herbivores

Types of plant eaters

1. Leaf-eating - folivorous insects,
browsing and grazing
vertebrates
- Examples:
- Larvae of butterflies
- Leaf cutter ants in tropics
- Wild and domesticated ungulates
- Leaf miners

2. granivores- seed eaters
3. frugivores - fruit eaters
4. phloem feeders
5. root feeders (nematodes,
beetle larvae etc.)

What herbivores make the biggest global
impact

Hypotheses:
1) it’s probably some subset of arthropods (given abundance, observed
impacts in some places)
2) Livestock have a disproportionate impact compared to wild mammals
– especially when you consider the plants fed them

Herbivores can consume, consistently:

forests - 5 to 15% of leaf area
grasslands- 25 to 30% of leaves
and up to 90% for a short period of time.

So with all these herbivores, why is the world still green?

Two general answers:
1) Plants are mostly poor food – hard-to-digest fiber, secondary
compounds, low percent N – this limits the growth rates of
individual herbivores and herbivore populations
2) Herbivores have enemies that prey on them and sometimes
regulate their populations

top-down and bottom-up control,,

“Top-down” regulation of herbivore abundance

• Predators can keep herbivores in check
  “from above”
  • Example: Yellowstone wolves and elk
  • Wolves reduce numbers of elk and also change their
  behavior
  • This can produce a “trophic cascade” that affects
  plants: willows and aspen recover and grow faster
  when wolves are present

Bottom-up” regulation of plant abundance: Plant quality can control herbivore populations from “below”

• Example: leaf-feeding scale insects – demographic rates of scale insects
  depend strongly on plant nutritional quality

types of plant defenses:

1. mechanical factorsspines
thorns
trichomes
tough leaves

2. Chemical defenses
a. Digestibility reducers

b. Toxins

Constitutive vs. Induced Defenses

Constitutive defenses are those that are present in a plant regardless of herbivore damage
Induced defenses (qualitative) are elicited by an attack of herbivores

Induced defense example: snail grazing on white clover (Trifolium repens) ---> plants produce
cyanogenic glycosides.
---> high snail density is correlated with high proportion of cyanogenic plants

Quantitative defenses”

a. Digestibility reducers“Quantitative defenses”
• Abundant -- can be 80-90% of dry
weight
• Immobile -- can’t be deployed
quickly after threat detected

cellulose
hemicellulose
tannins
lignins

“Qualitative defenses”

Toxic – effective at lower concentrations
Mobile -- can be deployed fast to area of
attack (especially the N-based toxins)

Nitrogen based:
alkaloids
toxic amino acids
cyanogenic glycosides

Carbon based:
terpenes
phenols
steroids
ecdysone- an insect hormone

generalist vs specialist induced defense

This experiment showed that induced defenses were effective against a
generalist herbivore, but not against a specialist herbivore (Pieris rapae)

• the selection pressure on

  a specialist herbivore is going to be way stronger than the

  selection pressure on a generalist herbivore to tolerate

  this type of toxin that this plant produces

What plant tissue is made of:

8 major types of
compounds and their energy costs:
– Lipids (highest)
– Soluble phenolics (high)
– Organic N compounds / protein (high)
– Lignin (medium)
– Structural carbohydrates (cellulose) (low)
– Non-structural carbohydrates (starch) (low)
– Organic acids (very low)
– Minerals (0)

Leaves are usually more or less expensive to make compared
to other plant tissues

more

So it’s costly to lose them

But defending leaves from herbivory also has costs
There’s a tradeoff between growth and defense

Growth/defense tradeoffs

Plants have limited resources (carbon/energy, nitrogen). They must choose
how much to invest in defense vs growth, storage, and reproduction

plants and herbivores have co-evolved

Specialist herbivores face strong selection pressure to overcome their
host’s defense. Specialists more likely to tolerate/overcome defenses

c. Growing bigger can itself be a defense

Sometimes tolerating some herbivory and growing larger is the best
defense

Plant apparency theory

a. Apparent (common, easily found) plants must invest heavily in defense

• 1. These defenses are quantitative – they take a lot of materials to work.
  tough leaves
  lignin, etc.

• 2. A classic example is oak trees that fill their leaves with tannins
  

b. Unapparent (rare, short-lived) can get by with less investment in defense

• 1. These defenses are qualitative toxins -- effective at low concentrations
  -- good against generalists but not specialists

• 2. escape specialist herbivores by being unapparent (that is, hiding)

• 3. short-lived meadow plants that may avoid herbivory by being around
  only a short time

Resource availability theory

Investment in herbivore protection depends
on the availability of resources
A. High resource availability in the environment
1. Fast-growing plants can quickly replace lost parts
2. their loss is less costly so less investment in defense is needed
- if a plant uses a defense, it’s often a qualitative, induced defense

• if plants have lots of resources,

  it may not pay off to

  spend a bunch on defense anyway.

B. Low resource availability in the environment
1. Plants grow more slowly and have longer lived leaves that are
relatively more costly to replace
2. Greater investment in defense is required.
Tends to favor quantitative, immobile, constitutive defenses
Though this is not always true: some slow-growers have N-based
qualitative defenses.

overall prediction for resource availability theory

if you are a plant

that has a slow maximum growth rate and depending how

much defense you make you'll actually grow fastest if you

make them invest as much as you can in defense whereas

for faster growing plants the optimal strategy is like,

spend very little on the fence and just keep growing fast.

Understory plants

low carbon
availability, so quantitative C-based
defense compounds too expensive.
Favors N-based, qualitative defense
(e.g. Piper plants in Central America)

Overstory trees with high
photosynthetic rates

abundance
carbohydrates relative to nitrogen.
Favors qualitative defense based on
Carbon-rich secondary compounds
(e.g. oaks, chaparral shrubs)