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Community Structure and Dynamics
37.1 A community includes all the organisms inhabiting a particular area
In the hierarchy of life, a population is a group of interacting individuals of a particular species.
o The next step up is a biological community, an assemblage of all the populations of organisms
living close enough together for potential interaction.
o Ecologists define the boundaries of the community according to the research questions they
want to investigate.
37.1 A community includes all the organisms inhabiting a particular area
o A community can be described by its species composition.
o Community ecologists
 seek to understand how abiotic factors and interactions between populations affect the
composition and distribution of communities and
 investigate community dynamics, the variability or stability in the species composition of
a community caused by biotic and abiotic factors.
37.1 A community includes all the organisms inhabiting a particular area
o Community ecology
 is necessary for the conservation of endangered species and the management of
wildlife, game, and fisheries,
 is vital for controlling diseases, such as malaria, bird flu, and Lyme disease, that are
carried by animals, and
 has applications in agriculture, where people attempt to control the species
composition of communities they have established.
37.2 Interspecific interactions are fundamental to community structure
o Interspecific interactions
o are relationships with individuals of other species in the community and
o greatly affect population structure and dynamics.
o In Table 37.2, interspecific interactions are classified according to the effect on the populations
concerned.
Table 37.2
37.2 Interspecific interactions are fundamental to community structure
o Interspecific competition occurs when populations of two different species compete for the
same limited resource.
o In mutualism, both populations benefit.
o In predation, one species (the predator) kills and eats another (the prey).
o In herbivory, an animal consumes plant parts or algae.
o In parasitism, the host plants or animals are victimized by parasites or pathogens.
37.3 Competition may occur when a shared resource is limited
o An ecological niche is the sum of an organism’s use of the biotic and abiotic resources in its
environment.
o Interspecific competition occurs when
o the niches of two populations overlap and
o both populations need a resource that is in short supply.
o In general, competition lowers the carrying capacity of competing populations because the
resources used by one population are not available to the other population.
37.5 EVOLUTION CONNECTION: Predation leads to diverse adaptations in prey species
o Predation benefits the predator but kills the prey.
o Prey adapt using protective strategies that include
o camouflage,
o mechanical defenses, and
chemical defenses.
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Figure 37.5a
37.6 EVOLUTION CONNECTION: Herbivory leads to diverse adaptations in plants
o A plant whose body parts have been eaten by an animal must expend energy to replace the loss.
o Thus, numerous defenses against herbivores have evolved in plants.
o Plant defenses against herbivores include
o spines and thorns and
o chemical toxins, often the substances that we use medicinally or for other purposes.
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37.6 EVOLUTION CONNECTION: Herbivory leads to diverse adaptations in plants
o Like the chemical defenses of animals, toxins in plants tend to be distasteful, and herbivores
learn to avoid them.
o Some plants even produce chemicals that cause abnormal development in insects that eat them.
37.7 Parasites and pathogens can affect community composition
o A parasite lives on or in a host from which it obtains nourishment.
o Internal parasites include nematodes and tapeworms.
o External parasites include mosquitoes, ticks, and aphids.
o Pathogens are disease-causing microscopic parasites that include bacteria, viruses, fungi, or
protists.
o Plants are also attacked by parasites, including nematodes and aphids, that tap into the phloem
and suck plant sap.
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37.8 Trophic structure is a key factor in community dynamics
o Producers are autotrophs that support all other trophic levels.
o Consumers are heterotrophs.
o Herbivores are primary consumers.
o Secondary consumers
o on land typically eat herbivores and
o in aquatic ecosystems typically eat zooplankton.
o Tertiary consumers typically eat secondary consumers.
o Quaternary consumers typically eat tertiary consumers.
Figure 37.8
37.8 Trophic structure is a key factor in community dynamics
o Detritivores derive their energy from detritus, the dead material produced at all the trophic
levels.
o Decomposers
o are mainly prokaryotes and fungi and
o secrete enzymes that digest molecules in organic materials and convert them into inorganic
forms in the process called decomposition.
