BIOS 5970: Plant-Herbivore Interactions
Dr. Stephen Malcolm, Department of Biological Sciences
•  D. POPULATION & COMMUNITY DYNAMICS
•  Week 13. Herbivory, predation & parasitism:
–  Lecture summary:
•  Predation:
–  Categories
–  Behavior
–  Optimal foraging theory
•  Herbivory
–  Categories
–  Effects on plants
–  Functional responses
•  Parasites and disease
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 1
2. Predation:
•  “Predation” usually includes considerations
of parasitism and herbivory.
•  Like parasitism and herbivory it is a
description of the interaction between
predator foraging behavior and prey defense
–  Literature reflects strong emphasis on predator
foraging behavior and prey-predator dynamics.
–  Defense is mostly relegated to the realms of
natural history description.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 2
3. Predator foraging behavior:
•  Description of:
– Where they feed.
– What they feed on.
– How they are influenced by other
predators.
– How they are influenced by prey
density.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 3
4. Categories of Predation:
•  Begon, Harper & Townsend (1996) use “predation” to
consider the following categories of “predator”:
–  Predators:
•  Kill and completely consume many prey items during their life.
–  Parasitoids:
•  Free-living adult insects that lay eggs in or on their single host ("prey")
in which the larva (or larvae) develops into a new free-living adult.
•  Host is always killed.
–  Parasites:
•  Most of their life is spent in close association in or on a single host and
usually do not kill the host.
–  Herbivores:
•  Most only partially consume individual plants, but they include a range
of plant feeders that act like true parasites (e.g. aphids), parasitoids
(e.g. fig wasps), predators (e.g. mice and seed beetles).
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 4
5. Diet composition and food preference:
•  Predators can be:
–  Monophagous:
•  Single prey type and have a large impact on prey
population dynamics.
–  Oligophagous:
•  Few prey types
–  Polyphagous:
•  Many prey types and probably have little impact on
the population dynamics of any one species.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 5
6. Prey choice and profitability:
•  Within basic diet breadths predators choose more
profitable prey preferentially (Fig. 9.1).
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 6
7. Herbivore choice (Table 9.1):
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 7
8. Assessment of profitability:
•  Food can be assessed by predators as
either:
– Ranked food resources:
•  Most valuable or “perfectly substitutable”
– Balanced food resources:
•  Integral or “complementary”
•  Usually necessary to balance required
nutrients that may be absent from high ranked
foods.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 8
9. Switching:
•  Predators can also “switch” their food preference.
•  Through learned abilities to handle prey more
profitably:
–  More efficient balance among, search, pursuit, and
handling behaviors before consumption.
–  May be facilitated by specific “search images”.
–  Such changes in diet may also be seasonal or on shorter
time scales that may be associated with the induction of
physiologies better suited to exploiting the food
resource.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 9
10. “Optimal foraging” and diet width:
•  Why are real diets “narrower” than potential diets?
•  If energy maximization is the primary criterion
that correlates with fitness then optimal foraging
theory is useful.
•  MacArthur & Pianka (1966) initiated the influential
optimal foraging theory approach for the
description of the evolutionary ecology of predatory
behavior based on maximization of the net rate of
energy intake:
–  Net rate of energy in take = gross energy intake energetic costs of obtaining that energy.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 10
11. Predator costs:
•  Energy and time costs of:
– Searching for prey
– Handling prey - includes:
•  Detection
•  Pursuit
•  Acceptance
•  Subjugation
•  Consumption & Digestion
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 11
12. Optimal foraging theory:
•  Aim is to predict the expected foraging
“strategy” under specified conditions:
–  Is it a "tactic" or a "strategy" ?
–  Generalists:
•  Have low time search costs but higher costs of
handling both unprofitable and profitable prey.
–  Specialists:
•  Have high time costs but lower costs of handling
profitable prey.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 12
13. Diet profitability:
•  MacArthur & Pianka argued that a prey item should
be included (and diet width expanded) if it is equal
to or more profitable than the average profitability
of the present diet.
•  Thus should eat ith prey if:
E
E
≥
h (s + h )
i
i
–  Where i is the next most profitable prey item.
–  E = energy content, h = handling time
(therefore E/h = profitability), s = search time
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 13
14. Diet profitability:
•  handling time < search time = generalists
– e.g. foliage gleaning bird guild
•  Guild is a group of individuals that exploit the
same resource in the same way
•  handling time > search time = specialists
– e.g. lions living near prey
– Note: handling time includes pursuit time!
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 14
15. Marginal Value Theorem:
•  Based on work of Charnov (1976) and
Parker & Stuart (1976) to predict behavior of
optimal forager in patches of food of different
profitabilities.
•  Forager should maximize overall intake of
resource (energy) per time spent foraging in
habitats with food distributed patchily.
•  How long should the forager spend in
patches of varying profitability?
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 15
16. Marginal
Value Theorem
Fig. 9.16:
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 16
17. Herbivory:
•  Problems: Herbivory is sometimes
considered as a subset of predation
because of 2 ways to classify
consumers:
– 1. Taxonomic classification:
•  Carnivores consume animals.
•  Herbivores consume plants.
