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Herbivory and Plant-Pathogen Interations. Chapter 11. Herbivory – the consumption of all or part of a living plant. 4 types of Herbivores Granivores : consume seeds or grains, killing the individual within. Grazers : eat grasses and other low-growing plants.

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Herbivory and Plant-Pathogen Interations

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herbivory the consumption of all or part of a living plant
Herbivory – the consumption of all or part of a living plant
  • 4 types of Herbivores
    • Granivores: consume seeds or grains, killing the individual within.
    • Grazers: eat grasses and other low-growing plants.
    • Browsers: eat leaves from trees or shrubs.
    • Frugivores: eat fruits, sometimes without damage to the seeds.
effects of herbivory
Effects of Herbivory
  • Ecological
  • Evolutionary
  • Important to keep balance in particular systems.
  • Depending on part of plant eaten
    • Roots
    • Leaves
    • Phloem
    • Meristems
    • Flowers, fruits, and seeds
  • Depending on stage in life cycle
adaptive responses
Adaptive Responses
  • Resistance – the ability of a plant to avoid being eaten.
  • Tolerance – the ability to minimize reduction in fitness due to herbivory.
    • Overcompensation: plants respond to herbivory by growing more rapidly.
herbivory and plant populations
Herbivory and Plant Populations

“Top-Down” Theory

“Bottom-Up” Theory

Plant populations are limited by such abiotic factors as water, light, and soil nutrients.

Therefore, plant populations are not limitid by biotic factors such as herbivores.

  • Predation causes herbivores to be found in small densities.
  • Thus, plant populations are not negatively affected by herbivores due to the small population size of the herbivores.
contrasting arguments of herbivory and plant populations
Contrasting Arguments of Herbivory and Plant Populations
  • Other researchers argue that herbivory does regulate plant populations.
  • It is probable that “top-down” or “bottom up” theories apply in some settings, while the herbivory regulation theory applies in other settings.
  • More research must be conducted to conclude the circumstances that each theory prevails under.
damage to conifers in western north america and southeastern united states due to bark beetles
Damage to conifers in western North America and Southeastern United States due to Bark Beetles
  • A hole is chewed through the bark and the actively growing cambium in order to lay eggs.
  • The larvae eat the cambium which destroys it and the vascular tissues.
  • The conifers have a defense mechanism of oozing sap in order to suffocate the bark beetle or push it out of the hole.
  • Large attacks by bark beetles overwhelm the conifers and decreases the conifers ability to defend itself. This can lead to mortality.

Figure 11.2

the effect of chronic herbivory on plant demography
The Effect of Chronic Herbivory on Plant Demography
  • Chronic Herbivory- herbivory that occurs over an extended period of time.
    • Chronic herbivory can drastically decrease fitness.
  • Demography- study of characteristics of a population such as size, growth, and density.
  • Examples
    • Chronic herbivory made pinyon pine trees have a distinct shape, decreased growth rates, and produce male cones almost always.
    • Eucalyptus trees protected with an insecticide produced much larger growth than those trees with no insecticide.
herbivory and spatial distribution in haplopappus squarrosus
Herbivory and Spatial Distribution in Haplopappussquarrosus

Figure 11.3 a

Figure 11.3 b

  • Most abundant in the transition climate between coastal and interior, but produced more seeds closer to the ocean.
  • The above pattern is a result of more insect granivory closer to the ocean.
  • Granivory can eradicate a large number of the seeds produced.
    • Bruchid beetles were responsible for 13%-38% of Tachigali versicolor granivory, and vertibrates were responsible for 0% to 59% of the granivory.
  • Granivory can be deterred or enhanced by chemicals expelled by the plant.
    • Coral bean trees can expel a strong neurotoxin that affects vertebrates.
biological control of prickly pear cactus by cactoblastic cactorum
Biological Control of Prickly Pear Cactus by Cactoblasticcactorum

Figure 11.4 a

Figure 11.4 b

Biological control- use of herbivores by humans to control populations of undesirable plant species.

