Chapter 17: Predation and Herbivory (and a bit of Chapter 20)

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Chapter 17 Predation and Herbivory (and a bit
of Chapter 20)

• Robert E. Ricklefs
• The Economy of Nature, Fifth Edition

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• The rabbit/myxoma story

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Interacting populations evolve in response to
each other
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Evolution of Resistance in Rabbits

• Decline in lethality of the myxoma virus in
  Australia resulted from evolutionary responses in
  both the rabbit and the virus populations
• genetic factors conferring resistance to the
  disease existed in the rabbit population prior to
  introduction of the myxoma virus
• the myxoma epidemic exerted strong selective
  pressure for resistance
• eventually most of the surviving rabbit
  population consisted of resistant animals

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Evolution of Hypovirulence in Myxoma Virus

• Decline in lethality of the myxoma virus in
  Australia resulted from evolutionary responses in
  both the rabbit and the virus populations
• less virulent strains of virus became more
  prevalent following initial introduction of the
  virus to Australia
• virus strains that didnt kill their hosts were
  more readily dispersed to new hosts (mosquitoes
  bite only living rabbits)

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The Rabbit-Myxoma System Today

• Left alone, the rabbit-myxoma system in Australia
  would probably evolve to an equilibrial state of
  benign, endemic disease, as in South America
• pest management specialists continue to introduce
  new, virulent strains to control the rabbit
  population
• Contagious diseases spread through the atmosphere
  or water are less likely to evolve hypovirulence,
  as they are not dependent on their hosts for
  dispersal.

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RABBITS AND MYXOMA

• is an example of a predator (the virus) and
  prey (the rabbits).

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Prickly Pear cactus were also introduced into
Australia.

• Like rabbits, they quickly spread over the
  continent.
• A predator of the cactus was introduced.
• The cactus moth.
• The cactus only survived in areas where the moth
  was absent.

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Comparing cactus before (a) and after (b) the
moth introduction.
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The cactus is an example of predator prey
interactions.

• Do predators limit prey population growth?
• Do prey limit predator population growth?
• The balance between the two depends on their
  adaptations.
• Some adaptations were already found in species.
• Some adaptations are a result of predator/prey
  interactions.

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All life forms are both consumers and victims of
consumers.

• There are many consumer-resource interactions
• Predator-prey
• Herbivore-plant
• Parasite-host
• Producers
• Consumers
• Predator Parasite Parasitoid Herbivore
  Detritivore

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Some Definitions

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• Predators catch individuals and consume them,
  removing them from the prey population.
• Parasites consume parts of a living prey
  organism, or host
• parasites may be external or internal
• a parasite may negatively affect the host but
  does not directly remove it from the population

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More Definitions

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• Parasitoids consume the living tissues of their
  hosts, eventually killing them
• parasitoids combine traits of parasites and
  predators
• Herbivores eat whole plants or parts of plants
• may act as predators (eating whole plants) or as
  parasites (eating parts of plants)
• grazers eat grasses and herbaceous vegetation
• browsers eat woody vegetation

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Detritivores occupy a special niche.

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• Detritivores consume dead organic material, the
  wastes of other species
• have no direct affect on populations that produce
  these resources
• do not affect the abundance of their food
  supplies
• do not influence the evolution of their resources
• are important in the recycling of nutrients
  within ecosystems

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An example of a parasitoid wasp.

• This was is laying its egg in the caterpillar.
• The egg will develop into larvae.
• The larvae will consume the caterpillar as it
  grows.
• A combination of predation, and parasitism.

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Predators have adaptations for exploiting their
prey.

• This lion has adaptations to capture fast prey.
• This whale is a filter feeder.
• Spiders make webs to subdue prey.

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Even predator adaptations take practice!
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Predators and prey are different sizes, and this
can pose problems.

• If a prey item is too small it may be too hard
  to handle.
• Imagine trying to capture mice with your hands.
• If a prey item is too large the predator may
  not be able to subdue.
• Imagine trying to tackle a elephant to eat.
• Blue whales weigh many tons, but eat tiny shrimp
  (use of filters).

