Community Ecology

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Community Ecology
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55 Community Ecology

• 55.1 What Are Ecological Communities?
• 55.2 What Processes Influence Community
  Structure?
• 55.3 How Do Species Interactions Cause Trophic
  Cascades?
• 55.4 How Do Disturbances Affect Ecological
  Communities?
• 55.5 What Determines Species Richness in
  Ecological Communities?

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55.1 What Are Ecological Communities?

• An ecological community consists of all the
  species that live and interact in a given area.
• In the early twentieth century, two plant
  ecologists debated the nature of communities.
• Henry Gleason argued that plant communities were
  loose associations of species each species was
  distributed based on its environmental
  requirements.

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55.1 What Are Ecological Communities?

• Frederick Clements argued that plant communities
  were tightly integrated superorganisms.
  Communities in similar areas would have the same
  species.
• Studies of plant species distributions showed
  that different combinations of plants occurred in
  different locations, supporting Gleasons view.

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Figure 55.1 Plant Distributions along an
Environmental Gradient
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55.1 What Are Ecological Communities?

• Thus, ecological communities are not assemblages
  that move together as a unit.
• Each species has unique interactions with its
  environment.
• Nevertheless, ecologists still wish to understand
  how these loose assemblages of species function.

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55.1 What Are Ecological Communities?

• Organisms in a community can be divided into
  trophic levels based on their source of energy.
• Photosynthesizers or primary producers are
  autotrophs that get their energy directly from
  sunlight.
• All heterotrophs consume, directly or indirectly,
  the energy-rich molecules made by the primary
  producers.

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55.1 What Are Ecological Communities?

• Herbivores eat plants, and constitute the primary
  consumer level.
• Organisms that eat herbivores are secondary
  consumers.
• Organisms that eat secondary consumers are
  tertiary consumers.
• Detritivores or decomposers eat dead bodies and
  waste products.

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Table 55.1 The Major Trophic Levels
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55.1 What Are Ecological Communities?

• Organisms that get their food from more than one
  trophic level are omnivores.
• Many species are omnivores, and trophic levels
  are not clearly distinct, but the concept is a
  useful way of thinking about energy flow in a
  community.

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55.1 What Are Ecological Communities?

• A sequence in which a plant is eaten by an
  herbivore, which is eaten by a secondary
  consumer, etc. can be diagrammed as a food chain.
• Food chains are interconnected to make food webs.

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Figure 55.2 Food Webs Show Trophic Interactions
in a Community
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55.1 What Are Ecological Communities?

• Most communities have only four to five trophic
  levels.
• Energy is lost between trophic levels. Diagrams
  showing energy or biomass (weight of living
  matter) at each trophic level show how energy
  decreases as it flows from lower to higher levels.

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Figure 55.3 Diagrams of Biomass and Energy
Distributions
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55.1 What Are Ecological Communities?

• Variations in the dimensions of the energy
  distribution diagrams depend on the nature of the
  organisms at each level.
• For example, a forest has high biomass in the
  producer level much energy is stored as wood, a
  difficult-to-digest material that is unavailable
  to most herbivores.

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55.1 What Are Ecological Communities?

• In aquatic systems, the primary producers are
  bacteria and protists.
• High rates of cell division support a high
  biomass of herbivores, and result in an inverted
  biomass distribution.

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55.1 What Are Ecological Communities?

• Detritivores include bacteria, fungi, worms,
  mites, and many insects. They transform detritus
  (dead remains and waste products) into free
  mineral nutrients that can be taken up and used
  again by plants.
• Continued ecosystem productivity depends on the
  decomposition of detritus.

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55.2 What Processes Influence Community Structure?

• Categories of species interactions
• Predation or parasitism one participant is
  harmed, the other benefits.
• Competition two organisms using same resource
  that is insufficient to supply needs of both.
• Mutualism both species benefit.

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55.2 What Processes Influence Community Structure?

• Commensalism one participant benefits and the
  other is unaffected.
• Amensalism one participant is harmed, and the
  other is unaffected.
• These interactions may increase or decrease the
  range of conditions over which a species can
  exist.

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Table 55.2 Types of Ecological Interactions
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55.2 What Processes Influence Community Structure?

• Predation and parasites
• Parasites are usually smaller than their hosts,
  and live inside or outside the host. They often
  feed on the host without killing it.
• Microparasites are much smaller bacteria,
  viruses, protists.

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55.2 What Processes Influence Community Structure?

• Predators are typically larger and live outside
  the bodies of their prey.
• Predators of animals typically kill their prey
  herbivores are predators of plants, and often do
  not kill the plants.
• Predators can reduce the size of prey
  populations, but predator-prey relationships are
  usually more complex.

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55.2 What Processes Influence Community Structure?

