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
•  Root feeders
•  Sap suckers
•  Gallers
sheep, bison, rabbits & grasshoppers.
deer, goats and hares.
many insects.
of leaves, stems, trunks, buds, seeds
and fruits (many insects).
nematodes, insects, mammals.
many insects, birds and mammals.
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
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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.
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
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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.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 11: slide 7
Figure 12.7 (3rd 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 (3rd ed.): Regrowth of 2 varieties of
Lolium multiflorum after defoliation.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 11: slide 9
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Figure 8.3 (3rd 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 (2nd 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 (3rd 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
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Figure 8.4 (3rd 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
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