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- Biodiversity is the variability among living organisms from all sources, including terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species, and of ecosystems.
- Biodiversity forms the foundation of the vast array of ecosystem services that critically contribute to human well-being.
- Biodiversity is important in human-managed as well as natural ecosystems.
- Decisions humans make that influence biodiversity affect the well-being of themselves and others.
Biodiversity is the foundation of ecosystem services to which human well-being is intimately linked. No feature of Earth is more complex, dynamic, and varied than the layer of living organisms that occupy its surfaces and its seas, and no feature is experiencing more dramatic change at the hands of humans than this extraordinary, singularly unique feature of Earth. This layer of living organisms—the biosphere—through the collective metabolic activities of its innumerable plants, animals, and microbes physically and chemically unites the atmosphere, geosphere, and hydrosphere into one environmental system within which millions of species, including humans, have thrived. Breathable air, potable water, fertile soils, productive lands, bountiful seas, the equitable climate of Earth’s recent history, and other ecosystem services (see Box 1.1 and Key Question 2) are manifestations of the workings of life. It follows that large-scale human influences over this biota have tremendous impacts on human well-being. It also follows that the nature of these impacts, good or bad, is within the power of humans to influence (CF2).
Defining Biodiversity
Biodiversity is defined as “the variability among living organisms from all sources including, inter alia, terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems.” The importance of this definition is that it draws attention to the many dimensions of biodiversity. It explicitly recognizes that every biota can be characterized by its taxonomic, ecological, and genetic diversity and that the way these dimensions of diversity vary over space and time is a key feature of biodiversity. Thus only a multidimensional assessment of biodiversity can provide insights into the relationship between changes in biodiversity and changes in ecosystem functioning and ecosystem services (CF2).
Biodiversity includes all ecosystems—managed or unmanaged. Sometimes biodiversity is presumed to be a relevant feature of only unmanaged ecosystems, such as wildlands, nature preserves, or national parks. This is incorrect. Managed systems—be they plantations, farms, croplands, aquaculture sites, rangelands, or even urban parks and urban ecosystems—have their own biodiversity. Given that cultivated systems alone now account for more than 24% of Earth’s terrestrial surface, it is critical that any decision concerning biodiversity or ecosystem services address the maintenance of biodiversity in these largely anthropogenic systems (C26.1).
Measuring Biodiversity: Species Richness and Indicators
In spite of many tools and data sources, biodiversity remains difficult to quantify precisely. But precise answers are seldom needed to devise an effective understanding of where biodiversity is, how it is changing over space and time, the drivers responsible for such change, the consequences of such change for ecosystem services and human well-being, and the response options available. Ideally, to assess the conditions and trends of biodiversity either globally or sub-globally, it is necessary to measure the abundance of all organisms over space and time, using taxonomy (such as the number of species), functional traits (for example, the ecological type such as nitrogen-fixing plants like legumes versus non-nitrogen-fixing plants), and the interactions among species that affect their dynamics and function (predation, parasitism, competition, and facilitation such as pollination, for instance, and how strongly such interactions affect ecosystems). Even more important would be to estimate turnover of biodiversity, not just point estimates in space or time. Currently, it is not possible to do this with much accuracy because the data are lacking. Even for the taxonomic component of biodiversity, where information is the best, considerable uncertainty remains about the true extent and changes in taxonomic diversity (C4).
