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WEEK TWO: HOW DOES WATER SUPPORT LIFE? | ESSAY TWO

Biodiversity and Ecosystem Services

by Eleanor Sterling, Nora Bynum, and Erin Vintinner

There is a profound connection between water and biodiversity. Biodiversity is the variety of life on Earth at all levels, from genes to ecosystems,and the ecological and evolutionary processes that sustain it. Species have adapted to the presence or absence of water, both salty and fresh, withan extraordinary range of forms and behaviors. Some species are composed mostly of water, some live in it, and still others avoid it. But all life onEarth needs water, and all organisms depend on their surrounding ecosystems for survival.

Freshwater ecosystems vary in terms of their size, the number and kinds of species they contain, and their relative proportion of abiotic (non-living) components. Their borders might be well-defined (a pond, for example) or less distinct (such as an area that transitions from wetland toforest). Freshwater ecosystems also change over time, both naturally and due to human disturbances.

World's Major Biomes

Terrestrial biomes differ according to temperature, precipitation, latitude, and altitude, and are defined by differences in characteristic plants andanimals. ©USDA

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Hierarchical Organization of Ecology, or Ecological Levels

Genetic: variety of genes found in living organismsIndividual: any living organismPopulation: members of the same species in an area at same timeCommunity: interacting populations in an area at same timeEcosystem: community of living and nonliving things that interact through cycling of matter and flow of energyLandscape: region that includes several interacting ecosystems

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Biome: large areas that contain ecosystems with similar conditions

Biosphere: region of Earth where life exists, along with the oceans, rivers, lakes, and soils and solid sediments that support living things

Key ecological factors affect the abundance and distribution of life in aquatic biomes. These include physical factors such as light, pressure, and

temperature, as well as the relative abundance of chemicals, both organic (carbon-based) and inorganic, such as oxygen, nitrogen, phosphorus, and

silicon. The pH and salt content of water, along with the flow, depth, and movement of water masses, also affect the distribution of life. So do

ecological interactions such as competition, predation, and the distribution of food. With the exception of the aquatic factors of pressure and flow,

many of these factors, especially temperature, also affect the distribution of life in terrestrial biomes. Terrestrial biomes are further affected by

abiotic factors such as topography and soil composition.

The Nitrogen Cycle

Nitrogen is essential to life, as a key component of proteins and nucleic acids such as DNA.

Free nitrogen (N2) makes up 80 percent of Earth's atmosphere, but living things cannot use this nitrogen directly. Bacteria must convert

inorganic molecular nitrogen into compounds such as ammonia or nitrate through the process of nitrogen fixation.

As plants then absorb these compounds, organic nitrogen becomes available to the food chain.

When organisms die, organic compounds containing nitrogen are broken down and released, in a process called denitrification.

Human activities can both release nitrogen to the atmosphere and "fix" nitrogen, transforming it into forms available for life. For example,

industrial processes are a major source of nitrogen fertilizer.

©AMNH

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Where is the Greatest Concentration of Life on Earth?

Although freshwater ecosystems occupy less than 1 percent of Earth's surface, they are home to 7 percent of all the species known to science—the

greatest concentration of life on the planet. Consequently, losing even relatively small portions of freshwater habitat significantly affects Earth's

biodiversity.

How Do Standing and Flowing Freshwater Systems Compare?

Freshwater ecosystems are divided into flowing and standing systems. Smaller, faster-flowing water ecosystems (springs and brooks) tend to be

cool, with more dissolved oxygen and fewer potential habitats for aquatic organisms. Larger and slower systems (streams and rivers) carry

sediment (clay, silt, and gravel) that is rich in nutrients. They also transport organic matter that supports plankton and riparian (riverbank) plants,

which anchor food chains. (Plankton are drifting organisms, including animals, plants, and bacteria, that provide food for larger animals such as

fishes. Phytoplankton use photosynthesis to obtain energy, while zooplankton consume other plankton for energy.) When sediment is deposited on

floodplains and river deltas, it creates fertile soils. The fertile soil and decreased flow create habitat for mussels, snails, crayfishes, insects, turtles,

and fishes, which in turn support an abundance of birds and mammals. Estuaries, where seawater combines with fresh water and is mixed by tides,

are transitional zones. This mixture of salt and freshwater makes estuaries far more biologically productive (nutrient-rich and supporting many

forms of life) than standing-water ecosystems.

Standing-water ecosystems contain varying quantities of sediment and organic matter. Ponds, which are small, completely enclosed bodies, are

shallow enough for light to penetrate to the bottom and can support rooted plants. Larger and deeper, lakes are characterized by the stratification of

light, temperature, and dissolved oxygen. Their upper layers receive more light, are generally warmer, and contain more dissolved oxygen. During

fall and spring, changes in surface temperature can cause these layers to mix in a process called turnover. Phytoplankton in the upper layers

produce food for a web that includes insects, amphibians, reptiles, crustaceans, fishes, birds and mammals.

