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Chapter 11
Hebivory
© 2002 by Prentice Hall, Inc.
Upper Saddle River, NJ 07458
Outline
• Plant chemical and mechanical
defenses against herbivores
• Mathematical models predict
polyphagous herbivores have a
greater effect on plants than
monophagous herbivores
Outline
• Herbivores remove 15-18% of
terrestrial plant biomass; over 50%
in aquatic systems
• Herbivore numbers are strongly
influenced by chemical and
mechanical plant defenses, and the
amount of nitrogen in the plant
• Herbivores can change plant
communities by preferentially
eating dominant plant species
Plant Defenses
• Example of plant chemical
defenses
–
–
–
–
–
Alkaloids
Mustard oils
Terpenoids
Phenylpropanes
Figure 11.1
Plant Defenses
• Two general classes of
compounds
– Carbon nutrient balance theory
– Nitrogen compounds
• Limited by carbon (due to shortages
of light or water)
– Carbon compounds
• Limited by nitrogen
– Table 11.1
Plant Defenses
• Secondary chemicals
– Not part of primary metabolic
pathways that plants use to obtain
energy
– Defense compounds
Plant Defenses
• Strategies of plant defenses
– Quantitative defenses
• Substances that are ingested in large
amounts by the herbivore
• Prevent digestion of food
• Ex. tannins and resins
• Figure 11.2
Number of caterpillars
per 50 leaf clusters
% Dry weight tannin
concentration of leaf
3
2
1
0
40
30
20
0
May
June
July
August
Sept.
Plant Defenses
• Strategies of plant defenses
(cont.).
– Qualitative defenses
• Highly toxic substances
• Very small doses can kill herbivore
• Ex. Atropine
• Figure 11.3
Plant Defenses
• Strategies of plant defenses
(cont.).
– Apparency
• Correlated to qualitative and
quantitative defense strategies
• Apparent plants
– Long-lived
Plant Defenses
• Apparent plants (cont.).
– Apparent to herbivores (mainly insects)
– Defenses are mainly quantitative
» Effective against specialist and
generalist
» Long history of association with these
K-selected plants
Plant Defenses
– Apparency (cont.).
• Unapparent plants
– Weeds
– Ephemeral and unavailable for long
periods of the year
– Defenses are mainly qualitative
– Herbivores are mainly generalist like
vertebrates
• Table 11.2
Plant Defenses
• Strategies of plant defenses
(cont.).
– Mechanical defenses
• Thorns and spines defer vertebrate
herbivores
Plant Defenses
– Mechanical defenses (cont.).
• Generalizations (Peter Grubb)
– In many open sites, plants are primarily
close to the ground and so are very
spinose to protect them
– Plants such as palms, with one or a few
apical meristems, are also likely to
protect them with spines
Plant Defenses
• Generalizations (Peter Grubb) (cont.).
– Evergreens, such as holly, in a deciduous
forest are likely to face severe herbivore
pressure in the winter and so are very
spinose
– Repellents
• Thistles produce compounds that
repel certain insect larvae
Plant Defenses
– Repellents (cont.).
• Potatoes synthesize a component of
an alarm pheromone released by
aphids when they are attacked by
predators
– Alarm pheromone causes aphids to flee
– As a result, the potatoes are free of the
aphids
Plant Defenses
• Strategies of plant defenses
(cont.).
– Reproductive inhibition
• Plants (e.g., firs) contain insect
hormone derivatives
– If digested, prevent insect metamorphosis
– Results in diminished reproductive output
Plant Defenses
– Reproductive inhibition (cont.).
• Plants (e.g., floss flower) produce a
chemical mimic of insect molting
hormone ecdysone
– Insects digest plant material
– Insects die when they molt prematurely
Plant Defenses
• Strategies of plant defenses
(cont.).
– Masting
• Occurs in a few tree species (e.g.,
some oaks)
Plant Defenses
– Masting (cont.).
• Production of more seeds in some
years
– Satiates herbivores
– Permits more seed to survive – Ex. Beech
» In mast years, 3.1% of seeds destroyed
by boring moths
» In non-mast years, 38% of seeds
destroyed
Plant Defenses
• Production of more seeds in some
years (cont.).
