining has had devastating impacts on the biota of inland waters (C20.4).
Source & ©: Millennium Ecosystem Assessment
Chapter 3, p.47
The source document for this Digest states:
Apparently stable areas of habitat may suffer from fragmentation, with significant impacts on their biodiversity (C4.3.1). Fragmentation is caused by natural disturbance (such as fires or wind) or by land use change and habitat loss, such as the clearing of natural vegetation for agriculture or road construction, which divides previously continuous habitats. Larger remnants, and remnants that are close to other remnants, are less affected by fragmentation. Small fragments of habitat can only support small populations, which tend to be more vulnerable to extinction. Moreover, habitat along the edge of a fragment has a different climate and favors different species to the interior. Small fragments are therefore unfavorable for those species that require interior habitat, and they may lead to the extinction of those species. Species that are specialized to particular habitats and those whose dispersal abilities are weak suffer from fragmentation more than generalist species with good dispersal ability (C4.3.1). Fragmentation affects all biomes, but especially forests (see Figure 3.15) and major freshwater systems (see Figure 3.16).
Figure 3.15 Click on any continent below to view maps which estimate the amount of:
Forest fragmentation induced by human activities
Source & ©: Millennium Ecosystem Assessment
Chapter 3, p.51
The source document for this Digest states:
Invasive alien species have been a major cause of extinction, especially on islands and in freshwater habitats, and they continue to be a problem in many areas. In freshwater habitats, the introduction of alien species is the second leading cause of species extinction, and on islands it is the main cause of extinction over the past 20 years, along with habitat destruction. Awareness about the importance of stemming the tide of invasive alien species is increasing, but effective implementation of preventative measures is lacking. The rate of introductions continues to be extremely high; for example, in New Zealand plant introductions alone have occurred at a rate of 11 species per year since European settlement in 1840 (C4.3.2).
Source & ©: Millennium Ecosystem Assessment
Chapter 3, p.52
The source document for this Digest states:
Overexploitation remains a serious threat to many species and populations. Among the most commonly overexploited species or groups of species are marine fish and invertebrates, trees, and animals hunted for meat. Most industrial fisheries are either fully or overexploited, and the impacts of overharvesting are coupled to destructive fishing techniques that destroy habitat, as well as associated ecosystems such as estuaries and wetlands. Even recreational and subsistence fishing has contributed to what is known as the “shifting baselines” phenomenon, in which what we consider the norm today is dramatically different from pre-exploitation conditions.
Many of the current concerns with overexploitation of bushmeat (wild meat taken from the forests by local people for income or subsistence) are similar to those of fisheries, where sustainable levels of exploitation remain poorly understood and where the offtake is difficult to manage effectively. Although the true extent of exploitation is poorly known, it is clear that rates of offtake are extremely high in tropical forests. The trade in wild plants and animals and their derivatives is poorly documented but is estimated at nearly $160 billion annually. It ranges from live animals for the food and pet trade to ornamental plants and timber. Because the trade in wild animals and plants crosses national borders, the effort to regulate it requires international cooperation to safeguard certain species from overexploitation (C4.3.4).
Source & ©: Millennium Ecosystem Assessment
Chapter 3, p.53
The source document for this Digest states:
Over the past four decades, nutrient loading has emerged as one of the most important drivers of ecosystem change in terrestrial, freshwater, and coastal ecosystems. While the introduction of nutrients into ecosystems can have both beneficial and adverse effects, the beneficial effects will eventually reach a plateau as more nutrients are added (for example, 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 the key driver for the remarkable increase in food production of the past 50 years (S7.3). (See Figure 3.17) The total amount of reactive, or biologically available, nitrogen created by human activities increased ninefold between 1890 and 1990, with most of that increase taking place in the second half of the century in association with increased use of fertilizers (C7.3.2).
More than half of all the synthetic nitrogen fertilizers ever used on Earth have been used since 1985 (R9.2). Humans now produce more reactive nitrogen than is produced by all natural pathways combined (R9.ES). Nitrogen application has increased fivefold since 1960, but as much as 50% of the nitrogen fertilizer applied may be lost to the environment. Phosphorus application has increased threefold since 1960, with steady increase until 1990, followed by leveling off at a level about equal to applications in 1980. (See Figure 3.18) These changes are mirrored by phosphorus accumulation in soils, which can serve as an indicator of eutrophication potential for freshwater lakes and phosphorus-sensitive estuaries. Potential consequences include eutrophication of freshwater ecosystems, hypoxia in coastal marine ecosystems, nitrous oxide emissions contributing to global climate change, and air pollution by NOx in urban areas. Occurrence of such problems varies widely in different regions (S7.3). (See Figure 3.19)
Source & ©: Millennium Ecosystem Assessment
Chapter 3, p.53
The source document for this Digest states:
Climate change in the past century has already had a measurable impact on biodiversity. Observed recent changes in climate, especially warmer regional temperatures, have already had significant impacts on biodiversity and ecosystems, including causing changes in species distributions, population sizes, the timing of reproduction or migration events, and an increase in the frequency of pest and disease outbreaks. Many coral reefs have undergone major, although often partially reversible, bleaching episodes when local sea surface temperatures have increased during one month by 0.5–1o Celsius above the R1average of the hottest months (R13.1.3). Precipitation patterns have changed spatially and temporally, and global average sea level rose 0.1–0.2 meters (S7.ES). By the end of the century, climate change and its impacts may be the dominant direct driver of biodiversity loss and changes in ecosystem services globally.
