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Ecosystems
Chapter 47
Impacts, Issues
Bye-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
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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