PREDATION

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PREDATION

• READINGS FREEMAN Chapter 53
• Students who wish to observe their religious
  holidays in lieu of attending class must notify
  Dr. Molumby (molumby_at_uic.edu).

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CONSUMPTION

• The consuming of one living thing by another.
• A basic eating relationship between populations
  of different species.
• Must be evaluated on the basis of its effects on
  populations, not on individuals.
• A (consumer) / - (consumed) interaction.

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MAJOR TYPES OF CONSUMPTION

• Herbivory --- Eating of plants by animals. May
  not result in death of individual plant.
• Parasitoidism --- Larvae of parasitoids consume
  hosts.
• Cannibalism --- The eater and eaten belong to the
  same species (intraspecific predation).
• Parasitism --- Host provides nutrition to one or
  many individual parasites. Host may or may not
  die.
• Predation --- Predator kills prey and consumes
  all or part.

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HERBIVORY

• Occurs when animals eat plants.
• Herbivores are those animals that exclusively or
  primarily eat plant tissue.
• Generally restricted to specific parts of the
  plant (leaves, flowers, fruits, roots, tubers,
  sap) thus, leaving the rest to regenerate.
• Resembles predation when seed (which contains
  plant embryo), seedling or whole plant is
  consumed.

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VERTEBRATE HERBIVORES

• Large ungulates are the most conspicuous native
  herbivores in North America.
• Those that feed primarily on grasses and forbs
  are grazers. Those that feed on tree leaves are
  browsers.

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INVERTEBRATE HERBIVORES

• Half of all insect species are thought to be
  herbivores. Groups such as butterflies, moths,
  weevils, leaf beetles, gall wasps, leaf-mining
  flies and plant bugs are almost exclusively plant
  eaters.
• Snails, slugs, mites and millipedes are largely
  herbivores.

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HERBIVORY

• Is thought to be ecologically important, but its
  impact is still debated. Suggested positive
  impacts include
• Increased production and nutrient uptake.
• Increased quality of leaf litter and soil.
• Increased chances of successful seedling
  establishment.
• Improved conditions for plant growth (pruning
  effect).

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Some Evolutionary Responses of Plants to Herbivory

• 1. Mechanical forms of protection. Microscopic
  crystals in tissues, thorns, hooks, spines.
• 2. Defensive chemicals. Strychnine, morphine,
  nicotine, digitoxin, etc.
• 3. Fruits. Attractive and tasty tissues
  surrounding seeds that promote dispersal.

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PARASITOIDISM

• Insects, usually flies and small wasps, that lay
  their eggs on living hosts. The larvae then feed
  within the body of the host, eventually causing
  death.
• Recent experimental evidence suggests that
  parasitoids locate their hosts by responding to
  airborne chemical signals from plants damaged by
  the host.

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PARASITOIDS

• A tachinid fly lays eggs on a hornworm (moth
  larva). The fly larvae develop by consuming the
  hornworm.
• Many species of ichneumon wasps are parasitoids.

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CANNIBALISM

• An individual consumes another individual of the
  same species.
• A form of intraspecific predation.
• Relatively common among insects when density is
  high. Usually involves adults consuming eggs and
  larvae.
• Demonstrated to be density-dependent factor
  regulating experimental insect populations.

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PARASITISM

• Occurs when a member of one species (parasite)
  consumes tissues or nutrients of another species
  (host).
• Parasites live on or in their hosts often for
  long periods of time.
• Parasites are most often much smaller than their
  hosts.
• It is not necessarily fatal to the host.

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A VERTEBRATE PARASITE

• The sea lamprey was introduced into the Great
  Lakes in 1921 through the Welland Canal.
• Contributed greatly to the decline of whitefish
  and lake trout (shown).
• Chemical control programs started in 1956 have
  reduced lamprey populations.

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INVERTEBRATE PARASITES

• Tapeworm is an intestinal parasite in many
  species of vertebrates, including humans.
• The deer tick (small one) and wood tick are
  common external parasites on mammals.

