Predation

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Predation

• Chapter 10

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Wolves in Yellowstone Park

• U.S. Fish and Wildlife Service, 1980s.
• Reintroduce in Yellowstone Park and stabilize
  wolf populations in Minnesota and Montana.

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Wolves in Yellowstone Park

• Concerns
• Cattle ranchers concerned Decimate herd?
• Are predators tied to the health of the main
  prey?
• Can predators switch prey?
• Ramifications to reestablishment.
• Results No major effects on cattle.

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Predation

• Traditional view carnivory predator feeding on
  another animal.
• Differences from herbivory
• Herbivory is often non-lethal.
• Differences from parasitism
• In parasitism, one individual is utilized for the
  development of more than one parasite.
• Omnivores feed on both plants and animals.

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Antipredator Adaptations

• The variety of strategies that organisms have
  evolved to avoid being eaten suggests that
  predation may be a strong selective force.

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Antipredator Adaptations

• Aposematic or warning coloration
• Advertises an unpalatable taste.
• Ex. Blue jays and monarch butterflies
• Caterpillar obtains poison from milkweed.
• Blue jays suffer violent vomiting from ingesting
  caterpillar.
• Ex. Tropical frogs
• Toxic skin poisons.

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Antipredator Adaptations

• Camouflage
• Blending of organism into background color.
• Grasshoppers.
• Stick insects mimic twigs and branches.
• Zebra stripes blend into grassy background.

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Antipredator Adaptations

• Mimicry
• Animals that mimic other animals.

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Antipredator Adaptations

• Müllerian mimicry
• Unpalatable species converge to look the same.
• Reinforce basic distasteful design.
• Ex. Wasps often have black yellow pattern.
• Mimicry ring a group of sympatric species, often
  different taxa, share a common warning pattern.

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Antipredator Adaptations

• Batesian mimicry
• Mimicry of unpalatable species by palatable
  species.
• Ex. hoverflies resemble stinging bees and wasps.

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Antipredator Adaptations

• Difficulty distinguishing type of mimicry
• Monarch butterflies and viceroy butterflies.
• Only recently(1991) was it shown that viceroy
  butterflies are also unpalatable, providing an
  example of Müllerian mimicry.

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Antipredator Adaptations

• Displays of intimidation
• Ex. Toads swallow air to make themselves appear
  larger.
• Ex. Frilled lizards extend their collars to
  produce the same effect.

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Antipredator Adaptations

• Polymorphism
• Two or more discrete forms in the same
  population.
• Color polymorphism
• Predator has a preference (usually the more
  abundant form).
• Prey can proliferate in the rarer form.
• Ex. leafhopper nymphs (orange and black).
• Ex. Pea aphids (red and green).

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Antipredator Adaptations

• Reflexive selection
• Every individual is slightly different.
• Examples brittle stars, butterflies, moths,
  echinoderms, and gastropods.
• Thwart predators learning processes.

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Antipredator Adaptations

• Prey phenologically separated from predator
• Ex. Fruit bats
• Either diurnal or nocturnal.
• Only nocturnal in the presence of predatory
  diurnal eagles.

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Antipredator Adaptations

• Chemical defense
• Used to ward off predators.
• Ex. bombardier beetles possess a reservoir of
  hydroquinone and hydrogen peroxide.
• When threatened, eject chemicals into explosion
  chamber.
• Mix with peroxidase enzyme.
• Mixture is violently sprayed at attacker.

Eisner, T. and D. J. Aneshansley. 1999. Spray
aiming in the bombardier beetle Photographic
evidence. Proc Natl Acad Sci U S A. August
17 96 (17) 97059709.
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Antipredator Adaptations

• Masting
• Synchronous production of many progeny by all
  individuals in population.
• Satiate predators.
• Allows for some progeny to survive.
• Common to seed herbivory.
• Ex. 17-year and 13-year periodical cicadas.

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Antipredator Adaptations

• Chemical defense is most common.

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Antipredator Adaptations

• Predators still manage to survive.
• The coevolution of defense and attack can be seen
  as an ongoing evolutionary arms race.
• Prey likely to be one step ahead.
• Life-dinner principle.

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Predator-Prey Models

• What effects do predators have on prey?
• Depends on such things as prey and predator
  densities, and predator efficiency.

