COMMUNITY STRUCTURE

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• COMMUNITY STRUCTURE
• The Influence of COMPETITION
• The Influence of PREDATION (HERBIVORY)
• Reading General through cpt 23, and pp485-501

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• COMMUNITY STRUCTURE
• The Influence of COMPETITION
• The Influence of PREDATION (HERBIVORY)
• Reading General through cpt 23, and pp485-501

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THE INFLUENCE OF COMPETITION ON COMMUNITY
STRUCTURE
  1.   Two species of Typha in Louisiana (Jim
Graces work)   2.   Reduce or remove number of
Meadow Mouse (Microtus), and the population size
of House Mouse (Mus) increases   3.   Two species
of Microtus, one in wetter parts, the other in
drier parts of meadows.  Remove one species from
the wetter area, and the other species moves in
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BullrushCattail
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THE INFLUENCE OF COMPETITION ON COMMUNITY
STRUCTURE
  1.   Two species of Typha in Louisiana (Jim
Graces work)   2.   Reduce or remove number of
Meadow Mouse (Microtus), and the population size
of House Mouse (Mus) increases   3.   Two species
of Microtus, one in wetter parts, the other in
drier parts of meadows. Remove one species from
the wetter area, and the other species moves in
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Microtus (meadow mouse)
Mus (house mouse)
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THE INFLUENCE OF COMPETITION ON COMMUNITY
STRUCTURE
  1.   Two species of Typha in Louisiana (Jim
Graces work)   2.   Reduce or remove number of
Meadow Mouse (Microtus), and the population size
of House Mouse (Mus) increases   3.   Two species
of Microtus, one in wetter parts, the other in
drier parts of meadows.  Remove one species from
the wetter area, and the other species moves in
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• 4.   Field bordered by trees and hedges
• a.   Lots of sparrows feeding on seeds on one
  part for the border
• b.   Juncos feeding on seeds on other parts of
  the border
• c.   Remove sparrows, and juncos spread and
  increased
• d.   Sparrows allowed to return and juncos
  displaced
• Connell (p94, 95, 137) photo on 449 on
  Chthamalus and Balanus
• The Ghost of Competition Past
• 7.   Piankas lizards

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Melospiza melodia (song sparrow)
Junco hyemalis (dark-eyed Junco)
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• 4.   Field bordered by trees and hedges
• a.   Lots of sparrows feeding on seeds on one
  part for the border
• b.   Juncos feeding on seeds on other parts of
  the border
• c.   Remove sparrows, and juncos spread and
  increased
• d.   Sparrows allowed to return and juncos
  displaced
• Connell (p94, 95, 137) photo on 449 on
  Chthamalus and Balanus
• The Ghost of Competition Past
• 7.   Piankas lizards

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Krebs Fig. 7.9 p94,95,137
Chthamalus
Balanus
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• 4.   Field bordered by trees and hedges
• a.   Lots of sparrows feeding on seeds on one
  part for the border
• b.   Juncos feeding on seeds on other parts of
  the border
• c.   Remove sparrows, and juncos spread and
  increased
• d.   Sparrows allowed to return and juncos
  displaced
• Connell (p94, 95, 137) photo on 449 on
  Chthamalus and Balanus
• The Ghost of Competition Past
• 7.   Piankas lizards

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Frequency
Environmental gradient
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Frequency
Environmental gradient
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GHOST OF COMPETITION PAST
Frequency
Environmental gradient
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The Ghost of competition past. is a term
coined by Joe Connell, to stress that inter
specific competition acting as an evolutionary
force in the past, has often left its mark on the
behaviour, distribution or morphology of species,
even when there is no present-day competition
between them. Current competition. is
happening in the present
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• 4.   Field bordered by trees and hedges
• a.   Lots of sparrows feeding on seeds on one
  part for the border
• b.   Juncos feeding on seeds on other parts of
  the border
• c.   Remove sparrows, and juncos spread and
  increased
• d.   Sparrows allowed to return and juncos
  displaced
• Connell (p94, 95, 137) photo on 449 on
  Chthamalus and Balanus
• The Ghost of Competition Past
• 7.   Piankas lizards