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Form and Function Match Diet

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• Form and function of predators are closely tied
  to diet
• vertebrate teeth are adapted to dietary items
• horses have upper and lower incisors used for
  cutting fibrous stems of grasses, flat-surfaced
  molars for grinding
• deer lack upper incisors, simply grasping and
  tearing vegetation, but also grinding it
• carnivores have well-developed canines and
  knifelike premolars to secure and cut prey

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A predators form and function are closely tied
to its diet. (a) upper incisors are used to cut
plant material (b) flat-surfaced molars for
grinding plant material (c) knifelike premolars
secure prey and tear flesh

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More Predator Adaptations

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• The variety of predator adaptations is
  remarkable
• consider grasping and tearing functions
• forelegs for many vertebrates
• feet and hooked bills in birds
• distensible jaws in snakes
• digestive systems also reflect diet
• plant eaters feature elongated digestive tracts
  with fermentation chambers to digest long,
  fibrous molecules comprising plant structural
  elements

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Distensible jaws shift the articulation of the
jaw with the skull from the quadrate bone to the
supratemporal

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Digestive tracts of consumers are adapted to
their diets. Digestive organs of herbivores gt
carnivores

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Large carnivores tend to pursue large prey.Size
of prey consumed is related to size of predator.
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What about the prey?

• How much energy do you have available for growth?
• If you are predated upon, your growth rates are
  affected.

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Prey have adaptations to avoid being consumed.

• Hiding
• If a predator cant see you, it cant eat you.
• Evolution of cryptic coloration.
• Escaping
• If you can outrun your predator, it cant eat
  you.
• Evolution of speed or maneuverability.
• Active defense mechanisms
• Animals with poison glands.
• Plants with thorns, toxic substances.

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Crypsis and Warning Coloration

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• Through crypsis, animals blend with their
  backgrounds such animals
• are typically palatable or edible
• match color, texture of bark, twigs, or leaves
• are not concealed, but mistaken for inedible
  objects by would-be predators
• Behaviors of cryptic organisms must correspond to
  their appearances.

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Cryptic appearances (a) mantid (b) stick insect
(c) lantern fly

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Chemical defenses.

• The production of chemicals which repel potential
  predators.
• Notice the colors of this insect.

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Warning Coloration aposematism

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• Why should a prey item evolve bright colors?
• It definitely brings attention to you.
• Black and yellow are the most common colors.
• Unpalatable animals may acquire noxious chemicals
  from food or manufacture these chemicals
  themselves
• such animals often warn potential predators with
  warning coloration or
• certain aposematic colorations occur so widely
  that predators may have evolved innate aversions
• If an animal eats a brightly colored prey item
• It may get sick.
• It may die.
• If it lives, it will remember.

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Unpalatable organisms

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Why arent all prey unpalatable?

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• Chemical defenses are expensive, requiring large
  investments of energy and nutrients.
• Some noxious animals rely on host plants for
  their noxious defensive chemicals
• not all food plants contain such chemicals
• animals utilizing such chemicals must have their
  own means to avoid toxic effects

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Batesian Mimicry

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• Certain palatable species mimic unpalatable
  species (models), benefiting from learning
  experiences of predators with the models.
• This relationship has been named Batesian mimicry
  in honor of discoverer Henry Bates.
• Experimental studies have demonstrated benefits
  to the mimic
• predators quickly learn to recognize color
  patterns of unpalatable prey
• mimics are avoided by such predators

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Harmless mantid (b) and moth (c) evolved to
resemble a wasp (a)

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Müllerian Mimicry

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• Müllerian mimicry occurs among unpalatable
  species that come to resemble one another
• many species may be involved
• each species is both model and mimic
• process is efficient because learning by predator
  with any model benefits all other members of the
  mimicry complex
• certain aposematic colors/patterns may be
  widespread within a particular region

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Costa Rican butterflies and moths

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Parasites!

• Parasites have adaptations to allow them to live
  in the host.
• The host has adaptations to fight off parasites.
• The parasite does not want to kill the host, but
  disperse its offspring to another host.

