Langues:
The source document for this Digest states:
The structure of the world’s ecosystems changed more rapidly in the second half of the twentieth century than at any time in recorded human history, and virtually all of Earth’s ecosystems have now been significantly transformed through human actions. The most significant change in the structure of ecosystems has been the transformation of approximately one quarter (24%) of Earth’s terrestrial surface to cultivated systems. (See Box 1.1.) More land was converted to cropland in the 30 years after 1950 than in the 150 years between 1700 and 1850.
Between 1960 and 2000, reservoir storage capacity quadrupled; as a result, the amount of water stored behind large dams is estimated to be three to six times the amount held by natural river channels (this excludes natural lakes) (See Figure 1.1.) In countries for which sufficient multiyear data are available (encompassing more than half of the present-day mangrove area), approximately 35% of mangroves were lost in the last two decades. Roughly 20% of the world’s coral reefs were lost and an additional 20% degraded in the last several decades of the twentieth century. Box 1.1. and Table 1.1 summarize important characteristics and trends in different ecosystems.
Click here for a comparative table of reporting systems as defined by the Millennium Assessment.
Although the most rapid changes in ecosystems are now taking place in developing countries, industrial countries historically experienced comparable rates of change. Croplands expanded rapidly in Europe after 1700 and in North America and the former Soviet Union, particularly after 1850. Roughly 70% of the original temperate forests and grasslands and Mediterranean forests had been lost by 1950, largely through conversion to agriculture. Historically, deforestation has been much more intensive in temperate regions than in the tropics, and Europe is the continent with the smallest fraction of its original forests remaining. However, changes prior to the industrial era seemed to occur at much slower rates than current transformations.
Source & ©: MA 
Chapter 1, p.26
The source document for this Digest states:
Box 1.1. Characteristics of the World’s Ecological Systems
We report assessment findings for 10 categories of the land and marine surface, which we refer to as “systems”: forest, cultivated, dryland, coastal, marine, urban, polar, freshwater, island, and mountain. Each category contains a number of ecosystems. However, ecosystems within each category share a suite of biological, climatic, and social factors that tend to be similar within categories and differ across categories. The MA reporting categories are not spatially exclusive; their areas often overlap. For example, transition zones between forest and cultivated lands are included in both the forest system and cultivated system reporting categories. These reporting categories were selected because they correspond to the regions of responsibility of different government ministries (such as agriculture, water, forestry, and so forth) and because they are the categories used within the Convention on Biological Diversity.
(click on the links below for further information and maps)
Source & ©: MA
Chapter 1, p.27-30
The source document for this Digest states:
The ecosystems and biomes that have been most significantly altered globally by human activity include marine and freshwater ecosystems, temperate broadleaf forests, temperate grasslands, Mediterranean forests, and tropical dry forests. (See Figure 1.2) Within marine systems, the world’s demand for food and animal feed over the last 50 years has resulted in fishing pressure so strong that the biomass of both targeted species and those caught incidentally (the “bycatch”) has been reduced in much of the world to one tenth of the levels prior to the onset of industrial fishing. Globally, the degradation of fisheries is also reflected in the fact that the fish being harvested are increasingly coming from the less valuable lower trophic levels as populations of higher trophic level species are depleted. (See Figure 1.3.)
Freshwater ecosystems have been modified through the creation of dams and through the withdrawal of water for human use. The construction of dams and other structures along rivers has moderately or strongly affected flows in 60% of the large river systems in the world. Water removal for human uses has reduced the flow of several major rivers, including the Nile, Yellow, and Colorado Rivers, to the extent that they do not always flow to the sea.
As water flows have declined, so have sediment flows, which are the source of nutrients important for the maintenance of estuaries. Worldwide, sediment delivery to estuaries has declined by roughly 30%.
Within terrestrial ecosystems, more than two thirds of the area of 2 of the world’s 14 major terrestrial biomes (temperate grasslands and Mediterranean forests) and more than half of the area of four other biomes (tropical dry forests, temperate broadleaf forests, tropical grassland, and flooded grasslands) had been converted (primarily to agriculture) by 1990, as Figure 1.3 indicated. Among the major biomes, only tundra and boreal forests show negligible levels of loss and conversion, although they have begun to be affected by climate change.
Source & ©: MA
Chapter 1, p.26
The source document for this Digest states:
Globally, the rate of conversion of ecosystems has begun to slow largely due to reductions in the rate of expansion of cultivated land, and in some regions (particularly in temperate zones) ecosystems are returning to conditions and species compositions similar to their pre-conversion states. Yet rates of ecosystem conversion remain high or are increasing for specific ecosystems and regions.
