herbivory - university of arizona
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Herbivory I. IntroductionA. Functional types of heterotrophs
Predators
Parasites
Herbivores
How do they differ?
Functional types of heterotrophs
Predators - kill and eat severalanimals (prey) over lifetime
Parasites - attack (but not necessarilykill) other animals, usually attack justone (host) over lifetime
Functional types of heterotrophs
Herbivores - may function likeparasites (e.g. aphids sucking plantsap) or predators (rabbit eatingannual plant). The factor thatdefines them is simply they all eatplants!
B. Adaptations for herbivoryThe three digestive challenges of herbivory1. Low levels of proteinCellulose and morphological defensesSecondary plant compounds
Xylem
Phloem Gym. leaves
Ang. leavesSeeds
Animals
0.0001 0.001 0.01 0.1 1 10
Percent nitrogen content (dry weight)
B. Adaptations for herbivoryThe three digestive challenges of herbivory1. Low levels of protein2. Cellulose and morphological defenses3. Secondary plant compounds
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The three digestive challenges of herbivory1. Low levels of protein2. Cellulose and morphological defenses3. Secondary plant compoundsMorphological defensesTrichomes (hairs) and spinesStructural defenses - leaf toughnessPlant waxesHolly leaf waxand thorns areobstacles toherbivores - ifyou cut into theedge, severalcaterpillars canfeed
The three digestive challenges ofherbivory1. Low levels of protein2. Cellulose and morphological defenses3. Secondary plant compounds
Surface defenses against largeherbivores obvious - e.g. cactusthorns
Surface defenses against insectherbivores may be less so
Morphological defenses on the plantsurface. Example - wild potatoes
Two types ofglandular hairs(“trichomes”)with stickyexudate thattraps insects ofdifferent weights
Longhairs
Shorthairs
Morphological defenses on the plantsurface. Example - wild potatoes
Long hairs withnaked exudatetrap small lightarthropodssuch as thisherbivorousmite
Morphological defenses on the plantsurface. Example - wild potatoes
Short hairs tarsus (foot)with exudate + claw of larger = !in membrane insect (Colorado
potato beetle) �
Morphological defenses on the plantsurface. Example - wild potatoes Why was someone particularly interested
in the hairs on wild potatoes?This species of wild potato is resistant to
insects - but tubers toxicInvestigators wanted a potato resistant to
insects (so would not need to use pesticides)but edible - made hybrids and selected themfor both traits.
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Morphological defenses on the plantsurface. Example - wild potatoes
Whiteflytarsus Coated hair
Not all insects are trapped. Whitefliesproduce lots of wax particles - coatthemselves, coat sticky hairs, go free…
Whitefly wax an example of an insectcountermeasure to a plant defense.
The three digestive challenges of herbivory1. Low levels of protein2. Cellulose and morphological defenses3. Secondary plant compounds
The chemicals that give plants theirdifferent flavors and smells aresecondary plant compounds
called ‘secondary’ because not ordinarilyinvolved in normal plant metabolism
evolved in response to herbivory
The three digestive challenges of herbivory1. Low levels of protein2. Cellulose and morphological defenses3. Secondary plant compounds
What do they do to the herbivore?
Some toxic, some deterrents, someinterfere with assimilation of nutrients,e.g. tannins
The three digestive challenges of herbivory- cellulose, low levels of protein andsecondary plant compounds. How doherbivores deal with them?
Some simple things, like they spend a lot oftime eating….
While predators are hanging out ….
They have long guts for longerprocessing time, and some special gutfeatures
For example ruminants: multiplestomachs, cud-chewing, andbacterial fermentation
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They enlist the help of bacteria,protists or fungi.
