Herbivory

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Herbivory

• Chapter 11

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Herbivory

• Why dont herbivores eat up all the plants?
• Maybe predators keep herbivores in check.
• Maybe plants can defend themselves.

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Plant Defenses

• Example of plant chemical defenses
• Alkaloids (nicotine in tobacco, morphine in
  poppies, caffeine in tea).
• Mustard oils.
• Terpenoids (in peppermint).
• Phenylpropanes (in cinnamon and cloves).

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Plant Defenses

• Two general classes of compounds
• Carbon nutrient balance theory.
• Nitrogen compounds
• Limited by carbon (due to shortages of light or
  water).
• Mainly alkaloids.
• Carbon compounds
• Limited by nitrogen.
• Mainly terpenoids and phenolics.

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Plant Defenses

• Secondary chemicals
• Not part of primary metabolic pathways that
  plants use to obtain energy.
• Defense compounds.

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Strategies of Plant Defenses

• Quantitative defenses
• Substances that are ingested in large amounts by
  the herbivore.
• Prevent digestion of food.
• Ex. tannins and resins.

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Strategies of Plant Defenses

• Qualitative defenses
• Highly toxic substances.
• Very small doses can kill herbivores.
• Many have selective toxicity toxic to
  herbivores, but not to birds or other seed
  dispersers.
• Ex. Atropine produced by the European deadly
  nightshade, Atropa belladonna.
• Poison stored in discreet glands, so the plant
  wont poison itself.

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Plant Defenses

• Apparency
• Correlated to qualitative and quantitative
  defense strategies.
• Apparent plants
• Long-lived (e.g. oak trees).
• Apparent to herbivores (mainly insects).
• Defenses are mainly quantitative.
• Effective against specialist and generalist.

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Plant Defenses

• Unapparent plants
• Weeds
• Ephemeral and unavailable for long periods of the
  year.
• Defenses are mainly qualitative.
• Herbivores are mainly generalists like
  vertebrates.

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Plant Defenses

• Mechanical defenses
• Thorns and spines defer vertebrate herbivores.
• Generalizations (Peter Grubb)
• In many open sites, plants are primarily close to
  the ground and so are very spinose to protect
  them.
• Plants such as palms, with one or a few apical
  meristems, are also likely to protect them with
  spines.
• Evergreens, such as holly, in a deciduous forest
  are likely to face severe herbivore pressure in
  the winter and so are very spinose.

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Plant Defenses

• Repellents
• Thistles produce compounds that repel certain
  insect larvae.
• Potatoes synthesize a component of an alarm
  pheromone released by aphids when they are
  attacked by predators.
• Alarm pheromone causes aphids to flee.
• As a result, the potatoes are free of the aphids.

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Plant Defenses

• Reproductive inhibition
• Plants (e.g., firs) contain insect hormone
  derivatives.
• If digested, prevent insect metamorphosis.
• Results in diminished reproductive output.
• Plants (e.g., floss flower) produce a chemical
  mimic of insect molting hormone ecdysone.
• Insects digest plant material.
• Insects die when they molt prematurely.

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Plant Defenses

• Masting
• Occurs in a few tree species (e.g., some oaks).
• Production of more seeds in some years
• Satiates herbivores.
• Permits more seed to survive Ex. Beech.
• In mast years, 3.1 of seeds destroyed by boring
  moths.
• In non-mast years, 38 of seeds destroyed.
• Other benefits
• Enhanced pollination.
• Enhanced seed dispersal.

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Plant Defenses

• Defensive associations
• Protection from herbivores through association
  with unpalatable neighbors.
• Ex. Chrysomelid beetle and the purple
  loosestrife.
• Loosestrife sometimes grows on its own, or in
  thickets of an aromatic shrub, Myrica gale.
• Myrica secretes a volatile chemical that deters
  insects from feeding on it.
• Chemical also interferes with beetle searching
  for loosestrife.