There are many measures of biodiversity; species richness (the number of species in a given area) represents a single but important metric that is valuable as the common currency of the diversity of life—but it must be integrated with other metrics to fully capture biodiversity. Because the multidimensionality of biodiversity poses formidable challenges to its measurement, a variety of surrogate or proxy measures are often used. These include the species richness of specific taxa, the number of distinct plant functional types (such as grasses, forbs, bushes, or trees), or the diversity of distinct gene sequences in a sample of microbial DNA taken from the soil. Species- or other taxon-based measures of biodiversity, however, rarely capture key attributes such as variability, function, quantity, and distribution—all of which provide insight into the roles of biodiversity. (See Box 1.2)
Ecological indicators are scientific constructs that use quantitative data to measure aspects of biodiversity, ecosystem condition, services, or drivers of change, but no single ecological indicator captures all the dimensions of biodiversity (C2.2.4). (See Box 1.3) Ecological indicators form a critical component of monitoring, assessment, and decision-making and are designed to communicate information quickly and easily to policy-makers. In a similar manner, economic indicators such as GDP are highly influential and well understood by decision-makers. Some environmental indicators, such as global mean temperature and atmospheric CO2 concentrations, are becoming widely accepted as measures of anthropogenic effects on global climate. Ecological indicators are founded on much the same principles and therefore carry with them similar pros and cons (C2.2.4). (See Box 1.4)."
Box 1.1 Linkages among Biodiversity, Ecosystem Services, and Human Well-being
Box 1.2: Measuring and Estimating Biodiversity: More than Species Richness
Box 1.3: Ecological Indicators and Biodiversity
Box 1.4: Criteria for Effective Ecological Indicators
Source & ©: Millennium Ecosystem Assessment
Chapter 1, p.18
The source document for this Digest states:
Biodiversity is essentially everywhere, ubiquitous on Earth’s surface and in every drop of its bodies of water. The virtual omnipresence of life on Earth is seldom appreciated because most organisms are small (<5 centimeters); their presence is sparse, ephemeral, or cryptic, or, in the case of microbes, they are invisible to the unaided human eye (CF2).
Documenting spatial patterns in biodiversity is difficult because taxonomic, functional, trophic, genetic, and other dimensions of biodiversity have been relatively poorly quantified. Even knowledge of taxonomic diversity, the best known dimension of biodiversity, is incomplete and strongly biased toward the species level, megafauna, temperate systems, and components used by people. (See Figure 1.1) This results in significant gaps in knowledge, especially regarding the status of tropical systems, marine and freshwater biota, plants, invertebrates, microorganisms, and subterranean biota. For these reasons, estimates of the total number of species on Earth range from 5 million to 30 million. Irrespective of actual global species richness, however, it is clear that the 1.7–2 million species that have been formally identified represent only a small portion of total species richness. More-complete biotic inventories are badly needed to correct for this deficiency (C4).
Source & ©: Millennium Ecosystem Assessment
Chapter 1, p.19
The source document for this Digest states:
Spatial Patterns of Biodiversity: Hotspots, Biomes,1 Biogeographic Realms, Ecosystems, and Ecoregions
While the data to hand are often insufficient to provide accurate pictures of the extent and distribution of all components of biodiversity, there are, nevertheless, many patterns and tools that decision-makers can use to derive useful approximations for both terrestrial and marine ecosystems. North-temperate regions often have usable data on spatial distributions of many taxa, and some groups (such as birds, mammals, reptiles, plants, butterflies, and dragonflies) are reasonably well documented globally. Biogeographic principles (such as gradients in species richness associated with latitude, temperature, salinity, and water depth) or the use of indicators can supplement available biotic inventories. Global and sub-global maps of species richness, several of which are provided in the MA reports Current State and Trends and Scenarios, provide valuable pictures of the distribution of biodiversity (C4, S10).
Most macroscopic organisms have small, often clustered geographical ranges, leading to centers of both high diversity and endemism, frequently concentrated in isolated or topographically variable regions (islands, mountains, peninsulas). A large proportion of the world’s terrestrial biodiversity at the species level is concentrated in a small part of the world, mostly in the tropics. Even among the larger and more mobile species, such as terrestrial vertebrates, more than one third of all species have ranges of less than 1,000 square kilometers. In contrast, local and regional diversity of microorganisms tends to be more similar to large-scale and global diversity because of their large population size, greater dispersal, larger range sizes, and lower levels of regional species clustering (C4.2.3).