• Predator and prey population densities can
  oscillate together.
• Growth of predator population nearly always lags
  growth of prey population.
• As predator population grows, it reduces size of
  prey population, then predators run out of food
  and population crashes.

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Figure 55.4 Hare and Lynx Populations Cycle in
Nature (Part 1)
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Figure 55.4 Hare and Lynx Populations Cycle in
Nature (Part 2)
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55.2 What Processes Influence Community Structure?

• To test whether oscillations of snowshoe hare and
  Canadian lynx populations were due only to the
  interactions between the species, enclosures were
  built to exclude lynx, but not hares.
• Population cycles of the hares were influenced by
  food supply as well as predators.

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Figure 55.5 Prey Population Cycles May Have
Multiple Causes (Part 1)
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Figure 55.5 Prey Population Cycles May Have
Multiple Causes (Part 2)
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55.2 What Processes Influence Community Structure?

• Predators may restrict range of prey species.
• Megapodes are birds that do not incubate their
  eggs but instead lay them in a large mound of
  decomposing plants heat from the decomposition
  warms the eggs.
• Megapodes have colonized many islands but are
  absent wherever there are Asian mammalian
  predators that eat eggs.

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Figure 55.6 Megapode Distributions are Limited by
Mainland Predators
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55.2 What Processes Influence Community Structure?

• Prey species have evolved many adaptations to
  make them more difficult to capture, subdue, or
  eat.
• Includes toxic hairs, tough spines, noxious
  chemicals, camouflage, and mimicry.
• Predators, in turn, have evolved more effective
  ways to capture prey.

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55.2 What Processes Influence Community Structure?

• Mimicry is well studied.
• Batesian mimicry a palatable species mimics an
  unpalatable or noxious species.
• Müllerian mimicry two or more unpalatable or
  noxious species converge to look alike.

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55.2 What Processes Influence Community Structure?

• Batesian mimicry can be maintained if the mimic
  is less common in the environment than the
  unpalatable species.
• In Müllerian mimicry all species in the system
  benefit when an inexperienced predator eats one
  individual, and learns to avoid individuals of
  all the species.

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Figure 55.7 Batesian and Müllerian Mimicry Systems
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55.2 What Processes Influence Community Structure?

• For microparasite populations to persist, new
  hosts must become infected before the current
  host dies.
• Microparasites can invade a host population with
  many susceptible individuals microparasite
  population decreases as more hosts become immune.

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55.2 What Processes Influence Community Structure?

• Microparasites can be transferred from host to
  host in various ways breath, body fluids, water,
  animal vectors.
• Infected hosts can sometimes continue to infect
  others, even after death.
• Cholera is caused by the bacterium Vibrio
  cholerae. The bacterium is ingested from water
  supplies thousands are released when the victim
  defecates.

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Figure 55.8 Filtering Water Can Help Combat
Cholera
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55.2 What Processes Influence Community Structure?

• Competition for resources can influence abundance
  and distribution of species.
• Interference competition one species interferes
  with the activities of another.
• Exploitation competition one species reduces the
  availability of a resource.

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55.2 What Processes Influence Community Structure?

• Competition can occur between individuals of the
  same species intraspecific competition. A
  primary cause of density-dependent birth and
  death rates.
• Interspecific competition occurs between
  individuals of different species.
• Competitive exclusion occurs when a superior
  competitor prevents another species from using a
  habitat.

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55.2 What Processes Influence Community Structure?

• Competition can restrict a species habitat.
• Plants compete for space. The shoots need
  sunlight and the roots compete for water and
  minerals.
• Sessile animals also compete for space.

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55.2 What Processes Influence Community Structure?

• Two barnacle species compete in the intertidal
  zone, ending up in two distinct bands. Chthamalus
  lives in the higher zone because it is more
  tolerant of desiccation.
• In the lower zone it is outcompeted by Balanus.
• If only one species of barnacle is present, it
  occupies a larger zone than when both are present.

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Figure 55.9 Competition Restricts the Intertidal
Ranges of Barnacles
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55.2 What Processes Influence Community Structure?

• One species may restrict another species range
  by reducing populations of a shared prey species.
• Two parasitoid wasps prey on scale insects both
  were introduced to control scale.
• When the second species was introduced, it
  reduced the scale population so much that it
  displaced the first species.

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Figure 55.10 A Parasitoid Wasp Outcompetes Its
Close Relative
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55.2 What Processes Influence Community Structure?

• Ammensal interactions are widespread.
• Examples herds of mammals trampling plants
  around a water hole tree branches falling on
  smaller plants or animals.
• A rhinoceros grazing on an African plain
  demonstrates several types of interactions.

