Ecosystems, and the plant and animal species that compose them, provide a host of services to all living things. These services include the regulation of atmospheric gases that affect global and local climates, including the air we breathe; maintenance of the hydrologic cycle; control of nutrient and energy flow through the planet, including waste decomposition and detoxification, soil renewal, nitrogen fixation, and photosynthesis; a genetic library, providing a source of information to create better agricultural crops or livestock; maintenance of reproduction, such as pollination and seed dispersal, in plants that we rely upon for food, clothing, or shelter; and control of agricultural pests. Often the values of ecosystem services are not considered in commercial market analyses, yet they are critically important to human survival. Humans can rarely replace these services—or, if they can, only at considerable cost. According to a study by Costanza et al. (1997), the earth provides a minimum of $16 to $54 trillion worth of "services" to humans per year.

Biodiversity plays a critical role in regulating the earth's physical, chemical, and geo logical properties, from influencing the chemical composition of the atmosphere to modifying the climate. Earth's atmosphere has a unique composition, being made up primarily of nitrogen (77 percent) and oxygen (21 percent)—unlike the atmospheres of Venus and Mars, which are almost entirely composed of carbon dioxide (95 percent). Initially, like those of Venus and Mars, the atmosphere of Earth lacked oxygen. About 3.5 billion years ago, early life forms (bacteria) helped to create an oxygenated atmosphere by means of photosynthesis, taking up carbon dioxide and releasing oxygen (Schopf, 1983). Eventually, these organisms altered the composition of the atmosphere and paved the way for organisms that use oxygen as an energy source (aerobic respiration). Thus, organisms and their environment evolved together, achieving a balance between living and nonliving things, a state known as homeostasis.

The atmosphere is continually influenced by biodiversity. Phytoplankton (microscopic marine plants) in our oceans play a central role in regulating atmospheric chemistry. The oceans are the major reservoir for carbon on the planet, and they regulate carbon levels in the atmosphere. Carbon is continually exchanged between the atmosphere and the oceans. Phytoplankton transform carbon dioxide into organic matter during photosynthesis. This carbon-laden organic matter settles either directly or indirectly (after it has been consumed) to the deep ocean, where it stays for centuries or even thousands of years. This movement of carbon through the oceans removes excess carbon from the atmosphere and regulates the earth's climate. Over the last century, humans appear to have affected the atmospheric balance by releasing large amounts of carbon dioxide. The excess carbon dioxide, along with similar so-called greenhouse gases, is believed to be heating up our atmosphere and changing the world's climate.

Besides influencing global climate by modifying the atmosphere's composition, biodiversity affects climate in other ways. The extent and distribution of different types of vegetation over the globe, for example, modify climate by affecting the reflectance of sunlight (radiation balance), through the release of water vapor (evapotranspiration), and by changing wind patterns and moisture loss (surface roughness). The amount of solar radiation reflected by a surface is known as its albedo; surfaces with low albedo reflect a small amount of sunlight; those with high albedo reflect a large amount. Different types of vegetation have different albedos: forests typically have low albedo, whereas deserts have high albedo. Thus vegetation cover influences the amount of energy that reaches the earth. Deciduous forests are a good example of the seasonal relationship between vegetation and radiation balance. In the summer, the leaves in deciduous forests absorb solar radiation through photosynthesis; in winter, after their leaves have fallen, deciduous forests tend to reflect more radiation. These seasonal changes in vegetation modify climate in complex ways by changing evapotranspiration rates and albedo.

Vegetation absorbs water from the soil and releases it back into the atmosphere through evapotranspiration, the major pathway for water to move from the soil to the atmosphere. This release of water lowers the air temperature. In the Amazon region, vegetation and climate are tightly coupled; evapotranspiration of plants is believed to contribute 50 percent of the annual rainfall. Deforestation in this region leads to a complex feedback mechanism: it reduces evapotranspiration, decreasing rainfall and increasing the area's vulnerability to fire (Laurance and Williamson, 2001). Deforestation is also influencing the cli mate of cloud forests in the mountains of Costa Rica. The Monteverde Cloud Forest is nominally well protected within a network of reserves, and it harbors a rich diversity of organisms, many of which are found nowhere else. However, deforestation in lower-lying lands is changing the local climate and lifting the clouds above the mountains, leaving the cloud forest cloudless. Removing the clouds from a cloud forest dries the forest, so that it can no longer support the same vegetation or provide appropriate habitat for many of the species originally found there. As these areas dry up, there is literally nowhere for the cloud forest species to go, and they may disappear permanently. Similar patterns may be occurring in other, less well known montane cloud forests around the world.

Different vegetation types and topographies have varying surface roughness—that is, average vertical relief; small-scale irregularities of a surface change the flow of winds in the lower atmosphere, which in turn influences climate. Lower surface roughness tends to reduce surface moisture and to increase evaporation. Models examining the conversion of African savanna to grassland and agriculture found that precipitation declined by 10 percent in the new landscape (Hoffmann and Jackson, 2000). This decline was caused equally by changes in the surface albedo and the surface roughness. Farmers apply this knowledge when they plant trees to create windbreaks. Windbreaks reduce wind speed and change the microclimate, increasing surface roughness, reducing soil erosion, and modifying temperature and humidity. For many field crops, windbreaks increase yield and production efficiency. They also minimize stress on livestock from cold winds.

