Community Ecology

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Community Ecology
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Symbioses

• Symbiosis means species living in close or
  intimate association
  
• parasitism ,- one species, the parasite,
  benefits at the expense of the host
  
• commensalism ,0 or 0,0 here there may be a
  positive effect for one species or neither, but
  neither is harmed
  
• mutualism , a situation in which two species
  interact with each other and both benefit

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Commensalism - ,0 or 0,0 - can have positive
effect for one species or for neither
Shark with remora and water buffalo with cattle
egret
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Mutualism

• Mutualism - the individuals in a population of
  each mutualist species grow and/or survive and/or
  reproduce at a higher rate when in the presence
  of individuals of the other. Each benefits
  (,)
• It is important to note that mutualistic
  interactions evolve because benefits to each
  partner outweigh any costs of the mutualism -
  species still act in "selfish" way

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Phlox family adaptations to many different pollin
ators
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Bumblebee covered with pollen
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Bumblebee pollinating beebalm
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Moth pollinating flower note proboscis
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Hummingbird Pollination
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Hummingbird pollinated flower
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Bat Pollination
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Mycorrhizae

• Endomycorrhizae - fungal hyphae penetrate the
  outer cells of the plant root and often form
  swellings, coils, or minute branches and extend
  into the surrounding soil. Endomycorrhizae are
  the most common type of mycorrhizae.
• Ectomycorrhizae - hyphae surround the root, but
  do not penetrate its cells

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Mutualisms involving behavior clownfish and
sea anemones
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Close-up of Anemone and Clownfish
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Mutualisms involving culturing other species
leaf cutter ants and fungi
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Mutualisms involving culturing other species
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Perhaps the most important mutualism of all
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Community Structure and Dynamics

• Often a small number of species in the community
  exert strong control on that communitys
  structure, especially on the composition,
  relative abundance, and diversity of species.

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Species Diversity

• The species diversity of a community is the
  variety of different kinds of organisms that make
  up the community.
  
• Species diversity has two components.
• Species richness is the total number of different
  species in the community.
  
• The relative abundance of the different species
  is the proportion each species represents of the
  total individuals in the community.
  
• Species diversity is dependent on both species
  richness and relative abundance.

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A comparison of species diversity in two
communities
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How do the niches differ for all these plants?
Konza Prairie, Kansas
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Stability and Disturbance

• Stability is the tendency of a community to reach
  and maintain a relatively constant composition of
  species despite disturbance. This is sometimes
  referred to as equilibrium.
• Many communities seem to be characterized by
  change rather than stability.
  
• The nonequilibrium model proposes that
  communities constantly change following a
  disturbance.
  
• A disturbance is any agent which causes complete
  or partial destruction of the ecological
  community resulting in the creation of bare
  space. Disturbance changes a community by
  removing organisms or altering resource
  availability.
• Storms, fires, floods, droughts, frosts, human
  activities, or overgrazing can be disturbances.

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Example of Intermediate Disturbance
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Human Disturbance Habitat Fragmentation
Habitat fragmentation the process by which a
natural landscape is broken up into small parcel
s of natural ecosystems, isolated from
one another by lands dominated by human
activities.
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Human Disturbance Habitat Fragmentation
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Succession

• Ecological succession is the transition in
  species composition in disturbed areas over
  ecological time.
  
• Primary succession begins in a lifeless area
  where soil has not yet formed, such as a volcanic
  island or the moraine left behind as a glacier
  retreats.
• Secondary succession occurs where an existing
  community has been removed by a disturbance such
  as a clear-cut or fire, while the soil is left
  intact.

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Primary Succession at Glacier Bay
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Secondary Succession an old field in upstate
New York
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In Succession

• Early arrivals and later-arriving species are
  linked in one of three key processes.
  
• Early arrivals may facilitate the appearance of
  later species by changing the environment.
  
• For example, early herbaceous species may
  increase soil fertility.
  
• Early species may inhibit establishment of later
  species.
  
• Early species may tolerate later species but
  neither hinder nor help their colonization.

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Trophic Structure

• The trophic structure of a community is
  determined by the feeding relationships between
  organisms.
  
• The transfer of food energy up the trophic levels
  from its source in autotrophs (usually
  photosynthetic organisms) through herbivores
  (primary consumers) and carnivores (secondary and
  tertiary consumers) and eventually to decomposers
  is called a food chain.
• In the 1920s, Oxford University biologist Charles
  Elton recognized that food chains are not
  isolated units but are linked together into food
  webs.

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Charles Elton 1900-1991
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Simple Food Chains
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Simple Food Web
Antarctic Ocean
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A More Complex Food Web
Arctic Ocean
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A Really Complex Food Web
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How long are food chains?

• Each food chain within a food web is usually only
  a few links long.
  
• Charles Elton pointed out that the length of most
  food chains is only four or five links.
  
• Research in the 1990s by Gary Polis indicated
  that desert food chains can be very long up to
  18 links.

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Gary Polis 1946-2000
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Some Complex Desert Food Chains
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Why limits the length of a food chain?

• The energetic hypothesis suggests that the length
  of a food chain is limited by the inefficiency of
  energy transfer along the chain.
  
• Only about 10 of the energy stored in the
  organic matter of each trophic level is converted
  to organic matter at the next trophic level.
  
• The energetic hypothesis predicts that food
  chains should be relatively longer in habitats
  with higher photosynthetic productivity.
  
• The dynamic stability hypothesis suggests that
  long food chains are less stable than short
  chains.
  
• Population fluctuations at lower trophic levels
  are magnified at higher levels, making top
  predators vulnerable to extinction.
  
• In a variable environment, top predators must be
  able to recover from environmental shocks that
  can reduce the food supply all the way up the
  food chain.
• The dynamic stability hypothesis predicts that
  food chains should be shorter in unpredictable
  environments.
  
• Most of the available data supports the energetic
  hypothesis.

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Dominant and Keystone Species

• Certain species have an especially large impact
  on community structure because they are highly
  abundant or because they play a pivotal role in
  community dynamics.
• The exaggerated impact of these species may occur
  through their trophic interactions or through
  their influences on the physical environment.
  
• Dominant species are those species in a community
  that are most abundant or have the highest
  biomass (the sum weight of all individuals in a
  population).
• Keystone species are not necessarily the most
  abundant in a community.
  
• They influence community structure by their key
  ecological niches.
  
• If keystone species are removed, community
  structure is greatly affected.

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Ponderosa Pine as Dominant Species
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Wolves as Keystone Predators - Dynamics of a
Trophic Cascade
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Actors in Trophic Cascade
Wolf Coyote
Big Sage Pronghorn
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Beaver As Keystone Species
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Beaver As Keystone Species
Stream before and after Beaver dam construction
Beaver pond succession