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Plant Defenses

• Opposite association Associational
  susceptibility
• The spilling over of herbivores from palatable
  neighbors.
• Fall cankerworms prefer to feed on box elder
  trees and rarely feed on isolated cottonwood
  trees.
• When cottonwoods occur under box elder, the
  cankerworms spill over and defoliate the
  cottonwoods.

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Plant Defenses

• Thus, in both associational resistance and
  associational susceptibility, herbivory is
  influenced by neighboring species.

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Plant Defenses

• Mutualism
• Plants that defend themselves through enlisting
  help from other animals.

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Plant Defenses

• Understanding plant defenses
• Important to agriculture.
• Use knowledge to defend crops against insects.
• Host plant resistance
• Problems
• Long time needed to develop resistances.
• Resistance to one pest may increase
  susceptibility to other pests.
• Sometimes pests can overcome resistance.

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Plant Defenses

• Benefits
• Once resistance is developed, requires minimal
  cost from farmer.
• Environmentally benign.

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Plant Defenses

• Insect response to resistance
• Certain chemicals that are toxic to generalist
  insects, actually increase the growth rates of
  adapted specialist insects.
• Specialization of herbivores to supposedly toxic
  plants evolutionary arms race.

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Plant Defenses

• The result would be a profusion of specialist
  herbivores on plants.
• Is this the case?

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Plant Defenses

• Study of the diversity of chemical defenses and
  the number of insect species in the carrot
  family.
• The most specialized feeders were found on the
  plants with the most complex chemical defenses.

Specialists!
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Modeling Herbivory

• In modeling herbivory, the outcomes of
  plant-herbivore interactions are determined by
  the degree of specialization of the herbivore.

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Modeling Herbivory

• Models depend on the degree of polyphagy of the
  herbivores.
• Monophagous herbivores are highly specialized and
  feed on only one species.
• Many insects tend to be specialized.
• Polyphagous herbivores feed on many species.
• Many vertebrates tend to be generalists.
• Exceptions Pandas and bamboo, and koala and
  eucalyptus.

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Effects of Herbivory on Plants

• Herbivore responses to plant defenses
• Detoxify plant poisons.
• Oxidation
• Conjugation

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Effects of Herbivory on Plants

• Measured effects of herbivores on plants in the
  field.
• In 93 cases, 7 of leaf area was consumed.
• In forested systems, 5-15 defoliation by
  insects.
• May underestimate effects due to leaf turnover.

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• Herbivory increases with plant productivity.

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Effects of Herbivory on Plants

• Removal experiments
• Best way to estimate effects of herbivory on
  plants.
• Remove herbivores.
• Examine effects on plant growth and reproduction.

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Effects of Herbivory on Plants

• Review of 246 experiments (Bigger and Marvier,
  1998).
• Metaanalysis statistical technique.
• The average plant that was protected from
  herbivores was significantly larger than the
  average exposed plant.
• Stronger effects in aquatic systems.

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Effects of Herbivory on Plants

• Invertebrate herbivores have a greater effect
  than vertebrates in terrestrial systems.
• Herbivory in aquatic systems stronger than
  terrestrial.

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Effects of Herbivory on Plants

• Which type of plants are most affected?
• Effect size of herbivores was greatest on algae.
  perhaps due to lack of chemical defenses also
  on grasses and shrubs.
• Effect size of herbivores was smallest on woody
  plants.

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Biological Control

• Evidence from biological control of weeds in
  agriculture.
• 50 of the 190 major weeds in the US are invaders
  from outside the country.
• Interest in biological control due to expense of
  chemical control.

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Biological Control

• Biological control success stories
• Klamath weed was controlled by two French
  beetles.
• A floating fern, Salvinia molesta was controlled
  by the weevil Cyrtobagus salvinae.
• Florida alligator weed was controlled by the
  alligator weed flea beetle.

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Biological Control

• Unsuccessful biological control stories
• Control of Lantana camara in Hawaii.
• In a review of 701 examples of biological weed
  control, only 26 were rated successful.

