Plant Ecology - Chapter 11

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Plant Ecology - Chapter 11

• Herbivory Pathogens

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Herbivory

• The consumption of all or part of a living plant
• A predator when it kills and eats an individual

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Herbivory

• Granivores - eat seeds or grains
• Grazers - eat grasses, low-growing plants
• Browsers - eat leaves from trees, shrubs
• Frugivores - eat fruits

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Herbivory

• How much do they eat?
• Estimate - 10 of leaves of forest trees eaten
  each year
• Least in temperate forest, most in dry tropical
  forest

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Is herbivory good for a plant?

• Reduce self-shading
• Remove leaves in excess of optimum LAI
• Reduce respiratory drag on plant

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Herbivory can cause death

• Girdling (ring-barking) of young trees by
  rabbits, squirrels, and rodents

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Herbivory can cause death

• Introduction of disease into plant by grazer
• Dutch elm disease
• Fungus carried by elm bark beetle
• Clogs circulatory system of American elm trees

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Herbivory can cause death

• Grazing on one species may be sufficient to sway
  competitive interaction in favor of another
  species

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Herbivory can cause death

• Large populations of fluid-suckers (e.g., aphids)
  can virtually stop growth and/or kill a plant

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Herbivory can affect survival

• Repeated defoliation often required to kill
  mature plant
• Large proportion of seedlings killed by single
  attack
• But some seedling plants have high tolerance -
  e.g., 75 survival after 5 defoliations

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Herbivory can affect growth

• Effects range from none to total cessation of
  growth
• Depends on
• Timing of defoliation
• Type of plant involved (grasses most tolerant
  because of basal meristem rather than apical
  meristem)

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Herbivory can affect fecundity

• 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

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Good herbivores

• Some pollen-eaters help pollinate
• Some fruit-eaters help distribute seeds
• Some seed-eaters store seeds in ground and forget
  them
• Mutualistic relationships

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Compensation for herbivory

• Temporarily mobilize stored carbohydrates until
  regrowth returns photosynthesis to normal

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Compensation for herbivory

• Reroute photosynthetic products to damaged areas
  to enhance regrowth
• To roots, or shoot, or leaves

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Compensation for herbivory

• Increase rate of photosynthesis in remaining leaf
  surface area

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Compensation for herbivory

• Stimulate dormant buds to grow, or reduce death
  rate among surviving parts
• Despite all these possible mechanisms,
  compensation is rarely perfect, so plants are
  harmed in the long-term

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Compensation for herbivory
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Defensive responses to grazers

• Grow bigger, sharper spines

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Defensive responses to grazers

• Produce more or new defensive chemicals

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Defensive responses to grazers

• Reduce palatability
• Tougher
• More fiber
• Lower nitrogen content

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Effect of grazing on whole population of plants

• Do they only prey on the weak?
• Reduction in intraspecific competition
• Can reduce high LAI to more optimal levels and
  improve plant productivity
• Typically only works in high-density populations
  little or no compensation in low-density
  populations

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Effect of grazing on whole population of plants

• Controversial and unresolved
• Two explanations on why herbivores are NOT
  important regulators of plant populations
• Top-down
• Bottom-up

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Effect of grazing on whole population of plants

• Top-down - herbivores usually at such low
  densities because of their predators, cannot have
  negative effects on entire plant population

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Effect of grazing on whole population of plants

• Bottom-up - plant populations are limited by
  abiotic factors (light, water, nutrients), not by
  herbivores

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Effect of grazing on whole population of plants

• On the other hand, there are some various obvious
  examples of population control by herbivores -
  e.g., gypsy moths and oaks

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Effect of grazing on whole population of plants

• Another example - bark beetles and conifers -
  widespread mortality in N. Amer.

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Effect of grazing on plant distribution

• Eating can limit distribution in some areas, or
  rodent/squirrel caches can enhance abundance

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Biological control

• Moth introduced into Australia to kill invasive
  prickly pear cactus - good there, but problems
  elsewhere

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Biological control

• Beetles introduced to control purple loosestrife

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Herbivory communities

• Vertebrate, invertebrate grazers can have
  dramatic effects on plant communities

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Herbivory communities

• E.g., rabbits and grasslands of southern England

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Herbivory communities

• Native and introduced grazers can have
  significant profound effects

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Herbivory communities

• Large herbivores in Yellowstone

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Parasitic Plants

• Obligate parasitic plants - obtain energy,
  nutrients, water from host plant
• E.g., mistletoes

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Parasitic Plants

• Hemiparasites - independent and photosynthetic,
  or parasite on other plants (e.g., roots)

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Plant Pathogens

• Fungi, water molds, bacteria, viruses cause
  diseases in plants
• Individual, population, and community effects

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Plant Pathogens

• Soybean rust - fungus from Asia, infects leaves
• Survives only on green tissue (eliminated each
  fall here, but kudzu in south is infested)

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Plant Pathogens

• Citrus canker - bacterium causes premature leaf,
  fruit drop

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Plant Pathogens

• Smuts - affect flowers, are caused by fungi
• Sexually transmitted

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Plant Pathogens

• Chestnut blight - fungal canker disease, kills
  cambium under bark
• American chestnut formerly dominated plant
  communities

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Plant Pathogens - people

• Irish potato famine resulted from potato blight
  caused by water mold
• Destroyed Irish potato crop in 1840s

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Plant Pathogens - people

• 1 million people died from famine and disease
• 1 million emigrated to U.S., Canada (especially
  New York, Boston)

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Plant Pathogens - people

• Population of Ireland has not recovered
• Remnants of former potato farms remain today

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Plant defense against pathogens

• Phytoalexins - secondary chemicals produced at
  site of infection to kill microbes
• Phloem plugging - phloem clogs in response to
  damage, prevents spread of infection through
  vascular system
• Localized tissue death - barrier to infection