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Parasites have adaptations to ensure their
dispersal.

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• Parasites are usually much smaller than their
  hosts and may live either externally or
  internally
• internal parasites exist in a benign environment
• both food and stable conditions are provided by
  host
• parasites must deal with a number of challenges
• host organisms have mechanisms to detect and
  destroy parasites
• parasites must disperse through hostile
  environments, often via complicated life cycles
  with multiple hosts, as seen in Plasmodium, the
  parasite that causes malaria

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Parasite-Host Systems A Balancing Act

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• The parasite-host interaction represents a
  balance between parasite virulence and host
  defenses
• immune system of host can recognize and disable
  parasites
• but parasites may multiply rapidly before an
  immune response can be deployed

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Parasites may defeat a hosts immune response.

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• Circumventing the hosts immune system is a
  common parasite strategy
• some parasites suppress the hosts immune system
  (AIDS virus)
• other parasites coat themselves with proteins
  that mimic the hosts own proteins (Schistosoma)
• some parasites continually coat their surfaces
  with novel proteins (trypanosomes)

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Cross-Resistance

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• Some parasites elicit an immune response from the
  host, then coat themselves with host proteins
  before the immune response is fully mobilized
• initial immune response by host may benefit the
  host later when challenged by related parasites
  in a phenomenon known as cross-resistance
• Once an immune response has been elicited,
  antibodies can persist for a long time,
  preventing reinfection.

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Many parasites have complex life cycles.Malaria
(Plasmodium) parasitic life cycle.
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Plants have antiherbivore defenses.

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• Plant-herbivore warfare is waged primarily
  through biochemical means.
• Full spectrum of plant defenses includes
• low nutritional content of plant tissues
• toxic compounds synthesized by the plants
• structural defenses
• spines and hairs
• tough seed coats
• sticky gums and resins

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Plant adaptations against predation.

• Nutritional value?
• It could be as simple as a spine.
• Ouchy bush!
• It could be as complicated as chemicals.
• Tannins.
• Secondary compounds.

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Spines protect the stems and leaves (a) cholla
cactus and (b) prickly pear cactus

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Digestibility

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• Animals typically select plant food according to
  its nutrient content
• especially important to young animals, which have
  high demands for protein
• Some plants deploy compounds that limit the
  digestibility of their tissues
• tannins produced by oaks and other plants
  interfere with the digestion of proteins
• some animals can overcome the effect of tannins
  through production of digestive dispersal agents

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Secondary Compounds

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• Secondary compounds are produced by plants for
  purposes (typically defensive) other than
  metabolism.
• Such compounds can be divided into three major
  classes
• nitrogen compounds (lignin, alkaloids, nonprotein
  amino acids, cyanogenic glycosides)
• terpenoids (essential oils, latex, plant resins)
• phenolics (simple phenols)

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Induced and Constitutive Defenses

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• Constitutive chemical defenses are maintained at
  high levels in the plant at all times.
• Induced chemical defenses increase dramatically
  following an attack
• suggests that some chemicals are too expensive to
  maintain under light grazing pressure
• plant responses to herbivory can reduce
  subsequent herbivory

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Herbivores control some plant populations.

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• Examples of control of introduced plant pests by
  herbivores provides evidence that herbivory can
  limit plant populations
• prickly pear cactus in Australia
• controlled by introduction of a moth,
  Cactoblastis
• Klamath weed in California
• controlled by introduction of a beetle, Chrysolina

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Effects of Grazers and Browsers on Vegetation

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• Herbivores consume 30-60 of aboveground
  vegetation in grasslands
• demonstrated by use of exclosures limiting access
  to vegetation by herbivores
• Occasional outbreaks of tent caterpillars, gypsy
  moths, and other insects can result in complete
  defoliation of forest trees.

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Imagine a plant being eaten, which stimulates
plant or chemical production.
Mite growth is inhibited if the plant was
previously eaten.
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Look at the impact of herbivores.
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Outbreaks of herbivorous insects can defoliate
forests.