Protists and grass in aruminant gut
They enlist the help of bacteria, protistsor fungi
BuchneraovaryAphid bacterial symbionts,Buchnera provide aphidswith amino acids scarce inphloem
They may have feeding strategies to dealwith cellulose and leaf toughness
Phloem & xylem feeders circumvent mostsurface defenses, cellulose
Aphidfeeding
Stylets
They may have feeding strategies todeal with cellulose and leaf toughness
The flexible stylets ofphloem- and xylem-feeding insects canthread around cells,find vascular tissue.
Food canal
Salivary canal
They may have feeding strategies to dealwith cellulose and leaf toughness
Leaf miners areinsects thatlive theirlarval lifebetween thetough upperand lowersurfaces ofthe leaf, thusavoidingsurfacewaxes, spinesetc.
They may have feeding strategies to dealwith cellulose and leaf toughness
Leafskeletonizinginsects avoidthe betterdefendedvasculartissue
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They may have feeding strategies to dealwith cellulose and leaf toughness
Galling insectssecrete substancethat induces theplant to growaround it in a veryspecific way -galling insectsfeed on tissue ofthe gall
gall wasp gall on rose
Feeding strategies may help with bothmorphological defenses and secondarycompounds
Example: Selective feeding of howlermonkeys.
3. Selective feeding of herbivores
Four ‘rules’ of howler monkey feeding
1 - fed on rare tree species2 - fed on few individuals(e.g. 12/ 149) of acceptabletree species3 - fed on young leavesonly4 - often ate the petiole,threw the leaf blade away!
3. Selective feeding of herbivores
What explains the ‘rules’ of howlermonkey feeding?1 - fed on rare tree species
The selected tree species had lower levelsof alkaloids (toxic) and tannins (inhibitprotein digestion)
2 - fed on few individual treesThe selected individual trees also hadlower levels of alkaloids and tannins thanothers in the population
3. Selective feeding of herbivores
What explains the ‘rules’ of howlermonkey feeding?
3 - fed on young leaves onlyYoung leaves had less non-nutritive fiber
4 - often ate the petiole, threw theleaf blade away!Petioles had lower concentrations ofalkaloids than the leaf blades
Feeding strategies may help with bothmorphological defenses and secondarycompounds
What explains the ‘rules’ of howlermonkey feeding?
Selective feeding reduced monkeysexposure to non-nutritive and toxicfoliage
Video clip of selective feeding byherbivores
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Herbivores may also evolve ability todetoxify secondary plant compounds
Most secondary compounds toxic ordeterrent to most herbivores…
But specialist herbivores may be ableto detoxify them, and even use themto help locate and identify foodsource
Plant- herbivore evolutionAn evolutionary arms race?
Resistance to toxic plantcompounds by the herbivores maylead to
selection acting on plantto develop greater quantities andmore kinds of toxins
Reciprocal evolution of this type: anevolutionary arms race
An evolutionary arms race between herbivoreand plant may lead to specialization of theherbivore.1) Evolution of novel toxic compound by
plant - plant escapes herbivores2) Evolution in herbivore allows
detoxification or sequestration of toxin -herbivores colonize plant, enjoy littlecompetition
3) Herbivores may start to use novelcompound to find plant
4) Cycle repeats leading to more complexchemistries, more specialization
An evolutionary arms race betweenherbivore and plant may lead tospecialization of the herbivore. An examplefrom 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 furanocoumarinsThe most complex and difficult to detoxify -
the angular furanocoumarins
If there was an evolutionary arms racebetween insect specialization andumbellifers developing more complexchemistries, what relationship would youexpect to see?
Insects Plant chemistry
Generalists None or linear furanocoumarins
Specialists Angular furanocoumarins
If there was an evolutionary arms racebetween insect specialization andumbellifers developing more complexchemistries, what relationship would youexpect to see?
Insects Plant chemistry
Generalists None or Linear furanocoumarins �
Specialists Angular furanocoumarins
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An example from insects feeding on wildumbellifers
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 (mostcomplex) andlinear 0.43 0.285 0.285
Plant- herbivore evolutionSelection on plant compounds may notonly come from response of herbivore.Natural enemies of herbivores may usecompounds to help find hosts.