Biodiversity is also important for global soil and water protection. Terrestrial vegetation in forests and other upland habitats helps to maintain the water quality and quantity of the hydrologic cycle, and it also helps to control soil erosion. Plant leaves slow the descent of raindrops, so that by the time the water reaches the ground it is less likely to wash away soil and more likely to percolate into the ground. Roots hold soil in place, which increases water absorption and decreases soil erosion during heavy rains. Plants pump water from the soil back into the atmosphere, completing the cycle. In watersheds (land areas drained by a river and its tributaries) where vegetation has been removed, flooding prevails in the wet season and drought in the dry season. Soil erosion is also more intense and rapid, causing a double effect: removal of nutrient-rich topsoil and siltation in downstream riverine or, ultimately, oceanic environments. This siltation can harm riverine and coastal fisheries as well as damage coral reefs. In the Mississippi River delta ecosystem, for example, a buildup of sediment and pesticides has created an anoxic area (that is, an area without oxygen), known as the dead zone, in the Gulf of Mexico (Turner and Rabalais, 1994). The source of these sediments and pesticides is upriver, far from the delta. Another example comes from East Africa, where sediment discharges caused significant damage to the Malindi-Watamu fringing reef complex along the Kenyan coast (van Katwijk et al., 1993), smothering the corals and leading to excessive algal growth.

Wetlands, natural communities linking land and water, are also instrumental for the maintenance of clean water and erosion control. Wetlands are defined as lands where water is present at or near the surface of the soil, or within the root zone, all year or for a period of time during the year; they are characterized by vegetation adapted for those conditions. Microbes and plants in wetlands, some of the most productive ecosystems on earth, absorb nutrients and in the process filter and purify water before pollutants can enter aquatic ecosystems.

Wetlands help reduce flood, wave, and wind damage. They slow the flow of flood waters and accumulate sediments that would otherwise be carried downstream or into coastal areas. Wetlands also serve as breeding grounds and nurseries for fish, and they support thousands of bird and other animal species.

Nutrient cycling is yet another critical service provided by nature. Fungi and microbes in the soil help to break down dead plants and animals. This process converts elements and compounds—such as nitrogen and phosphate—into nutrients that most plants use and thus enriches the soil. Nitrogen-fixing bacteria, for example, transform atmospheric nitrogen into nitrates or nitrites. Nitrogen is essential for plant growth, and an insufficient quantity limits biomass production in both natural and agricultural ecosystems. In addition to decomposition, microbes also detoxify waste, changing waste products into forms less harmful to humans.

Humans cultivate only a small fraction of the plant and animal species on earth. To ensure that we can sustain these systems, we depend on biodiversity, especially the wild counterparts of cultivated foods and domesticated animals, as a genetic library that we can use to create new varieties or breeds better able to combat pests or disease, more suited to certain environmental conditions. Thus biodiversity acts as a kind of insurance for agriculture. For instance, corn (Zea mays), along with wheat and rice, is one of the world's most important cultivated plants. The annual global market for corn is nearly $60 billion, yet this crop is susceptible to several viral diseases. In the late 1970s, teosinte (Zea diploperennis), the closest wild relative of corn, was discovered and found to be resistant to viral diseases that infect Z. mays. The new species has the same chromosome number as Z. mays and can therefore hybridize with it. When that occurs, some of the viral resistance is transferred to domestic corn. Four viral-resistant commercial strains have since been produced, highlighting the importance of wild counterparts to cultivated food crops.

As a further example of ecosystem services, an estimated 90 percent of flowering plants depend on pollinators, such as wasps, birds, bats, and bees, to reproduce. Without these pollinators, many plant species would face extinction. Plants and their pollinators are increasingly threatened around the world, however (Buchmann and Nabhan, 1995). Yet pollination is critical to most major crops and virtually impossible to replace. For instance, imagine how costly orange juice would be (and how little would be available) if its natural pollinators no longer existed, and each orange flower had to be fertilized by hand.

Agricultural pests (principally insects, plant pathogens, and weeds) destroy an estimated 37 percent of U.S. crops (Pimentel and Levitan, 1986). The level of destruction varies depending on the crop, where it is grown, and the type of pest. According to Oerke et al. (1994), production losses caused by pests, pathogens, and weeds amount to 15 percent, 14 percent, and 13 percent on average, respectively, for the principal cereals and potatoes. Without natural predators that keep pests under control, these figures would be much higher. Natural pest control saves farmers billions each year, and pesticides are no replacement for the services provided by these crop-friendly predators.

Some animal species are important dispersers of plant and tree seeds. Loss of these species may have a "domino effect," leading to the loss of those plants and trees that depend upon them for reproduction. In the pine forests of western North America, for example, corvids (including jays, magpies, and crows), squirrels, and bears play a role in seed dispersal. The Clark's nutcracker (Nucifraga columbiana) is particularly well adapted to the dispersal of whitebark pine (Pinus albicaulis) seeds. The nutcracker removes the wingless seeds from the cones, which otherwise would not open on their own. Nutcrackers hide the seeds in clumps. When the uneaten seeds eventually grow, they are clustered, accounting for the typical distribution pattern of white-bark pine in the forest.

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