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Figure 55.11 A Single Small Community
Demonstrates Many Interactions (Part 1)
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Figure 55.11 A Single Small Community
Demonstrates Many Interactions (Part 2)
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55.2 What Processes Influence Community Structure?

• Many organisms participate in mutualistic
  interactions.
• Plants and mycorrhizae, plants and N-fixing
  bacteria, corals and photosynthetic protists,
  termites and protists in their guts that digest
  cellulose, plants and their pollinators.

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Figure 55.12 PlantAnimal Mutualisms Are
Important in Pollination (A)
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55.2 What Processes Influence Community Structure?

• Many plants provide nectar on the vegetative
  parts of the plant to attract ants.
• Ants provide protection by attacking herbivores.
• Many species of Acacia trees have associations
  with ants. The ants receive food and a place to
  live.

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55.2 What Processes Influence Community Structure?

• Interactions between plants and their pollinators
  and seed dispersers are not always mutualistic.
• Seed dispersers are also seed predators that
  destroy some of the seeds.
• Some animals cut holes in petals to gain access
  to nectar without transferring any pollen.

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55.2 What Processes Influence Community Structure?

• Some plants exploit pollinators
• Certain orchid flowers mimic female insects,
  enticing male insects to copulate with them. The
  males transfer pollen which benefits the plant,
  but the male insect has no nectar reward, and no
  offspring.

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Figure 55.12 PlantAnimal Mutualisms Are
Important in Pollination (B)
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55.3 How Do Species Interactions Cause Trophic
Cascades?

• Interactions of a predator species can cause a
  cascade of effects on lower trophic levels.
• In Yellowstone National Park, wolves were
  extirpated by 1926. Elk were culled each year to
  prevent them from exceeding carrying capacity,
  until 1968. Elk population then rapidly increased.

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55.3 How Do Species Interactions Cause Trophic
Cascades?

• When wolves were absent, elk browsed aspen trees
  so heavily that no young aspens could get a
  start.
• Elk also browsed streamside willows to the point
  that beavers were nearly exterminated.
• Wolves were reintroduced in 1995, and preyed
  primarily on elk. Aspen and willow regrew, and
  beaver population increased.

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Figure 55.13 Wolves Initiated a Trophic Cascade
(Part 1)
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Figure 55.13 Wolves Initiated a Trophic Cascade
(Part 2)
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Figure 55.13 Wolves Initiated a Trophic Cascade
(Part 3)
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55.3 How Do Species Interactions Cause Trophic
Cascades?

• Trophic cascades may affect multiple ecosystems.
• Example dragonfly larvae are more abundant in
  ponds without fish. In a study of ponds with and
  without fish, researchers found that adult
  dragonflies were more common around fish-free
  ponds, and the insect pollinators they preyed on
  were less common.

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55.3 How Do Species Interactions Cause Trophic
Cascades?

• Flowers of St. Johns wort near fish-free ponds
  were visited by insect pollinators much less, and
  produced fewer seeds.
• Thus, fish predation on dragonfly larvae in one
  habitat influenced another habitat where fish do
  not live.

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Figure 55.14 Trophic Cascades May Cross Habitats
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55.3 How Do Species Interactions Cause Trophic
Cascades?

• Beavers can also cause trophic cascades by
  preferentially cutting some species of trees they
  alter the species composition of the vegetation.
• They also create aquatic habitat for many other
  species by building dams.
• Organisms that build such structures are called
  ecosystem engineers.

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55.3 How Do Species Interactions Cause Trophic
Cascades?

• A species that exerts influence out of proportion
  with its abundance is called a keystone species.
• They may influence species richness and the flow
  of energy and materials.

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55.3 How Do Species Interactions Cause Trophic
Cascades?

• Sea stars in the rocky intertidal zones of the
  Pacific northwest prey on mussels.
• If sea stars are absent, the mussels crowd out
  all other competitors. By eating mussels, the sea
  stars create space that other species can
  colonize.

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Figure 55.15 Some Sea Stars are Keystone Species
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55.3 How Do Species Interactions Cause Trophic
Cascades?

• Keystone species may not be predators.
• Fig trees in tropical forests may act as keystone
  species.
• The fruits ripen at a time of year when fruit is
  otherwise scarce. Dozens of fruit-eating species
  depend on the figs at this time of year.

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55.4 How Do Disturbances Affect Ecological
Communities?

• A disturbance is an event that changes the
  survival rate of one or more species.
• For example, a windstorm that blows down a tree,
  crushing other plants, can open up space and
  resources for other species.
• Effects of disturbance depend on the size and
  duration of the disturbance.
• Fires in Yellowstone in 1988 created a mosaic of
  burned and unburned patches.

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Figure 55.16 Fires Create Mosaics of Burned and
Unburned Patches
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55.4 How Do Disturbances Affect Ecological
Communities?

