Chapter 7
Community Ecology
AP Environmental Science
Edinburg North High School
Discussion Questions
• What determines the number of species in
a community?
• How can we classify species according to
their roles in a community?
• How do species interact with one another?
• How do communities respond to changes
in environmental conditions?
• Does high species biodiversity increase
the stability and sustainability of a
community?
Core Case Study: Why Should We
Care about the American Alligator?
• Plays a number of roles in the ecosystems
where they are found.
• Dig deep depressions (gator holes).
• Hold water during dry spells, serve as refuges for
aquatic life.
• Build nesting mounds.
• provide nesting and feeding
sites for birds.
• Keeps areas of open
water free of vegetation.
• Alligators are a keystone species:
• Help maintain the structure and function of the
communities where it is found.
• Survived for at least 200 m.y.
• By 1960s, was near extinction in Florida
and decreased significantly in other
southern states.
• In 1967, federally listed as endangered.
• In 1977, reclassified as threatened in FL,
LA, and TX; in 1987, was upgraded in
seven other states.
Community Structure And
Species Diversity
• Community Structure: Appearance Matters
• Physical appearance – relative sizes,
stratification, and distribution of populations in a
community.
• Terrestrial communities (Fig. 7-2)
• Aquatic communities
• Oceans, rocky shores, sandy beaches, lakes, rivers,
wetlands
• Structure is patchy because physical
conditions, resources, and species vary from
place to place.
Fig. 7-2. Generalized Terrestrial Communities
• Edges and ecotones
• Habitat fragmentation increases edge area.
• Makes species more vulnerable to physical a
biological stress.
• Creates barriers to gene flow and dispersal
• Species Diversity and Niche Structure:
Different Species Playing Different Roles
• Communities shaped by species, species
interactions, and species interactions with
physical environment.
• Species diversity: the number of different
species it contains (species richness) combined
with the abundance of individuals within each of
those species (species evenness).
• Niche structure – how many potential
ecological niches occur, how they resemble or
differ, and how the species occupying different
niches interact.
• Geographic location – species diversity is
highest in the tropics and declines as we move
from the equator toward the poles.
• Low latitudes = Constant climate and reliable food
supply  greater specialization w/ narrow niches
• Higher latitudes = More variable climate and food
availability  generalist species w/ wide niches
• Coral reefs, rainforest, deep seas, and tropical
lakes are high in species richness but have
fewer members of each species (low
evenness).
• Case Study: Species Diversity on Islands.
• Robert MacArthur and E.O. Wilson proposed
the species equilibrium model or theory of
island biogeography in the 1960’s.
• Model projects that at some point the rates of
immigration and extinction should reach an
equilibrium based on:
• Island size
• Distance to nearest mainland
Types of Species
• Types of Species in Communities
• Native, nonnative, indicator, keystone, and foundation
species play different ecological roles in communities.
• A given species may play more than one role in an
ecosystem.
• Native species - those that normally live and thrive in a
particular community.
• Non-native, or alien species - those that migrate into or
are deliberately or accidentally introduced into a
community.
• Invasive species
• Indicator Species: Biological Smoke Alarms
• Species that serve as early warning systems of
damage to a community or ecosystem are
called indicator species.
• The presence or absence of trout in a waters that
are within their thermal requirements (Fig. 3-11)
• Birds have been affected by habitat fragmentation
and pesticides.
• Butterflies because many have specific host plants
for their larvae.
• Old idea: the canary in a coal mine
• Case Study: Why Are Amphibians Vanishing?
• Frogs serve as indicator species because different
parts of their life cycles can be easily disturbed.
• Reasons for Amphibian Declines
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Habitat loss and fragmentation.
Prolonged drought.
Pollution.
Increases in ultraviolet radiation.
Parasites.
Viral and Fungal diseases.
• Chytrid fungus that attacks the skin of frogs
• In 2005, Correlation between climate change and harlequin
frogs in Central and South America.
• In 2008, some new evidence casts doubt on this
hypothesis.