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Predator-Prey Models

• Graphical method to monitor relationship
• Prey isoclines have characteristic hump shape.
• In the absence of predators, prey density would
  be equal to the carrying capacity, K1.
• Lower limit, individuals become too rare to meet
  for reproduction.
• Between these two values, prey population can
  either increase or decrease depending on predator
  density.
• Above the isocline, prey populations decline.
• Below the isocline, prey populations increase.

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Predator-Prey Models

• Predator isoclines
• Threshold density, where predator population will
  increase.
• Predator population can increase to carrying
  capacity.
• If there is mutual interference or competition
  between predators
• More prey required for a given density of
  predators.
• Predator isoclines slopes toward the right.

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Predator-Prey Models

• Superimpose prey and predator isoclines
• One stable point emerges the intersection of the
  lines.
• Three general cases
• Inefficient predators require high densities of
  prey.
• A moderately efficient predator leads to stable
  oscillations of predator and prey populations.
• A highly efficient predator can exploit a prey
  nearly down to its limiting rareness.

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Predator-Prey Models

• All based on how efficient predator is.
• Shift in isoclines
• Prey starvation (shift to left).
• Food enrichment (shift to right).
• Carrying capacity changes.
• Predator isocline changes paradox of
  enrichment Increases in nutrients or food
  destabilizes the system.

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Predator-Prey Models

• While some models seem to be just an abstraction
  of nature, we need every predictive tool possible
  to help us decide what harvest is sustainable for
  many economically valuable species.

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Predator-Prey Models

• Functional response
• How an individual predator responds to prey
  density can affect how predators interact with
  prey.
• Three types
• Type I Individuals consume more prey as prey
  density increases.
• Type II Predators can become satiated and stop
  feeding, or limited by handling time.
• Type III Feeding rate is similar to logistic
  curve low at low prey densities, but increases
  quickly at high densities.

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Predator-Prey Models

• Changes in prey consumption
• Functional response changes.
• Dictates how individual predators respond to prey
  population.
• Numerical response changes
• Governs how a predator population migrates into
  and out of areas in response to prey densities.

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Field Studies of Predator-Prey Interactions

• Field comparisons to models
• Do predators control prey populations?

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Field Studies of Predator-Prey Interactions

• Importance of predators in controlling prey
  density.
• Kaibab deer herd (Northern Arizona)
• Kaibab Plateau declared a national park around
  1900.
• All big predators were removed and deer hunting
  was prohibited.
• Estimates of 10 fold increase in deer population.
• Reevaluated by Graham Caughley (1970)
• Predator control had some impact cessation of
  hunting and removal of competing sheep and cattle
  also had an impact.

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Field Studies of Predator-Prey Interactions

• Serengeti plains of eastern Africa
• Large predators have little effect on large
  mammal prey.
• Most prey taken are either injured or very old.
• Contribute little to future generations.
• Prey are migratory while predators are residents
  and are more likely to be limited by prey
  available during the dry season.

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Field Studies of Predator-Prey Interactions

• Moose population on Michigan's Isle Royale
• Wolf-free existence until 1949.
• Durwood Allen (1958) began to track wolf and
  moose populations.

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Field Studies of Predator-Prey Interactions

• Trends in populations
• Wolf population
• Peaked at 50 in 1980.
• Severe nosedive in 1981.
• Small recovery in the late 1990s.

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Field Studies of Predator-Prey Interactions

• Moose population
• Increased steadily in the 1960s and 1970s.
• Declined as the wolf population increased until
  1981.
• A record population of 2500 was reached in 1995,
  when the wolf population was low.
• Good evidence of prey population control by
  predators.
• Confounded in 1996 when the moose population
  crashed starvation.
• Wolves influence moose density, but ultimately,
  moose population levels are set by available food.

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• Canada lynx and snowshoe hare
• Populations show dramatic cyclic oscillations
  every 9 to 11 years.
• Cycle has existed as long as records have existed
  (over 200 years).
• An example of intrinsically stable predator-prey
  relationship.

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Field Studies of Predator-Prey Interactions

• The lynx cycle is dependent on the abundance of
  hares.
• The hare cycle is dependent on the abundance of
  food.
• Overgrazed grasses produce toxins for 2-3 years.

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Introduced Predators

• Dingo predations on kangaroos in Australia
• Dingo is an introduced species and the largest
  Australian carnivore.
• Predator of imported sheep.
• Eliminated from certain areas
• Spectacular increases in native species
• 160 fold increase in red kangaroos.
• Over 20 fold increase in emus.