Biomes and biogeographic realms provide broad pictures of the distribution of functional diversity. Functional diversity (the variety of different ecological functions in a community independent of its taxonomic diversity) shows patterns of associations (biota typical of wetlands, forests, grasslands, estuaries, and so forth) with geography and climate known as biomes (see Figure 1.2), with ecosystems and ecoregions being smaller divisions within biomes (see Figure 1.3). These can be used to provide first-order approximations of both expected functional diversity as well as possible changes in the distribution of these associations should environmental conditions change.
Source & ©: Millennium Ecosystem Assessment
Chapter 1, p.20
The source document for this Digest states:
Temporal Patterns of Biodiversity: Background Rates of Extinction and Biodiversity Loss
Knowledge of patterns of biodiversity over time allow for only very approximate estimates of background rates of extinction or of how fast species have become extinct over geological time. Except for the last 1,000 years, global biodiversity has been relatively constant over most of human history, but the history of life is characterized by considerable change. The estimated magnitude of background rates of extinction is roughly 0.1–1.0 extinctions per million species per year. Most measurements of this rate have come from assessing the length of species’ lifetimes through the fossil record: these range over 0.5–13 million years, and possibly 0.2–16 million years. These data probably underestimate background extinction rates because they are necessarily largely derived from taxa that are abundant and widespread in the fossil record (C4.4.2). Current rates of extinction are discussed in Key Question 3.
A mismatch exists between the dynamics of changes in natural systems and human responses to those changes. This mismatch arises from the lags in ecological responses, the complex feedbacks between socioeconomic and ecological systems, and the difficulty of predicting thresholds. Multiple impacts (especially the addition of climate change to the mix of forcing functions) can cause thresholds, or rapid and dramatic changes in ecosystem function even though the increase in environmental stress has been small and constant over time. Understanding such thresholds requires having long-term records, but such records are usually lacking or monitoring has been too infrequent, of the wrong periodicity, or too localized to provide the necessary data to analyze and predict threshold behavior (C28, S3.3.1).
Shifts to different regimes may cause rapid substantial changes in biodiversity, ecosystem services, and human well-being. Regime shifts have been commonly documented in pelagic systems due to thresholds related to temperature regimes and overexploitation (C19.2.1, C18). Some regime shifts are essentially irreversible, such as coral reef ecosystems that undergo sudden shifts from coral-dominated to algal-dominated reefs (C19.5). The trigger for such phase shifts usually includes increased nutrient inputs leading to eutrophic conditions and removal of herbivorous fishes that maintain the balance between corals and algae. Once the thresholds (both an upper and a lower threshold) for the two ecological processes of nutrient loading and herbivory are passed, the phase shift occurs quickly (within months), and the resulting ecosystem—though stable—is less productive and less diverse. Consequently, human well-being is affected not only by reductions in food supply and decreased income from reef-related industries (diving and snorkeling, aquarium fish collecting, and so on), but also by increased costs due to diminished ability of reefs to protect shorelines. (Algal reefs are more prone to being broken up in storm events, leading to shoreline erosion and seawater breaches of land) (C19.3). Such phase shifts have been documented in Jamaica, elsewhere in the Caribbean, and in Indo-Pacific reefs (C19, S3.3.1).
Introduced invasive species can act as a trigger for dramatic changes in ecosystem structure, function, and delivery of services. For example, the introduction of the carnivorous ctenophore Mnemiopsis leidyi (a jellyfish-like animal) in the Black Sea caused the loss of 26 major fisheries species and has been implicated (along with other factors) in the subsequent growth of the oxygen-deprived “dead” zone (C19.2.1).
Source & ©: Millennium Ecosystem Assessment
Chapter 1, p.21
The source document for this Digest states:
Biodiversity plays an important role in ecosystem functions that provide supporting, provisioning, regulating, and cultural services. These services are essential for human well-being. However, at present there are few studies that link changes in biodiversity with changes in ecosystem functioning to changes in human well-being. Protecting the Catskill watersheds that provide drinking water for New York City is one case where safeguarding ecosystem services paid a dividend of several billion dollars. Further work that demonstrates the links between biodiversity, regulating and supporting services, and human well-being is needed to show this vital but often unappreciated value of biodiversity (C4, C7, C11).