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Lake Stratification

Lakes are divided into three zones stratified by light, temperature, and the level of dissolved oxygen. The warmest surface layer is referred to as

the littoral zone. The epilimnion, or upper layer, receives more light, is generally warmer, and contains more dissolved oxygen than the

hypolimnion, or lower layer. The thermocline, in green, represents the zone that separates the lake's upper and lower layers. ©USGS / AMNH

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What Services do Freshwater Ecosystems Provide?Freshwater ecosystems provide a range of important environmental benefits, some obvious and some less so. These include water and water

purification, food (subsistence, commercial, and sport fisheries), fish and wildlife habitat, fuel and raw materials such as wood for fuel and grasses

for thatching roofs, hydropower, transportation, flood regulation and drought mitigation, erosion prevention and coastal protection, groundwater

recharge, pollution dilution, soil stabilization and fertilization, nutrient cycling, climate regulation, recreation and tourism, spiritual and aesthetic

benefits, and non-material resources like security and safety.

The Case Study examines the many ecosystem services provided by the Mekong River basin. Millions of farmers and fishermen along this Asian

waterway depend on its rich sediment and fish-filled waters. The delta is a major rice-growing region, and fish and shrimp are farmed in its vast

swampy waters. The coast is lined with mangroves trees that provide habitat for thousands of species, protect the coastline, and provide local

people with wood and many other valuable commodities.

Water Quality

These are the key assessment factors that affect water quality:

Temperature: Temperature influences the rate of many biological and chemical processes, the oxygen content of water, and the metabolic

processes of aquatic organisms.

pH: The relative acidity or alkalinity of a body of water is determined by the concentration of hydrogen ions, which affects many biological

and chemical processes.

Turbidity: A measure of suspended sediment, influencing how much light can pass through water.

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Nutrients: Organic and inorganic chemicals that organisms need in order to live and grow. Nitrogen, phosphorus, carbon dioxide, and salt

are key nutrients.

Dissolved oxygen (DO): Oxygen in water is dissolved and is needed for respiration by organisms, decomposition, and various chemical

processes.

Biological Oxygen Demand (BOD): Demand for oxygen by organisms for respiration. If BOD exceeds the level of dissolved oxygen,

oxygen is depleted. Few organisms can survive in oxygen-depleted environments.

How Should We Value Those Services?

Despite the indisputable importance of these services, people seldom agree on how to assess their worth. This is largely because the debate reflects

underlying human values, which vary dramatically among both societies and individuals.

Economists typically divide biodiversity services into two categories: direct or indirect. Fish constitute a direct service, while soil fertilization is an

example of an indirect service. We also value services that are not utilitarian but that we would miss if they were to disappear. These include

existence value (the value of knowing that something exists) and bequest value (the value of knowing that something will persist for future

generations). Potential or option value refers to the use that something may have in the future. Often we realize the worth of an ecosystem service

only after it is lost. Many such services cannot be replaced, or only partially and at considerable cost. Although many are essential to our survival,

they have no market value. For example, how can we quantify the value of the services provided by the Colorado River Delta? The challenge lies

in translating these values, as they apply to freshwater ecosystems, for decision makers.

Street trees in New York City have been valued at $122 million per year because of the many services they provide: shade, reducing stormwater

runoff, increasing property values, and removing carbon dioxide from the atmosphere. In 1997, a group of researchers estimated the value of 17

essential ecosystem goods and services at an average of $33 trillion per year. That sum equals twice the total annual gross national product of all

the countries in the world. Rivers, lakes, and wetlands provided $6.6 trillion of this total. Some scientists and social scientists have disputed the

quantification, citing, among other things, the difficulty of "scaling up" across ecosystems and types of services. Others note that there may be an

inherent contradiction in placing a finite value on an irreplaceable life-support system. For example, while humans can undertake to provide some

services, such as raising fish in aquaculture, substitutions may not work as well. For example, a sea wall may protect against storm surges as

coastal wetlands do, but it must be constantly maintained against erosion and may not protect against all surges. And these substitutes are always

more costly. The scientific debate on how to classify and value ecosystem services, and how this knowledge should inform environmental

management and decision-making, is constantly evolving.

Because many of the benefits of ecosystem services have not been valued in the marketplace, many human populations have been spending this

capital without even stopping to tally the losses. While valuing ecosystem services is an imprecise science, it is a crucial tool for shaping policy

that protects and conserves natural ecosystems.

Related Links

EPA: National Wetlands Mitigation Action Plan »

Learn about current practices in wetland mitigation, and about the federal plan to improve the ecological performance and results of compensatory

mitigation in the United States.

USGS: Water Resources of the United States »

Information on current water resource programs across the United States, from stream flow and water quality to water use and water budgets.

Action Bioscience Ecosystem Services »

Learn more about the benefits of ecosystem services and gain access to educator resources and articles on the value of ecosystems.