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  • BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism Slide - 1

    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 - 2

    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 - 3

    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 - 4

    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 - 5

    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 - 6

    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 - 7

    7. Herbivore choice (Table 9.1):

  • BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism Slide - 8

    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 - 9

    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 - 10

    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 - 11

    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 - 12

    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 - 13

    13. Diet profitability:

    E i hi

    ≥ E (s +h )

    •  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: –  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 - 14

    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 - 15

    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 - 16

    16. Marginal Value Theorem

    Fig. 9.16:

  • BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 13: Herbivory, predation & parasitism Slide - 17

    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 - 18

    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 - 19

    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

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