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Introduced Predators

• Effects on feral pigs
• Shortage of young pigs.
• Considerable impact on recruitment of pigs.

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Introduced Predators

• European foxes and feral cats in Australia
• Hunt chickens and introduced rabbits.
• Effects when removed

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Introduced Predators

• Lamprey and the Great Lakes
• Construction of Welland Canal allowed lamprey to
  enter the Great Lakes.
• Dramatic reduction in lake trout.
• Trout recovered after lamprey population was
  reduced.

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Humans As Predators - Whaling

• Exploitation necessary?
• Is harvesting at any level sustainable?
• History of Antarctic whaling
• 1930s, blue whales primarily harvested.
• 1950s, blue whale population depleted, replaced
  with fin whale.
• 1960s, fin whale population collapsed.
• 1960s, humpback whale population collapsed.

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Humans As Predators - Whaling

• Prior to 1958, Sei whales hardly ever harvested.
• Reduction in other whales made Sei whale
  attractive.
• Peak harvest of about 20,000 by 1964-65.
• Catches declined thereafter due to limitations.
• The relatively small minke whale was ignored in
  the southern oceans until 1971-72.
• Began to be taken, and is now the largest
  component of the southern baleen whale catch.

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Humans As Predators - Whaling

• Whale ban proposed in 1985-86, took effect in
  1988.
• Iceland, Norway, and Japan, 1994
• Argued for resumption of limited commercial
  whaling.
• Should we ban commercial whaling?
• Whale populations are recovering.
• Ex. Blue whale populations have increased four
  fold.
• Ex. California grey whales have recovered to
  pre-whaling levels?

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Humans As Predators - Whaling

• A new study that appeared in Science by Joe Roman
  Steve Palumbi (2003) used molecular data to
  estimate pre-whaling population sizes for
  humpback, fin, and minke whales.

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Humans As Predators - Whaling

• The large amount of genetic diversity found
  indicates that pre-whaling population sizes were
  MUCH larger than what had been assumed in the
  past.
• Good plan to hold off on whaling!

From Roman Palumbi. 2003. Whales before
whaling in the North Atlantic. Science. 301
508-510.
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Field Experiments with Natural Systems

• Lions in South Africa
• Kruger National Park, 1903.
• Lions Shot Number of large prey increased.
• Shooting of lions ended in 1960.
• Wildebeast increased so much that their numbers
  had to be culled from 1965 to 1972.

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Field Experiments with Natural Systems

• Gray partridge, European game bird.
• Over 20 million shot annually in Great Britain in
  the 1930s.

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Field Experiments with Natural Systems

• Only 3.8 million shot in the mid-1980s.
• High chick mortality due to starvation.
• Reduced insects due to introduction of herbicides
  in the 1950s was suspected.
• Trials confirmed this was the case.

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Field Experiments with Natural Systems

• Also, smaller populations in areas where there
  was no control of predators by gamekeepers.
• Predation control increased
• The number of partridges that bred successfully.
• The average size of the broods.
• Partridge populations by 75.

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Field Experiments with Natural Systems

• Predators and rodents in Finland
• Large scale removal of predators, April 1992 and
  1995 over 2-3 km2.
• Large increase in rodent population by June
  (compared to control plots).

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Summary

• Predation is a strong selective force in nature.
• Many antipredator mechanisms have evolved
• Aposematic coloration
• Camouflage
• Batesian and Müllerian mimicry
• Intimidation displays
• Polymorphisms
• Chemical defenses

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Summary

• Modeling predator-prey interactions
• Even simple predator-prey models show
• Stable cycles.
• Wildly increasing and unstable oscillations.
• Difficulty in predicting or modeling how
  predators and prey interact.
• Mutual interference between predators.
• Existence of specific predator territory sizes.
• Ability of predators to feed on more than one
  type of prey.

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Summary

• Some large-scale field observations support
• Predators only take weak and sickly individuals.
• Prey populations influence predator numbers, not
  vice versa.

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Summary

• Accidental or deliberate introductions of exotic
  predators
• Profound effects on native prey populations.
• Predators have important regulatory effects on
  prey.
• May not be indicative of natural systems.

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Summary

• Evidence from natural systems
• Most studies have concluded that predators have a
  significant effect on prey.