Understanding Ecological Systems: Species Interactions and Herbivory Effects

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

2. Species InteractionsCh 17 Herbivory

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

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

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

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

7. In food chains, all life forms are both consumers and victims of consumers. • Predators • Parasites • Parasitoids • Pathogens • Herbivores

8. Herbivory: Effects on plants

9. Outbreaks of herbivorous insects can defoliate forests. Spruce budworm

10. 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

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

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

13. Does herbivore control plant species?If….then… • Natural enemies hypothesis: • Biological control: Figure 2

14. In spite of plant defenses, herbivores can control plant population size. Klamath weed + beetle (biological control agent)

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

16. Chemical defenses (secondary compounds) are toxic to herbivores.

17. 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

18. 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.

19. What is evidence that plant defenses are induced by herbivory? #2 • Mite sp 1 attacks. • Plant responds by making defense chemical. • Mite sp 2 attacks but in much lower numbers. to #1

20. ***Are herbivores ‘lawnmowers’ or selective feeders?What 3 factorsmay explainresults? Figure 4

21. ***Describe the major pattern in this figure.Generate a WHY ? Develop an ‘If…then’. Herbivores of oak leaves April October Figure 5

22. 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

23. ***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 Figure 7 Leaf age

24. ***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? ‘Figure’ 8

25. ***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

26. 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.

27. Top-down control Tri-trophic predators interactions herbivores plants nutrients/lightBottom-up control “Figure” 10

28. 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

29. Experimental set-up…caged tree saplings

30. “Figure” 12 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?