Plant defenses against herbivores

1. Plant defenses against herbivores • Plants can’t run away from herbivores • Plants can’t hide – leaves must be exposed too catch light for photosynthesis. • Plants are usually too abundant to be cryptically colored! • Therefore, plant defenses operate in situ, either directly or indirectly

2. Types of defense • Mechanical • Chemical • Biotic • Phenological A species may use more than one of these lines of defense, and may use different ones at different stages of its life

3. Mechanical defenses • Hairs on leaf surface • Spines – modified leaves • Thorns – modified shoots • Toughness Other modifications – e.g. fake eggs on Passiflora tendrils fool ovipositing Heliconius butterflies

4. Hairiness - multipurpose

5. spiny thistles and Eeyore

6. Thorns to prevent biting, climbing

7. Chemical defenses • Palatability/acceptability influencers: feeding inhibitors • Digestibility reducing compounds • Toxins

8. Palatability influencers Pre-chomping: volatile compounds, may repel herbivores, but specialized herbivores may use them as a cue Post-chomping: Tannins (astringency) – oaks; Oxalic acid – sorrels; Calcium oxalate – Araceae; photosensitization; cyanogenic glycosides

9. Digestibility reducers e.g. Tannins – reduce assimilation of plant proteins by herbivores by interfering with proteolytic enzymes Herbivores eat a lot, assimilate little Grow more slowly, may fail to develop in suitable season Longer in larval stage, vulnerable to enemies

10. Toxins – Cabbages and glucosinolates Peter J. Bryant Cabbage white Common green-eyed white

11. Giant swallowtail and Rutaceae hostplants Fireflyforest.net

12. Toxins e.g. cardiac glycosides in Asclepias Cows and sheep eating plants get sick, have abortions, etc… Most insects avoid foliage except monarch butterflies. Their caterpillars are toxic due to sequestration of the milkweed toxins! Certain specialists are adapted to feed despite toxins.

13. Milkweed and monarch caterpillar

14. Monarch butterfly life cycle Adults benefit from sequestered cardenolides too!

15. Naïve birds will remember…

16. Idea leuconoe (Lepidoptera subfamily Danainae) on Parsonsia alboflavescens (Apocynaceae) c/o Tatyana Livshulz Fieldwork in Taiwan

17. Tithorea pinthias Nymphalidaeon Prestonia portobellensis Dan Janzen – Santa Rosa National Park

18. Plumeria and Pseudosphinx tetrio Squirrel cuckoos whack and snack http://biological-diversity.info Belize wildlife site

19. Biotic protection • Ants tending extrafloral nectaries and/or residing in plant body protect plants • Parasitoids visiting nectaries may also provide plant protection • Plants can benefit from ants tending certain herbivores as well, but not always • Beneficial mites can eat herbivorous mites – plants may have domatia to house them

20. Senna mexicana foliar nectaries and ant

21. Turnera ulmifolia in greenhouse

22. Vicia sativa with stipular nectaries

23. Phenological defense • Timing of production of parts susceptible to herbivory • Leaf flushing • Masting – flowering/fruiting every 2-5 years synchronously with others (predator satiation)

24. New leaf flushing – phenological defense?

25. Mast fruitingmast = beech A “mast year” occurs when the number of acorns or other nuts (like Beech nuts Hickory nuts or Hazel nuts) produced by a tree or shrub in a single season is much higher than usual. 

26. Jargon of antiherbivore defense

27. Qualitative vs. Quantitative Qualitative defenses (e.g. toxins) – very effective against non-adapted species, effective in small amounts on all but specialist herbivores Quantitative defenses – more effective in larger doses, general-purpose, e.g. toughness, low nutrients, digestibility-reducers

28. Plants and their parts

29. Optimal defense theory • Considers costs to plants of antiherbivore defenses • Qualitative defenses cheaper than quantitative defenses? • Maybe just more appropriate – act quickly against specific herbivores • Lots of debate over the years…

30. Physiological ecologists to the rescue… • The cost of defense are one of the constraints on leaf form and function • Photosynthetic capacity, nitrogen, longevity, and susceptibility to herbivores are all related • Quantify these things for a cost-benefit analysis

31. Benefit from a leaf = [rate of carbon gain * carbon gain period] minus [carbon cost of growth and maintenance + losses to herbivory] • CO2 exchange can take care of all except herbivory losses • Amount of nitrogen in leaf correlates with photosynthetic capacity

32. Effects of herbivores can be dramatic! Hawaiian landscape protected from cattle grazing on Mauna Loa

33. Odocoileus virginianus ssp. clavium

34. Earlier deer exclosures on Key Deer Refuge – at least 25 years old. No fire in that time (note wooden posts). Woody plant cover substantially greater inside fenced areas Long term protection from deer browsing?

35. Fire and Key Deer Herbivory • Fire clears dense understory • Promotes resprouting and seed germination • New foliage more palatable to deer • Preferential grazing may affect understory plants: • Growth • Reproduction • Recruitment and/or persistence