– Other benefits
» Enhanced pollination
» Enhanced seed dispersal
Plant Defenses
• Strategies of plant defenses
(cont.).
– Defensive associations
• Protection from herbivores through
association with unpalatable
neighbors
Plant Defenses
– Defensive associations (cont.).
• Ex. Chrysomelid beetle and the
purple loosestrife
– Loosestrife sometimes grows on its own,
or in thickets of an aromatic shrub, Myrica
gale
– Myrica secretes a volatile chemical that
deters insects from feeding on it
Plant Defenses
• Ex. Chrysomelid beetle and the
purple loosestrife (cont.).
– Chemical also interferes with beetle
searching for loosestrife
– Figure 11.4
Proportion leaf damage
Number larvae/plant
4
3
2
1
0
0.08
0.06
0.04
0.02
0
Outside
Myrica
Inside
Myrica
Plant Defenses
– Defensive associations (cont.).
• Opposite association: Associational
susceptibility
– The spilling over of herbivores from
palatable neighbors
– Fall cankerworms prefer to feed on box
elder trees and rarely feed on isolated
cottonwood trees.
Plant Defenses
• Opposite association: Associational
susceptibility (cont.).
– When cottonwoods occur under box elder,
the cankerworms spill over and defoliate
the cottonwoods
– Figure 11.5
Canker worm density
(larvae/ shoot)
a)
1.50
1.25
1.00
0.75
0.50
0.25
0
80
Percent defoliation
b)
60
40
20
10
0
Cottonwood Cottonwood Cottonwood
under
under
In open
elder box Cottonwood
Plant Defenses
• Strategies of plant defenses
(cont.).
– Mutualism
• Plants that defend themselves
through enlisting help from other
animals
Plant Defenses
• Understanding plant defenses
– Important to agriculture
• Use knowledge to defend crops
against insects
• Problems
– Long-time needed to develop resistances
– Resistance to one pest may increase
susceptibility to other pests
Plant Defenses
– Important to agriculture (cont.).
• Benefits
– Once resistance is developed, requires
minimal cost from farmer
– Environmentally benign
• Insect response to resistance
– Certain chemicals that are toxic to
generalist insects, actually increase the
growth rates of adapted specialist insects
Plant Defenses
• Insect response to resistance (cont.).
– Specialization of herbivores to supposedly
toxic plants – evolutionary “arms race”
(Diagram)
– Figure 11.6
Host plant chemistry
0.50
Complex:
with angular and linear
Proportion of total insects
furanocoumarins
0.25
0
0.50
Intermediate:
with linear
furanocoumarins only
0.25
0
0.75
Simple:
without furanocoumarins
0.50
0.25
0
Extreme
Generalists
(feed on 1-3
Genera)
Intermediate
Species
(feed on 4-20
Genera)
Polyphagous
(feed on
>3 families)
Modeling Herbivory
• Series of plant-herbivore
interactions models developed
by Mick Crawley (1997)
Modeling Herbivory
• Models depend on the degree
of polyphagy of the herbivores
– Monophagous herbivores are
highly specialized and feed on
only one species
• Many insects tend to be specialized
Modeling Herbivory
• Models depend on the degree
of polyphagy of the herbivores
(cont.).
– Polyphagous herbivores feed on
many species
• Many vertebrates tend to be
generalists
Modeling Herbivory
– Polyphagous herbivores feed on
many species (cont.).
• Exceptions: Pandas and bamboo, and
koala and eucalyptus
Modeling Herbivory
• Models for Monophagous
herbivores
– Simplest model assumes a
carrying capacity – K, for the
plants
Modeling Herbivory
• Models for Monophagous
herbivores (cont.).
– Rate of change in plant population
• dV/dt = A – B
– dV/dt = rate of plant increases (numbers
or biomass)
– A = gains by growth
– B = losses by herbivory
Modeling Herbivory
– Rate of change in plant population
(cont.).
