Ecosystems

Chapter 47

Impacts, IssuesBye-Bye, Blue Bayou

Among many effects of global warming, coastal marshes in Louisiana are disappearing under rising water – along with habitat and revenues

47.1 The Nature of Ecosystems

Ecosystem• An array of organisms and a physical

environment, all interacting through a one-way flow of energy and a cycling of nutrients

• Sustained by ongoing inputs of energy and nutrients (open system)

Overview of Participants

Primary producers (autotrophs)• Obtain energy from nonliving sources (sunlight)

• Build organic compounds from CO2 and water

Consumers (heterotrophs)• Get energy and carbon from organic sources

• Carnivores, herbivores, parasites, omnivores

Overview of Participants

Detritivores, such as earthworms and crabs, eat small particles of organic matter (detritus)

Decomposers, such as bacteria and fungi, feed on organic wastes and remains and break them down into inorganic building blocks

Energy and Nutrients

Energy flows one way • Producers capture light energy and convert it to

bond energy in organic molecules (photosynthesis)

• Metabolic reactions break bonds (aerobic respiration) and give off heat, which is not recycled

Nutrients are cycled• Producers take up inorganic compounds from the

environment; decomposers return them

Trophic Structure of Ecosystems

Trophic levels• Hierarchy of feeding relationships in which

energy is transferred when one organism eats another

• Each trophic level is a number of transfers away from the system’s original energy input

Food Chain

Food chain• A sequence of steps by which some energy

captured by primary producers is transferred to organisms at successively higher tropic levels

• Omnivores feed at several levels

A number of food chains cross-connect with each other as food webs

47.1 Key Concepts Organization of Ecosystems

An ecosystem consists of a community and its physical environment

A one-way flow of energy and a cycling of raw materials among its interacting participants maintain it

It is an open system, with inputs and outputs of energy and nutrients

47.2 The Nature of Food Webs

Food webs • Multiple interconnecting food chains, including

grazing and detrital food chains

Grazing food chain• Energy stored in producers flows to herbivores,

which tend to be large animals

Detrital food chain• Energy in producers flows to decomposers and

detritivores, which tend to be small

Land Versus Aquatic Food Chains

In land ecosystems, most of the energy stored in producers moves through detrital food chains

In aquatic ecosystems, most of the energy in producers flows to grazers rather than detritivores

How Many Transfers?

Cumulative energy losses from energy transfers between trophic levels limits the length of food chains to four or five trophic levels• Food chains tend to be shortest in variable

habitats, longer in stable habitats

• Food webs with more carnivores have fewer connections; herbivores have more connections

47.2 Key Concepts Food Webs

Food chains are linear sequences of feeding relationships

Food chains cross-connect as food webs

Most energy that enters a food web returns to the environment, mainly as metabolic heat

Nutrients are recycled within the food web

47.3 Energy Flow Through Ecosystems

Primary producers capture energy and take up nutrients, which move to other trophic levels

Primary production• Rate at which producers capture and store energy

• Gross primary production – amount captured

• Net primary production – amount used in growth

Ecological Pyramids

A biomass pyramid depicts dry weight of organisms at each trophic level in an ecosystem• Largest tier is usually producers

• For some aquatic systems, pyramid is inverted

An energy pyramid depicts the energy that enters each trophic level in an ecosystem• Largest tier is always producers

A Biomass Pyramid

An aquatic ecosystem: Silver Springs, Florida

Ecological Efficiency

Between 5 and 30 percent of energy in tissues of organisms at one trophic level ends up in tissues of those at the next trophic level • Some energy is lost as heat

• Some biomass is not digested

Efficiency of transfers tends to be greatest in aquatic systems (less lignin, more ectotherms)

47.4 Biological Magnification

Biological magnification• Some harmful substances, such as DDT,

become increasingly concentrated in tissues of organisms as they move up the food chain

DDT and Silent Spring

DDT is a synthetic pesticide used extensively in the 1940s and 1950s; it also killed other organisms such as songbirds and fishes

Biological magnification of DDT in bird top carnivores weakened eggs; populations shrank

Rachel Carson’s book Silent Spring exposed dangers of DDT, prompting its ban in the US

The Mercury Menace

Mercury from coal-burning power plants, mines and industries washes into aquatic habitats

Mercury accumulates in fish top carnivores, and people who eat them

Mercury damages developing human nervous systems; children and women who are pregnant or nursing should avoid eating contaminated fish

47.3-47.4 Key Concepts Energy and Materials Flow

Ecosystems differ in how much energy their producers capture and how much is stored in each trophic level

Some toxins that enter an ecosystem can become increasingly concentrated as they pass from one trophic level to another

47.5 Biogeochemical Cycles

In a biogeochemical cycle, an essential element moves from nonliving environmental reservoirs, into living organisms, then back to the reservoirs

Elements essential to life (nutrients) include oxygen, hydrogen, carbon, nitrogen, phosphorus

Biogeochemical Cycles

Nutrients move from inorganic reservoirs (rocks, sediments, water, atmosphere) to living systems through primary producers

Photosynthetic organisms take up dissolved ions and carbon dioxide; bacteria fix nitrogen gas

