Plant defenses against herbivores

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Plant defenses against herbivores

• Plants cant run away from herbivores
• Plants cant hide leaves must be exposed too
  catch light for photosynthesis.
• Plants are usually too abundant to be cryptically
  colored!
• Therefore, plant defenses operate
• in situ, either directly or indirectly

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Types of defense

• Mechanical
• Chemical
• Biotic
• Phenological
• A species may use more than one of these lines of
  defense, and may use different ones at different
  stages of its life

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Mechanical defenses

• Hairs on leaf surface
• Spines modified leaves
• Thorns modified shoots
• Toughness
• Other modifications e.g. fake eggs on
  Passiflora tendrils fool ovipositing Heliconius
  butterflies

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Hairiness - multipurpose
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spiny thistles and Eeyore
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Thorns to prevent biting, climbing
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Chemical defenses

1. Palatability/acceptability influencers feeding
   inhibitors
2. Digestibility reducing compounds
3. Toxins

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Palatability influencers

• Pre-chomping volatile compounds, may repel
  herbivores, but specialized herbivores may use
  them as a cue
• Post-chomping
• Tannins (astringency) oaks Oxalic acid
  sorrels Calcium oxalate Araceae
  photosensitization cyanogenic glycosides

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Digestibility reducers

• e.g. Tannins reduce assimilation of plant
  proteins by herbivores by interfering with
  proteolytic enzymes
• Herbivores eat a lot, assimilate little
• Grow more slowly, may fail to develop in suitable
  season
• Longer in larval stage, vulnerable to enemies

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Toxins Cabbages and glucosinolates
Peter J. Bryant
Cabbage white
Common green-eyed white
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Giant swallowtail and Rutaceae hostplants
Fireflyforest.net
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Toxins

• e.g. cardiac glycosides in Asclepias
• Cows and sheep eating plants get sick, have
  abortions, etc
• Most insects avoid foliage except monarch
  butterflies. Their caterpillars are toxic due to
  sequestration of the milkweed toxins!
• Certain specialists are adapted to feed despite
  toxins.

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Milkweed and monarch caterpillar
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• Monarch butterfly life cycle
• Adults benefit from sequestered cardenolides too!

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Naïve birds will remember
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Idea leuconoe (Lepidoptera subfamily Danainae) on
Parsonsia alboflavescens (Apocynaceae)
c/o Tatyana Livshulz Fieldwork in Taiwan
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Tithorea pinthias Nymphalidaeon Prestonia
portobellensis
Dan Janzen Santa Rosa National Park
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Plumeria and Pseudosphinx tetrio
Squirrel cuckoos whack and snack
http//biological-diversity.info Belize
wildlife site
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Biotic protection

• Ants tending extrafloral nectaries and/or
  residing in plant body protect plants
• Parasitoids visiting nectaries may also provide
  plant protection
• Plants can benefit from ants tending certain
  herbivores as well, but not always
• Beneficial mites can eat herbivorous mites
  plants may have domatia to house them

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Senna mexicana foliar nectaries and ant
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Turnera ulmifolia in greenhouse
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Vicia sativa with stipular nectaries
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Phenological defense

• Timing of production of parts susceptible to
  herbivory
• Leaf flushing
• Masting flowering/fruiting every 2-5 years
  synchronously with others (predator satiation)

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New leaf flushing phenological defense?
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Mast fruitingmast beech

• A mast year occurs when the number of acorns or
  other nuts (like Beech nuts Hickory nuts or Hazel
  nuts) produced by a tree or shrub in a single
  season is much higher than usual. 

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Jargon of antiherbivore defense
Plant types Feeny Rhoades Cates
Short-lived (rare /or ephemeral) hard to find Qualitative defenses Unapparent
Long-lived (abundant /or persistent) bound to be found Quantitative Apparent
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Qualitative vs. Quantitative

• Qualitative defenses (e.g. toxins) very
  effective against non-adapted species, effective
  in small amounts on all but specialist herbivores
• Quantitative defenses more effective in larger
  doses, general-purpose, e.g. toughness, low
  nutrients, digestibility-reducers

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Plants and their parts
Unapparent Apparent
Annual species Woody perennial spp
Early successional spp Climax spp
Rare spp Common spp
Young leaves Mature leaves
Leaves Bark, stem
Deciduous leaves Evergreen leaves
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Optimal defense theory

• Considers costs to plants of antiherbivore
  defenses
• Qualitative defenses cheaper than quantitative
  defenses?
• Maybe just more appropriate act quickly against
  specific herbivores
• Lots of debate over the years

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Physiological ecologists to the rescue

• The cost of defense are one of the constraints on
  leaf form and function
• Photosynthetic capacity, nitrogen, longevity, and
  susceptibility to herbivores are all related
• Quantify these things for a cost-benefit analysis

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• Benefit from a leaf
• rate of carbon gain carbon gain period
• minus
• carbon cost of growth and maintenance
• losses to herbivory
• CO2 exchange can take care of all except
  herbivory losses
• Amount of nitrogen in leaf correlates with
  photosynthetic capacity

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Effects of herbivores can be dramatic!
Hawaiian landscape protected from cattle grazing
on Mauna Loa
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Odocoileus virginianus ssp. clavium
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Earlier deer exclosures on Key Deer Refuge at
least 25 years old. No fire in that time (note
wooden posts). Woody plant cover substantially
greater inside fenced areas Long term
protection from deer browsing?
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Fire and Key Deer Herbivory

• Fire clears dense understory
• Promotes resprouting and seed germination
• New foliage more palatable to deer
• Preferential grazing may affect understory
  plants
• Growth
• Reproduction
• Recruitment and/or persistence