• dN/dt = C – D
– dN/dt = rate of herbivore increase
– C = gains by growth
– D = losses by death
Modeling Herbivory
– Rate of change in plant population
(cont.).
• In the absence of herbivores, plant
populations increase logistically
– dV/dt = rV [(K – V) / K]
– r = plant’s intrinsic rate of increase
Modeling Herbivory
– Rate of change in plant population
(cont.).
• Losses for plants
– B = bNV
» b = feeding rate of herbivores
» bNV = functional response
» Type I: accurately modeled by bNV
Modeling Herbivory
– Rate of change in plant population
(cont.).
• Herbivore population growth
– C = cNV
» c = numerical response of herbivores;
their degree of attraction is affected
by plant density
Modeling Herbivory
– C = cNV (cont.).
» cNV is essentially a measure of
efficiency with which herbivores turn
food into progeny
• Losses for herbivores
– D = dN
– d = herbivore death rate
– Assumption: in the absence of plants, the
herbivores starve and their numbers
exponentially decline
Modeling Herbivory
– Rate of change in plant population
(cont.).
• At equilibrium;
– DV/dt and dN/dt are zero
– No population changes
– A = B and C = D
Modeling Herbivory
• At equilibrium; (cont.).
– V* = d/c
» Paradoxical results: plant equilibrium
abundance has nothing to do with
plant growth rate or carrying capacity
» Figure 11.7a
a)
Specialist herbivores
Plant abundance (V*)
1000
800
600
0
0.2
0.4
0.6
r
per capita rate of increase
0.8
1.0
Modeling Herbivory
• Models for polyphagous
vertebrates
– Constant level of herbivory – h
• Herbivore densities are not tied to
any one plant
– Plant growth
• dV/dt = rV [(K – V)/ K] –h
Modeling Herbivory
– Plant growth (cont.).
• For equilibrium plant biomass, V*
– V = (aK √r2K2 – 4rhK) / 2r
– Figure 11.7b
Generalist herbivores
b)
Plant abundance (V*)
1000
800
600
0
0.2
0.4
0.6
0.8
r
per capita rate of increase
1.0
Modeling Herbivory
• Caveats to models
– Misleading: c and d are treated
like independent parameters.
Could lead to unrealistic results
– There are at least 3 different
types of functional response
Effects of Herbivory on
Plants
• Herbivore responses to plant
defenses
– Detoxify plant poisons
• Oxidation
• Conjugation
Effects of Herbivory on
Plants
• Herbivore responses to plant
defenses (cont.).
– Measured effects of herbivores on
plants in the field
• In 93 cases, 7% of leaf area was
consumed
Effects of Herbivory on
Plants
– Measured effects of herbivores on
plants in the field (cont.).
• In forested systems, 5-15%
defoliation by insects
• May underestimate effects due to
leaf turnover
Effects of Herbivory on
Plants
– Measured effects of herbivores on
plants in the field (cont.).
• Herbivory increases with plant
productivity (Figure 11.8)
Aquatic
104
Terrestrial
3
10
Herbivory rate (gC m-2 year -1 )
2
10
10
1
-1
10
-2
10
5
10
2
103
10
Total net primary productivity (gC-2m
year -1 )
Effects of Herbivory on
Plants
• Removal experiments
– Best way to estimate effects of
herbivory on plants
• Remove herbivores
Effects of Herbivory on
Plants
– Best way to estimate effects of
herbivory on plants (cont.).
• Examine effects on plant growth and
reproduction
Effects of Herbivory on
Plants
– Best way to estimate effects of
herbivory on plants (cont.).
• Review of 246 experiments (Bigger
and Marvier, 1998)
– Metaanalysis statistical technique
Effects of Herbivory on
Plants
– Metaanalysis statistical technique (cont.).
» Mean effect size d was 0.47, which is a
medium size effect
» Meaning: the average size of plant
protected from herbivores was
significantly larger than those that
were not
Effects of Herbivory on
Plants
• Review of 246 experiments (Bigger
and Marvier, 1998) (cont.).
