Transcript Predation & Grazing

Predation
Chapter 8
Predation
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Consumption of one organism (prey) by
another (predator), in which the prey is
alive when first attacked by the predator
Functional classification of
predators
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True predators
Grazers
Parasites
Parasitoids
True predators
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Kill prey immediately after attacking it,
attack many
Grazers
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Attack many prey, but “kill” only part of
each individual
Parasites
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Attack only one or few prey, “kill” only
part of it
Parasitoids
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Attack only one prey, cause no immediate
death, but eventually kills prey
Effects of predators on prey
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True predators and parasitoids kill prey
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Grazers and parasites do not kill prey
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Affect both individual prey as well as prey
populations
Compensation for herbivory
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Reduce self-shading
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Remove leaves in excess of optimum LAI
Reduce respiratory “drag” on plant
Compensation for herbivory
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Temporarily mobilize stored carbohydrates
until regrowth returns photosynthesis to
normal
Compensation for herbivory
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Reroute photosynthetic products to
damaged areas to enhance regrowth
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To roots, or shoot, or leaves
Compensation for herbivory
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Increase rate of photosynthesis in
remaining leaf surface area
Compensation for herbivory
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Stimulate dormant buds to grow, or reduce
death rate among surviving parts
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Despite all these possible mechanisms,
compensation is rarely perfect, so plants
are harmed in the long-term
Herbivory can cause death
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Girdling (ring-barking) of young trees by
rabbits, squirrels, and rodents
Herbivory can cause death
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Introduction of disease
into plant by grazer
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Dutch elm disease
Fungus carried by elm
bark beetle
Clogs “circulatory”
system of American elm
trees
Herbivory can cause death
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Grazing on one species may be sufficient to
sway competitive interaction in favor of
another species
Herbivory can cause death
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Large populations of fluid-suckers (e.g.,
aphids) can virtually stop growth and/or
kill a plant
Herbivory can affect survival
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Repeated defoliation often required to kill
mature plant
Large proportion of seedlings killed by
single “attack”
Herbivory can affect growth
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Effects range from none to total cessation
of growth
Depends on:
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Timing of defoliation
Type of plant involved (grasses most tolerant
because of basal meristem)
Herbivory can affect fecundity
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Grazed plants tend to be smaller and bear
fewer seeds
Herbivory can delay flowering (move it
into inhospitable season), reduce, or totally
inhibit flowering
Some eat flowers, fruits, and seeds and
reduce fecundity
Good herbivores
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Some pollen-eaters help pollinate
Some fruit-eaters help distribute seeds
Some seed-eaters store seeds in ground and
forget them
Mutualistic relationships
Defensive responses to grazers
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Grow bigger, sharper spines
Defensive responses to grazers
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Produce more or new defensive chemicals
Defensive responses to grazers
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Reduce palatability
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Tougher
More fiber
Lower nitrogen content
Effect of grazing on whole
population of plants
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Do they only prey on the weak?
Reduction in intraspecific competition
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Can reduce high LAI to more optimal levels
and improve plant productivity
Typically only works in high-density
populations; little or no compensation in lowdensity populations
Effects of grazing on grazer
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Survival, growth, fecundity dependent on food
availability
Rises as availability increases over a certain range
Must be some minimum availability to keep
grazer alive (threshold)
Above certain level, grazers become satiated and
do not respond to increasing levels
threshold
satiation
Grazer
response
Food
availability
Trees and grazer satiation
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All trees of one species within a region
produce large crops of seeds at odd
intervals - mast years
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Tied to climatic variables
Trees and grazer satiation
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Seed predators cannot respond fast enough
reproductively, so many seeds survive and sprout
Only a predator with short generation time could
take advantage of mast years
Importance of food quality
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Large availability of food not helpful if it is
all of poor quality
Cannot eat enough to get the required
nourishment
Predator behavior
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Food selection - monophagous to
polyphagous
Parasites and parasitoids tend to have the
most specialized diets
True predators generally have broad diets
Grazers fit into all groups fairly equally
(specialized to unspecialized)
Diet preferences
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Ranked preferences - foods have similar
composition, but vary in size or
accessibility
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Energy gained per unit handling time
Balanced preferences - consume mixed
diet to meet specific requirements
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Items appear in constant proportion in diet
regardless of their proportional availability
Fixed preferences
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Specific food items preferred at all levels
of food availability
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Preferred when it makes up majority of foods
available
Still preferred when it makes up small
proportion of foods available
Switching preferences
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Some predators may switch preferences at
different levels of availability
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Eat disproportionately more when common
Ignored disproportionately when rare
Switching can occur when….
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Different types of prey occur in different
habitats and predators concentrate on most
profitable ones
Predator becomes more efficient/successful
in dealing with more abundant food
(learning)
Predator develops specific search image
for abundant foods
Diets and natural selection
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Natural selection can act to restrict diets
Prey can exert pressures demanding
specialized morphological or physiological
responses from the predator
Selection favors specialization as long as
prey species remains abundant, accessible,
predictable
Diets and natural selection
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Natural selection can broaden diets
Diets will be broad if individual food items
are inaccessible, unpredictable, or lacking
in certain nutrients
If diet is broad, food is easy to find, search
costs are low, and fluctuations in
abundance of one prey type are unlikely to
cause starvation
Coevolution of predator-prey
relations?
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Improvement in predator ability leads to
improvement in prey’s ability to
avoid/resist predator leads to improvement
in predator ability leads to ….
No real supportive evidence, but
Asplanchna and Brachionus example
Coevolution of predator-prey
relations?
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Asplanchna and Brachionus: rotifers in
lakes
Optimal foraging theory
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Predict foraging strategy under specified
conditions
Predictions based on search time and
handling time
Optimal foraging theory
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Predators with short handling times
relative to search times should be
generalists
Optimal foraging theory
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Predators with long handling times relative
to search times should be specialists
Optimal foraging theory:
“decision rules”
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1) prefer the more profitable prey
2) feed more selectively when profitable
prey are abundant
3) include less profitable prey in the diet
when most profitable prey are relatively
scarce
4) ignore unprofitable items regardless of
their abundance
Problem with optimal foraging in
nature?
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Predator avoidance by the predator
Mutual interference reduces efficiency
Partial refuges for prey in some habitats
Ideal free distribution of predators balance between attractive and repellent
forces
Predator-prey Patches
Cumulative
energy
extracted
Search time to
find patch
Slope = optimum
energy extracted
per time spent
Optimum time to
spend in patch
Time
Predator-prey Patches
Cumulative
energy
extracted
Differing
productivities
Time
Predator-prey Patches
Cumulative
energy
extracted
Differing
search times
Time
Predator-prey Cycles
Predator
numbers
Prey numbers
Predator-prey Cycles
Prey
Predator
Population
size
Time
Time lag = 1/4 cycle
Predator-prey Cycles
Predator-prey Relations
New individuals
added to
population per
time period
Human harvest of
wild populations
-trees
-fish
-ducks
-deer
K
Population size