37.9 VISUALIZING THE CONCEPT: Food chains interconnect, forming food webs
o A more realistic view of the trophic structure of a community is a food web, a network of
interconnecting food chains.
o A consumer may eat more than one type of producer, and several species of primary consumers
may feed on the same species of producer.
o Some animals weave into the food web at more than one trophic level.
Figure 37.9-1
Figure 37.9-2
Figure 37.9-3
Figure 37.9-4
37.10 Species diversity includes relative abundance and species richness
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Plant species diversity in a community often has consequences for the species diversity of
animals in the community.
o Species diversity also impacts pathogens.
o Most pathogens infect a limited range of host species or may even be restricted to a single host
species.
o When many potential hosts are living close together, it is easy for a pathogen to spread from one
to another.
37.10 Species diversity includes relative abundance and species richness
o Low species diversity is characteristic of most modern agricultural ecosystems.
o To combat potential losses, many farmers and forest managers rely heavily on chemical methods
of controlling pests.
o Modern crop scientists have bred varieties of plants that are genetically resistant to common
pathogens, but these varieties can suddenly become vulnerable, too.
37.11 SCIENTIFIC THINKING: Some species have a disproportionate impact on diversity
o The word “keystone” comes from the wedge-shaped stone at the top of an arch that locks the
other pieces in place.
o If the keystone is removed, the arch collapses.
o A keystone species occupies a niche that holds the rest of its community in place.
37.12 Disturbance is a prominent feature of most communities
o Disturbances
o are events that damage biological communities and
o include storms, fires, floods, drought, and human activity.
o The types of disturbances and their frequency and severity vary from community to community.
37.12 Disturbance is a prominent feature of most communities
o Small-scale disturbances often have positive effects.
o However, communities change drastically following a severe disturbance that strips away
vegetation and removes significant amounts of soil.
37.12 Disturbance is a prominent feature of most communities
o The disturbed area may be colonized by a variety of species, which are gradually replaced by a
succession of other species, in a process called ecological succession.
37.12 Disturbance is a prominent feature of most communities
o Primary succession begins in a virtually lifeless area with no soil.
o Examples of such areas are the rubble left by a retreating glacier or fresh volcanic lava flows.
Figure 37.12a
37.12 Disturbance is a prominent feature of most communities
o Secondary succession occurs when a disturbance destroys an existing community but leaves the
soil intact.
o Whenever human intervention stops, secondary succession begins.
o Numerous studies have documented the stages by which an abandoned farm field returns to
forest.
Figure 37.12b
37.13 CONNECTION: Invasive species can devastate communities
o Invasive species
o are organisms that have been introduced into non-native habitats by human actions and
o have established themselves at the expense of native communities.
o The absence of natural enemies often allows rapid population growth of invasive species.
37.13 CONNECTION: Invasive species can devastate communities
o Examples of invasive species include the deliberate introduction of
o rabbits into Australia and
o cane toads into Australia.
Figure 37.13a
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Figure 37.13b
Ecosystem Structure and Dynamics
37.14 Ecosystem ecology emphasizes energy flow and chemical cycling
o An ecosystem consists of
o all the organisms in a community and
o the abiotic environment with which the organisms interact.
o In an ecosystem,
o energy flow moves through the components of an ecosystem and
o chemical cycling is the transfer of materials within the ecosystem.
37.16 Energy supply limits the length of food chains
o A caterpillar represents a primary consumer.
o Of the organic compounds a caterpillar ingests, about
o 50% is eliminated in feces,
o 35% is used in cellular respiration, and
o 15% is converted to caterpillar biomass.
Figure 37.16a
37.16 Energy supply limits the length of food chains
o A pyramid of production illustrates the cumulative loss of energy with each transfer in a food
chain.
o Only about 10% of the energy stored at each trophic level is available to the next level. The
efficiencies of energy transfer usually range from 5 to 20%.