Cactoblastiscactorum in Argentina

the effect herbivory at the community level
The Effect Herbivory at the Community Level
  • Two main types of herbivory effects on a plant community
    • Direct Effects
      • Floral Phenotypic Effects
      • Floral Genotypic Effects
    • Indirect Effects
      • Attractiveness to Pollinators
      • Number of flowers visited via pollinators
      • Pollinators time spent on each flower
plant species diversity and richness are greater in grazed grasslands in yellowstone national park
Plant Species Diversity, and Richness are Greater in Grazed Grasslands in Yellowstone National Park
plant defenses against herbivory
Plant Defenses Against Herbivory
  • Plant immobility results in natural selection for being able to defend themselves from damage or death from herbivory
  • Selection results in plants that are tougher, less palatable, and better defended
physical defenses
Physical Defenses
  • Thorns and Spines are the obvious structures
  • Trichomes
  • Thick bark on trunks and roots
  • Tough coats that protect seeds, fruits, and nuts
  • Leaf Toughness
plant secondary chemistry
Plant Secondary Chemistry
  • Primary metabolites
    • Sugars, amino acids, and DNA
    • Necessary for basic functions of plants such as cellular respiration and photosynthesis
plant secondary chemistry19
Plant Secondary Chemistry
  • Secondary Chemicals
    • Broad group of chemicals
    • Defense and attraction of pollinators
    • Found only in particular species or groups of species
    • Often found only in specific organs or tissues
    • 3 major categories
      • Phenolics
      • Alkaloids
      • Terpenes
  • Include a large variety of chemicals that have an aromatic ring with an attached hydroxyl group (-OH)
  • Tannins: reduce digestibility of plant tissues
  • Lignins: impregnate woody cell walls, giving them structural strength
  • Relatively small molecules that contain Nitrogen
  • Bitter taste and many are toxic to herbivores
  • Effective in small quantities
    • Cocaine
    • Nicotine
    • Caffeine
  • Composed of multiple units of hydrocarbon isoprene (C5H8)
  • Oils responsible for flavors and scents of the mints (Lamiaceae) are terpenes and they deter herbivores and reduce growth of bacteria and fungi
constitutive vs induced defenses
Constitutive vs. Induced Defenses
  • Constitutive defenses are present in plants regardless of herbivore damage.
  • Induced responses are elicited by an attack by herbivores.
  • If these responses serve to protect, they are called induced defenses.
  • If they have a negative effect on attacking herbivores, they are called induced resistances.
evolutionary consequences of plant herbivore interactions
Evolutionary Consequences of Plant-Herbivore Interactions
  • If plants are so well defended, then how do herbivores survive?
  • Plants vary in their defenses due to differences in species, individuals and life stages.
  • Herbivores also have adaptations that allow them to overcome plant defenses.
  • Coevolution between plants and herbivores allows both species to survive under moderate conditions
mutual adaptation grasses and grazers
Mutual Adaptation : Grasses and Grazers
  • In most mammals the teeth cease to grow in adults, but in grazers the teeth continue to grow.
  • This is beneficial because the silica in grasses erodes teeth. The new growth in grazers’ teeth replaces the worn material.
  • Grass meristems are usually located at ground level to prevent them from being destroyed by grazers. As a result, they are able to quickly regrow the tissue that is lost to herbivores.
  • Specialists: Monarch Butterfly Larvae accumulate the toxins from Asclepiassyriacato deter their own predators.
parasitic plants
Parasitic Plants
  • Obligate Parasites require a host to survive
  • Hemiparasites may live in or on a host or independently
  • Parasites decrease the fitness of the host by reducing its ability to compete and reproduce.
  • Examples include Mistletoes and dodders
  • The amount of parasites on a particular host varies among individuals
  • Parasitic plants can also utilize the anti-herbivore defenses of the host.

Fruit infected with a fungus

Flowers of Saponaria ocymoides

with anthers infected with a smut


physiological and evolutionary responses to pathogens
Physiological and Evolutionary Responses to Pathogens
  • Responses to Pathogens

Chemical defenses – Phytoalexins (may see localized and even systemic resistance)

Physical defenses - Phloem plugging

  • Pathogenicity

Ability to cause a diseases in a host – dependent on resistance genes and avirulence genes

  • Virulence

The ability of and infectious agent to produce disease and a measure of the degree of damage inflicted

  • Tolerance

The ability of a plant to maintain its fitness when infected with a pathogen