Our ecological ‘footprint’…1)

The hierarchical nature and processes of different levels of ecological systems:

Species InteractionsCh 17 Herbivory

Objectives

• Herbivory Effects on plants How demonstrate herbivory effects Herbivore selectivity Plant deterrents to herbivory What limits herbivory?•

In food chains, all life forms are both consumers and victims of consumers.

• Predators• Parasites• Parasitoids• Pathogens • Herbivores

Herbivory: Effects on plants

Spruce budworm

Outbreaks of herbivorous insects can defoliate forests.

Herbivory has great effects on plants:1) individual/ecosystem2) population 3) community

• If prefer dominant species--->

• What is a keystone• herbivore?

• If prefer subdominant species--->

Figure 1

Does herbivore control plant species?If….then…

• Natural enemies hypothesis: • Biological control:

Figure 2

Klamath weed + beetle(biological control agent)

In spite of plant defenses, herbivores can control plant population size.

***Are herbivores ‘lawnmowers’ or selective feeders? What 3 factorsmay explainresults?

Figure 3

***What are plant deterrents to herbivory?

• Structural defense• Low nutritional content; sequester nutritious parts• Mutualistic defense (ant-acacia)• Secondary compounds

Types of secondary compounds

• growth regulators

• toxins against generalist herbivores specialists evolve to detoxify toxin often N-based lignin, alkaloids, non-protein amino acids, cyanogenic glycosides--->HCN

• digestive inhibitors against specialists often C-based

tannins, phenolics, terpenoids

Chemical defenses

• Constitutive: high levels at all times• Induced: increase greatly after • attack • Theory: Cost of defense is too high to

maintain under light herbivory.• (but how quickly can they make them?

• Hypothesis: Plants ‘eavesdrop on neighbors - signal to make defense.

What is evidence that plant defenses are induced by herbivory?

• Mite sp 1 attacks.• Plant responds by making defense

chemical.• Mite sp 2 attacks but in much lower

numbers.

#2

to #1

Sample exam ?

1. Contrast the main difference in these types of plant chemical defenses:

A. Constitutive B. Inducible

2. Relative to a constitutive defense, how does an inducible defense benefit the plant?

What is one potential negative aspect of an inducible defense?

Question cont.3. Showshoe hares eat young shoots of trees in winter.

Ecologists found that trees with a high % shoots eaten in Year 1 had a high concentration of chemical defense in Year 2.

The question is: Did herbivory in Yr 1 CAUSE the high defense in yr 2?

A. Develop an “if…then” that addresses this observation.

B. Complete a figure to illustrate the expected results that would support your hypothesis. Label axes.

C. What is a key assumption underlying your hypothesis?

***Describe major pattern in figure. Generate a WHY ? Develop an ‘If…then’.

April October

Herbivores of oak leaves

Figure 5

Hypothesis/prediction:

• If oak leaves become less suitable insect food as they age,

• then caterpillars fed young leaves will grow better than if fed slightly older leaves.

• Diet larval weight % adults emerge• young leaves 45 76• old leaves 18 0

• ***What is conclusion?• Do data support the hypothesis?

“Figure” 6

***What are three changes as a leaf ages that could account for the previous results? (3 alternative hypotheses) • H1: Increase in toughness

• H2: Increase in secondary chemicals

• H3: Decrease in nutrient quality

Toughness index

Leaf ageFigure 7

***Develop predictions for H1 toughness and H2 chemical defense.

• If leaf toughness explains seasonal feeding pattern of oak insects,

• then larvae should grow equally well when eating ground-up old vs. young leaves.

• If chemical defenses have increased with leaf age, • then larvae should grow better on ground-up leaves of

young than old leaves.

• ***Are predictions ‘operational’?• Do they contain independent and dependent

variables?

***Results

• Larvae fed ground-up leaves Larval weight

• Young leaves 37• Old leaves 35

• ***Which hypothesis is supported?• Why hasn’t NS favored insect mouth parts able

to cope with tough leaves?• 3rd alternative hypothesis is still possible;

Maybe poorer nutrition in later summer; then NS toward early feeding.

“Figure” 9

• Herbivores consume only @ 10% of plant productivity (up to 30-60% in grasslands). Why so little?

• ***What factors limit herbivory? • Predators• Herbivores• Plants• Nutrients• Abiotic factors

• The ‘world is green’ hypothesis: Herbivores consume a small % of vegetation because they are held in check by a variety of factors.

Top-down control Tri-trophic predators interactions

herbivores

plants

nutrients/light Bottom-up control

“Figure” 10

Observation/question: Despite many potential herbivores, why do leaves lose low leaf area?• Observation: Birds eat insect herbivores.

• ***Hypothesis:• If bird predation on insect herbivores indirectly reduces the amount of leaf area consumed,

• ***Prediction: then leaf area consumed will be greater for plants

with bird-exclusion cages than those without cages.

Experimental set-up…caged tree saplings

Results:1) number of insects: 70% greater on saplings without birds than with birds. 2) % leaf area missing: 35% without birds 22% with birds

• *** What’s conclusion?• Support for hypothesis?• Tri-trophic interaction; top-down control.• Bird predation: directly reduces # of herbivores indirectly reduces leaf damage by herbivores• New questions: Will 1) decreases in bird

populations due to forest fragmentation or 2) change in phenology increase insect damage?

“Figure” 12