Under the aegis of the MA the first systematic examination of the status and trends in terrestrial and coastal land cover was carried out using global and regional datasets. The pattern of deforestation, afforestation, and dryland degradation between 1980 and 2000 is shown in Figure 1.4. Opportunities for further expansion of cultivation are diminishing in many regions of the world as most of the land well-suited for intensive agriculture has been converted to cultivation. Increased agricultural productivity is also diminishing the need for agricultural expansion.
As a result of these two factors, a greater fraction of land in cultivated systems (areas with at least 30% of land cultivated) is actually being cultivated, the intensity of cultivation of land is increasing, fallow lengths are decreasing, and management practices are shifting from monocultures to polycultures. Since 1950, cropland areas have stabilized in North America and decreased in Europe and China. Cropland areas in the Former Soviet Union have decreased since 1960. Within temperate and boreal zones, forest cover increased by approximately 2.9 million hectares per year in the 1990s, of which approximately 40% was forest plantations. In some cases, rates of conversion of ecosystems have apparently slowed because most of the ecosystem has now been converted, as is the case with temperate broadleaf forests and Mediterranean forests.
Source & ©: MA
Chapter 1, p.26-32
The source document for this Digest states:
Ecosystem processes, including water, nitrogen, carbon, and phosphorus cycling, changed more rapidly in the second half of the twentieth century than at any time in recorded human history. Human modifications of ecosystems have changed not only the structure of the systems (such as what habitats or species are present in a particular location), but their processes and functioning as well. The capacity of ecosystems to provide services derives directly from the operation of natural biogeochemical cycles that in some cases have been significantly modified.
Source & ©: MA
Chapter 1, p.32
The source document for this Digest states:
Water withdrawals from rivers and lakes for irrigation or for urban or industrial use doubled between 1960 and 2000. (Worldwide, 70% of water use is for agriculture.) Large reservoir construction has doubled or tripled the residence time of river water—the average time, that is, that a drop of water takes to reach the sea. Globally, humans use slightly more than 10% of the available renewable freshwater supply through household, agricultural, and industrial activities, although in some regions such as the Middle East and North Africa, humans use 120% of renewable supplies (the excess is obtained through the use of groundwater supplies at rates greater than their rate of recharge).
Source & ©: MA
Chapter 1, p.32
The source document for this Digest states:
Since 1750, the atmospheric concentration of carbon dioxide has increased by about 34% (from about 280 parts per million to 376 parts in 2003). Approximately 60% of that increase (60 parts per million) has taken place since 1959. The effect of changes in terrestrial ecosystems on the carbon cycle reversed during the last 50 years. Those ecosystems were on average a net source of CO2 during the nineteenth and early twentieth centuries (primarily due to deforestation, but with contributions from degradation of agricultural, pasture, and forestlands) and became a net sink sometime around the middle of the last century (although carbon losses from land use change continue at high levels) (high certainty). Factors contributing to the growth of the role of ecosystems in carbon sequestration include afforestation, reforestation, and forest management in North America, Europe, China, and other regions; changed agriculture practices; and the fertilizing effects of nitrogen deposition and increasing atmospheric CO2 (high certainty).
Source & ©: MA
Chapter 1, p.33
The source document for this Digest states:
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. (See Figures 1.5 and 1.6.) A recent study of global human contributions to reactive nitrogen flows projected that flows will increase from approximately 165 teragrams of reactive nitrogen in 1999 to 270 teragrams in 2050, an increase of 64%. More than half of all the synthetic nitrogen fertilizer (which was first produced in 1913) ever used on the planet has been used since 1985.
Human activities have now roughly doubled the rate of creation of reactive nitrogen on the land surfaces of Earth. The flux of reactive nitrogen to the oceans increased by nearly 80% from 1860 to 1990, from roughly 27 teragrams of nitrogen per year to 48 teragrams in 1990. (This change is not uniform over Earth, however, and while some regions such as Labrador and Hudson's Bay in Canada have seen little if any change, the fluxes from more developed regions such as the northeastern United States, the watersheds of the North Sea in Europe, and the Yellow River basin in China have increased by ten- to fifteenfold.)
Source & ©: MA
Chapter 1, p.33
The source document for this Digest states:
The use of phosphorus fertilizers and the rate of phosphorus accumulation in agricultural soils increased nearly threefold between 1960 and 1990, although the rate has declined somewhat since that time. The current flux of phosphorus to the oceans is now triple that of background rates (approximately 22 teragrams of phosphorus per year versus the natural flux of 8 teragrams).
Source & ©: MA
Chapter 1, p.33-35
The source document for this Digest states:
A change in an ecosystem necessarily affects the species in the system, and changes in species affect ecosystem processes.
The distribution of species on Earth is becoming more homogenous. By homogenous, we mean that the differences between the set of species at one location on the planet and the set at another location are, on average, diminishing. The natural process of evolution, and particularly the combination of natural barriers to migration and local adaptation of species, led to significant differences in the types of species in ecosystems in different regions. But these regional differences in the planet’s biota are now being diminished.
