Herbivory
I. Introduction
A. Functional types of heterotrophs
Predators
Parasites
Herbivores
How do they differ?
Functional types of heterotrophs
Functional types of heterotrophs
Predators - kill and eat several
animals (prey) over lifetime
Herbivores - may function like
parasites (e.g. aphids sucking plant
sap) or predators (rabbit eating
annual plant). The factor that
defines them is simply they all eat
plants!
Parasites - attack (but not necessarily
kill) other animals, usually attack just
one (host) over lifetime
B. Adaptations for herbivory
B. Adaptations for herbivory
The three digestive challenges of herbivory
1. Low levels of protein
Cellulose and morphological defenses
Secondary plant compounds
The three digestive challenges of herbivory
1. Low levels of protein
2. Cellulose and morphological defenses
3. Secondary plant compounds
Animals
Seeds
Ang. leaves
Gym. leaves
Phloem
Xylem
0.0001
0.001
0.01
0.1
1
10
Percent nitrogen content (dry weight)
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The three digestive challenges of herbivory
1. Low levels of protein
2. Cellulose and morphological defenses
3. Secondary plant compounds
Morphological defenses
Trichomes (hairs) and spines
Structural defenses - leaf toughness
Plant waxes
The three digestive challenges of
herbivory
1. Low levels of protein
2. Cellulose and morphological defenses
3. Secondary plant compounds
Surface defenses against large
herbivores obvious - e.g. cactus
thorns
Surface defenses against insect
herbivores may be less so
Holly leaf wax
and thorns are
obstacles to
herbivores - if
you cut into the
edge, several
caterpillars can
feed
Morphological defenses on the plant
surface. Example - wild potatoes
Morphological defenses on the plant
surface. Example - wild potatoes
Long
hairs
Short
hairs
Two types of
glandular hairs
(“trichomes”)
with sticky
exudate that
traps insects of
different weights
Long hairs with
naked exudate
trap small light
arthropods
such as this
herbivorous
mite
Morphological defenses on the plant
surface. Example - wild potatoes
Morphological defenses on the plant
surface. Example - wild potatoes
Short hairs
with exudate
in membrane
tarsus (foot)
+ claw of larger
insect (Colorado
potato beetle)
Why was someone particularly interested
in the hairs on wild potatoes?
=!
This species of wild potato is resistant to
insects - but tubers toxic
Investigators wanted a potato resistant to
insects (so would not need to use pesticides)
but edible - made hybrids and selected them
for both traits.
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The three digestive challenges of herbivory
1. Low levels of protein
2. Cellulose and morphological defenses
3. Secondary plant compounds
Morphological defenses on the plant
surface. Example - wild potatoes
Not all insects are trapped. Whiteflies
produce lots of wax particles - coat
themselves, coat sticky hairs, go free…
Whitefly wax an example of an insect
countermeasure to a plant defense.
The chemicals that give plants their
different flavors and smells are
secondary plant compounds
called ‘secondary’ because not ordinarily
involved in normal plant metabolism
Whitefly
tarsus
Coated hair
evolved in response to herbivory
The three digestive challenges of herbivory
- cellulose, low levels of protein and
secondary plant compounds. How do
herbivores deal with them?
The three digestive challenges of herbivory
1. Low levels of protein
2. Cellulose and morphological defenses
3. Secondary plant compounds
Some simple things, like they spend a lot of
time eating….
What do they do to the herbivore?
Some toxic, some deterrents, some
interfere with assimilation of nutrients,
e.g. tannins
While predators are hanging out ….
They have long guts for longer
processing time, and some special gut
features
For example ruminants: multiple
stomachs, cud-chewing, and
bacterial fermentation
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They enlist the help of bacteria, protists
or fungi
They enlist the help of bacteria,
protists or fungi.
Aphid bacterial symbionts,
Buchnera provide aphids
with amino acids scarce in
phloem
Protists and grass
in a
ruminant gut
They may have feeding strategies to deal
with cellulose and leaf toughness
ovary
Buchnera
They may have feeding strategies to
deal with cellulose and leaf toughness
Phloem & xylem feeders circumvent most
surface defenses, cellulose
Food canal
Aphid
feeding
The flexible stylets of
phloem- and xylemfeeding insects can
thread around cells,
find vascular tissue.
