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General Ecology: Lecture 12
November 10, 2003
Topic: Species Interactions: Herbivory and Mutualism
I.
II.
III.
IV.
Overview: Why treat herbivory separately from general predation topics (since we earlier
defined herbivory as a type of predation)?
Defense mechanisms in plants
A.
Types of defenses
1.
Structural defenses: spines, tough tissues, etc…
2.
Chemical defenses: secondary plant substances [Fig. 14.1]
B.
Two hypotheses about the origin of secondary plant substances used to defend plants
1.
These compounds were originally adaptations of the plant to avoid autointoxication
(i.e. these substances are less toxic than their precursor metabolites) later became
important in plant defense
2.
These compounds specifically evolved to thwart herbivores.
a)
Predictions that support the current, primary function of these compounds as
protection against herbivores (second hypothesis)—See text p. 236

NOTE: Sloppy logic in text… Your book states, “Much evidence to
support these four predictions has now accumulated. The
autointoxication view is now thought to be incorrect…”
However…Support of these predictions in no way actually tests
whether these compounds developed primarily to rid the plant of
toxins or as plant defense. To disprove that hypothesis, you would
have to show that these compounds are no less toxic than their
metabolic precursors.
C.
Which types of plants are more likely to invest strongly in defense mechanisms?
1.
Plant apparency hypothesis (Paul Feeny, 1976)
2.
Resource availability hypothesis
D.
Inducible defenses
1.
Advantage?
E.
Coevolution of plants and herbivores
1.
Example 1: Common oak (Quercus robur) and its Lepidopteran predators
(butterflies, moths) in western Europe.
2.
Example 2: Ants and acacia (mutualistic relationship)
Grazing facilitation?: Herbivores on the Serengeti Plains
A.
Timing and feeding ecology of three major ungulate species feeding on plains grasses
[Fig. 14.9]
B.
Focus: Impact of wildebeest
1.
Wildebeest exclusion experiments [Fig. 14.11]
C.
Prediction of hypothesis (according to text): “If grazing facilitation is mutualistic and
obligatory, wildebeest numbers should not increase if zebra numbers do not increase,
and if wildebeest numbers increase, gazelle numbers should also increase.”
1.
Not supported by data [Fig. 14.12]
2.
Does this lack of support mean that grazing facilitation doesn’t really occur?
Can grazing benefit plants?: The overcompensation hypothesis
A.
Possibly, moderate levels of grazing actually increase plant growth rate,
overcompensating for grazing loss [Fig. 14.13]
1.
Note that this is a theoretical idea without experimental evidence, but has been used
to support grazing on federal lands.
Study Questions
1.
What are the two key hypotheses put forth to explain the origin of secondary plant substances?
Are these hypotheses mutually exclusive? Explain.
2.
What key predictions support the hypothesis that the current, major function of secondary plant
compounds is plant defense? If supported, do these predictions also show that the secondary
compounds originally evolved for plant defense? If not, what type of evidence could disprove the
hypothesis that the secondary compounds developed not for defense, but to reduce toxic effects of
other metabolites to plants?
3.
Describe/explain the “plant apparency hypothesis”. In particular, what are non-apparent vs.
apparent plants? What types/level of plant defenses would be expected to develop in non-apparent
vs. apparent plants?
4.
Describe/explain the “resource availability hypothesis”, and compare its predictions to that of the
plant apparency hypothesis.
5.
What are inducible defenses, and what might be their advantage?
6.
What is coevolution? Provide at least 2 examples of coevolution of plants and herbivores. Be sure
to describe the particular adaptations of both the plant and the herbivore in response to the other.
Are these relationships always +/-? Explain.
7.
Describe the patterns of feeding by the three major ungulates of the Serengeti Plains. Your answer
should describe both temporal differences as well as differences in the plants/plant parts utilized
by these different grazers.
8.
What is meant by “grazing facilitation”? Provide an example where grazing by one species
actually appears to facilitate grazing by another species.
9.
According to the text, what would the data in Figure 14.2 look like if grazing facilitation was
responsible for population sizes of the major ungulates on the Serengeti Plains? Does the data
shown in Figure 14.2 disprove the hypothesis that grazing facilitation occurs on the Serengeti
Plain? Explain.
10.
In your own words, state the “overcompensation hypothesis”. Is there much evidence for this
hypothesis? Why mention it, then?