Figure 37.16b
37.16 Energy supply limits the length of food chains
o An important implication of this stepwise decline of energy in a trophic structure is that the
amount of energy available to top-level consumers is small compared with that available to
lower-level consumers.
o Only a tiny fraction of the energy stored by photosynthesis flows through a food chain all the way
to a tertiary consumer.
o This explains why top-level consumers such as lions and hawks require so much geographic
territory.
37.17 CONNECTION: A pyramid of production explains the ecological cost of meat
o As omnivores, people eat plant material and meat.
o When humans eat
o grain or fruit, we are primary consumers,
o beef or other meat from herbivores, we are secondary consumers, and
o fish like trout or salmon, we are tertiary or quaternary consumers.
37.17 CONNECTION: A pyramid of production explains the ecological cost of meat
o Only about 10% of the chemical energy available in a trophic level is passed to the next higher
trophic level.
o Therefore, the human population has about 10 times more energy available to it when people
eat plants than when they eat the meat of herbivores.
o Eating meat of any kind is expensive
o economically and
o environmentally.
Figure 37.17
37.18 Chemicals are cycled between organic matter and abiotic reservoirs
o Ecosystems are supplied with a continual influx of energy from the
o sun and
o Earth’s interior.
o Except for meteorites, there are no extraterrestrial sources of chemical elements.
o Thus, life also depends on the recycling of chemicals.
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37.18 Chemicals are cycled between organic matter and abiotic reservoirs
o Biogeochemical cycles include
o biotic components,
o abiotic components, and
o abiotic reservoirs, where chemicals accumulate or are stockpiled outside of living organisms.
o Biogeochemical cycles can be
o local or
o global.
Figure 37.18
37.19 The carbon cycle depends on photosynthesis and respiration
o Carbon is
o the major ingredient of all organic molecules and
o found
o in the atmosphere,
o in fossil fuels, and
o dissolved in carbon compounds in the ocean.
o The reciprocal metabolic processes of photosynthesis and cellular respiration are mainly
responsible for the cycling of carbon between the biotic and abiotic worlds.
Figure 37.19
37.19 The carbon cycle depends on photosynthesis and respiration
o On a global scale, the return of CO2 to the atmosphere by cellular respiration closely balances its
removal by photosynthesis, but the increased burning of wood and fossil fuels (coal and
petroleum) is raising the level of CO2 in the atmosphere.
37.20 The phosphorus cycle depends on the weathering of rock
3−
o Organisms require phosphorus, usually in the form of the phosphate ion (PO4 ), for nucleic acids,
phospholipids, ATP and as a mineral component of bones and teeth.
37.20 The phosphorus cycle depends on the weathering of rock
o In contrast to the carbon cycle and the other major biogeochemical cycles, the phosphorus cycle
does not have an atmospheric component.
o Rocks are the only source of phosphorus for terrestrial ecosystems.
o Plants absorb phosphate ions in the soil and build them into organic compounds.
o Phosphates are returned to the soil by decomposers.
o Phosphate levels in aquatic ecosystems are typically low enough to be a limiting factor.
Figure 37.20
37.20 The phosphorus cycle depends on the weathering of rock
o Because phosphates are transferred from terrestrial to aquatic ecosystems much more rapidly
than they are replaced, the amount in terrestrial ecosystems gradually diminishes over time.
o Soil erosion from land cleared for agriculture or development accelerates the loss of phosphates.
o Farmers and gardeners often use crushed phosphate rock, bone meal (finely ground bones from
slaughtered livestock), or guano, the droppings of seabirds and bats, to add phosphorus to the
soil.
37.21 The nitrogen cycle depends on bacteria
o Nitrogen is
o an ingredient of proteins and nucleic acids,
o essential to the structure and functioning of all organisms, and
o a crucial and often limiting plant nutrient.
o Nitrogen has two abiotic reservoirs:
o the atmosphere, of which about 80% is nitrogen gas, and
o soil.
37.21 The nitrogen cycle depends on bacteria
Nitrogen fixation
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converts N2 to compounds of nitrogen that can be used by plants and
is carried out by some bacteria.
Figure 37.21 illustrates the actions of two types of nitrogen-fixing bacteria.