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VIRAL PARASITES

• The common influenza virus (top) has inhabited
  every host in this room! It has caused more
  deaths than any other pathogen.
• The bird flu virus (bottom) is a potential threat
  to humans.

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Freeman Figure 52.9 Part 1
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Freeman Figure 52.9 Part 2
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PREDATION

• The most conspicuous interaction is when an
  individual of one species (predator) eats all or
  most of an individual of another species (prey).
• The most thoroughly studied consumptive
  relationship between species.
• Of high ecological and evolutionary significance.
• An everyday occurrence in nature.

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Possible Outcomes of Predation

• 1. Predator population has little effect on
  abundance of prey population.
• 2. Predator population eradicates prey
  population this may contribute to extinction of
  predator population due to lack of food.
• 3. Predator and prey populations coexist in
  dynamic equilibrium.

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A Dynamic Equilibrium Model of Predator/Prey
Populations

• 1. Assume an exponential growth model for a prey
  population living in the absence of predators.
• 2. Assume an exponential decline model for a
  predator population living in the absence of
  prey.
• 3. Assume density of predators is a function of
  density of prey and vice versa.

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Prey Population Living Alone

• Assume a constant rate of increase in absence of
  predators.
• dN/dt r1 N
• where N number of prey
• t time
• r1 reproductive capacity of prey
  (births exceed deaths)

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Predator Population Living Alone

• Assume a constant rate of decline in absence of
  predators.
• dP/dt - r2 P
• where P number of predators
• t time
• - r2 reproductive capacity of
  predators (deaths exceed births)

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Predator and Prey Populations Living Together

• Assume a constant rate of increase in prey
  population is slowed by an amount depending on
  the number of predators. dN/dt (r1 - K1) N
  where K1 a constant related to the effect of
  predation on prey.
• Assume a constant rate of decrease in predator
  population is slowed by an amount depending on
  the number of prey. dP/dt ( -r2 K2) P
  where K2 a constant related to the effect of
  predation on predators.

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A Model Predator/Prey Cycle
This graph shows a limit cycle of predators and
prey.
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Description of Dynamic Equilibrium

• When predator numbers are low, prey numbers
  increase rapidly.
• As prey numbers increase, predators begin to
  increase.
• When predators numbers are high, prey numbers
  decrease rapidly.
• As prey numbers decrease, predator numbers fall.

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The Hare Lynx Predator/Prey Relationship

• Snowshoe hare and Canadian lynx show classic
  population cycles with a 10-11 year periodicity.
• Hare are herbivores and feed on twigs under the
  snow in winter lynx feed primarily on snowshoe
  hare.

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The Hare/Lynx CycleBased on Pelt Sales
Similar data is provided in Figure 53.10
(Freeman, 2005).
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Are Hare/Lynx Populations Dynamically Linked?

• Evidence For Lynxes usually have large
  populations at the same time or just after hares
  do. Prey abundance often has a dramatic effect on
  predator abundance. Snowshoe hare abundance has a
  strong influence on lynx abundance.
• Evidence Against Snowshoe hare populations show
  cycles on islands where lynxes are absent. Do
  lynx populations have a strong influence on hare
  populations?

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What is the impact of food and predation on the
snowshoe hare density?

• Hypothesis Food or predator or both will
  influence hare density, thus contributing to the
  snowshoe hare cycle?
• Predictions 1. Food addition (rabbit chow) will
  increase hare density. 2. Predator exclusion
  (enclosure by electric fence that excludes
  lynxes) will increase hare density. 3. Food
  addition and predator exclusion will interact to
  increase hare density. 4. Fertilizer (NPK plant
  nutrients) addition will stimulate plant growth
  that will act as hare food and thus increase hare
  density.

Reported in SCIENCE 8-25-95
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What is the impact of food and predation on the
snowshoe hare density?