Salivary canal
Stylets
They may have feeding strategies to deal
with cellulose and leaf toughness
Leaf miners are
insects that
live their
larval life
between the
tough upper
and lower
surfaces of
the leaf, thus
avoiding
surface
waxes, spines
etc.
They may have feeding strategies to deal
with cellulose and leaf toughness
Leaf
skeletonizing
insects avoid
the better
defended
vascular
tissue
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They may have feeding strategies to deal
with cellulose and leaf toughness
Feeding strategies may help with both
morphological defenses and secondary
compounds
Galling insects
secrete substance
that induces the
plant to grow
around it in a very
specific way galling insects
feed on tissue of
the gall
Example: Selective feeding of howler
monkeys.
gall wasp gall on rose
3. Selective feeding of herbivores
3. Selective feeding of herbivores
What explains the ‘rules’ of howler
monkey feeding?
Four ‘rules’ of howler monkey feeding
1 - fed on rare tree species
The selected tree species had lower levels
of alkaloids (toxic) and tannins (inhibit
protein digestion)
1 - fed on rare tree species
2 - fed on few individuals
(e.g. 12/ 149) of acceptable
tree species
3 - fed on young leaves
only
4 - often ate the petiole,
threw the leaf blade away!
2 - fed on few individual trees
The selected individual trees also had
lower levels of alkaloids and tannins than
others in the population
Feeding strategies may help with both
morphological defenses and secondary
compounds
3. Selective feeding of herbivores
What explains the ‘rules’ of howler
monkey feeding?
What explains the ‘rules’ of howler
monkey feeding?
3 - fed on young leaves only
Young leaves had less non-nutritive fiber
Selective feeding reduced monkeys
exposure to non-nutritive and toxic
foliage
4 - often ate the petiole, threw the
leaf blade away!
Petioles had lower concentrations of
alkaloids than the leaf blades
Video clip of selective feeding by
herbivores
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Plant- herbivore evolution
An evolutionary arms race?
Herbivores may also evolve ability to
detoxify secondary plant compounds
Resistance to toxic plant
compounds by the herbivores may
lead to
Most secondary compounds toxic or
deterrent to most herbivores…
But specialist herbivores may be able
to detoxify them, and even use them
to help locate and identify food
source
selection acting on plant
to develop greater quantities and
more kinds of toxins
Reciprocal evolution of this type: an
evolutionary arms race
An evolutionary arms race between herbivore
and plant may lead to specialization of the
herbivore.
1) Evolution of novel toxic compound by
plant - plant escapes herbivores
2) Evolution in herbivore allows
detoxification or sequestration of toxin herbivores colonize plant, enjoy little
competition
3) Herbivores may start to use novel
compound to find plant
4) Cycle repeats leading to more complex
chemistries, more specialization
An evolutionary arms race between
herbivore and plant may lead to
specialization of the herbivore. An example
from insects feeding on wild umbellifers
(plants related to carrots, celery)
Furanocoumarins are a class of secondary
compounds found in many plant families.
Within the Umbelliferae there are different
forms:
Less complex - the linear furanocoumarins
The most complex and difficult to detoxify the angular furanocoumarins
If there was an evolutionary arms race
between insect specialization and
umbellifers developing more complex
chemistries, what relationship would you
expect to see?
If there was an evolutionary arms race
between insect specialization and
umbellifers developing more complex
chemistries, what relationship would you
expect to see?
Insects
Plant chemistry
Generalists
None or linear
furanocoumarins
Specialists
Angular furanocoumarins
Insects
Generalists
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Specialists
Plant chemistry
None or Linear
furanocoumarins
Angular furanocoumarins
An example from insects feeding on wild
umbellifers
Plant- herbivore evolution
Selection on plant compounds may not
only come from response of herbivore.
Natural enemies of herbivores may use
compounds to help find hosts.
Prop.
Prop.
