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 ?
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Mutualism
Commensalism +,0
Amensalism
-,0
Herbivory
Predation
Parasitism
Disease
Competition
Pairwise interspecific interactions
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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.
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Predators
Parasites
Parasitoids
Pathogens
Herbivores
Herbivory: Effects on plants
Outbreaks of herbivorous insects can
defoliate forests.
Spruce budworm
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
In spite of plant defenses, herbivores can
control plant population size.
Klamath weed + beetle
(biological control agent)
***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?
#2
to
#1
• Mite sp 1 attacks.
• Plant responds by making defense
chemical.
• Mite sp 2 attacks but in much lower
numbers.
***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’.
Herbivores
of oak
leaves
April
October
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
• young leaves
45
• old leaves
18
% adults emerge
76
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
Figure 7
Leaf age
***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
***Results
• Larvae fed ground-up leaves
• Young leaves
• Old leaves
Larval
weight
37
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?
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***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
predators
Tri-trophic
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
“Figure” 12
1) number of insects: 70% greater on saplings
without birds than with birds.
2) % leaf area missing:
35% without birds
22% with birds
Results:
• *** 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?