– Stronger effects in aquatic systems
» Figure 11.9
Terrestrial vertebrate herbivores
Terrestrial invertebrate herbivores
Aquatic vertebrate herbivores
Aquatic invertebrate herbivores
2
1.5
1
0.5
Effect size
0
-0.5
Effects of Herbivory on
Plants
• Review of 246 experiments (Bigger
and Marvier, 1998) (cont.).
– Which type of plants are most affected?
» Effect size of herbivores was greatest
on algae (Figure 11.10), also on
grasses and shrubs.
Effect size
1.5
1
0.5
Algae
Grasses
0
Shrubs
Trees
Effects of Herbivory on
Plants
– Which type of plants are most affected?
(cont.).
» Effect size of herbivore was smallest
on woody plants
• Biological control
– Evidence from biological control
of weeds in agriculture
Effects of Herbivory on
Plants
• Biological control (cont.).
– 50% of the 190 major weeds in the
US are invaders from outside the
country
– Interest in biological control due
to expense of chemical control
Effects of Herbivory on
Plants
• Biological control (cont.).
– Biological control success stories
• Klamath weed was controlled by two
French beetles
Effects of Herbivory on
Plants
– Biological control success stories
(cont.).
• A floating fern, Salvinia molesta was
controlled by the weevil Cyrtobagus
salvinae
Effects of Herbivory on
Plants
– Biological control success stories
(cont.).
• The Florida alligator weed was
controlled by the alligatorweed flea
beetle
• Figure 11.11
Effects of Herbivory on
Plants
• Biological control (cont.).
– Unsuccessful biological control
stories
• Control of Lantana camara in Hawaii
Effects of Herbivory on
Plants
– Unsuccessful biological control
stories (cont.).
• In a review of 701 examples of
biological weed control, only 26%
were rated successful
Effects of Herbivory on
Plants
• Beneficial herbivory
– Review of herbivory (Bigger and
Marvier, 1998)
• 60 comparsions demonstrated a
reduction in plant size due to natural
levels of herbivory
Effects of Herbivory on
Plants
– Review of herbivory (Bigger and
Marvier, 1998) (cont.).
• 10 comparisons demonstrated an
increase in plant size
– Plants are stimulated to regrow after
damage, and may overcompensate
Effects of Herbivory on
Plants
• Beneficial herbivory (cont.).
– Benefits of root herbivory (Figure
11.12)
D
D
D
D
D
D DD D
D D
D
D
D
D
D
D
A
D
D
D
D
D
D
D
D
D
D
D D
High- tide level
D D
D
D
D
D
D
D
D
A A A
D
D
Soil
D
D
D
D
A
D
D
A
D
A
D D
A
A
D
A A
Effects of Herbivory on
Plants
• Beneficial herbivory (cont.).
– Benefits of grazing and regrowth
• More flowers and fruits on grazed
plants (Gronemeyer et al. 1997)
• More likely to occur in perennials
than annuals
Effects of Plants on
Herbivores
• Herbivores select those plants
that have the most nutrition (in
terms of nitrogen)
Effects of Plants on
Herbivores
• Animal tissue contains 10
times more nitrogen than plant
tissue
Effects of Plants on
Herbivores
• Red deer prefer to feed on
grasses that were defecated
upon by herring gulls
– As the amount of droppings
increased, the nitrogen content of
the grasses also increased
Effects of Plants on
Herbivores
• Plant fertilization (i.e., nitrogen)
had a positive effect on
population size, survivorship,
growth and fucundity of insects
(Figure 11.13)
Number of caterpillars
per 50 leaf clusters
% Dry weight tannin
concentration of leaf
3
2
1
0
40
30
20
0
May
June
July
August
Sept.
Effects of Plants on
Herbivores
• Plant fertilization (cont.).
– Response was greater in
cultivated versus wild plants, and
broadleaf trees versus conifers
(Figure 11.14)
Percentage of cases
0
20
40
60
80
Conifers
Broadleaf trees
Herbs
Herbivore response:
Positive
Negative
100
Effects of Plants on
Herbivores
• Two variations of the nitrogen
limitation theory
– Stress hypothesis (White 1993)
Effects of Plants on
Herbivores
– Stress hypothesis (White 1993)
(cont.).
