Introduction Foraging theory is a branch of behavioural ecology that deals with the foraging behaviour of the organisms with respect to the environment where the organism lives. Types of foraging : solitary and group Foraging strategies : sit and wait , active Optimal Foraging Theory (OFT): Formulated by MacAorthur-Pianka (1966). It is a behavioral ecology model that helps predict how an animal behaves when searching for food. It states “to maximize fitness, an animal adopts a foraging strategy that provides the most benefit (energy) for the lowest cost, maximizing the net energy gained.” Although obtaining food provides the animal with energy, for searching and capturing the food require both energy and time. OFT helps predict the best strategy that an animal can use to achieve this goal. It is an ecological application of the optimality model. This theory assumes that the most economically advantageous foraging pattern will be selected for a species through natural selection. Optimal foraging model Generates quantitative predictions of how animals maximize their fitness while they forage. The model building process involves identifying the currency, constrains and appropriate decision rule for the forager (organism’s best foraging strategy). Optimal Decision Rule EXAMPLES optimal number of food items that an animal should carry back to its nesting site. the optimal size of a food item that an animal should feed on. Optimal Diet Model: Energy (E): amount of energy required for searching the food Handling time (H): amount of time the predator takes to handle the food Search time (S): amount of time the predator takes to find a prey and this is dependent on the abundance of the food and the ease of locating it Patch departure rule : The foragers changes the track in patch and habitat quality to save time to invest time more effectively on other patches. Departure from a prey patch is one of the key factors determining its foraging success. ‘W’ representing the time a predator is ‘willing’ to invest in the patch. As long as no prey are captured, W’ declines and when it drops below critical level the patch is abandoned. Marginal value theorem The MARGINAL VALUE THEOREM is a type of optimality model that is often applied to optimal foraging. This theorem is used to describe a situation in which an organism searching for food in a patch must decide when it is economically favorable to leave. When more energy is required for an animal to search food in this patch then the animal should leave the patch and go to another patch where more food/ resources are available. It is prediction when to leave the patch. While the animal is within a patch, it experiences the law of diminishing returns, where it becomes harder and harder to find prey as time goes on. Optimal foraging in bees: Wolf and Schmid-hempel (1989) showed that the cost of heavy nectar is great that it shortens the bees lifespan. CONCLUSION

1. ENT-602
Insect ecology and diversity
SUBMITTED TO:
Dr. SC Verma
Dr. PL Sharma
SUBMITTED BY:
Simran Bhatia
H-2021-06-D

2. Contents
♦Introduction
♦Types of foraging
♦Foraging strategies
♦Optimal Foraging Theory (OFT)
♦Optimal Decision Rule
♦Optimal Diet Model
♦Patch departure rule
♦Marginal value theorem
♦Optimal foraging in bees
♦Conclusion

3. INTRODUCTION
◦ Foraging is searching for wild food resources. It affects an animal’s
fitness because it plays an important role in an organisms ability to survive and
reproduce.
◦ Foraging theory is a branch of behavioural ecology that deals with the foraging
behaviour of the organisms with respect to the environment where the organism
lives.

4. Natural selection may favor 'efficient' foragers, and efficiency means that :
• Individual maximize energy intake or intake of some nutrient
• Individual minimize fluctuations in energy intake, or
• Individual maximize energy intake during certain periods . . .
What is maximized? Let's assume for now that an 'optimal' foragers is attempting to maximize energy intake. If so,
• what decisions must a forager or predator make?
• What types of food to eat?
• Where & how long to search for food?
• What type of search path to use?
Any food (energy) item has both a cost (time & energy) & a benefit (net food value).
 The relative value of each of these determines how much 'profit' a particular item represents.
 In other words: 'Profit' = net food (energy) value divided by time required to obtain & handle the food item, and
Efficient foragers should select most profitable prey!