Species composition matters as much or more than species richness when it comes to ecosystem services. Ecosystem functioning, and hence ecosystem services, at any given moment in time is strongly influenced by the ecological characteristics of the most abundant species, not by the number of species. The relative importance of a species to ecosystem functioning is determined by its traits and its relative abundance. For example, the traits of the dominant or most abundant plant species—such as how long they live, how big they are, how fast they assimilate carbon and nutrients, how decomposable their leaves are, or how dense their wood is—are usually the key species drivers of an ecosystem’s processing of matter and energy. Thus conserving or restoring the composition of biological communities, rather than simply maximizing species numbers, is critical to maintaining ecosystem services (C11.2.1, C11.3).
Local or functional extinction, or the reduction of populations to the point that they no longer contribute to ecosystem functioning, can have dramatic impacts on ecosystem services. Local extinctions (the loss of a species from a local area) and functional extinctions (the reduction of a species such that it no longer plays a significant role in ecosystem function) have received little attention compared with global extinctions (loss of all individuals of a species from its entire range). Loss of ecosystem functions, and the services derived from them, however, occurs long before global extinction. Often, when the functioning of a local ecosystem has been pushed beyond a certain limit by direct or indirect biodiversity alterations, the ecosystem-service losses may persist for a very long time (C11).
Changes in biotic interactions among species—predation, parasitism, competition, and facilitation—can lead to disproportionately large, irreversible, and often negative alterations of ecosystem processes. In addition to direct interactions, such as predation, parasitism, or facilitation, the maintenance of ecosystem processes depends on indirect interactions as well, such as a predator preying on a dominant competitor such that the dominant is suppressed, which permits subordinate species to coexist. Interactions with important consequences for ecosystem services include pollination; links between plants and soil communities, including mycorrhizal fungi and nitrogen-fixing microorganisms; links between plants and herbivores and seed dispersers; interactions involving organisms that modify habitat conditions (beavers that build ponds, for instance, or tussock grasses that increase fire frequency); and indirect interactions involving more than two species (such as top predators, parasites, or pathogens that control herbivores and thus avoid overgrazing of plants or algal communities) (C11.3.2).
Many changes in ecosystem services are brought about by the removal or introduction of organisms in ecosystems that disrupt biotic interactions or ecosystem processes. Because the network of interactions among species and the network of linkages among ecosystem processes are complex, the impacts of either the removal of existing species or the introduction of new species are difficult to anticipate (C11). (See Table 1.1)
Table 1.1: Ecological Surprises Caused by Complex Interactions
As in terrestrial and aquatic communities, the loss of individual species involved in key interactions in marine ecosystems can also influence ecosystem processes and the provisioning of ecological services. For example, coral reefs and the ecosystem services they provide are directly dependent on the maintenance of some key interactions between animals and algae. As one of the most species-rich communities on Earth, coral reefs are responsible for maintaining a vast storehouse of genetic and biological diversity. Substantial ecosystem services are provided by coral reefs—such as habitat construction, nurseries, and spawning grounds for fish; nutrient cycling and carbon and nitrogen fixing in nutrient-poor environments; and wave buffering and sediment stabilization. The total economic value of reefs and associated services is estimated as hundreds of millions of dollars. Yet all coral reefs are dependent on a single key biotic interaction: symbiosis with algae. The dramatic effects of climate change and variability (such as El Niño oscillations) on coral reefs are mediated by the disruption of this symbiosis (C11.4.2)."