• Change in community composition following a
  disturbance is called succession.
• Primary succession begins on sites that lack
  living organisms.
• Secondary succession begins on sites where some
  organisms have survived.

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55.4 How Do Disturbances Affect Ecological
Communities?

• Species that colonize first often set up
  environmental conditions that allow other species
  to follow (facilitation).
• Example of primary succession a series of
  moraines left by retreating glacier in Glacier
  Bay, Alaska. Ecologists study moraines of
  different ages to understand the stages of
  succession over the last 200 years.

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Figure 55.17 Primary Succession on a Glacial
Moraine (Part 1)
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Figure 55.17 Primary Succession on a Glacial
Moraine (Part 2)
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55.4 How Do Disturbances Affect Ecological
Communities?

• Succession is caused in part by changes in the
  soil brought about by the plants themselves.
• Moraines are deficient in nitrogen the first
  plants to colonize have N-fixing bacteria in root
  nodules.
• The soil is improved enough for spruces to grow.

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55.4 How Do Disturbances Affect Ecological
Communities?

• Example of secondary succession Fungal species
  succession on decomposing pine needles in pine
  forest litter.
• Each group of fungi uses different compounds in
  the pine needles, converting them to compounds
  that the next fungal group can use.

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Figure 55.18 Secondary Succession on Pine Needles
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55.4 How Do Disturbances Affect Ecological
Communities?

• The intermediate disturbance hypothesis
• Communities with intermediate levels of
  disturbance tend to have more species that those
  with high or low levels of disturbance.

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55.4 How Do Disturbances Affect Ecological
Communities?

• Only species with great dispersal capabilities
  and high reproductive rates can survive in areas
  with high disturbance.
• Where disturbance levels are low, competitive
  species replace other species, reducing species
  richness.

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55.4 How Do Disturbances Affect Ecological
Communities?

• This was tested using boulders of different sizes
  in an intertidal zone.
• Small boulders were often dislodged by waves
  (high disturbance) large boulders were seldom
  disturbed.
• Intermediate sized boulders had more species than
  small boulders. When small boulders were glued in
  place, they accumulated more species.

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Figure 55.19 Species Richness Depends on Level of
Disturbance (Part 1)
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Figure 55.19 Species Richness Depends on Level of
Disturbance (Part 2)
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55.4 How Do Disturbances Affect Ecological
Communities?

• In some cases, established species inhibit
  colonization by other species.
• Also tested on rocks in the intertidal zone.
• If all organisms are removed from rocks, a
  succession of species ensues. But if the first
  colonizers are removed, the next species have
  much greater population densities.

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55.4 How Do Disturbances Affect Ecological
Communities?

• Inhibition may have been occurring when the
  Central American land bridge was established.
• Many mammal species were able to colonize South
  America from North America, but only a handful
  were able to move from South America to North
  America.

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55.5 What Determines Species Richness in
Ecological Communities?

• Species richness is the number of species living
  in a community.
• An observed biogeographic pattern is that more
  species are found in the low latitudes than high
  latitudes.
• Example gradient in mammal species richness in
  Central and North America

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Figure 55.20 The Latitudinal Gradient of Species
Richness of North American Mammals
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55.5 What Determines Species Richness in
Ecological Communities?

• The mountainous regions also have greater species
  richness. More vegetation types and climates
  exist in these topographically diverse areas.
• Islands and peninsulas generally have fewer
  species than a similar area on the mainland
  (theory of island biogeography).

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55.5 What Determines Species Richness in
Ecological Communities?

• Species richness is correlated with ecosystem
  productivity, but the relationship is complex.
• Richness often increases with productivity, but
  only to a point, then declines.
• Interspecific competition may be more intense
  when productivity is high, resulting in
  competitive exclusion.

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Figure 55.21 Species Richness Peaks at
Intermediate Productivity
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55.5 What Determines Species Richness in
Ecological Communities?

• Species richness might enhance productivity,
  because more species would be using all possible
  resources.
• If environment changes, a species-rich ecosystem
  is more likely to have species already adapted to
  new conditions.
• The species-rich ecosystem would be more stable,
  or change less over time.

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55.5 What Determines Species Richness in
Ecological Communities?

• This hypothesis was tested by planting grasses in
  controlled plots. Species number ranged from a
  few to 25 species.
• Over 11 years, plots with more species were more
  productive, and productivity varied less from
  year to year.
• Population densities were not stable because
  different species performed better in drought
  years and wet years.

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Figure 55.22 Species Richness Enhances Community
Productivity (Part 1)
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Figure 55.22 Species Richness Enhances Community
Productivity (Part 2)
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Figure 55.22 Species Richness Enhances Community
Productivity (Part 3)