Figure 37.21
37.21 The nitrogen cycle depends on bacteria
Human activities are disrupting the nitrogen cycle by adding more nitrogen to the biosphere each year
than natural processes.
Combustion of fossil fuels in motor vehicles and
coal-fired power plants produces nitrogen oxides
(NO and NO2).
Modern agricultural is another major source of nitrogen.
Some nitrogen escapes to the atmosphere, where
it forms NO2 or nitrous oxide (N2O), an inert gas that lingers in the atmosphere and contributes to global
warming.
37.22 CONNECTION: A rapid inflow of nutrients degrades aquatic ecosystems
In aquatic ecosystems, primary production is limited by low nutrient levels of
phosphorus and
nitrogen.
Over time, standing water ecosystems gradually accumulate nutrients from the decomposition of organic
matter and fresh influx from the land, and primary production increases in a process known as
eutrophication.
37.22 CONNECTION: A rapid inflow of nutrients degrades aquatic ecosystems
Eutrophication of lakes, rivers, and coastal waters
depletes oxygen levels and
decreases species diversity.
In many areas, phosphate pollution leading to eutrophication comes from
agricultural fertilizers,
pesticides,
sewage treatment facilities, and
runoff of animal waste from feedlots.
Figure 37.22a
37.23 CONNECTION: Ecosystem services are essential to human well-being
Ecosystems that we create are also essential to our well-being. Agricultural ecosystems
supply most of our food and fibers,
make use of ecosystem services, such as control of agricultural pests by natural predators and pollination
of crops, and
rely upon nutrient cycling to maintain soil fertility, the foundation for crop growth.
37.23 CONNECTION: Ecosystem services are essential to human well-being
But agricultural methods introduced over the past several decades have pushed croplands beyond their
natural capacity to produce food.
Nutrient runoff affects freshwater and marine ecosystems as fertilizer use increases.
Pesticides may kill beneficial organisms as well as pests.
Clearing and cultivation expose land to wind and water that erode the rich topsoil. In China, for example,
overgrazing and other poor agricultural practices are turning 900 square miles of land—an area the size of
Rhode Island—into desert each year.
37.23 CONNECTION: Ecosystem services are essential to human well-being
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Human activities also threaten many forest ecosystems and the services they provide.
Forests are also cut down to provide timber and fuel wood.
Many people in nonindustrialized countries use wood for heating and cooking.
37.23 CONNECTION: Ecosystem services are essential to human well-being
The growing demand of the human population for food, fibers, and water has largely been satisfied at the
expense of other ecosystem services, but these practices cannot continue indefinitely.
Sustainability is the goal of developing, managing, and conserving Earth’s resources in ways that meet the
needs of people today without compromising the ability of future generations to meet theirs.
You should now be able to
Define a biological community. Explain why the study of community ecology is important.
Define interspecific competition, mutualism, predation, herbivory, and parasitism and provide examples
of each.
Define an ecological niche. Explain how interspecific competition can occur when the niches of two
populations overlap.
Describe the mutualistic relationship between corals and dinoflagellates.
Define predation. Describe the protective strategies potential prey employ to avoid predators.
You should now be able to
Explain why many plants have chemical toxins, spines, or thorns. Define coevolution and describe an
example.
Explain how parasites and pathogens can affect community composition.
Identify and compare the trophic levels of terrestrial and aquatic food chains.
Explain how food chains interconnect to form food webs.
Describe two components of species diversity.
Define a keystone species.
You should now be able to
Explain how disturbances can benefit communities. Distinguish between primary and secondary
succession.
Explain how invasive species can affect communities.
Compare the movement of energy and chemicals within and through ecosystems.
Compare the primary production of tropical rain forests, coral reefs, and open ocean. Explain why the
differences between them exist.
Describe the movement of energy through a food chain.
You should now be able to
Explain how carbon, nitrogen, and phosphorus cycle within ecosystems.
Explain how rapid eutrophication of aquatic ecosystems affects species diversity and oxygen levels.
Explain how human activities are threatening natural ecosystems.