Source & ©: Millennium Ecosystem Assessment
Chapter 1, p.22
The source document for this Digest states:
Biodiversity affects key ecosystem processes in terrestrial ecosystems such as biomass production, nutrient and water cycling, and soil formation and retention—all of which govern and ensure supporting services (high certainty). The relationship between biodiversity and supporting ecosystem services depends on composition, relative abundance, functional diversity, and, to a lesser extent, taxonomic diversity. If multiple dimensions of biodiversity are driven to very low levels, especially trophic or functional diversity within an ecosystem, both the level and stability (for instance, biological insurance) of supportive services may decrease (CF2, C11). (See Figure 1.4)
Region-to-region differences in ecosystem processes are driven mostly by climate, resource availability, disturbance, and other extrinsic factors and not by differences in species richness (high certainty). In natural ecosystems, the effects of abiotic and land use drivers on ecosystem services are usually more important than changes in species richness. Plant productivity, nutrient retention, and resistance to invasions and diseases sometimes grow with increasing species numbers in experimental ecosystems that have been reduced to low levels of biodiversity. In natural ecosystems, however, these direct effects of increasing species richness are usually overridden by the effects of climate, resource availability, or disturbance regime (C11.3).
Even if losses of biodiversity have small short-term impacts on ecosystem function, such losses may reduce the capacity of ecosystems for adjustment to changing environments (that is, ecosystem stability or resilience, resistance, and biological insurance) (high certainty). The loss of multiple components of biodiversity, especially functional and ecosystem diversity at the landscape level, will lead to lowered ecosystem stability (high certainty). Although the stability of an ecosystem depends to a large extent on the characteristics of the dominant species (such as life span, growth rate, or regeneration strategy), less abundant species also contribute to the long-term preservation of ecosystem functioning. There is evidence that a large number of resident species, including those that are rare, may act as “insurance” that buffers ecosystem processes in the face of changes in the physical and biological environment (such as changes in precipitation, temperature, pathogens) (C11.3.2). As tragically illustrated by social conflict and humanitarian crisis over droughts, floods, and other ecosystem collapses, stability of ecosystems underpins most components of human well-being, including health, security, satisfactory social relations, and freedom of choice and action (C6; see also Key Question 2).
Source & ©: Millennium Ecosystem Assessment
Chapter 1, p.21
The source document for this Digest states:
Invasion resistance
The preservation of the number, types, and relative abundance of resident species can enhance invasion resistance in a wide range of natural and semi-natural ecosystems (medium certainty). Although areas of high species richness (such as biodiversity hot spots) are more susceptible to invasion than species-poor areas, within a given habitat the preservation of its natural species pool appears to increase its resistance to invasions by non-native species. This is also supported by evidence from several marine ecosystems, where decreases in the richness of native taxa were correlated with increased survival and percent cover of invading species (C11.3.1, C11.4.1).
Pollination
Pollination is essential for the provision of plant-derived ecosystem services, yet there have been worldwide declines in pollinator diversity (medium certainty). Many fruits and vegetables require pollinators, thus pollination services are critical to the production of a considerable portion of the vitamins and minerals in the human diet. Although there is no assessment at the continental level, documented declines in more-restricted geographical areas include mammals (lemurs and bats, for example) and birds (hummingbirds and sunbirds, for instance), bumblebees in Britain and Germany, honeybees in the United States and some European countries, and butterflies in Europe. The causes of these declines are multiple, but habitat destruction and the use of pesticide are especially important. Estimates of the global annual monetary value of pollination vary widely, but they are in the order of hundreds of billions of dollars (C11.3.2, Box C11.2).
Climate regulation
Biodiversity influences climate at local, regional, and global scales, thus changes in land use and land cover that affect biodiversity can affect climate. The important components of biodiversity include plant functional diversity and the type and distribution of habitats across landscapes. These influence the capacity of terrestrial ecosystems to sequester carbon, albedo (proportion of incoming radiation from the Sun that is reflected by the land surface back to space), evapotranspiration, temperature, and fire regime—all of which influence climate, especially at the landscape, ecosystem, or biome levels. For example, forests have higher evapotranspiration than other ecosystems, such as grasslands, because of their deeper roots and greater leaf area. Thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems. In the Amazon, for example, 60% of precipitation comes from water transpired by upwind ecosystems (C11.3.3).
