2. Basic feeding guilds of...

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Dr. S. Malcolm BIOS 3010: Ecology Lecture 11: slide 1

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.


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.


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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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.


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.


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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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.


Figure 12.7 (3rd ed.):

•  Plant niche diversity for insect and fungal exploiters separated into feeding guilds


Figure 8.2 (3rd ed.): Regrowth of 2 varieties of Lolium multiflorum after defoliation.

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Figure 8.3 (3rd ed.): Compensatory flower production by secondary and tertiary umbels in damaged parsnip


Figure 8.4 (2nd ed.): Reduced competitive dominance of oats susceptible to a root-feeding nematode.

resistant

susceptible


Figure 8.7 (3rd ed.): Negative effect of competition + beetle herbivory on leaf area of the dock Rumex crispus

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Figure 8.4 (3rd ed.): Decreased Phyllonorycter leaf miner survivorship with increased oak defoliation.


Figure 9.4: Effects of beetle herbivory on waterlily leaf survivorship


Figure 9.5: Effects of flea beetle herbivory on sand-dune willow growth rate

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