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- 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


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.


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


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?


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


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 + 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 + 1h1 + 2h2

>1e1 + 2e2


Mathematically, a predator should eat prey 1 if the following is true

1e1

1 + 1h1

Holds true when

e1h2 - e2h1

> e21



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)


Are there any data to support this?

Work with great tit - Parus major

mealworm bits

conveyor belt



Proportion encountered

Predicted proportion in diet

Observed proportion in diet

Low density

Large prey Small prey






Low density High density







Low density High density High density



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


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


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).



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



Feeding area Nesting area

Davoren & Berger ‘99

Rhinoceros auklet (Cerorhinca monocerata))




Hypothesis: Birds should feed differently if feeding themselves or taking food to offspring

Forage for self Forage for chicks




Hypothesis: Birds should feed differently if feeding themselves or taking food to offspring

Size in mm

100

50

0

Self

Chicks


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.


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



Feeders

Every visit

OR

Juncos behave as if they are risk adverse



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



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



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




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




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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PREDATORS AND PREY