Chapter 54 Ecosystems
Overview: Ecosystems, Energy, and Matter
• An ecosystem consists of all the organisms living in a community as
well as all the abiotic factors with which they interact.
• The dynamics of an ecosystem involve two processes that cannot be
fully described by population or community processes and
phenomena: energy flow and chemical cycling.
• Energy enters most ecosystems in the form of sunlight.
• It is converted to chemical energy by autotrophs, passed to
heterotrophs in the organic compounds of food, and dissipated
• Chemical elements are cycled among abiotic and biotic components
of the ecosystem.
• Energy, unlike matter, cannot be recycled.
• An ecosystem must be powered by a continuous influx of
energy from an external source, usually the sun.
• Energy flows through ecosystems, while matter cycles within them.
Concept 54.1 Ecosystem ecology emphasizes energy flow and
• Ecosystem ecologists view ecosystems as transformers of energy
and processors of matter.
• We can follow the transformation of energy by grouping the species
in a community into trophic levels of feeding relationships.
Ecosystems obey physical laws.
• The law of conservation of energy states that energy cannot be
created or destroyed but only transformed.
• Plants and other photosynthetic organisms convert solar energy
to chemical energy, but the total amount of energy does not
change. • The total amount of energy stored in organic molecules plus the
amounts reflected and dissipated as heat must equal the total
solar energy intercepted by the plant.
• The second law of thermodynamics states that some energy is lost as
heat in any conversion process.
• We can measure the efficiency of ecological energy
• Chemical elements are continually recycled.
• A carbon or nitrogen atom moves from one trophic level to
another and eventually to the decomposers and back again.
Trophic relationships determine the routes of energy flow and
chemical cycling in ecosystems.
• Autotrophs, the primary producers of the ecosystem, ultimately
support all other organisms.
• Most autotrophs are photosynthetic plants, algae or bacteria
that use light energy to synthesize sugars and other organic
• Chemosynthetic prokaryotes are the primary producers in
deep-sea hydrothermal vents.
• Heterotrophs are at trophic levels above the primary producers and
depend on their photosynthetic output.
• Herbivores that eat primary producers are called primary
• Carnivores that eat herbivores are called secondary
• Carnivores that eat secondary producers are called tertiary
• Another important group of heterotrophs is the detritivores, or
decomposers. • They get energy from detritus, nonliving organic material such
as the remains of dead organisms, feces, fallen leaves, and
• Detritivores play an important role in material cycling.
Decomposition connects all trophic levels.
• The organisms that feed as detritivores form a major link between the
primary producers and the consumers in an ecosystem.
• Detritivores play an important role in making chemical elements
available to producers.
• Detritivores decompose organic material and transfer chemical
elements in inorganic forms to abiotic reservoirs such as soil,
water, and air.
• Producers then recycle these elements into organic compounds.
• An ecosystem’s main decomposers are fungi and prokaryotes.
Concept 54.2 Physical and chemical factors limit primary production
• The amount of light energy converted to chemical energy by an
ecosystem’s autotrophs in a given time period is an ecosystem’s
An ecosystem’s energy budget depends on primary production.
• Most primary producers use light energy to synthesize organic
molecules, which can be broken down to produce ATP.
• The amount of photosynthetic production sets the spending limit of
the entire ecosystem.
• A global energy budget can be analyzed.
• Every day, Earth is bombarded by approximately 1023 joules of
• The intensity of solar energy striking Earth varies with
latitude, with the tropics receiving the greatest input. • Most of this radiation is scattered, absorbed, or reflected
by the atmosphere.
• Much of the solar radiation that reaches Earth’s surface
lands on bare ground or bodies of water that either
absorb or reflect the energy.
• Only a small fraction actually strikes algae, photosynthetic
prokaryotes, or plants, and only some of this is of
wavelengths suitable for photosynthesis.
• Of the visible light that reaches photosynthetic organisms,
only about 1% is converted to chemical energy.
• Although this is a small amount, primary producers produce
about 170 billion tons of organic material per year.
• Total primary production in an ecosystem is known as gross primary
• This is the amount of light energy that is converted into
chemical energy per unit time.
• Plants use some of these molecules as fuel in their own cellular
• Net primary production (NPP) is equal to gross primary production
minus the energy used by the primary producers for respiration (R):
NPP = GPP - R
• To ecologists, net primary production is the key measurement,
because it represents the storage of chemical energy that is available
to consumers in the ecosystem.
• Primary production can be expressed as energy per unit area per unit
time, or as biomass of vegetation added to the ecosystem per unit
area per unit time.
