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PREDATORS AND PREY

PREDATORS AND PREY. LOOK AT THREE ASPECTS:. 1. Decisions made by animals in collecting food. 2. Behaviour involved in collecting food. 3. Ways to avoid being food. Optimality Theory. Optimality models

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PREDATORS AND PREY

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  1. PREDATORS AND PREY

  2. LOOK AT THREE ASPECTS: 1. Decisions made by animals in collecting food 2. Behaviour involved in collecting food 3. Ways to avoid being food

  3. Optimality Theory Optimality models - predict what an animal should do (course of action it should take) under a specific set of conditions to maximize its fitness Three parts: (1) Decisions - strategies available to the animal (2) Currency - criteria upon which decision is made (3) Constraints - limits of the animal

  4. OPTIMAL FORAGING THEORY HOW TO BE A GOOD PREDATOR

  5. Foraging Models Two major types : (1) Diet selection or prey models (2) Patch models

  6. Diet Selection Models Meadow Vole (Microtus pennsylvanicus) Barn Owl (Tyto alba)

  7. How is the owl selecting prey? Voles Proportion in fauna Other rodents Proportion in diet Other rodents Voles

  8. Diet Selection Models ASK THE FOLLOWING QUESTION: 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Imagine a predator seeking prey: Finds either prey type Eat?? Move on?? Currency: Maximize rate of energy intake

  9. The RULES!!! 1. We can measure some standard currency 2. There is a cost in handling prey 3. A predator can’t handle one prey and search for another at the same time. Predator knows all this 4. Prey are encountered sequentially 5. Prey are recognized instantly and accurately

  10. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? ei = energy provided by prey type i hi = handling time and effort associated with prey type i li = encounter rate with prey type i Ts = amount of time devoted to searching for prey type i T = total time For this example, we will assume that there are two prey types.

  11. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Assume predator always take prey with the higher ei/hi value i.e. a more favourable energy gain : handling effort ratio Low ei/hi value Higher ei/hi value

  12. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Assume predator always take prey with the higher ei/hi value Assume that the higher ei/hi value is prey type 1 (or e1/h1) Question : Should forager take prey 1 alone or take prey 1 and 2 as they are encountered?

  13. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Begin by calculating the total energy (E) per unit time associated with prey 1 Total energy obtained from prey 1 Tsl1e1 E = T Ts + Tsl1h1 Total handling time + Search time l1e1 E = Simplifies to T 1+ l1h1

  14. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Now calculate the total energy (E) per unit time associated both prey 1 and 2 Ts (l1e1 + l2e2) E = T Ts + Tsl1h1 + Tsl2h2 l1e1 + l2e2 E = Simplifies to T 1+ l1h1 + l2h2

  15. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Should a predator each both types of prey or just prey 1? Mathematically, a predator should eat prey 1 if the following is true l1e1 l1e1 + l2e2 > 1+ l1h1 1+ l1h1 + l2h2 Energy gain from eating prey 1 Energy gain from eating prey 1 + 2

  16. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Should a predator each both types of prey or just prey 1? Mathematically, a predator should eat prey 1 if the following is true l1e1 l1e1 + l2e2 > Holds true when 1+ l1h1 1+ l1h1 + l2h2 e2 > l1 e1h2 - e2h1

  17. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Should a predator each both types of prey or just prey 1? e2 > l1 e1h2 - e2h1 Two predictions: 1. Once a critical encounter rate with prey 1 is reached, it alone should be taken 2. The decision about whether or not to take prey 2 does not depend on how common it is (i.e. it’s encounter rate – l2 is missing from the equation)

  18. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Are there any data to support this? Work with great tit - Parus major mealworm bits conveyor belt

  19. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Are there any data to support this? Low density Proportion encountered Predicted proportion in diet Observed proportion in diet Large prey Small prey

  20. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Are there any data to support this? Low density High density Proportion encountered Predicted proportion in diet Observed proportion in diet Large prey Small prey

  21. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Are there any data to support this? Low density High density High density Proportion encountered Predicted proportion in diet Observed proportion in diet Large prey Small prey

  22. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? What can affect this model? Have we chosen the right currency? -maybe animal is making more complex judgements about food Berteaux et al, ‘98 - Deer Calorie Level High Low High Protein Level Low

  23. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? What can affect this model? 2) Probability of finding prey is not proportional to its density Tinbergen - warblers - eat caterpillars -develop a ‘search image’ = food Equally palatable ≠ food (or any other colour)

  24. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? = food Tinbergen - warblers - eat caterpillars ≠ food In population In diet Frequency of Caterpillars Time

  25. Foraging Models Two major types : (1) Diet selection or prey models (2) Patch models

  26. Patch Models Most food has a clumped distribution (or exists in patches)

  27. PATCH MODELS HOW LONG SHOULD A FORAGER STAY IN A CERTAIN PATCH? Problem : Imagine a hummingbird on a flower ? ? ? ? ?