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Effects of Herbivory on Plants

• Beneficial herbivory
• Review of herbivory (Bigger and Marvier, 1998).
• 60 comparisons demonstrated a reduction in plant
  size due to natural levels of herbivory.
• 10 comparisons demonstrated an increase in plant
  size.
• Plants are stimulated to regrow after damage, and
  may overcompensate.

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Effects of Herbivory on Plants

• Benefits of root herbivory
• The actions of root-boring isopods stimulates
  formation of new prop roots at the point of
  attack.
• This leads to increased stability.

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Effects of Herbivory on Plants

• Benefits of grazing and regrowth
• More flowers and fruits on grazed plants
  (Gronemeyer et al. 1997).
• More likely to occur in perennials than annuals.

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Applied Ecology Pest Control

• Huge monocultures are ideal targets for insect
  pests and diseases.
• Wide variety of control techniques.

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Applied Ecology

• Pesticides in the US
• 50,000 pesticides are registered for use in the
  US.
• Five to six hundred million kg of pesticides used
  each year.
• 70 for agriculture, 23 for forestry, 7 for
  home and garden.

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Applied Ecology

• Many are strong poisons.
• 60 of herbicides and 30 of insecticides are
  potentially oncogenic.
• 95 of human tissue samples contain pesticide
  residues.
• Pesticides affect non-target organisms.

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Effects of Plants on Herbivores

• Nitrogen Limitation Theory
• Herbivores select those plants that have the most
  nutrition (in terms of nitrogen).
• Animal tissue contains 10 times more nitrogen
  than plant tissue.

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Effects of Plants on Herbivores

• Red deer prefer to feed on grasses that were
  defecated upon by herring gulls.
• As the amount of droppings increased, the
  nitrogen content of the grasses also increased.

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Effects of Plants on Herbivores

• Plant fertilization (i.e., nitrogen) had a
  positive effect on population size, survivorship,
  growth and fecundity of herbivorous insects.

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Effects of Plants on Herbivores

• Plant fertilization
• Response was greater in cultivated versus wild
  plants, and in herbaceous plants and broadleaf
  trees versus conifers.

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Effects of Plants on Herbivores

• Two variations of the nitrogen limitation theory.
• Stress hypothesis (White 1993)
• Plant stresses tend to increase the availability
  of nitrogen because many nitrogen-rich compounds
  are mobilized in response to stress.
• Ex. drought stressed plants accumulate high
  numbers of herbivores.

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Effects of Plants on Herbivores

• Plant vigor hypothesis (Price 1991)
• Herbivores select fast growing parts of plants or
  fast growing plants because these are higher in
  nitrogen.
• Ex. Many attacks by insects are on young trees.

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Effects of Plants on Herbivores

• Herbivore density correlated with plant quality.
• Correlation does not always indicate causation!
• Herbivore population patterns are dependent on
  other factors.
• Predation
• Parasitism

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Herbivory Affects Community Structure

• Darwin observed that competitively dominant
  grasses were kept in check by grazing, allowing
  more species to occur in areas that have been
  grazed.
• United States and bison.

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Herbivory Affects Community Structure

• Herbivory can affect a plant communitys
  composition, then it has the potential to affect
  its succession how that composition changes
  over time.

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Summary

• A variety of plant defenses demonstrate the
  strength and frequency of herbivory in nature.
• Chemical defenses
• Mechanical defenses
• Hormone mimics
• Mutualism

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Summary

• Chemical defenses
• Quantitative
• Build up in the gut of the herbivore preventing
  digestion.
• Qualitative
• Toxic compounds that can be lethal in small
  doses.

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Summary

• Mathematical models
• Effects of herbivores on plants depends whether
  they are monophagous or polyphagous.

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Summary

• Effects of herbivores
• Herbivores remove 15-18 of terrestrial plant
  tissue.
• Herbivores remove 51 of aquatic plant tissue.
• Impact of biological control.
• Impact in agriculture.

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Summary

• Population densities of herbivores are strongly
  influenced by plant quality, particularly plant
  nitrogen.
• Herbivory may have substantial effects on plant
  communities.