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CH 37: COMMUNITY STRUCTURE AND
DYNAMICS
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37.1 Community Ecology
• The next step up is a
biological community,
an assemblage of all
the populations of
organisms living close
enough together for
potential interaction.
• A community can be
described by its
species composition.
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37.1 Community Ecology
• 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.
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37.2 Interspecific Interactions
• Interspecific interactions
• are relationships with individuals of other species in
the community
• greatly affect population structure and dynamics.
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Competition: populations of
two different species compete
for the same limited resource.
Mutualism: both
populations benefit.
Predation: one species
(the predator) kills and eats
another (the prey).
Herbivory: an animal
consumes plant parts or
algae.
Parasitism: the host plants
or animals are victimized
by parasites or pathogens.
37.3 Competition
• An ecological niche is the sum of an organism’s
use of the biotic and abiotic resources in its
environment.
• Interspecific competition occurs when
• the niches of two populations overlap and
• both populations need a resource that is in short
supply.
• In general, competition lowers the carrying capacity
of competing populations
• the resources used by one population are not
available to the other population.
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Mutualism Video: Clownfish and Anemone
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37.5 EVOLUTION CONNECTION: Predation
• Predation benefits the predator but kills the prey.
• Prey adapt using protective strategies that include
• camouflage,
• mechanical defenses, and
• chemical defenses.
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37.6 EVOLUTION CONNECTION: Herbivory
• A plant whose body parts have
been eaten by an animal must
expend energy to replace the
loss. This has lead to the
evolution of many defenses.
• Plant defenses include:
• spines and thorns and
• chemical toxins
• Toxins have bad tastes or can
cause abnormal growth in the
organisms that eat them.
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37.7 Parasites and pathogens
• A parasite lives on or in a host
from which it obtains nourishment.
• Internal parasites include
nematodes and tapeworms.
• External parasites include
mosquitoes, ticks, and aphids.
• Pathogens are disease-causing
microscopic parasites that include
bacteria, viruses, fungi, or protists.
Parasitic aphids
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37.8 Trophic structure
• Every community has a trophic structure, a
pattern of feeding relationships consisting of
several different levels.
• The sequence of food transfer up the trophic levels
is known as a food chain.
• This transfer of food moves chemical nutrients
(matter) and energy from producers up through the
trophic levels in a community.
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37.8 Trophic structure
• Producers are autotrophs
that support all other trophic
levels.
• Consumers are heterotrophs.
• Herbivores are primary
consumers.
• Secondary consumers
• on land typically eat herbivores
and
• in aquatic ecosystems typically
eat zooplankton.
• Tertiary consumers typically
eat secondary consumers.
• Quaternary consumers
typically eat tertiary consumers.
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37.8 Trophic structure
• Detritivores derive their energy from detritus, the
dead material produced at all the trophic levels.
• Decomposers
• are mainly prokaryotes (bacteria) and fungi and
• secrete enzymes that digest molecules in organic
materials and convert them into inorganic forms in
the process called decomposition.
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37.9 Food chains interconnect, forming food
webs
• A more realistic view of the
trophic structure of a
community is a food web, a
network of interconnecting
food chains.
• A consumer may eat more than
one type of producer, and
several species of primary
consumers may feed on the
same species of producer.
• Some animals weave into the
food web at more than one
trophic level.
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37.10 Species diversity includes relative
abundance and species richness
• Species diversity is defined by two components:
1. species richness, the number of species in a
community, and
2. relative abundance, the proportional
representation of a species in a community.
A
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B
37.10 Species diversity includes relative
abundance and species richness
• Plant species diversity in a community often has
consequences for the species diversity of animals.
• Species diversity also impacts pathogens.
• Diverse communities will be less impacted by hostspecific pathogens.
• Low species diversity is characteristic of most
modern agricultural ecosystems.
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37.11 SCIENTIFIC THINKING: Some species
have a disproportionate impact on diversity
• Keystone species, defined as a
species
• whose impact on its community is
larger than its biomass or
abundance indicates and
• that occupies a niche that holds
the rest of its community in place.
• For example, in the tropics, figs bear fruit year around. In the dry
season, this is the only food available for many species. If figs
were removed from the forest, many fruit-eating animals would
disappear; and this in turn would affect many other plants that
depend upon these frugivores for pollination. So, the fig is key
to the survival of the community.
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37.12 Disturbance
• Disturbances
• are events that damage biological communities and
• include storms, fires, floods, drought, and human
activity.
The disturbed area may be
colonized by a variety of
species, which gradually
replace other species over
time, in a process called
ecological succession.
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37.12 Disturbance
• Primary succession begins in a virtually lifeless
area with no soil.
• Examples: rubble left by a retreating glacier or
fresh volcanic lava flows.
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37.12 Disturbance
• Secondary succession occurs when a
disturbance destroys an existing community but
leaves the soil intact.
• Examples:
• Wildfires
• Whenever human intervention (farming, logging, etc.) stops,
secondary succession begins.
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37.13 CONNECTION: Invasive species
• Invasive species
• are organisms that have been introduced into nonnative habitats by human actions and
• have established themselves at the expense of
native communities.
• The absence of natural enemies often allows rapid
population growth of invasive species.
• http://www.cbsnews.com/8301-3445_16220127579/the-threat-of-invasivespecies/?tag=contentBody;cbsCarousel
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37.13 CONNECTION: Invasive species
• Examples of invasive species include the
deliberate introduction of
• rabbits into Australia and
• cane toads into Australia.
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