• Plant stresses tend to increase the
availability of nitrogen because many
nitrogen-rich compounds are
mobilized in response to stress
Effects of Plants on
Herbivores
• Plant stresses tend to increase the
availability of nitrogen because many
nitrogen-rich compounds are
mobilized in response to stress
(cont.).
– Ex. drought stressed plants accumulate
high numbers of herbivores
Effects of Plants on
Herbivores
• Two variations of the nitrogen
limitation theory (cont.).
– Plant vigor hypothesis (Price
1991)
Effects of Plants on
Herbivores
– Plant vigor hypothesis (Price
1991) (cont.).
• Herbivores select fast growing parts
of plants or fast growing plants
because these are higher in nitrogen
Effects of Plants on
Herbivores
• Herbivores select fast growing parts
of plants or fast growing plants
because these are higher in nitrogen
(cont.).
– Ex. Many attacks by insects are on young
trees
Effects of Plants on
Herbivores
• Herbivore density correlated
with plant quality
• Herbivore population patterns
are dependent on other factors
– Predation
Effects of Plants on
Herbivores
• Herbivore population patterns
are dependent on other factors
(cont.).
– Parasitism
Herbivory Affects
Community Structure
• Darwin observed that
competitively dominant
grasses were kept in check by
grazing
Herbivory Affects
Community Structure
• United States and bison (Figure
11.15)
Species richness per 50m 2
6.6
Control
39.7
9.4
Grazed
54.6
Plant type:
C4
C3
Applied Ecology
• Pest control around the world
• Huge monocultures are ideal
targets for insect pests and
diseases
Applied Ecology
• Wide variety of control
techniques (Table 1)
Applied Ecology
• Pesticides in the US
– 50,000 pesticides are registered
for use in the US
– Five to six hundred million kg of
pesticides used each year
– 70% for agriculture, 23% for
forestry, 7% for home and garden
Applied Ecology
• Pesticides in the US
– Many are strong poisons
– 60% of herbicides and 30% of
insecticides are potentially
oncogenic
– 95% of human tissue samples
contain pesticide residues
Applied Ecology
• Effects on non-target
organisms
Summary
• A variety of plant defenses
demonstrate the strength and
frequency of herbivory in
nature.
– Chemical defenses
– Mechanical defenses
Summary
• A variety of plant defenses
demonstrate the strength and
frequency of herbivory in
nature. (cont.).
– Hormone mimics
– Mutualism
Summary
• Chemical defenses
– Quantitative
• Build up in the gut of the herbivore
preventing digestion
– Qualitative
• Toxic compounds that can be lethal
in small doses
Summary
• Mathematical models
– Effects of herbivores on plants
depends whether they are
monophagous or polyphagous
• Effects of herbivores
– Herbivores remove 15-18% of
terrestrial plant tissue
Summary
• Effects of herbivores (cont.).
– Herbivores remove 51% of aquatic
plant tissue
– Impact of biological control
– Impact in agriculture
Summary
• Population densities of
herbivores are strongly
influenced by plant quality,
particularly plant nitrogen
• Herbivory may have substantial
effects on plant communities
Discussion Question #1
• Cannabin and cocaine are
secondary chemicals produced by
the hemp plant (Cannibis sativa)
and the coca plant, respectively.
What do you suppose are the roles
of these substances in the lives of
their plants?
Discussion Question #2
• From a theoretical standpoint,
think about some differences
that might occur between
monophagous and polyphagous
herbivores. Why are the majority
of mammals polyphagous and yet
many insects are monophagous?
Discussion Question #3
• Given the advantages and
disadvantages of different
forms of pest control, which do
you think would be more
suitable for annual crops? For
orchards? For forests? Why?
Discussion Question #4
• Do you think chemical
defenses would be more likely
to occur in lush tropical leaves
that have no other defenses or
in tough desert plants that can
ill afford the losses to
herbivores?
Discussion Question #5
• Do you think that if plants need
to protect their valuable tissue
from losses to herbivores,
flowers and fruits would have
high levels of secondary
chemicals? Explain.