Download Community Composition and Predation • Predators selecting

Community Composition and Predation
Community Composition and Predation
• Predators selecting specific
prey will tend to increase
diversity
– reducing the abundance
of dominant species that
exclude others
– Potential explanation for
higher diversity in tropical
systems
• Apparent competition: indirect
interaction between two species via
shared predator
Predator
Prey 1
Prey 2
Predator
• Trophic cascade: predation alters
abundance of consumer at one trophic
level, cascades through food web.
Prey 1
Predator
Prey 1
Prey 2
Prey 1
Competition, 1 winner
Non-consumptive cascade
Prey 2
Producer
Both populations
reduced, less
competition
Prey Responses
• Prey responses to the
presence of predators
(nonconsumptive effects)
Fish
Diet selectivity
of grazers
• Changes in movement or
habitat
Grazers
Algae
No clear
trophic
cascade…shift
in community
structure.
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Habitat Shifts
• Prey will shift habitats to
mediate the tradeoff between
acquiring resources (increasing
fitness) and avoiding predation.
• Zooplankton diel migration
– More algae available at the
surface where predation is
high.
– Most predators are visual,
predator efficiency decreases
at night.
Morphological Shifts
Evolutionary Trends in Life Histories
• Plastic responses (e.g.
Daphnia)
• Strong selection on
specific phenotypes.
– Morphologies to
avoid predation
through increased Ts
or Th
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Intra vs. interspecific competition
• Predation gradient:
– C – large predators
targeting adults
– R – moderate sized
predators targeting
juveniles
– A – low overall predation
on all size classes
• RA = reproductive allotment
• Bd. Interval – time between
broods
• As N approaches K resources are more limiting, this is
intraspecific competition
• Interspecific competition = competition among two species
using the same resources
• Ecological equivalents:
– α12 - Number of individuals of species 2 that are equivalent
to one individual of species 1.
– α21 - Number of individuals of species 1 that are equivalent
to one individual of species 2.
Review density dependent (intraspecific)
• Asymmetric competition - α12 not equal to α21
• symmetric competition - α12 roughly equal to α21
• Use α12 to calculate affect of one species on another.
– K1 =1000
– N1 = 600
– N2 = 300
– α12 = 0.8; 0.8 * 300 = 240
– N1 = 600 + 240 equivalent competitors = 840
dN
 rN
dt
dN
 K  N
 rN 

 K 
dt
• K can be incorporated into our model by simply
modifying the rate of increase (rN) by a measure of how
close N is to K (Equation 4.7).
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Lotka-Volterra Models of Interspecific Competition
 K1  N 1  a12 N 2 
dN 1
 r1 N 1 

dt
K1


• Models change in population size of species 1,
accounting for impact of species 2.
• Similarly, affect of species 1 on species 2:
 K  N 2  a 21 N 1 
dN 2
 r2 N 2  2

dt
K2


• Stable equilibrium or
the winner of
competitive
interactions defined by
K 1 and 2 and α1 and 2
• Some definitive
winners, some
winners depend on
starting points
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