Prop.
specialists intermed.
generalists
(1-3 genera) (4-20 gen.) (>3 families)
Furanocoumarins
None
0.00
0.36
0.64
Linear only
0.30
0.30
0.40
Angular (most
complex) and
linear
0.43
0.285
0.285
An example of a tritrophic (three
trophic levels) interaction
An example of a tritrophic (three trophic
levels) interaction
First more background:
An example of a tritrophic (three
trophic levels) interaction
First more background:
Some inducible plant compounds are
volatile (i.e. low molecular weight,
diffuse in air)
Parasitic wasps and predators of the
herbivores have been found to use
these volatile chemical cues to find
their prey.
How are plant chemical defenses
deployed?
May be present in tissues all the time constitutive plant defenses.
May be produced only in response to
herbivore feeding - inducible plant
defenses.
Are plants calling for help?
C. Plant- herbivore evolution
3. Are plants calling for help? An example.
An example of a tritrophic (three
trophic levels) interaction
One of the caterpillars (HV) is the host of a
species of parasitic wasp. HZ is not a
host.
Two related moths, HV and HZ, are
pests of tobacco and cotton.
HV
HV
HZ
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Wasp arriving
to lay eggs
in HV
An example of a tritrophic (three trophic
levels) interaction
An example of a tritrophic (three trophic
levels) interaction
Percent of visits by wasps
Experimenters let caterpillars feed upon
some plants and then cut off the damaged
leaves. They then measured visits of
wasps to 1) undamaged tobacco plants, 2)
plants which had had HV feeding on them,
3) plants which had had HZ feeding on
them.
The
host of
the
wasp
Non-host
HV
HZ Control
HV
Amount of volatiles
The wasp is most likely to visit plants that
had been fed upon by its host caterpillar.
How can it distinguish different types of
plants?
HZ
Plants fed upon by the
different herbivores
release different
amounts and kinds of
volatile compounds
Undamaged
Volatiles
C. Plant- herbivore evolution
3. Are plants calling for help? An example.
III. Defenses of herbivores against
predators
Summary:
1. Aposematic coloration:
warning coloration
• Plants release a specific blend of chemicals in
response to HV feeding.
• Wasp responds to chemicals, even if
caterpillars are gone.
• The plant benefits if wasp attack prevents
further feeding.
• But did plant signals evolve to attract
herbivores enemies?
Signals toxicity to
predators
Many toxic animals
have adopted red
and black or yellow
and black
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III. Defenses of herbivores against
predators
Where do those aposematic herbivores
get their toxins?
1. Aposematic coloration:
warning coloration
Instead of metabolizing plant toxins,
some herbivores wall them off from
sensitive tissues within their bodies, use
them as a defense
Where do those aposematic herbivores
get their toxins?
called sequestering plant toxins
III. Defenses of herbivores against
predators
2. Mimicry - Muellerian
III. Defenses of herbivores against
predators
Why do those aposematic herbivores
look alike?
2. Mimicry - Batesian
Convergence on the same signal benefits
all toxic animals that have it
When non-toxic animals mimic toxic or
defended species: Batesian mimicry
When toxic animals resemble each other:
Muellerian mimicry
Mimicry
Batesian
mimicry
can
include
adopting
the form
of a very
different
animal
Moth mimicking a tarantula
Moth mimicking a wasp
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III. Defenses of herbivores against
predators
Mimicry - Muellerian and Batesian
mimics may coexist in “mimicry rings”
D. Defenses of herbivores against
predators
2. Mimicry - Batesian
Condition for evolution of Batesian
mimics:
*Palatable
There must be many more individuals of
the toxic or defended species than of the
mimic species.
®
®
*
®
Why?
®
Batesian
mimics
®
*
®
*
*
® Unpalatable
Models and
Muellerian
mimics
III. Defenses of herbivores against
predators
III. Defenses of herbivores against
predators
3. Crypsis
3. Crypsis
Cryptic behaviors -
Butterfly ‘leaf’
motionless (or ‘swaying in wind’)
feed on the leaf underside
feed at night
conceal damage (cut off or trim damaged
part)
Katydid ‘leaf’
Bug ‘thorn’
III. Defenses of herbivores against
predators
4. Feed in groups
Odds of being first
eaten fall with group
size, and if you’re
bad tasting, odds of
being second eaten
small
Other possible
benefits - group defense, e.g. in caterpillars that
regurgitate
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