In addition to biodiversity within habitats, the diversity of habitats in a landscape exerts additional impacts on climate across multiple scales. Landscape-level patches (>10 kilometers in diameter) that have lower albedo and higher surface temperature than neighboring patches create cells of rising warm air above the patch (convection). This air is replaced by cooler moister air that flows laterally from adjacent patches (advection). Climate models suggest that these landscape-level effects can substantially modify local-to-regional climate. In Western Australia, for example, the replacement of native heath vegetation by wheatlands increased regional albedo. As a result, air tended to rise over the dark (more solar-absorptive and therefore warmer) heathland, drawing moist air from the wheatlands to the heathlands. The net effect was a 10% increase in precipitation over heathlands and a 30% decrease in precipitation over croplands (C11.3.3).
Some components of biodiversity affect carbon sequestration and thus are important in carbon-based climate change mitigation when afforestation, reforestation, reduced deforestation, and biofuel plantations are involved (high certainty). Biodiversity affects carbon sequestration primarily through its effects on species characteristics, which determine how much carbon is taken up from the atmosphere (assimilation) and how much is released into it (decomposition, combustion). Particularly important are how fast plants can grow, which governs carbon inputs, and woodiness, which enhances carbon sequestration because woody plants tend to contain more carbon, live longer, and decompose more slowly than smaller herbaceous plants. Plant species also strongly influence carbon loss via decomposition and their effects on disturbance. Plant traits also influence the probability of disturbances such as fire, windthrow, and human harvest, which temporarily change forests from accumulating carbon to releasing it (C11.3.3).
The major importance of marine biodiversity in climate regulation appears to be via its effect on biogeochemical cycling and carbon sequestration. The ocean, through its sheer volume and links to the terrestrial biosphere, plays a huge role in cycling of almost every material involved in biotic processes. Of these, the anthropogenic effects on carbon and nitrogen cycling are especially prominent. Biodiversity influences the effectiveness of the biological pump that moves carbon from the surface ocean and sequesters it in deep waters and sediments. Some of the carbon that is absorbed by marine photosynthesis and transferred through food webs to grazers sinks to the deep ocean as fecal pellets and dead cells. The efficiency of this trophic transfer and therefore the extent of carbon sequestration is sensitive to the species richness and composition of the plankton community (C11.4.3).
Pest, disease, and pollution control
The maintenance of natural pest control services, which benefits food security, rural household incomes, and national incomes of many countries, is strongly dependent on biodiversity. Yields of desired products from agroecosystems may be reduced by attacks of animal herbivores and microbial pathogens, above and below ground, and by competition with weeds. Increasing associated biodiversity with low-diversity agroecosystems, however, can enhance biological control and reduce the dependency and costs associated with biocides. Moreover, high-biodiversity agriculture has cultural and aesthetic value and can reduce many of the externalized costs of irrigation, fertilizer, pesticide, and herbicide inputs associated with monoculture agriculture (C11.3.4, Boxes C11.3 and C11.4).
The marine microbial community provides critical detoxification services, but how biodiversity influences them is not well understood. There is very little information on how many species are necessary to provide detoxification services, but these services may critically depend on one or a few species. Some marine organisms provide the ecosystem service of filtering water and reducing effects of eutrophication. For example, American oysters in Chesapeake Bay were once abundant but have sharply declined—and with them, their filtering ecosystem services. Areas like the Chesapeake might have much clearer water if large populations of filtering oysters could be reintroduced. Some marine microbes can degrade toxic hydrocarbons, such as those in an oil spill, into carbon and water, using a process that requires oxygen. Thus this service is threatened by nutrient pollution, which generates oxygen deprivation (C11.4.4).
Source & ©: Millennium Ecosystem Assessment
Chapter 1, p.25
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