An example of a tritrophic (threetrophic levels) interaction
An example of a tritrophic (threetrophic levels) interactionFirst more background:
How are plant chemical defensesdeployed?
May be present in tissues all the time -constitutive plant defenses.
May be produced only in response toherbivore feeding - inducible plantdefenses.
An example of a tritrophic (three trophiclevels) interactionFirst more background:
Some inducible plant compounds arevolatile (i.e. low molecular weight,diffuse in air)
Parasitic wasps and predators of theherbivores have been found to usethese volatile chemical cues to findtheir prey.
Are plants calling for help?
An example of a tritrophic (threetrophic levels) interaction
Two related moths, HV and HZ, arepests of tobacco and cotton.
HV HZ
C. Plant- herbivore evolution3. Are plants calling for help? An example.
One of the caterpillars (HV) is the host of aspecies of parasitic wasp. HZ is not ahost.
Wasp arrivingto lay eggsin HV
HV
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An example of a tritrophic (three trophiclevels) interaction
Experimenters let caterpillars feed uponsome plants and then cut off the damagedleaves. They then measured visits ofwasps to 1) undamaged tobacco plants, 2)plants which had had HV feeding on them,3) plants which had had HZ feeding onthem.
An example of a tritrophic (three trophiclevels) interaction
Perc
ent o
f vis
its b
y w
asps
HV HZ Control
Thehost ofthewasp
Non-host
The wasp is most likely to visit plants thathad been fed upon by its host caterpillar.How can it distinguish different types ofplants? Plants fed upon by the
different herbivoresrelease differentamounts and kinds ofvolatile compounds
Volatiles
Amou
nt o
f vol
atile
sHV
HZ
Undamaged
C. Plant- herbivore evolution3. Are plants calling for help? An example.
Summary:• 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?
III. Defenses of herbivores againstpredators
1. Aposematic coloration:warning coloration
Signals toxicity to predators
Many toxic animalshave adopted red and black or yellowand black
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III. Defenses of herbivores againstpredators
1. Aposematic coloration:warning coloration
Where do those aposematic herbivoresget their toxins?
Where do those aposematic herbivoresget their toxins?
Instead of metabolizing plant toxins,some herbivores wall them off fromsensitive tissues within their bodies, usethem as a defense
called sequestering plant toxins
III. Defenses of herbivores againstpredators
2. Mimicry - Muellerian
Why do those aposematic herbivoreslook alike?
Convergence on the same signal benefitsall toxic animals that have it
When toxic animals resemble each other:Muellerian mimicry
III. Defenses of herbivores againstpredators
2. Mimicry - Batesian
When non-toxic animals mimic toxic ordefended species: Batesian mimicry
MimicryBatesian
mimicrycanincludeadoptingthe formof a verydifferentanimal
Moth mimicking a tarantula Moth mimicking a wasp
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D. Defenses of herbivores againstpredators
2. Mimicry - Batesian
Condition for evolution of Batesianmimics:
There must be many more individuals ofthe toxic or defended species than of themimic species.
Why?
III. Defenses of herbivores againstpredatorsMimicry - Muellerian and Batesianmimics may coexist in “mimicry rings”
*PalatableBatesianmimics
® UnpalatableModels andMuellerianmimics
*
*
**
®
®
®
®®
®
III. Defenses of herbivores againstpredators
3. Crypsis
Bug ‘thorn’
Butterfly ‘leaf’ Katydid ‘leaf’
III. Defenses of herbivores againstpredators
3. Crypsis
Cryptic behaviors -
motionless (or ‘swaying in wind’)feed on the leaf undersidefeed at nightconceal damage (cut off or trim damagedpart)
III. Defenses of herbivores againstpredators
4. Feed in groups
Odds of being firsteaten fall with groupsize, and if you’rebad tasting, odds ofbeing second eatensmall
Other possiblebenefits - group defense, e.g. in caterpillars thatregurgitate
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