• This should not be confused with the total biomass of
photosynthetic autotrophs present in a given time, which is
called the standing crop. • Primary production is the amount of new biomass added in a
given period of time.
• Although a forest has a large standing cross biomass, its
primary production may actually be less than that of some
grasslands, which do not accumulate vegetation because
animals consume the plants rapidly.
• Different ecosystems differ greatly in their production as well as in
their contribution to the total production of the Earth.
• Tropical rain forests are among the most productive terrestrial
• Estuaries and coral reefs also are very productive, but they
cover only a small area compared to that covered by tropical
• The open ocean has a relatively low production per unit area
but contributes more net primary production than any other
single ecosystem because of its very large size.
• Overall, terrestrial ecosystems contribute two-thirds of global net
primary production, and marine ecosystems contribute approximately
In aquatic ecosystems, light and nutrients limit primary production.
• Light is a key variable controlling primary production in oceans, since
solar radiation can only penetrate to a certain depth known as the
• The first meter of water absorbs more than half of the solar
• If light were the main variable limiting primary production in the
ocean, we would expect production to increase along a gradient from
the poles toward the equator, which receives the greatest intensity of
• There is no such gradient. • There are parts of the ocean in the tropics and subtropics that
exhibit low primary production, while some high-latitude ocean
regions are relatively productive.
• More than light, nutrients limit primary production in aquatic
• A limiting nutrient is an element that must be added for production to
increase in a particular area.
• The nutrient most often limiting marine production is either nitrogen or
• In the open ocean, nitrogen and phosphorous levels are very
low in the photic zone but are higher in deeper water where
light does not penetrate.
• Nitrogen is the nutrient that limits phytoplankton growth in many parts
of the ocean.
• This knowledge can be used to prevent algal blooms by limiting
pollution that fertilizes phytoplankton.
• Some areas of the ocean have low phytoplankton density despite
their relatively high nitrogen concentrations.
• For example, the Sargasso Sea has a very low density of
• Nutrient-enrichment experiments showed that iron availability
limits primary production in this area.
• Marine ecologists carried out large-scale field experiments in the
Pacific Ocean, spreading low concentrations of dissolved iron over 72
km2 of ocean.
• A massive phytoplankton bloom occurred, with a 27-fold
increase in chlorophyll concentration in water samples from test
• Why are iron concentrations naturally low in certain oceanic areas? • Windblown dust from the land delivers iron to the ocean, and
relatively little dust reaches the central Pacific and Atlantic
• The iron factor in marine ecosystems is related to the nitrogen factor.
• When iron is limiting, adding iron stimulates the growth of
cyanobacteria that fix nitrogen.
• Phytoplankton proliferate, once released from nitrogen limitation.
• Iron --> cyanobacteria --> nitrogen fixation--> phytoplankton
• In areas of upwelling, nutrient-rich deep waters circulate to the ocean
• These areas have exceptionally high primary production,
supporting the hypothesis that nutrient availability determines
marine primary production.
• Areas of upwelling are prime fishing locations.
• Nutrient limitation is also common in freshwater lakes.
• Sewage and fertilizer pollution can add nutrients to lakes.
• Additional nutrients shifted many lakes from phytoplankton
communities dominated by diatoms and green algae to communities
dominated by cyanobacteria.
• This process is called eutrophication and has a wide range of
ecological impacts, including the loss of most fish species.
• David Schindler of the University of Alberta conducted a series of
whole lake experiments that identified phosphorus as the nutrient that
limited cyanobacteria growth.
• His research led to the use of phosphate-free detergents and
other water quality reforms.
In terrestrial ecosystems, temperature and moisture are the key
factors limiting primary production. • Tropical rain forests, with their warm, wet conditions, are the most
productive of all terrestrial ecosystems.
• By contrast, low-productivity ecosystems are generally dry (deserts)
or dry and cold (arctic tundra).
• Between these extremes lie temperate forest and grassland
ecosystems with moderate climates and intermediate productivity.
• These contrasts in climate can be represented by a measure called
actual evapotranspiration, which is the amount of water annually
transpired by plants and evaporated from a landscape.
• Actual evapotranspiration increases with precipitation and with
the amount of solar energy available to drive evaporation and
• On a more local scale, mineral nutrients in the soil can play a key role
in limiting primary production in terrestrial ecosystems.
• Primary production removes soil nutrients.
• A single nutrient deficiency may cause plant growth to slow and
• Nitrogen and phosphorus are the soil nutrients that most commonly
limit terrestrial production.
• Scientific studies relating nutrients to terrestrial primary production
have practical applications in agriculture.
• Farmers can maximize crop yields with the right balance of
nutrients for the local soil and type of crop.