•  Omnivores consume plants and animals.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 17
18. Herbivory:
•  2. Functional classification:
–  True predators:
•  Kill and consume prey immediately; kill many prey
–  Grazers:
•  Attack many "prey"; rarely lethal; only partially consume
–  Parasitoids:
•  Attack single "prey", always lethal, complete consumption
–  Parasites (micro and macro):
•  Attack few or single "prey"; rarely lethal; only partially consume
–  Caveat: modularity is an important consideration
(modular versus unitary organisms)!
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 18
19. Basic kinds of herbivores:
•
•
•
•
•
•
•
•
•
Grazers - sheep, bison, rabbits and 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.
Vary according to scale of host resources (modularity) and
herbivore exploiter…
In addition frugivores, seed predators, pollinators, and
nectarivores all feed on plant parts.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 19
20. Herbivore feeding
guilds (Fig. 12.7):
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 20
21. Six effects of herbivores on plant
distribution and abundance:
•  1. Compensation:
–  Despite some compensation herbivores almost always harm plants
(Figs. 8.2 (grass growth) & 8.3 (parsnip flowers).
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 21
22. Six effects of herbivores on plant
distribution and abundance:
•  2. Enhance negative competitive effects:
–  Fig. 8.4, 2nd ed. (barley/oats) & 8.7 (beetle on Rumex)
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 22
23. Six effects of herbivores on plant
distribution and abundance:
•  3. Defense:
–  Repeated defoliation by herbivores can kill plants or make them more
susceptible to death but they can defend:
•  e.g. Fig 8.4 West's leaf miners and inducible defenses.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 23
24. Six effects of herbivores on plant
distribution and abundance:
•  4. Survivorship:
–  Mature plants are usually not killed:
•  Although repeated herbivory can increase mortality.
–  Recruitment can be slowed by herbivores killing seeds
or seedlings.
–  Charles Darwin found 83% mortality.
•  5. Growth:
–  Herbivory can slow or stop plant growth.
–  Timing is important.
–  But grasses tend to be resistant to the effects of grazing
because the low meristem is unaffected.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 24
25. Six effects of herbivores on plant
distribution and abundance:
•  6. Fecundity can be reduced:
–  Growth related:
•  Smaller plants produce fewer or less viable seeds.
–  Plants may flower later:
•  e.g. can turn annuals into perennials by repeated
grazing or mowing (Poa annua).
–  Herbivores can eat reproductive parts (flowers)
directly:
•  Excluding mutualistic pollen, or nectar, feeding, or
exploitative seed predation.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 25
26. Effects of herbivory on plant
populations:
•  Impact greatest on stressed individuals.
•  Compensation by unaffected individuals because of
reduced intraspecific competition.
–  Thus herbivory and competition can balance each other out and
result in similar densities before & after the event.
–  Because net recruitment/productivity increase.
•  Negative effects of herbivory are modified to some extent in
modular plants:
–  Thus compensation is important.
•  Threshold for compensation is important to consider for
repeated harvests or exploitation by natural herbivores:
–  e.g. locust plagues and herbivore mobility.
•  Herbivore compensation.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 26
27. Functional responses of herbivores:
•  To plant biomass:
–  Implies satiation at high levels of food availability.
•  May explain unpredictable masting by trees
subject to high levels of herbivory to swamp
herbivore recruitment and ensure high seed/
seedling survivorship..…
–  but should plants also enhance dispersion to
reduce the impact of intraspecific competition in
these years?
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 27
28. Masting (Figs. 8.11 & 8.12):
•  Herbivore life histories cannot respond?
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 28
29. Cost of masting:
•  Masting is expensive for the plant!
– Perhaps not as costly though as the
impact of severe herbivory!
– Temporal scaling of life histories:
•  Herbivores with short generation times can
track resource quantity fluctuations more
effectively than herbivores with long
generation times.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 29
30. The effect of grazing herbivores on
community structure:
•  Herbivory, predation, disease and parasitism, competition,
earthquakes, fire, rain, wind, temperature etc. can all
disturb communities (Fig. 21.1).
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 30
31. Disturbance:
•  “any relatively discrete event in time that removes
organisms or otherwise disrupts the community by
influencing the availability of space or food
resources, or by changing the physical
environment.”
•  “A general consequence is likely to be the opening
up of space, or freeing up of resources, that can be
taken over by new individuals.”
–  For example, a predator, or herbivore, or lawn mower, or
a strong wave, or a strong wind can open gaps in
communities.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 31
32. Intermediate grazing and exploitermediated coexistence:
•  Grazing by rabbits can
strongly influence plant
community structure.
•  Intermediate grazing
promotes most
diversity through
influence on
competition:
–  “exploiter-mediated
coexistence”
(Fig. 21.2.).
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 32
33. The effects of parasites and disease:
•  These exploiters can also affect other interactions
and facilitate coexistence of species as in Fig. 21.4.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 33
34. Effects of disease on community
structure:
•  Strongly negative effects on communities can also
occur for highly pathogenic invading diseases such
as malaria and bird pox in Hawaii which may have
exterminated 50% of the endemic bird species.
•  Also the destruction of chestnut and elm forests in
North America by introduced pathogens.
•  These effects are also likely to be frequency
dependent:
–  Influenced by the frequency of encounter in high density
populations.
BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism
Slide - 34