Page ‹#›

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

Page ‹#›

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.

Page ‹#›

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

Page ‹#›

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

Page ‹#›

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

Page ‹#›

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

Page ‹#›

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

Page ‹#›

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

Page ‹#›

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

Page ‹#›

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