Two factors are responsible for this trend. First, the extinction of species or the loss of populations results in the loss of the presence of species that had been unique to particular regions. Second, the rate of invasion or introduction of species into new ranges is already high and continues to accelerate apace with growing trade and faster transportation. (See Figure 1.7.) For example, a high proportion of the roughly 100 nonnative species in the Baltic Sea are native to the North American Great Lakes, and 75% of the recent arrivals of about 170 nonnative species in the Great Lakes are native to the Baltic Sea.
When species decline or go extinct as a result of human activities, they are replaced by a much smaller number of expanding species that thrive in human-altered environments. One effect is that in some regions where diversity has been low, the biotic diversity may actually increase—a result of invasions of nonnative forms. (This is true in continental areas such as the Netherlands as well as on oceanic islands.)
Source & ©: MA
Chapter 1, p.35
The source document for this Digest states:
Across a range of taxonomic groups, either the population size or range or both of the majority of species is currently declining. Studies of amphibians globally, African mammals, birds in agricultural lands, British butterflies, Caribbean corals, and fishery species show the majority of species to be declining in range or number. Exceptions include species that have been protected in reserves, that have had their particular threats (such as overexploitation) eliminated, or that tend to thrive in landscapes that have been modified by human activity.
Between 10% and 30% of mammal, bird, and amphibian species are currently threatened with extinction (medium to high certainty), based on IUCN–World Conservation Union criteria for threats of extinction. As of 2004, comprehensive assessments of every species within major taxonomic groups have been completed for only three groups of animals (mammals, birds, and amphibians) and two plant groups (conifers and cycads, a group of evergreen palm-like plants). Specialists on these groups have categorized species as “threatened with extinction” if they meet a set of quantitative criteria involving their population size, the size of area in which they are found, and trends in population size or area.
Under the widely used IUCN criteria for extinction, the vast majority of species categorized as “threatened with extinction” have approximately a 10% chance of going extinct within 100 years, although some long-lived species will persist much longer even though their small population size and lack of recruitment means that they have a very high likelihood of extinction.) Twelve percent of bird species, 23% of mammals, and 25% of conifers are currently threatened with extinction; 32% of amphibians are threatened with extinction, but information is more limited and this may be an underestimate. Higher levels of threat have been found in the cycads, where 52% are threatened. In general, freshwater habitats tend to have the highest proportion of threatened species.
Source & ©: MA
Chapter 1, p.35-37
The source document for this Digest states:
Over the past few hundred years, humans have increased the species extinction rate by as much as 1,000 times background rates typical over the planet’s history (medium certainty). (See Figure 1.8.) Extinction is a natural part of Earth’s history. Most estimates of the total number of species today lie between 5 million and 30 million, although the overall total could be higher than 30 million if poorly known groups such as deep-sea organisms, fungi, and microorganisms including parasites have more species than currently estimated. Species present today only represent 2–4% of all species that have ever lived. The fossil record appears to be punctuated by five major mass extinctions, the most recent of which occurred 65 million years ago.
The average rate of extinction found for marine and mammal fossil species (excluding extinctions that occurred in the five major mass extinctions) is approximately 0.1–1 extinctions per million species per year. There are approximately 100 documented extinctions of birds, mammal, and amphibians over the past 100 years, a rate 50–500 times higher than background rates. Including possibly extinct species, the rate is more than 1,000 times higher than background rates. Although the data and techniques used to estimate current extinction rates have improved over the past two decades, significant uncertainty still exists in measuring current rates of extinction because the extent of extinctions of undescribed taxa is unknown, the status of many described species is poorly known, it is difficult to document the final disappearance of very rare species, and there are time lags between the impact of a threatening process and the resulting extinction.
Source & ©: MA
Chapter 1, p.37
The source document for this Digest states:
Genes
Genetic diversity has declined globally, particularly among cultivated species. The extinction of species and loss of unique populations has resulted in the loss of unique genetic diversity contained by those species and populations. For wild species, there are few data on the actual changes in the magnitude and distribution of genetic diversity, although studies have documented declining genetic diversity in wild species that have been heavily exploited. In cultivated systems, since 1960 there has been a fundamental shift in the pattern of intra-species diversity in farmers’ fields and farming systems as the crop varieties planted by farmers have shifted from locally adapted and developed populations (landraces) to more widely adapted varieties produced through formal breeding systems (modern varieties). Roughly 80% of wheat area in developing countries and three quarters of the rice area in Asia is planted with modern varieties. (For other crops, such as maize, sorghum and millet, the proportion of area planted to modern varieties is far smaller.) The on-farm losses of genetic diversity of crops and livestock have been partially offset by the maintenance of genetic diversity in seed banks.
Source & ©: MA
Chapter 1, p.37
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