5. Types of foraging
Solitary foraging
•Animals find, capture and
consume their prey alone
•Foraging alone can result in
less interaction with other
foragers, which can decrease the
amount of competition
Group foraging
• Animals find, capture and
consume prey in the presence
of other individuals
• Success depends not only on
your own foraging behaviors
but the behaviors of others as
well

6. MacArthur breaks foraging down into four phases:
1) deciding where to search
2) searching for palatable food items
3) upon locating a potential food item, deciding whether or not to pursue it
4) pursuit itself, with possible capture and eating

7. FORAGING STRATEGIES
SIT AND WAIT : conditions required to support a sit- and-wait strategy (1 or more)
1) relatively high prey density
2) high prey mobility
3) low predator energy requirements
Predators find a suitable patch and wait for mobile prey to come within striking range.
They remain motionless in order to be unobserved by its prey
Some shows camouflage
Advantages:
less energy is spent
hidden from predators
Disadvantages:
need to wait for food to come and it may take a long time

8. Examples
Malaysian mantid Hymenopus
bicornis resembling orchid
Robber fly waiting for prey
Antlion waiting for prey in pits

9. 2) Active foraging
◦ More energetic foraging involving active searching for suitable patches, and once there, for prey or for hosts
Advantages:
◦ less time consuming, finds food itself
Disadvantages
◦ more energy is spent

10. The foraging
of aphidophagous larval,
coccinellid beetles and
syrphid flies amongst their
clumped prey illustrates
several features of random
food searching.
Random,
or non-
directional
foraging
• Several more specific
directional means of host
finding can be recognized,
including the use of chemicals,
sound and light of the variety
of cues available, many insects
probably use more than one,
depending upon distance
or proximity to the resource
sought
Non-random,
or directional
foraging

11. Non-random, or directional foraging
CHEMICAL
ability to detect the chemical odors and messages produced by prey or hosts allows
specialist predators and parasitoids to locate these resources.
eg. Trichogramma wasps locate the eggs laid by their preferred host moth by the sex
attractant pheromones released by the moth
SOUND
the sound signals produced by some insects to attract mates, have been utilized by some
parasites to locate their preferred hosts acoustically
eg. Biosteres longicaudatus, a braconid hymenopteran endoparasitoid of a larval tephritid
fruit fly, detects vibrations made by the larvae moving and feeding within fruit.

12. 3. Phoresy
Phoresy is a phenomenon in which an individual is transported
by a larger individual of another species.
This relationship benefits the carried species and does not
directly affect the carrier
 It provides a means of finding a new host or food source
Eg. Ischnoceran lice (phthiraptera) transported by the winged
adults of Ornithomyia
 Egg-parasitizing hymenopterans (scelionidae,
trichogrammatidae, and torymidae), some attach themselves
to adult females of the host species, thereby gaining immediate
access to the eggs at oviposition.

13. Optimum foraging theory (OFT)
Formulated by MacAorthur-Pianka (1966).
It is a behavioral ecology model that helps predict how an animal behaves when searching for food.
It states “to maximize fitness, an animal adopts a foraging strategy that provides the most benefit (energy) for the lowest cost,
maximizing the net energy gained.”
Although obtaining food provides the animal with energy, for searching and capturing the food require both energy and time.
OFT helps predict the best strategy that an animal can use to achieve this goal.
It is an ecological application of the optimality model.
This theory assumes that the most economically advantageous foraging pattern will be selected for a species through natural
selection.
Optimal foraging model
Generates quantitative predictions of how animals maximize their fitness while they forage.
The model building process involves identifying the currency, constrains and appropriate decision rule for the forager
(organism’s best foraging strategy).

14. Currency
 Defined as the unit that is
optimized by the animal.
 It is the net energy gain per unit
time.
 It can also be net energy gain per
digestive turnover time instead of
net energy gain per unit time
 By identifying the currency, one
can construct a hypothesis about
which benefits and costs are
important to the forager in
question.
Constrains
 Hypotheses about the limitations
that are placed on an animal.
 Due to features of the
environment or the physiology of
the animal could limit their
foraging efficiency.
EXAMPLE-:
The time that it takes for the
forager to travel from the nesting
site to the foraging site is an
example of a constraint.
The maximum number of food
items a forager is able to carry
back to its nesting site is another
example of a constraint

15. •Building an Optimal Foraging
Model
Optimal foraging model generates
quantitative predictions of how animals
maximize their fitness while they forage.
The model building process involves
identifying:
 the currency (maximum food per unit
time),
 constrains (environmental) and
 appropriate decision rule for the forages
(organism’s best foraging strategy).

16. Model's prediction of what will be the animal's best foraging strategy
or set of choices under the organism's control
EXAMPLES
optimal number of food items that an animal should carry back to its
nesting site.
the optimal size of a food item that an animal should feed on.
◦ Figure, shows an example of how an optimal decision rule could be
determined from a graphical model. The curve represents the energy
gain per cost (E) for adopting foraging strategy X. Energy gain per cost
is the currency being optimized. The constraints of the system
determine the shape of this curve.
◦ The optimal decision rule (x*) is the strategy for which the currency,
energy gain per costs, is the greatest.

