Page # Herbivory. I. Introduction A. Functional types of heterotrophs. Predators. Parasites. Herbivores. How do they differ?

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1 Herbivory I. Introduction A. Functional types of heterotrophs Predators Parasites Herbivores How do they differ? Functional types of heterotrophs Predators - kill and eat several animals (prey) over lifetime Parasites - attack (but not necessarily kill) other animals, usually attack just one (host) over lifetime Functional types of heterotrophs Herbivores - may function like parasites (e.g. aphids sucking plant sap) or (rabbit eating annual plant). The factor that defines them is simply they all eat plants!

2 B. Adaptations for herbivory The three digestive challenges of herbivory 1. Low levels of protein Cellulose and morphological defenses Secondary plant compounds B. Adaptations for herbivory The three digestive challenges of herbivory 1. Low levels of protein 2. Cellulose and morphological defenses 3. Secondary plant compounds Animals Phloem Seeds Ang. leaves Gym. leaves Xylem Percent nitrogen content (dry weight) 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 Holly leaf wax and thorns are obstacles to herbivores - if you cut into the edge, several caterpillars can feed 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

3 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.

4 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 three digestive challenges of herbivory 1. Low levels of protein 2. Cellulose and morphological defenses 3. Secondary plant compounds The chemicals that give plants their different flavors and smells are secondary plant compounds Whitefly tarsus Coated hair called secondary because not ordinarily involved in normal plant metabolism evolved in response to herbivory The three digestive challenges of herbivory 1. Low levels of protein 2. Cellulose and morphological defenses 3. Secondary plant compounds What do they do to the herbivore? Some toxic, some deterrents, some interfere with assimilation of nutrients, e.g. tannins The three digestive challenges of herbivory - cellulose, low levels of protein and secondary plant compounds. How do herbivores deal with them? Some simple things, like they spend a lot of time eating.

5 While 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 They enlist the help of bacteria, protists or fungi They enlist the help of bacteria, protists or fungi. Protists and grass in a ruminant gut Aphid bacterial symbionts, Buchnera provide aphids with amino acids scarce in phloem ovary Buchnera

6 They may have feeding strategies to deal with cellulose and leaf toughness They may have feeding strategies to deal with cellulose and leaf toughness Phloem & xylem feeders circumvent most surface defenses, cellulose Aphid feeding Food canal Salivary canal The flexible stylets of phloem- and xylemfeeding insects can thread around cells, find vascular tissue. 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

7 They may have feeding strategies to deal with cellulose and leaf toughness 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 Feeding strategies may help with both morphological defenses and secondary compounds Example: Selective feeding of howler monkeys. gall wasp gall on rose 3. Selective feeding of herbivores 3. Selective feeding of herbivores Four rules of howler monkey feeding 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! What explains the 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) 2 - fed on few individual trees The selected individual trees also had lower levels of alkaloids and tannins than others in the population

8 3. Selective feeding of herbivores What explains the rules of howler monkey feeding? 3 - fed on young leaves only Young leaves had less non-nutritive fiber 4 - often ate the petiole, threw the leaf blade away! Petioles had lower concentrations of alkaloids than the leaf blades Feeding strategies may help with both morphological defenses and secondary compounds What explains the rules of howler monkey feeding? Selective feeding reduced monkeys exposure to non-nutritive and toxic foliage Video clip of selective feeding by herbivores Herbivores may also evolve ability to detoxify secondary plant compounds 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 Plant- herbivore evolution An evolutionary arms race? Resistance to toxic plant compounds by the herbivores may lead to selection acting on plant to develop greater quantities and more kinds of toxins Reciprocal evolution of this type: an evolutionary arms race

9 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 Specialists Plant chemistry None or Linear furanocoumarins Angular furanocoumarins

10 An example from insects feeding on wild umbellifers Furanocoumarins Prop. Prop. Prop. specialists intermed. generalists (1-3 genera) (4-20 gen.) (>3 families) None Linear only Angular (most complex) and linear 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. An example of a tritrophic (three trophic levels) interaction An example of a tritrophic (three trophic levels) interaction First more background: 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. 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 of the herbivores have been found to use these volatile chemical cues to find their prey. Are plants calling for help?

11 An example of a tritrophic (three trophic levels) interaction Two related moths, HV and HZ, are pests of tobacco and cotton. C. Plant- herbivore evolution 3. Are plants calling for help? An example. One of the caterpillars (HV) is the host of a species of parasitic wasp. HZ is not a host. HV Wasp arriving to lay eggs in HV HV HZ An example of a tritrophic (three trophic levels) interaction 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. An example of a tritrophic (three trophic levels) interaction Percent of visits by wasps HV The host of the wasp Non-host HZ Control

12 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? Amount of volatiles HV HZ Undamaged Plants fed upon by the different herbivores release different amounts and kinds of volatile compounds Volatiles C. Plant- herbivore evolution 3. 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? 1. Aposematic coloration: warning coloration Signals toxicity to Many toxic animals have adopted red and black or yellow and black

13 1. Aposematic coloration: warning coloration Where do those aposematic herbivores get their toxins? Where do those aposematic herbivores get their toxins? Instead of metabolizing plant toxins, some herbivores wall them off from sensitive tissues within their bodies, use them as a defense called sequestering plant toxins 2. Mimicry - Muellerian Why do those aposematic herbivores look alike? Convergence on the same signal benefits all toxic animals that have it 2. Mimicry - Batesian When non-toxic animals mimic toxic or defended species: Batesian mimicry When toxic animals resemble each other: Muellerian mimicry

14 Mimicry Batesian mimicry can include adopting the form of a very different animal Moth mimicking a wasp Moth mimicking a tarantula Mimicry - Muellerian and Batesian mimics may coexist in mimicry rings D. Defenses of herbivores against 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

15 3. Crypsis 3. Crypsis Butterfly leaf Katydid leaf Cryptic behaviors - motionless (or swaying in wind ) feed on the leaf underside feed at night conceal damage (cut off or trim damaged part) Bug thorn 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