Chapter 11 Herbivory

Harned Hall 301

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Sections

Herbivory and Population Regulation

The green world hypothesis makes the observation that there are green leaves aplenty and asks the question "how can this be so if every plant has at least one species of herbivore that eats it?"

Predator/Parasite limitation

Testable in that one can exclude or eliminate the predators/parasites and, if the hypothesis is true, the herbivore population should expand

Plant defenses

Problem is that every plant has a herbivore that has beaten the defense

Herbivore self-regulation

Brings up the question of altruistic behavior

Why limit yourself if your neighbor is limiting himself?

Plant Defenses

Secondary Chemicals - those that are synthesized by specialized pathways in plant cells, not part of general plant cell metabolism

Some believe that they are synthesized as ante-herbivore toxins

Some believe that these molecules are waste products from plant metabolism and that they remain in the plant because plants are not motile, like animals, and it is safer to store the chemicals in the cells than to excrete them

Anti-herbivore activity is, if this is true, an unintended, but useful, outcome for a molecule that would have to be there in any case

Contraindications that secondary chemicals are waste products

  • Waste molecules should not be costly to produce in terms of energy
  • Secondary chemicals often are costly
  • Level of herbivory often positively correlated with level of defense (more secondary chemicals produced where herbivore damage is greatest
  • Some secondary chemicals are induced
  • More secondary chemicals are allocated to valuable tissues (leaves or storage areas) than to other areas
  • Fruit often cleared of secondary chemicals or their levels reduced just prior to ripening
    • Why store waste in a place it can't stay unless it has some function there

Two types of secondary chemicals (based on their effect on the herbivores, not on the structure of the chemicals)

Quantitative defense

Chemicals build up their toxic effect, so that they often deter the herbivore or slow its growth rather than outright poison it

These are often a large proportion of the dry weight of living plant tissue (non-woody tissue)

Example: Tannins

Polyphenolic compounds

Hydrolyzable tannins (can be attacked enzymatically)

Bind to and inactivate proteins in gut of herbivore (this is why we use them to tan hides into leather, as the binding makes the proteins tough and indigestible)

Interfere with digestion, can slow growth or kill

Condensed tannins

Bind to cell walls of plants and make the cell walls indigestible by fungi and bacteria

Qualitative defenses

Toxic in small doses

Usually present in low concentrations

Examples: Cardiac glycosides in milkweed, Alkaloids in cacti,

Some link the choice of secondary chemical type to type of plant

Apparent plants (K-selected, long-lived, large plants) tend to have quantitative defense (tannins in oaks) because their herbivores can build up over many generations on one plant

Unapparent plants (r-selected, weedy plants) tend to have qualitative defenses

Problem is that we do not often know what is an apparent versus an unapparent plant, so it is difficult to decide if this generalization is reasonable

Mechanical defenses

Hairs on leaves, spines on stems, thorns on stems

Hairs on leaves are to protect from small insects such as leave miners or those  that eat pits into leaves

Spines (modified leaves), prickles (sharp extensions of a plant's epidermis) and thorns (modified plant stems) are protection against larger herbivores

Silica bodies in leaves and stems, Calcium deposits in stems, and Lignified Collenchyma along vascular bundles

Grind down teeth of vertebrates and invertebrates

insect adults will never molt again and when their mandibles grind down enough, they starve

Failure to attract

Missing chemical odor or visual color cue used by herbivore to find its food

This can be loss of a protein from the cell membrane or cell wall of a microbe that makes it impossible for another microbe or a virus to detect its presence

Reproductive inhibition

Production of a hormone-mimic by plant that interferes with insect development

Strategy now used by many man-made herbicides

We do not know the long-term effect of saturating some environments with hormone-mimics

Anti-herbivore mutualisms

Ant - Bull Thorn Acacia

Ants live in bases of unusual thorns and eat Beltsian bodies

Ants remove other insects from the tree and even attack large herbivores (or humans)

Below-ground storage

Many plants survive intense grazing because much of photosynthate produced is stored below ground

plants with significant below-ground storage are adapted to grazing by large herbivores.

