Class 14 Populations 6 Herbivory, Mutualism Reading Assignment Krebs Chapter 14

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Class 14Populations 6Herbivory, Mutualism
Reading Assignment Krebs Chapter 14
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Herbivory Outline

• Adaptations of plants that reduce herbivory and
  increase fitness
• Co-evolution
• Mutualism
• Population dynamics of herbivore/plant systems

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Why so much vegetation?Why do herbivores not
consume more?

• Self-regulation in herbivore populations
• Already discussed
• Predation holds herbivore populations in check
• Already discussed
• Defense mechanisms by plants
• Plants cannot move to escape
• gt50 macroscopic spp are plants
• Conditions met for selection and adaptation

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1. Plant Defense Mechanisms

• Plant quality low nutrient, high lignin
  discourages herbivory
• Structural attributes thorns, hard seed
  coverings
• Chemical content secondary plant substances

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Why are Holly leaves spiny?
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Holly leaf and thin sections of edge
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Biochemical origins of secondary plant
substances
Peppermint oil and catnip
Juglone
cinnamon and clove
Nicotine, Caffeine, morphine
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Cyanogenesis production of enzyme and substrate
in bracken fern
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Evolutionary significance of secondary plant
substances

• Hypothesis 1 avoidance of autointoxication
• Plants produce toxins as metabolic by-products
  need to excrete them increases fitness
• (Now rejected-SPS turnover fast, do
  not
• accumulate passively like an
  excretion product)
• Hypothesis 2 defense against herbivores
• Production of SPS increases fitness primarily
  through reducing herbivory
• Produced as defenses against herbivores, involves
  cost to plant

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Fitness costs of self-defense mechanisms-
implications of argument

• Plants may evolve more defenses if exposed to
  much damage, but fewer defenses if costs are high
• Plants may be expected to allocate more defense
  cost to valuable tissues at risk
• Defenses may be reduced when enemies are absent,
  increased when plants attacked
• Because of costs, defense mechanisms might be
  reduced if plants are under severe environmental
  stress

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Resource Availability Hypothesis

• Takes into account
• resources available to plant in plant defenses
• ability to replace tissues taken by herbivores
• Depends on plant growth rate
• Fast growth rate, the less selection for costly
  plant defenses
• Slower growth rate, the greater the selection for
  costly defenses

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Inducible Defenses

• Defenses that are produced only when herbivory is
  high
• Avoids the cost of continuously producing a
  defense
• Is an inducible response a defense?
• Do changes influence rates of herbivory?
• Do changes result in less damage or do plants
  have greater fitness?

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Examples of inducible defenses

• Increase of toxic chemicals in cotton plants in
  response to mite invasions
• Spine production in marine animals
• Spine production in plants ?

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Effects of pre-exposure of cotton to mites on
number and species of mites. Increase of toxic
chemicals in cotton plants in response to
previous exposure.
Control Pre-exposed to same mite
Control Pre-exposed to different mite species
Cotton release chemicals when exposed to mites
that are toxic to mites, but effect varies with
mite species.
T total E eggs I immature M male F
female
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Membranipora membranacea (marine Bryozoan
colony)

• When attacked by nudibranch predator, within 36
  hrs, this marine Bryozoan increases the number of
  spines
• Nearby bryozoans also responded
• Those with induced spines grew at only 85 rate
  of non-induced colonies.

20mm
100mm (x 200)
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Spine production in Opuntia stricta cactus. A
rare case of correlation of plant spine
production and herbivore presence
On Australian islands with grazing, those plants
that are surviving have spines. On islands
without grazing, the production of spines has a
metabolic cost to plant.
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Herbivory Outline

• Adaptations of plants that reduce herbivory and
  increase fitness
• Co-evolution
• Mutualism
• Population dynamics of herbivore/plant systems

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2. Co-evolution of plant/herbivore characteristics

• Animals respond to plant defense through
  evolution of characteristics that overcome
  mechanisms
• Some plants and animals have co-evolved to the
  point where their relationship is mutualistic
• Interaction between two species in which both
  species benefit from the association, and cannot
  live separately

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Wild tobacco defense in Great Basin desert of
southwestern Utah.
Defensive Function of Herbivore-Induced Plant
Volatile Emissions in Nature. A. Kessler I. T.
Baldwin. Science 2001 291 2141-2144.
(2) This attack induces the tobacco plants to
emit volatile organic compounds that attract
natural enemies of the caterpillars such as
big-eyed bugs (Geocoris pallens).

• Wild tobacco
• plants are attacked by
• caterpillars of the
• moth Manduca
• quinquemaculata.

(3) The airborne volatiles also stop adult M.
quinquemaculata moths from laying their eggs
on tobacco plant leaves.
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Herbivory Outline

• Adaptations of plants that reduce herbivory and
  increase fitness
• Co-evolution
• Mutualism
• Population dynamics of herbivore/plant systems

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3. Mutualism

• Examples
• Ant/Acacia populations
• Root mycorrhizae
• Seed dispersal
• Ant/scale relationships
• Grazing in the Serengeti Plain

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Ants and Acacia
Swollen thorns containing immature ants
Surrounding plants removed by ants
15,000 worker ants on 4m tall tree
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Ants nests within the thorns of Acacia
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Root mycorrhizae

• In many plant species, especially forest trees,
  myccorhizal fungi grow around roots
• These mycchorizae get sugar from the plant, but
  extract nitrogen from the soil and exudes
  nitrogen and in some cases phosphorus that can be
  taken up by the plants
• Gives plants higher degree of fitness because of
  provision of N and P which allows greater root
  and shoot growth

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Seed Dispersal

• Seed dispersal by birds and animals can be a form
  of mutualism
• Many plants attract bird species to brightly
  colored, edible fruits
• Herbivores may thus provide greater dispersal
  capabilities to the plants
• trade off with grazing loss

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Herbivory Outline

• Adaptations of plants that reduce herbivory and
  increase fitness
• Co-evolution
• Mutualism
• Population dynamics of herbivore/plant systems

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Animal Irruptions

• A situation where an animal population rapidly
  increases to high densities, and then collapses
• Generally a failure of establishment of an
  equilibrium of herbivore and vegetation
• Often stabilizes over time in a convergent
  oscillation

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Caribou introduced in different areas of S Georgia
Each area has its own carrying capacity
Animals increased rapidly, but then decreased
rapidly
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Pattern of Irruptions in ungulates
Ungulate population size
carrying capacity of habitat
Stage 1 rapid increase in population, slight
decrease in CC due to reduction of food Stage 2
Population gt CC, damage to the food resource
resulting in a decrease in CC Stage 3, population
crashes because of food shortage, often triggered
by other factors such as poor weather and
consequent reduced CC Stage 4 population
recovers, then reaches an equilibrium.