  28. 2. HOW LONG SHOULD A FORAGER STAY IN A CERTAIN PATCH? Charnov - Marginal Value Theorem - to determine how long an animal should stay in a patch Net food intake • • T2 T1 t1 t2 Time between patches Time in patch

  29. 2. HOW LONG SHOULD A FORAGER STAY IN A CERTAIN PATCH? Charnov - Marginal Value Theorem - to determine how long an animal should stay in a patch From previous graph: If there is a longer time between patches, you should spend more time in a patch (the t1: T1 situation). If there is a shorter time between patches, you should spend less time in a patch (thet1: T1situation).

  30. 1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Are there any data to support this? Great tit - Parus major • • • Expected • • Observed Time in Patch • • • • • • • Travel Time

  31. Modifications to Optimal Foraging Models Central Place Foraging Cost - energy returning from feeding area -carrying load of food Nesting area Feeding area Cost - energy getting to feeding area

  32. Modifications to Optimal Foraging Models Central Place Foraging Davoren & Berger ‘99 Rhinoceros auklet (Cerorhinca monocerata)) Feeding area Nesting area

  33. Modifications to Optimal Foraging Models Central Place Foraging Davoren & Berger ‘99 Hypothesis: Birds should feed differently if feeding themselves or taking food to offspring Forage for self Forage for chicks

  34. Modifications to Optimal Foraging Models Central Place Foraging Davoren & Berger ‘99 Hypothesis: Birds should feed differently if feeding themselves or taking food to offspring 100 50 0 Chicks Self Size in mm

  35. Modifications to Optimal Foraging Models Nutrient Constraints (Belovsky, ‘78) Salt rich, energy poor Salt poor, energy rich Constraints: acquire maximum energy/time + ingest some amount of sodium Model pred. Field obs. 0 20 40 60 80 100

  36. Modifications to Optimal Foraging Models Risk Sensitive Foraging Patch 1 Patch 2 Mean = 8 food items Mean = 8 food items Variance = 0 Variance = 140.3 Problem for Forager: Go to Patch 1 and be guaranteed 8 food items OR Go to Patch 2 and risk getting either 0 or 16 food items

  37. Caraco et al (1980’s – 1990’s) Juncos - Junco phaenotus Feeders Every visit Constant reward Variable reward OR NOTE: Same average reward

  38. Caraco et al (1980’s – 1990’s) Juncos - Junco phaenotus Feeders Every visit OR Juncos behave as if they are risk adverse

  39. Caraco et al (1980’s – 1990’s) Juncos - Junco phaenotus Second question: Is there a level of food at which juncos start to become risk prone? Add food to variable feeder < OR Reward = 3 Average reward = 6

  40. Caraco et al (1980’s – 1990’s) Juncos - Junco phaenotus OR When Reward constant = ½ Reward variable 50% of juncos chose the variable

  41. Shrews fixed variable Tested shrews in times of satiation and hunger Barnard & Brown 1985

  42. Shrews Risk prone Risk adverse animal is getting enough food to satisfy it’s basic requirements % Visits to Variable Feeding Station 0.0 1.0 1.5 2.0 Intake Relative to Energy Requirement = selected variable source = selected fixed source

  43. Modifications to Optimal Foraging Models Risk Sensitive Foraging Consider 3 foragers: Forager A - values every food item equally Forager B - full, sated, stuffed - each additional food item is valued less and less Forager C - starving - each additional food item is valued more and more

  44. Modifications to Optimal Foraging Models Risk Sensitive Foraging Consider 3 foragers: Forager A - values every food item equally Utility or value of food Forager B - full, sated, stuffed - each additional food item is valued less and less Forager C - starving - each additional food item is valued more and more Food item

  45. Modifications to Optimal Foraging Models Risk Sensitive Foraging Consider 3 foragers: Forager A - should show no preference for either type of patch Forager B - should be risk averse (forage in patch 1) Forager C - should be risk prone (forage in patch 2)

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