• Overhunting.
• Natural immigration or deliberate introduction of
nonnative predators and competitors.
• Why care if we lose amphibians?
• First, declining trend suggests that the life support
system is deteriorating
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Habitat loss and degradation
Air an water pollution
Increased UV exposure
Climate change
• Second, they play important ecological roles in
biological communities.
• Eat insects (including mosquitoes)
• Other animals such as fish, reptiles, birds, and mammals
eat them
• Third, a genetic storehouse of pharmaceutical
products waiting to be discovered.
• Compounds isolated from amphibian skin have been used
as painkillers, antibiotics, and as treatment for burns and
heat disease.
• Keystone Species: Major
Players
• Keystone species – help
determine the types and
numbers of species in a
community.
• Pollination
• Insects, birds, and bats
• Top predators
• Wolf, bobcat, alligator, shark,
Piaster orchaceus
• Have you thanked a dung beetle?
• Loss of keystone species can
lead to population crashes and
extinctions of other species in a
community that depends on
them for certain services.
• Foundation Species: Other Major Players
• Foundation species – and expansion of the
keystone species concept; a species which
plays a major role in communities by creating
and enhancing habitats in a way that benefits
others species.
• Elephants push over, break, or uproot trees, creating
forest openings promoting grass growth for other
species to utilize.
• Seed dispersers – frugivores
• Beavers
• Mussels on the rocky shores of the Pacific
Northwest.
Species Interactions: Competition
and Predation
• How do species interact?
• When species in a community have common activities
or resources in common they interact.
• Five basic types of interactions:
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Interspecific competition
Predation
Parasitism
Mutualism
Commensalism
• Interactions between populations of different species
can influence abilities of individuals within those
populations to survive and reproduce; interacting
populations thus serve as agents of natural selection.
• The most common interaction is interspecific
competition.
• No two species can occupy the same niche
otherwise there will be intense competition.
• One of the competing species must then migrate, shift
resource requirements or behaviors by natural selection, or
suffer sharp decline or extinction.
• Humans have become serious competitors for
resources.
• As our ecological footprints grow, what can be
conclude about the species that share this planet
with us?
• Reducing or Avoiding Competition: Sharing
Resources
• Over a large timescale natural selection can act
to reduce competition
• Resource partitioning – when species
competing for similar resources evolve more
specialized traits that allow them to use shared
resources at different times, in different ways,
or in different places (Fig. 7-6).
• Leopards and lions
• Hawks and owls
• Insectivorous birds in the Northeast (Fig. 7-7)
Fig. 7-6. Resource partitioning and niche
specialization as a result of competition
between two species.
Fig. 7-7. Resource partitioning of Five Warblers
In Maine, USA.
• Predators and Prey: Eating and Being
Eaten
• Predation – when member of one species (the
predator) feed directly on all or part of a living
organism of another species (the prey).
• Predator-prey relationship
• Some may be surprising: grizzly-army cutworm moth
• Clearly individual prey lose, but at the
population level, predation plays a key role in
evolution by natural selection.
• Perception of predation by some people?
• Sensing the Environment to Find Food and
Mates
• EM radiation
• Birds and bees
• Pit-vipers
• Acute night vision
• Sound
• Pinnae
• Sonar
• Volatile chemicals
• Olfaction
• How do predators increase their chance fo
getting a meal?
• Herbivores
• Walk, swim, or fly up to plants to feed on them.
• Carnivores
• Pursuit
• Ambush
• Chemical Warfare
• Spiders
• Snakes
• How do prey defend against or avoid predators?
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Run, swim, or fly away
Highly developed senses of sight, hearing and/or smell
Protective shells, thick bark, spines, and thorns
Camouflage (Fig. 7-8a,b))
Chemical warfare (Fig. 7-8c-e)
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Poisons and venoms
Irritating
Foul smelling
Bad tasting
Warning coloration (Fig. 7-8d,e)
Mimicry (Fig. 7-8f)
Deceptive looks and behavior (Fig. 7-8g,h)
Gregarious behavior
Fig. 7-8. Some predator avoidance strategies.