• Method 1 kilometer 2 areas of boreal
  (coniferous) were managed for 8 years by
• 1. Food addition (rabbit chow)
• 2. Predator exclusion (mammals
  only, not birds)
• 3. Food addition and predator
  exclusion
• 4. Fertilizer (NPK plant
  nutrients) addition
• 5. Control areas (nothing was
  done in these areas)
• The 5 different management areas were
  selected as random from a larger area that had a
  relatively uniform community structure.
• Snowshoe hare density was monitored at
  various periods throughout the 8 year study.

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What is the impact of food and predation on the
snowshoe hare density?

• Result Relative to control areas
• 1. Food addition tripled (3x)
  hare density.
• 2. Predator exclusion doubled
  (2X) hare density.
• 3. Food addition and predator
  exclusion increased hare
• density eleven-fold (11X).
• 4. Fertilizer addition had hare
  density equivalent to
• control areas (no effect).
• Conclusion The snowshoe hare population cycles
  results from
• FOOD - HARE - LYNX INTERACTION
• Also see Figure 53.11 in Freeman (2005). He
  reports the results of the study at the end of 11
  years.

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What Drives the 10-year Cycle of Snowshoe Hares?

• Food Hypothesis
• Test 1. Twig consumption increases as hare
  density increases, but 60-80 of available food
  is not consumed.
• Test 2. Unlimited added rabbit chow does not
  stop cycle.
• Test 3. Added natural food does not stop hare
  decline.

Bioscience 1/01 HYPOTHESIS REJECTED
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What Drives the 10-year Cycle of Snowshoe Hares?

• Predator Hypothesis
• Test 1. 95 of radio-collared hare deaths were
  due to predation.
• Test 2. There were few deaths of radio-collared
  hare where predators were excluded.
• Test 3. Predator exclusion nearly eliminated the
  decline phase of the snowshoe hare cycle.

Bioscience 1/01 HYPOTHESIS ACCEPTED
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Moose and Wolf of Isle Royale

• The worlds longest running predator/prey
  research project. The 47th year of wolf and moose
  monitoring was completed in the winter of 2006.
• Winter provides the best opportunities for aerial
  surveying of the wolf and moose populations, with
  leaves off the trees and snow on the ground.

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Moose Population(Early History)

• Prior to 1900 there were no moose on the island.
• Sometime between then and 1905 a moose population
  was established.
• By 1929, the population was estimated to be
  around 2,000.
• During the early 1930s the moose destroyed their
  own food supply and numbers declined.
• A fire in 1936 burned browse over a quarter of
  the island, and by 1937 the moose population was
  around 400. Many predicted extinction of the
  population.
• The fire stimulated sapling production (browse),
  so by 1948 the population increased to around 800.

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Moose Population (Recent History)

• First scientific surveys of the moose population
  began in 1959.
• Since that time the population has fluctuated
  from a low of around 500 to a high of around 2500.

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Wolf Population

• The first wolf tracks on Isle Royale were
  observed in 1949.
• Annual monitoring began in 1959.
• Numbers have been as low as 12 and as high as 50.

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Moose and Wolf Populations of Isle Royale

• Significant fluctuations have been observed in
  both the moose and wolf populations since 1959.
• The significant increase in wolf population
  during the 1970s corresponds to the decline in
  moose. Wolves prey on very young, very old, sick
  or injured moose.
• Evidence that wolves impact the moose population
  is lacking.

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Predators as Agents of Biocontrol

• Predators have been used in attempts to control a
  variety of plant and animal pests. Often called
  biocontrol.
• Ladybird beetles and ant lions (lacewing larvae)
  have been used.

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Parasites as Agents of Biocontrol

• European rabbits were introduced into Australia
  in 1859 and became a major pest.
• In late 1950, the myxoma virus, spread by
  mosquitoes, began killing rabbits in large
  numbers. By 1953, rabbit immunity was detected.
  Today, the virus may kill only 50 of the rabbit
  population during an epidemic.
• Another virus (calicivirus), native to China, was
  found and testing as a potential biocontrol agent
  began in 1995 and continues to the present.

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PREDATION

• READINGS FREEMAN Chapter 53
• Students who wish to observe their religious
  holidays in lieu of attending class must notify
  Dr. Molumby (molumby_at_uic.edu).