Recent studies, using the climate envelope/species-area technique, estimated that the projected changes in climate by 2050 could lead to an eventual extinction of 15–52% of the subset of 1,103 endemic species (mammals, birds, frogs, reptiles, butterflies, and plants) analyzed (R13.1.3). While the growing season in Europe has lengthened over the last 30 years, in some regions of Africa the combination of regional climate changes and anthropogenic stresses has led to decreased cereal crop production since 1970. Changes in fish populations have been linked to large-scale climate oscillations; El Niño events, for instance, have affected fisheries off the coasts of South America and Africa, and decadal oscillations in the Pacific have affected fisheries off the west coast of North America (R13.1.3).
The scenarios developed by the Intergovernmental Panel on Climate Change project an increase in global mean surface temperature of 2.0–6.4o Celsius above preindustrial levels by 2100, increased incidence of floods and droughts, and a rise in sea level of an additional 8–88 centimeters between 1990 and 2100. (See Figure 3.20)
Harm to biodiversity will grow worldwide with increasing rates of change in climate and increasing absolute amounts of change. In contrast, some ecosystem services in some regions may initially be enhanced by projected changes in climate (such as increases in temperature or precipitation), and thus these regions may experience net benefits at low levels of climate change. As climate change becomes more severe, however, the harmful impacts on ecosystem services outweigh the benefits in most regions of the world. The balance of scientific evidence suggests that there will be a significant net harmful impact on ecosystem services worldwide if global mean surface temperature increases more than 2o Celsius above preindustrial levels or at rates greater than 0.2o Celsius per decade (medium certainty).Climate change is projected to further adversely affect key development challenges, including providing clean water, energy services, and food; maintaining a healthy environment; and conserving ecological systems and their biodiversity and associated ecological goods and services (R13.1.3).
- Climate change is projected to exacerbate the loss of biodiversity and increase the risk of extinction for many species, especially those already at risk due to factors such as low population numbers, restricted or patchy habitats, and limited climatic ranges (medium to high certainty).
- Water availability and quality are projected to decrease in many arid and semiarid regions (high certainty).
- The risk of floods and droughts is projected to increase (high certainty).
- The reliability of hydropower and biomass production is projected to decrease in some regions (high certainty).
- The incidence of vector-borne diseases such as malaria and dengue and of waterborne diseases such as cholera is projected to increase in many regions (medium to high certainty), and so too are heat stress mortality and threats of decreased nutrition in other regions, along with severe weather traumatic injury and death (high certainty).
- Agricultural productivity is projected to decrease in the tropics and sub-tropics for almost any amount of warming (low to medium certainty), and there are projected adverse effects on fisheries.
Projected changes in climate during the twenty-first century are very likely to be without precedent during at least the past 10,000 years and, combined with land use change and the spread of exotic or alien species, are likely to limit both the capability of species to migrate and the ability of species to persist in fragmented habitats.
Source & ©: Millennium Ecosystem Assessment
Chapter 3, p.56
The source document for this Digest states:
Present-day threats are often multiple and of greater intensity than historical threats. The susceptibility of an ecological community to a given threat will depend on the events of the past that have shaped the current biota. If the current threats are novel, they will have dramatic effects on populations, since species will lack adaptations. Even if drivers are similar to past drivers (climate, for example, has always been variable to some degree), the intensity of some current-day drivers is unprecedented (such as the rates and extent of habitat change). Furthermore, today’s drivers of extinction are often multiple—land use change, emerging disease, and invasive species are all occurring together, for instance. Because exposure to one threat type often makes a species more susceptible to a second, exposure to a second makes a species more susceptible to a third, and so on, consecutive, multiple threats to species may have unexpectedly dramatic impacts on biodiversity (S7.4, C4.3).
Each driver has a characteristic spatial and temporal scale at which it affects ecosystem services and human well-being. Climate change may operate on a spatial scale of a large region; 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, while economic forces tend to occur more rapidly. Because of the variability in ecosystems, their services, and human well-being in space and time, there may be mismatches or lags between the scale of the driver and the scale of its effects on ecosystem services (S7, SG7.3.5).
The fate of declining species and habitats will depend on sources of inertia and the speed of their response to management interventions. Natural sources of inertia correspond to the time scales inherent to natural systems; for example, recovery of a population cannot proceed more quickly than the average turnover or generation time, and established recovery will often take several generations. On top of this is anthropogenic inertia resulting from the time scales inherent in human institutions for decision-making and implementation. For most systems, these two sources of inertia will lead to delays of years, and more often decades, in slowing and reversing a declining biodiversity trend. This analysis assumes that the drivers of change could indeed be halted or reversed in the near term. Yet currently there is little evidence that any of the direct or indirect drivers are slowing or that any are well controlled at the large to global scale. More significantly, we have net yet seen all of the consequences of changes that occurred in the past (C4, R5, S7, S10).
The delay between a driver affecting a system and its consequences for biodiversity change can be highly variable. In the relatively well studied case of species extinctions, habitat loss is known to be a driver with particularly long lag times. In studies of tropical forest bird species the time from habitat fragmentation to species extinction has been estimated to have a half-life of decades to hundreds of years. Overall, these results suggest that about half of the species losses may occur over a period of 100 to 1,000 years. Therefore, humans have the opportunity to deploy active habitat restoration practices that may rescue some of the species that otherwise would have been in a trajectory toward extinction. Notwithstanding this, habitat restoration measures will not be likely to save the most sensitive species, which will become extinct soon after habitat loss (C4.5.2).
Source & ©: Millennium Ecosystem Assessment
Chapter 3, p.57
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