BIOS 3010: Ecology Lecture 11: Processes: Herbivory • Lecture summary: – Feeding guilds. – Effects of herbivores on plants: • Distribution and abundance. • Compensation. • Recruitment. • Fecundity. – Plant defense. • Diversity. • Plant defense theory.
Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 1
2. Basic feeding guilds of herbivores:
• Grazers sheep, bison, rabbits & grasshoppers. • Browsers deer, goats and hares. • Leaf miners many insects. • Borers of leaves, stems, trunks, buds, seeds and fruits (many insects). • Root feeders nematodes, insects, mammals. • Sap suckers many insects, birds and mammals. • Gallers many insects, mites, nematodes and bacteria. – Frugivores, seed predators, pollinators and nectarivores also feed on plant parts.
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3. Effects of herbivores on plants:
• Influence the distribution and abundance of plants: – Through effects on: • 1) Plant parts (Figure 12.7) – Roots, leaves, flowers etc. • 2) Timing in plant development – Seedlings vs mature plants etc. • 3) Post-attack effects – Induction of chemical defenses/trichomes/spines etc. – Redistribution of nutrients etc.
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1 4. Effects of herbivores on plants:
• 1) Compensation - despite some compensation herbivores almost always harm plants - although this may look like benefits – Figs 8.2 & 8.3 • 2) Enhanced negative competitive effects – Fig 8.4 Begon, 2nd ed. & Fig. 8.7 • 3) Increased mortality - repeated defoliation by herbivores can kill plants or make them more susceptible to death but they can defend – e.g. Fig 8.4 Begon, 3rd ed., and module death in Fig. 9.4) • 4) Reduced recruitment by killing seedlings – In a 6ft2 area cultivated by Charles Darwin (1859) he found that 295/357 seedlings (83%) were killed by slugs and insects (Begon et al., p. 322). • 5) Reduced growth - Herbivory can slow or stop plant growth (Fig. 9.5) - but grasses tend to be resistant to the effects of grazing because the low meristem is unaffected.
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5. Plant fecundity reduction:
• Smaller plants produce fewer or less viable seeds. • Plants may flower later: – Can turn annuals into perennials by repeated grazing or mowing. • Herbivores can eat reproductive parts (flowers) directly: – Excluding mutualistic, pollen or nectar feeding. – Seed predation.
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6. Plant defense:
• Herbivory is the process that describes the interaction between: – Plant defense, and, – Herbivore foraging. • See Figure 20.1 from Malcolm (1992) In, M.J. Crawley (editor), Natural Enemies: The population biology of predators, parasites and diseases. Blackwell Scientific Publications, Oxford.
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2 7. Plant defense theory (Table):
• “Apparency” theory (Feeny, 1976). • “Optimal Defense” theory (Rhoades & Cates 1976). • “Toxins” effective against abundant generalist herbivores, and may account for the effectiveness of some specialist herbivores (“Qualitative” of Feeny). – E.g. alkaloids, furanocoumarins, saponins, cardenolides and cyanogenic glycosides. • “Digestibility reducers” effective against both specialists and generalists by making nutrients less available to herbivores (“Quantitative” of Feeny). – E.g. tannins.
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Figure 12.7 (3rd ed.):
• Plant niche diversity for insect and fungal exploiters separated into feeding guilds
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Figure 8.2 (3rd ed.): Regrowth of 2 varieties of Lolium multiflorum after defoliation.
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3 Figure 8.3 (3rd ed.): Compensatory flower production by secondary and tertiary umbels in damaged parsnip
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Figure 8.4 (2nd ed.): Reduced competitive dominance of oats susceptible to a root-feeding nematode.
resistant susceptible
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Figure 8.7 (3rd ed.): Negative effect of competition + beetle herbivory on leaf area of the dock Rumex crispus
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4 Figure 8.4 (3rd ed.): Decreased Phyllonorycter leaf miner survivorship with increased oak defoliation.
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Figure 9.4: Effects of beetle herbivory on waterlily leaf survivorship
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Figure 9.5: Effects of flea beetle herbivory on sand-dune willow growth rate
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