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 - 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

OPTIMAL FORAGING THEORY HOW TO BE A GOOD PREDATOR

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

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

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

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

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. 4. Prey are encountered sequentially 5. Prey are recognized instantly and accurately Predator knows all this

1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? e i = energy provided by prey type i h i = handling time and effort associated with prey type i i = encounter rate with prey type i T s = 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.

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

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

1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Begin by calculating the total energy (E) per unit time associated with prey 1 E T s 1 e 1 T s + T s 1 h 1 T = Total energy obtained from prey 1 Total handling time + Search time E 1 e h 1 T = Simplifies to

1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Now calculate the total energy (E) per unit time associated both prey 1 and 2 E T s ( 1 e e 2 ) T s + T s 1 h 1 + T s 2 h 2 T = E h h 2 T = Simplifies to 1 e e 2

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

1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? h h 2 > 1 e e 2 Should a predator each both types of prey or just prey 1? Mathematically, a predator should eat prey 1 if the following is true 1 e h 1 Holds true when e 1 h 2 - e 2 h 1 > e2e2 1

1. WHAT FOOD ITEMS SHOULD A FORAGER EAT? Should a predator each both types of prey or just prey 1? e 1 h 2 - e 2 h 1 > e2e2 1 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 – 2 is missing from the equation)

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

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

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

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

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

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 (or any other colour) ≠ food Equally palatable

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

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

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

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

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 Time in patch Net food intake Time between patches  T1T1  T2T2

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     situation). If there is a shorter time between patches, you should spend less time in a patch (the      situation ).

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

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

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

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 selfForage for chicks

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

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

Modifications to Optimal Foraging Models Risk Sensitive Foraging Patch 1Patch 2 Mean = 8 food items Variance = 0 Mean = 8 food items Variance = 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

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

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

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

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

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

Intake Relative to Energy Requirement % Visits to Variable Feeding Station Shrews Risk proneRisk adverse animal is getting enough food to satisfy it’s basic requirements = selected variable source = selected fixed source

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

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 Utility or value of food Food item

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)