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

Species Interactions Ch 17 Herbivory

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

Pairwise interspecific interactions ***Which is: +/-, +/0, +/+, -/-, -/0 ? Mutualism Commensalism +,0 Amensalism -,0 Herbivory Predation Parasitism Disease Competition

Pairwise interspecific interactions Mutualism (+/+) Commensalism (+, 0) Amensalism (-, 0) Herbivory (+/-) Predation (+/-) Parasitism Disease Competition (-/-)

Fluidity of interspecific relationships: Can evolve from one type to another. Switch + and - signs of interaction, e.g. +/+ to +/-.

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/ecosystem 2) population 3) community If prefer dominant species---> What is a keystone herbivore? If prefer subdominant species---> Figure 1

How do we test the hypothesis that herbivores control plant populations? Describe, then explain these results.

Herbivores feed on plants - and also inoculate them with pathogens and rot-causing microbes.

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.

***What are plant deterrents to herbivory? Structural defense Low nutritional content; sequester nutritious parts Mutualistic defense (ant-acacia) Secondary compounds Figure 3

Chemical defenses (secondary compounds) are toxic to herbivores.

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

***Are herbivores ‘lawnmowers’ or selective feeders? What 3 factors may explain results? Figure 4

***Describe the major pattern in this 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 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) H 1 : Increase in toughness H 2 : Increase in secondary chemicals H 3 : Decrease in nutrient quality Toughness index Leaf age Figure 7

***Develop predictions for H 1 toughness and H 2 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? ‘Figure’ 8

***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 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. ‘Figure ‘ 11

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