47.6 The Water Cycle

The water cycle moves on a global scale• Water moves slowly from the world ocean (the

main reservoir) through the atmosphere (by evaporation and transpiration), onto land (by condensation and precipitation), then back to the ocean

Where Water Moves

Watershed• An area from which all precipitation drains into a

specific waterway

Groundwater• Water in soil and aquifers (permeable rock layers

that hold water)

Runoff • Flows over saturated ground into streams

A Global Water Crisis

Plenty of saltwater, little freshwater• Two-thirds of freshwater use sustains agriculture

Salinization• Buildup of mineral salts in soil

• Stunts crop plants and decreases yields

Aquifers are becoming polluted and depleted

Desalinization

Desalinization• The removal of salt from seawater to increase

freshwater supplies

• Requires a lot of fossil fuel

• Used mainly in places with large fuel reserves and small populations

47.7 Carbon Cycle

In the carbon cycle, carbon moves though all food webs, to and from its major reservoirs • Earth’s crust: 66 to 100 million gigatons

• World ocean: 38,000 to 40,000 gigatons

• Fossil fuel reserves: 4,000 gigatons

• Detritus in soil: 1,500 to 1,600 gigatons

• Air: 766 gigatons (mostly as CO2)

• Biomass: 540 to 610 gigatons

Movements of Carbon

Most of the annual cycling of carbon occurs between the ocean and the atmosphere• Some CO2 in surface waters is converted to

bicarbonate (carbon-oxygen cycling) and moved by ocean currents to deep ocean reservoirs

Photosynthesis, aerobic respiration, decomposition, and formation of carbonate sediments all contribute to the carbon cycle

Humans Are Altering the Carbon Cycle

Each year we withdraw 4 to 5 gigatons of fossil fuel from environmental reservoirs; and put 6 gigatons more carbon into the air than can be recycled to ocean reservoirs

Excess CO2 entering the atmosphere may be a factor in global climate change

47.8 Greenhouse Gases, Global Warming

Greenhouse effect• Radiant energy from the sun is absorbed by

Earth’s surface and radiated back as heat

• Gases in the upper atmosphere trap heat like a greenhouse, and radiate it back to Earth

• Greenhouse gases: carbon dioxide, water, nitrous oxide, methane, chlorofluorocarbons (CFCs)

Atmospheric Observations

Atmospheric CO2 fluctuates annually with patterns of photosynthesis

Average concentrations of CO2 and other greenhouse gases are increasing

Human activities – mainly burning of fossil fuels – increase greenhouse gases

Global Warming

Global warming• A long-term increase in temperature near Earth’s

surface, currently about 1.8°C (3.2°F) per century

Scientists expect far-reaching effects • Melting glaciers and rising sea levels

• Altered global precipitation patterns, droughts and flooding, more intense hurricanes

47.9 Nitrogen Cycle

Gaseous nitrogen (N2) makes up about 80 percent of the lower atmosphere• Most organisms can’t use gaseous nitrogen

The nitrogen cycle starts with nitrogen fixation• Nitrogen-fixing bacteria convert N2 in the air to

ammonia (NH3), then to ammonium (NH4+) and

nitrate (NO3-), which plants easily take up

Other Nitrogen Inputs Into Ecosystems

Ammonification• Bacteria and fungi make additional ammonium

available to plants when they break down nitrogen-rich wastes and remains

Nitrification• Bacteria convert ammonium to nitrite (NO2

-), and then to nitrate, which plants easily take up

Losing Nitrogen from Ecosystems

Denitrification• Denitrifying bacteria convert nitrate or nitrite to

gaseous nitrogen (N2) or nitrogen oxide (NO2)

Ammonium, nitrite, and nitrate are also lost from land ecosystems in runoff and by leaching

Disruptions by Human Activities

Deforestation and conversion of grassland to farmland causes nitrogen loss• Plant removal increases erosion and leaching

Synthetic ammonium fertilizers increase soil acidity and encourage ion exchange• Calcium and magnesium ions are washed away

Burning fossil fuels releases nitrogen oxides• Contribute to global warming and acid rain

47.10 The Phosphorus Cycle

Phosphorus cycle• A sedimentary cycle that moves phosphorus from

its main reservoir (Earth’s crust) through soils and sediments, aquatic habitats, and bodies of living organisms

Phosphate and the Phosphorus Cycle

Phosphorus in rocks is mainly phosphate (PO43-)

• Water moves phosphate through ecosystems

Phosphorus is a limiting factor on plant growth• Taken up by plants only in ionized form

• Required for ATP, phospholipids, nucleic acids

• Depleted when forest is converted to farmland

Eutrophication: Too Many Nutrients

Eutrophication• Nutrient enrichment of any ecosystem that is

otherwise low in nutrients; often a form of nutrient pollution from agricultural runoff or sewage

Eutrophication of a lake can cause excessive algal growth, oxygen depletion, and fish kills

Eutrophication

Phosphorus is often the limiting factor in aquatic ecosystems

47.5-47.10 Key Concepts Cycling of Water and Nutrients

The availability of water, carbon, nitrogen, phosphorus, and other substances influences primary productivity

These substances move slowly in global cycles, from environmental reservoirs, into food webs, then back to reservoirs