grasses have extensive root systems

many herbaceous plants  have thicken roots or separate tubers for root storage (beet, potato, carrot)

Induced Defense versus Constitutive defense

Constitutive defenses are produced whether or not plants are attacked by herbivores

  • Constitutive defenses are valuable if the herbivore attack might be fatal
  • induced defenses are only good if the plant survives the initial attack

Induced defenses are produced only after attack by herbivores or are  produced in greater amount after attack by herbivore

  • Many secondary chemicals are induced
  • Reduction of quality of leaves as food can be induces (removal of valuable materials or storage of minerals)
  • Increase in mechanical defense for leaves and stems grown after herbivore attack

Herbivore response

Arms race between plant and herbivore

Herbivores may attack plants only before defense are completely developed or before they are induced

Specialist herbivores have often completely adapted to presence of defense

Some actually need the defensive chemical for their own defense

  • Monarch butterfly protected from bird predation by presence of cardiac glycosides
  • cardiac glycosides present because the are eaten by larvae and the chemicals persist throughout pupal stage of development

Detoxification

  • Oxidation - mixed-function oxidases alter chemical structure through addition of oxygen
  • Reduction - addition of electrons used to modify or split apart toxins
  • Hydrolysis - splitting apart toxins through addition of water, which is itself split.
  • Conjugation - linking of toxin molecules or toxin and other molecules to produce a less toxic compound

Studies of Herbivory

Agriculture studies can have herbivores take 50% of production

Most studies of herbivory in nature show that herbivores take less than 10%

Can vary greatly (1% in some years, 100% in others)

Years of high herbivory often caused by "insect outbreaks"

Native herbivore that has peak years

Spruce budworm, Armyworm, Locust

Not to be confused with damage done by introduced insects (gypsy moth)

In the long term, outbreak years do not reduce wood production in forests because the bad year is often followed by increased production in subsequent years by young plants

Can be more important if that 10 % reduces seed output by a greater percent

Can be ecologically minor, but can be a major evolutionary force

Ecologically, a 1 % change in seeds may mean little

The same difference over a long time can produce profound changes via natural selection

Herbivores often very important when introduced into area

Pest insects on crops herbivores escape predators through introduction or because the predator does not search the field

Biological control of plants (introduced herbivore effective because there is no native predator to control it)

Opuntia - Cactoblastis

  • cactus spread in cattle grazing rangeland in Australia
  • moth lays eggs on cactus and larvae eat stems from inside out
  • reduced cactus to small, isolated stands
  • system is stable with establishment of new stands of cactus balanced by discovery and elimination of older stands
    • new stands start through long-distance transport of seed or stem segment
    • don't last forever as eventually found by moth

Salvinia - Cyrtobagus

  • plant overgrows ponds and streams
  • blocks light for algae, dying plant settles to bottom and decomposition strips oxygen from the water so that fish die
  • beetle effectivly eats plants from surface of water

Beneficial Herbivory

In some heavily grazed grasslands, some plants produce more leaves and grow larger after being grazed than before

this has been interpreted as beneficial to the plants

Controversial in that there is no mechanism known for the phenomenon

Selection by Herbivores

Herbivores often select plants of highest quality

Protein usually most valuable

Proteins high in nitrogen (amino group), thus the book's stress on the nitrogen content of plant tissue

Oils often more valuable as energy source than carbohydrates

Seeds often most valuable

High in both proteins and oils

Terms

Green world hypothesis, Predator/Parasite limitation, Plant defenses, Herbivore self-regulation, Secondary Chemicals, Quantitative defense, Hydrolyzable tannins, Condensed tannins, Qualitative defenses, Cardiac glycosides, Alkaloids, Apparent, Unapparent, Mechanical defenses, Silica bodies, Reproductive inhibition, hormone-mimic, Anti-herbivore mutualisms, Induced Defenses, Constitutive defense, Arms race, Generalist herbivores, Specialist herbivores, Detoxification, Oxidation, Reduction, Hydrolysis, Conjugation, insect outbreaks, Spruce budworm, Armyworm, Locust, Biological control, Opuntia - Cactoblastis

Last Updated September 19, 2006