Transcript If…. then…

Our ecological ‘footprint’…
1)
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?
•
In food chains, all life forms are both
consumers and victims of consumers.
•
•
•
•
•
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
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)
***Are herbivores ‘lawnmowers’ or selective
feeders?
What 3 factors
may explain
results?
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?
#2
to
#1
• Mite sp 1 attacks.
• Plant responds by making defense
chemical.
• Mite sp 2 attacks but in much lower
numbers.
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’.
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?
***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?
•
•
•
•
•
•
***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.
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?