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 Browsers Leaf miners Borers sheep, bison, rabbits & grasshoppers. deer, goats and hares. many insects. 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. Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 2 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. Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 3 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 6ft 2 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. Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 4 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. Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 5 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. Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 6 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. Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 7 Figure 12.7 (3 rd ed.): Plant niche diversity for insect and fungal exploiters separated into feeding guilds Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 8 Figure 8.2 (3 rd ed.): Regrowth of 2 varieties of Lolium multiflorum after defoliation. Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 9 3
Figure 8.3 (3 rd ed.): Compensatory flower production by secondary and tertiary umbels in damaged parsnip Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 10 Figure 8.4 (2 nd ed.): Reduced competitive dominance of oats susceptible to a root-feeding nematode. resistant susceptible Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 11 Figure 8.7 (3 rd ed.): Negative effect of competition + beetle herbivory on leaf area of the dock Rumex crispus Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 12 4
Figure 8.4 (3 rd ed.): Decreased Phyllonorycter leaf miner survivorship with increased oak defoliation. Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 13 Figure 9.4: Effects of beetle herbivory on waterlily leaf survivorship Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 14 Figure 9.5: Effects of flea beetle herbivory on sand-dune willow growth rate Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 15 5