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Three species of lizards eating prey of different
sizes
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Some Conclusions on the Effects of Competition
on Community Structure
1.   Interspecific competition is a plausible
explanation for many aspects of community
organization.   2.   Current competition can
easily be demonstrated.   3.   As an alternative
to current competition, the ghost of competition
past can be invoked to account for present-day
patterns - but it can be readily invoked because
it is impossible to observe and difficult to
prove!  
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Some Conclusions on the Effects of Competition
on Community Structure
 4.   The patterns uncovered, often support the
competition hypothesis, but often have
alternative explanations e.g. chance! Species
may have evolved independently into specialists,
and their specialized niches happen to be
different the species may never have competed.
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THE INFLUENCE OF PREDATION ON COMMUNITY STRUCTURE
1. KEYSTONE PREDATORS (Krebs 471-474) a.
removing rabbits from the English chalk
grasslands                        42 spp
community to 3 spp in 7 years   b.   Removing
Pisaster (starfish) from intertidal
zones                        15 spp community to
8 spp in 1 year (p 471)   c.   Sea otters off
the coast of Alaska (p471,2)                     
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A Keystone Species. is a species (usually having
a low frequency) in a community whose removal has
strong effects on community diversity and
composition they are often top predators. Krebs
p471, 621
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Krebs p448?
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Krebs Fig. 23.12 p472
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Comparative status of nearshore communities in
Rat Islands and Near Islands, western Aleutian
Islands, Alaska
  (from Simenstad et al. 1978. Science 200405)
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Sea Urchin
Sea Otter
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Limpet
Chiton
Barnacles
Mussels
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Macro algae
Harbour seal
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Alternative or Multiple Stable States. exist
in a stable community if, when perturbed
sufficiently, this stable community moves to a
second equilibrium state, at which point it
remains after the disturbance has been
removed. For details see Krebs p508
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2.   INTERMEDIATE DISTURBANCE HYPOTHESIS (Krebs
452-3) a.   Rabbits on sand dunes b.   Coral
reefs and Tropical rain Forests   3.  
INTERACTIONS INVOLVING PREDATION a.   Chondrus,
Enteromorpha, Littorina snail, crabs and gulls
(p453) b.   Littorina has strong preference
for Enteromorpha c.   But typically dont find
many Littorina in Enteromorpha pools
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2.   INTERMEDIATE DISTURBANCE HYPOTHESIS a.  
Rabbits on sand dunes b.   Coral reefs and
Tropical rain Forests   3.   INTERACTIONS
INVOLVING PREDATION a.   Chondrus, Enteromorpha,
Littorina snail, crabs and gulls (p453) b.  
Littorina has strong preference for
Enteromorpha c.   But typically dont find many
Littorina in Enteromorpha pools
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Periwinkle Littorina
crabs gulls
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Enteromorpha
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Gulls Crabs Periwinkle Enteromorpha
Gulls Crabs Periwinkle Chondrus
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Some Conclusions on the Effects of Predation on
Community Structure
1.   Selective predation (e.g. Starfish on
Mytilus) are likely to enhance community
diversity if their preferred prey are
competitively dominant.   2.   Even more
generalist predators (e.g. Rabbits on grasses)
may be expected to cause an increase in community
diversity.   3.   An intermediate intensity of
predation is most likely to be associated with
'prey' diversity (e.g. rabbits on sand dunes),
because too low an intensity may not prevent
competitive exclusion of inferior prey species,
and too high an intensity may drive preferred
prey to extinction.
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Some Conclusions on the Effects of Predation on
Community Structure
1.   Selective predation (e.g. Starfish on
Mytilus) are likely to enhance community
diversity if their preferred prey are
competitively dominant.   2.   Even more
generalist predators (e.g. Rabbits on grasses)
may be expected to cause an increase in community
diversity.   3.   An intermediate intensity of
predation is most likely to be associated with
'prey' diversity (e.g. rabbits on sand dunes),
because too low an intensity may not prevent
competitive exclusion of inferior prey species,
and too high an intensity may drive preferred
prey to extinction.
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Some Conclusions on the Effects of Predation on
Community Structure
1.   Selective predation (e.g. Starfish on
Mytilus) are likely to enhance community
diversity if their preferred prey are
competitively dominant.   2.   Even more
generalist predators (e.g. Rabbits on grasses)
may be expected to cause an increase in community
diversity.   3.   An intermediate intensity of
predation is most likely to be associated with
'prey' diversity (e.g. rabbits on sand dunes),
because too low an intensity may not prevent
competitive exclusion of inferior prey species,
and too high an intensity may drive preferred
prey to extinction.
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Some Conclusions on the Effects of Predation on
Community Structure
 4.   An intermediate level (frequency or
intensity) of disturbance is also most likely to
be associated with higher diversity communities
(e.g. Tropical rain forests and coral
reefs).   5.   The role of predators in shaping
community structure is likely to be least
significant in communities where physical
conditions are more severe or unpredictable.
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Some Conclusions on the Effects of Predation on
Community Structure
 4.   An intermediate level (frequency or
intensity) of disturbance is also most likely to
be associated with higher diversity communities
(e.g. Tropical rain forests and coral
reefs).   5.   The role of predators in shaping
community structure is likely to be least
significant in communities where physical
conditions are more severe or unpredictable.
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HARSH
BENIGN
INTERMEDIATE
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• ACACIA in Australia
• produces seed every year
• some establish in higher rainfall years
• insects destroy most seedlings
• ODD ONE GETS THROUGH
• need 3 successive wet years for success
• typically about every 40-50 years
• i.e. Widely-spaced age classes

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Acacia
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• LAMINARIA in sub-tidal
• deeper populations usually a single age class
  (low light)
• shallow populations usually multi-age
• i.e. in harsher conditions (low light) the ODD
  INDIVIDUAL survives in the unusual favorable year.

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Laminaria
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CHANCE
PREDATION
COMPETITION
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A. CARNIVORES
B. HERBIVORES
Krebs Fig. 24.8 p495
C. PLANTS