Species Interactions: Parasitism,
Mutualism, and Commensalism
• Parasites: Sponging Off Others
• Parasitism – when one species (the parasite)
feeds on part of another organism (the host).
• One benefits, the other harmed
• Enodparasites, some pathogenic
• Ectoparasites
• Some parasite have little contact with there
hosts
• Parasites can promote biodiversity and control
populations by helping keep some species from
becoming so plentiful that they eliminate others
• Mutualism: Win-Win
Relationship
• Mutualism – when two
species or a network
of species interact in a
way that both benefit.
• Pollination and seed
dispersal
• Being supplied with
food
• Receiving protection
• Each species benefits
by exploiting the other.
• Commensalism:
Using without
Harming
• Commensalism –
interaction that
benefits one species
but has little, if any
effect on the other
species.
• Silverfish and army
ants
• Remoras and sharks
• Epiphytes
Ecological Succession:
Communities in Transition
• Ecological Succession: How Communities
Change Over Time
• Ecological succession – gradual change in
species structure and composition of a
community in response to changing
environmental conditions
• Primary succession
• Secondary succession
• Pioneer (colonizing or early succession
species) species arrive first.
• Primary Succession: Starting from Scratch
• Primary succession begins with an essentially
lifeless are where there is no soil in a terrestrial
ecosystem (Fig. 7-11) or no bottom sediment in
aquatic systems.
• Early successional, or pioneer species: lichens and
mosses
• Midsuccession species: herbs, grasses and low
shrubs. Trees that need lots of sunlight replace
these.
• Late successional species: mostly trees that can
tolerate shade.
• Newly created ponds also go through succession.
Fig. 7-11. Primary Ecological
Succession
• Secondary Succession: Starting Over with
Some Help
• Secondary succession begins in an area where
the natural community has been disturbed,
removed, or destroyed (Fig. 7-12).
• Numbers and types of animals and
decomposers also change.
• Intermediate disturbance hypothesis – fairly
frequent but moderate disturbances lead to the
greatest species diversity.
Fig. 7-12. Secondary Succession of a
Plant Communities on a Farm Field in
North Carolina.
• Can we predict the path of succession, and
is nature in balance?
• Traditional view: succession proceeds until a
climax community is established – one
dominated by a few long-lived plant species
and is in balance with the environment.
• Most ecologists now recognize that mature
late-successional communities are not in a
state of permanent equilibrium
• In a state of continual disturbance and change.
• Not preordained to progress to an ideal climax
community
• Succession reflects the ongoing struggle by different
species for enough light, nutrients, food, and space.
Ecological Stability and Sustainability
• Stability of Living Systems: Surviving by Changing
• Communities like other living systems constantly
change in response to changing environmental
conditions.
• Negative and positive feedback loops interact to provide some
degree of stability over each system’s expected life span.
• Three aspects of stability and sustainability:
• Inertia, or persistence – resists disturbance or change
• Constancy – keep within the limits imposed by environment
• Resilience – bounce back and repair damage that is not too
drastic
• Community Productivity and Sustainability
• Research suggests that communities with more
species tend to have a higher NPP and can be
more resilient than simpler ones.
• Perhaps because species diversity allow species to
exploit a different portion of available resources
• It is difficult to assess just how much diversity is
necessary to achieve greater stability or
sustainability.
• Why should we bother to protect natural systems
• Developers say that because biodiversity does not
necessarily lead to increased ecological stability,
• Cut down old-growth forests, replace with tree plantations
• Convert grasslands to croplands
• Drain and develop wetlands, dump toxic and radioactive
wastes into deep oceans
• No worries about premature extinctions
• Ecologists say that just because natural systems are
not in balance does not mean these systems are an
unimportant part of earth’s natural capital in
unimportant.
• Human activities are disrupting, destroying, degrading
and simplifying the world’s ecosystems which
threatened the ecosystem services that support and
sustain all life and all economies.