Community Ecology

1
Community Ecology
Chapter 53
2
Community Ecology

• A community is an assemblage of species
  (populations) living close enough together for
  potential interaction in a habitat

3
Community Ecology

• The boundaries of a community can be set by the
  experimental question (the decomposers on a
  rotting log, the benthic community of Lake Erie,
  the trees and shrubs in Yellowstone National
  Park)
• Communities vary in their species richness and
  relative abundance

4
Interspecific Interactions

• Interspecific competition competition for
  resources between different species when
  resources are in short supply

5
Interspecific Competition

• Competitive exclusion principle The concept
  that when populations of two similar species
  compete for the same limited resources, one
  population will use the resources more
  efficiently and have a reproductive advantage
  that will
  eventually lead
  to the
  
  elimination of
  the other
  population

6
The Ecological Niche

• An organisms ecological niche is described as
  its place in an ecosystem its habitat and use
  of biotic and abiotic resources
• The competitive exclusion principle holds that
  two
  species with
  identical
  niches cannot
  coexist in a
  community

7
Resource Partitioning

• Resource partitioning slight variations in
  niche that allow ecologically similar species to
  coexist competition is a selection factor in
  evolution

8
Predation

• Predation a predator eating prey
• Includes herbivory and parasitism
• Eating and being eaten are prerequisite to
  reproductive success
• Adaptations to increase success in predation may
  include acute senses, speed and agility,
  camouflage coloration, and physical structure
  such as claws, fangs, teeth, and stingers

9
Predation

• Plants may defend themselves with mechanical
  devices, such as thorns, or chemical compounds
• Animals can defend against predation by hiding,
  fleeing, or defending
• Cryptic coloration allows animals to blend in to
  their environment
• Mechanical and chemical defenses discourage
  predation
• Aposematic coloration warns predators not to eat
  animals with chemical defenses

10
Predation
11
Predation

• Mimicry may be used by prey to exploit the
  warning coloration of other species
• Batesian mimicry a palatable or harmless
  species mimics an unpalatable or harmful model
• Müllerian mimicry two or more unpalatable
  species resemble each other

Cuckoo Bee
Yellow Jacket
12
Parasites

• A parasite derives its nourishment from another
  organism, the host, which is harmed in the
  process
• Parasites that live within the host are
  endoparasites those that feed on the surface of
  a host are ectoparasites
• In parasitoidism, insects lay eggs
  on hosts, on which their larvae
  then feed.
• Parasites do not usually cause
  lethal harm to the host on which
  they feed

13
Mutualism

• In mutualism, interactions between species
  benefit both participants
• Mutualistic interactions require the coevolution
  of adaptations in both species
• Many cases of mutual
  symbiosis may have evolved
  when organisms became able
  to derive some benefits
  from their predator or
  parasite

Mutualism between acacia trees and ants
14
Commensalism

• In commensalism, only one member appears to
  benefit from the interaction
• Examples include hitchhiking species and
  species that feed on food incidentally exposed by
  another

The Patella gets it food from the plant, the
Euklonia, which is not harmed or damaged in the
process.
Example of Mutualism
15
Trophic Structure

• The trophic structure
  of a community is its
  feeding relations
• A food chain shows
  the transfer of food
  energy from one
  trophic level to the
  next

16
Food Webs

• A food web
  diagrams the
  complex trophic
  relationships
  within a
  community

17
What Limits the Length of a Food Chain?

• Within a food web, each food chain consists of
  only a few links (usually five or fewer)
• Two hypotheses
• Energetic hypothesis food chains are limited by
  inefficiency of energy transfer (only about 10)
  from one trophic level to the next
• Dynamic stability hypothesis suggests that
  short food chains are more stable than long ones
  because an environmental disruption that reduces
  production at lower levels will be magnified at
  higher trophic levels as food supply is reduced
  all the way up the chain

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Dominant Species

• Species in a community that have the highest
  abundance or largest biomass are a major
  influence on the distribution and abundance of
  other species
• A species may become a dominant species due to
  its competitive use of resources or success at
  avoiding predation
• What happens if the dominant specie is removed
  from a habitat such as the American chestnut
  trees?

19
Keystone Species

• A keystone species has a large impact on
  community structure as a result of its ecological
  role
• Experiments with a predatory sea star (Pisaster)
  demonstrates the importance of keystone species

20
Sea Otters as Keystone Predators in the North
Pacific
21
Structures of Communities

• The structure of a community may be controlled
  bottom-up by nutrients or top-down by predators
• According to the bottom-up model of community
  organization, N ? V ? H ? P, an increase in
  nutrients yields an increase in biomass at each
  succeeding trophic level
• The top-down model, N ? V ? H ? P, assumes that
  predation controls community organization also
  called the trophic cascade model

22
Disturbance and Community Structure

• The nonequilibrium model emphasizes the
  nonstable, changing structure and composition of
  communities as a result of disturbances
• Disturbances such as fire, drought, storms,
  overgrazing, or human activity change resource
  availability, reduce or eliminate some
  populations, and may create opportunities for new
  species
• Small scale disturbances may enhance
  environmental patchiness and help maintain
  species diversity

23
Disturbance and Community Structure
24
Disturbance and Community Structure
25
Disturbance and Community Structure
26
Ecological Succession

• Ecological succession is the transition in
  species composition in a community, usually
  following some disturbance
• Primary succession occurs in areas previously
  devoid of life (new volcanic island or the
  moraine left by a retreating glacier)
• Autotrophic bacteria ? lichens ? mosses ? grasses
  ? shrubs ? trees
• Secondary succession occurs in existing
  communities that have been disturbed by fire,
  logging, or farming, but the soil remains intact
• Herbaceous species ? woody shrubs ? trees

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Biodiversity

• A communities biodiversity correlates with its
  size and geographic location
• Biodiversity is greatest in the tropics and on
  large versus smaller islands
• Biodiversity is measured by both species richness
  and relative abundance
• Ecologists measure biodiversity as heterogeneity,
  which considers both diversity factors richness
  and relative abundance

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Which Forest is More Diverse?
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Biodiversity

• Tropical habitats
  support much larger
  numbers of species
  of plants, animals,
  and other organisms
  than do temperate
  and polar regions

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Biodiversity

• A speciesarea curve illustrates the correlation
  between the size of a community and the number of
  species found there

31
Biodiversity

• Island biogeography states that the size of the
  island and its closeness to the mainland (or
  source of dispersing species) are important
  variables directly correlated with species
  diversity