PREDATORS AND PREY

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 cant 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
Ts l1e1
E

T
Ts Ts l1h1
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 Ts l1h1 Ts l2h2
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 l2e2
l1e1
gt
1 l1h1 l2h2
1 l1h1
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 l2e2
l1e1
gt
Holds true when
1 l1h1 l2h2
1 l1h1
e2
gt
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
gt
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.
its encounter rate)
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
Chosen most often

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?
Tinbergen - warblers - eat caterpillars
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 (the t1 T1
situation).
30
1. WHAT FOOD ITEMS SHOULD A FORAGER EAT?
Are there any data to support this?
Great tit - Parus major
Expected
Time in Patch
Travel Time
31
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
32
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
33
Modifications to Optimal Foraging Models
Central Place Foraging
Davoren Berger 99
Rhinoceros auklet (Cerorhinca monocerata))
Feeding area
Nesting area
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
Forage for self
Forage for chicks
35
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
36
Modifications to Optimal Foraging Models
Nutrient Constraints (Belovsky, 78)
Salt poor, energy rich
Salt rich, energy poor
Constraints acquire maximum energy/time
ingest some amount of sodium
Model pred.
Field obs.
0 20 40 60 80 100
37
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
38
Caraco et al (1980s 1990s)
Juncos - Junco phaenotus
Feeders
Every visit
Constant reward
Variable reward
OR
NOTE Same average reward
39
Caraco et al (1980s 1990s)
Juncos - Junco phaenotus
Feeders
Every visit
OR
Juncos behave as if they are risk adverse
40
Caraco et al (1980s 1990s)
Juncos - Junco phaenotus
Second question Is there a level of food at
which juncos start to become risk prone?
Add food to variable feeder
lt
OR
Reward 3
Average reward 6
41
Caraco et al (1980s 1990s)
Juncos - Junco phaenotus
OR
When Reward constant ½ Reward variable
50 of juncos chose the variable
42
Shrews
fixed
variable
Tested shrews in times of satiation and hunger
Barnard Brown 1985
43
Shrews
Risk prone
Risk adverse
animal is getting enough food to satisfy its
basic requirements
Visits to Variable Feeding Station
0.0 1.0 1.5 2.0
Intake Relative to Energy Requirement
44
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
45
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
46
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)