Concept 54.3 Energy transfer between trophic levels is usually less
than 20% efficient
• The amount of chemical energy in consumers’ food that is converted
to their own new biomass during a given time period is called the
secondary production of an ecosystem.
• We can measure the efficiency of animals as energy transformers
using the following equation: • production efficiency = net secondary production / assimilation
of primary production
• Net secondary production is the energy stored in biomass
represented by growth and reproduction.
• Assimilation consists of the total energy taken in and used for growth,
reproduction, and respiration.
• Production efficiency is thus the fraction of food energy that is not
used for respiration.
• This differs among organisms.
• Birds and mammals generally have low production
efficiencies of between 1% and 3% because they use so
much energy to maintain a constant body temperature.
• Fishes have production efficiencies of around 10%.
• Insects are even more efficient, with production
efficiencies averaging 40%.
• Trophic efficiency is the percentage of production transferred from
one trophic level to the next.
• Trophic efficiencies must always be less than production
efficiencies because they take into account not only the energy
lost through respiration and contained in feces, but also the
energy in organic material at lower trophic levels that is not
• Trophic efficiencies usually range from 5% to 20%.
• In other words, 80–95% of the energy available at one trophic
level is not transferred to the next.
• This loss is multiplied over the length of a food chain.
• If 10% of energy is transferred from primary producers to
primary consumers, and 10% of that energy is transferred to
secondary consumers, then only 1% of net primary production
is available to secondary consumers. • Pyramids of net production represent the multiplicative loss of energy
in a food chain.
• The size of each block in the pyramid is proportional to the new
production of each trophic level, expressed in energy units.
• Biomass pyramids represent the ecological consequences of low
• Most biomass pyramids narrow sharply from primary producers
to top-level carnivores because energy transfers are so
• In some aquatic ecosystems, the pyramid is inverted and
primary consumers outweigh producers.
• Such inverted biomass pyramids occur because the
producers—phytoplankton—grow, reproduce, and are
consumed by zooplankton so rapidly that they never develop a
large standing crop.
• They have a short turnover time, which means they have a
small standing crop biomass compared to their production.
• turnover time = standing crop biomass (mg/m2) /
• Because the phytoplankton replace their biomass at such a
rapid rate, they can support a biomass of zooplankton much
greater than their own biomass.
• Because of the progressive loss of energy along a food chain, any
ecosystem cannot support a large biomass of top-level carnivores.
• With some exceptions, predators are usually larger than the
prey they eat.
• Top-level predators tend to be fairly large animals.
• As a result, the limited biomass at the top of an ecological
pyramid is concentrated in a small number of large individuals.
• In a pyramid of numbers, the size of each block is proportional to the
number of individuals present in each trophic level. • The dynamics of energy through ecosystems have important
implications for the human population.
• Eating meat is an inefficient way of tapping photosynthetic
• Worldwide agriculture could feed many more people if humans
all fed as primary consumers, eating only plant material.
Herbivores consume a small percentage of vegetation: the green
• According to the green world hypothesis, herbivores consume
relatively little plant biomass because they are held in check by a
variety of factors, including predators, parasites, and disease.
• How green is our world?
• 83 × 1010 metric tons of carbon are stored in the plant biomass
of terrestrial ecosystems.
• Herbivores annually consume less than 17% of the total net
• The green world hypothesis proposes several factors that keep
herbivores in check:
• Plants have defenses against herbivores.
• Nutrients, not energy supply, usually limit herbivores.
• Animals need certain nutrients that plants tend to supply
in relatively small amounts.
• The growth and reproduction of many herbivores are
limited by availability of essential nutrients.
• Abiotic factors limit herbivores.
• Temperature and moisture may restrict carrying
capacities for herbivores below the level that would strip
• Intraspecific competition can limit herbivore numbers. • Territorial behavior and competitive behaviors may
reduce herbivore population density.
• Interspecific interactions check herbivore densities.
• Parasites, predators, and disease limit the growth of
• This applies the top-down model of community structure.
Concept 54.4 Biological and geochemical processes move nutrients
between organic and inorganic parts of the ecosystem
• Chemical elements are available to ecosystems only in limited
• Life on Earth depends on the recycling of essential chemical
• Nutrient circuits involve both biotic and abiotic components of
ecosystems and are called biogeochemical cycles.
• There are two general categories of biogeochemical cycles: global
• Gaseous forms of carbon, oxygen, sulfur, and nitrogen occur in
the atmosphere, and cycles of these elements are global.
• Elements that are less mobile in the environment, such as
phosphorus, potassium, calcium, and trace elements generally
cycle on a more localized scale in the short term.
• Soil i