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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 - PowerPoint PPT Presentation
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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?
ei = energy provided by prey type i
hi = handling time and effort associated with prey type i
i = 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.
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
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?
1. WHAT FOOD ITEMS SHOULD A FORAGER EAT?
Begin by calculating the total energy (E) per unit time associated with prey 1
E Ts 1e1
Ts + Ts 1h1T
=Total energy obtained from prey 1
Total handling time + Search time
E 1e1
1 + 1h1T
=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 Ts (1e1 + 2e2)
Ts + Ts 1h1 + Ts 2h2 T
=
E
1 + 1h1 + 2h2T
=Simplifies to1e1 + 2e2
1. WHAT FOOD ITEMS SHOULD A FORAGER EAT?
1 + 1h1 + 2h2
>1e1 + 2e2
Should a predator each both types of prey or just prey 1?
Mathematically, a predator should eat prey 1 if the following is true
1e1
1 + 1h1
Energy gain from eating prey 1
Energy gain from eating prey 1 + 2
1. WHAT FOOD ITEMS SHOULD A FORAGER EAT?
1 + 1h1 + 2h2
>1e1 + 2e2
Should a predator each both types of prey or just prey 1?
Mathematically, a predator should eat prey 1 if the following is true
1e1
1 + 1h1
Holds true when
e1h2 - e2h1
> e21
1. WHAT FOOD ITEMS SHOULD A FORAGER EAT?
Should a predator each both types of prey or just prey 1?
e1h2 - e2h1>
e21
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 prey Small 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 density High density
Large prey Small 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 density High density High density
Large prey Small 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 oftenProtein Level
Calorie Level
High
Low
High Low
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
foo
d in
take
Time between patches
•
T1
•
T2
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 spendmore 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 self Forage 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
100
50
0
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
0 20 40 60 80 100
Model pred.
Field obs.
Modifications to Optimal Foraging Models
Risk Sensitive Foraging
Patch 1 Patch 2
Mean = 8 food items
Variance = 0
Mean = 8 food items
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
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 = 3 Average 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
0.0 1.0 1.5 2.0
Risk prone Risk 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
Uti
lity
or
valu
e of
foo
dFood 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)