17. ◦ Also known as the prey choice model or the contingency model.
◦ In this model, the predator encounters different prey items and decides whether to eat what it
has or search for a more profitable prey item. The model predicts that foragers should ignore
low profitability prey items when more profitable items are present and abundant.
◦ Profitability is dependent on ecological variables
Energy (E): amount of energy required for searching the food
Handling time (H): amount of time the predator takes to handle the food
Search time (S): amount of time the predator takes to find a prey and this is dependent on the
abundance of the food and the ease of locating it
Profitability (P):
𝑬 (𝒆𝒏𝒆𝒓𝒈𝒚)
𝑯 (𝑯𝒂𝒏𝒅𝒍𝒊𝒏𝒈 𝒕𝒊𝒎𝒆)+𝑺 (𝒔𝒆𝒂𝒓𝒄𝒉𝒊𝒏𝒈 𝒕𝒊𝒎𝒆)
Currency: energy intake per unit time
Constrains: actual values of E, H and S

18.  Given that foragers may want to maximize 'profit', what should they do when less than optimal prey are
encountered?
 Predator - searching for 2 types of prey (1 & 2) that require search times of S1 & S2
 Prey - 2 types that yield E1 & E2 units of 'reward' (e.g., energy) AND take h1 & h2 seconds to handle
 So, their profitabilities = E1/h1 & E2/h2
 Let PREY TYPE 1 be more profitable than PREY TYPE 2:
 What should a predator's strategy be to maximize energy intake/unit time?
 Should a predator take only PREY TYPE 1 & always ignore PREY TYPE 2? Should a predator always take both?
 Here it depends on S1 (Search time for Type 1 prey)
 If :
 E2/h2 > E1/(h1+S1),(which would be true if S1 is large), then take both
 E2/h2 < E1/(h1+S1),(which would be true if S1 is small), then take only Type 1 prey
CLASSICAL MODEL OF PREY CHOICE

19. •Assumptions of this model:
•Prey value is measurable net energy or some other comparable single dimension
•Handling time is fixed
•Handling & searching cannot be done at the same time
•Prey are recognized instantaneously (with no errors)
•Prey are encountered sequentially & randomly
•Energetic costs of handling are the same for different prey
•Predators wish to maximize rate of energy (or some other measure of value) intake
•Predictions of model:
•Most profitable prey should never be ignored
•Less profitable prey should be ignored according to preceding equation
•Exclusion of less profitable prey should be all-or-nothing (depending on direction of inequality)
•Exclusion of less profitable prey does not depend on S2 (when Type 1 prey are sufficiently abundant
using time to handle Type 2 prey is not profitable)

20. Patch Departure Rule
 The foragers changes the track in patch and habitat quality to save time to
invest time more effectively on other patches.
 Departure from a prey patch is one of the key factors determining its
foraging success.
 ‘W’ representing the time a predator is ‘willing’ to invest in the patch.
 As long as no prey are captured, W’ declines and when it d r o p s b e l o w
critical level the patch is abandoned.

21. ◦ The MARGINAL VALUE THEOREM is a type of optimality model that is often applied to optimal foraging. This theorem is used to
describe a situation in which an organism searching for food in a patch must decide when it is economically favorable to leave.
◦ When more energy is required for an animal to search food in this patch then the animal should leave the patch and go to another patch
where more food/ resources are available. It is prediction when to leave the patch. While the animal is within a patch, it experiences
the law of diminishing returns, where it becomes harder and harder to find prey as time goes on.
◦ This may be because the prey is being depleted, the prey begins to take evasive action and becomes harder to catch, or the predator starts
crossing its own path more as it searches. This law of diminishing returns can be shown as a curve of energy gain per time spent in a
patch.
◦ Patch residence time : Time that animal spend within a patch
◦ Value of current patch : Resources available
◦ Value of other patches in environment
◦ Travel time : move to next closest path

22. Curve of energy gain per time spent in a patch.
1.The theorem predicts that foragers will remain a shorter time
in patches with little food than in patches with more food.
2. Patches will be abandoned more quickly when they’re close
together than when they’re scattered
MVT predicts that animal should leave the patch when the
energy intake within the patch diminishes to the average
energy harvesting time within the environment Time
travelled
btw
patches
Optimal
Time
spent in
patches
Old patch
new patch
Time
Resources
gained

23. The curve starts off with a steep slope and gradually levels off as
prey becomes harder to find. Another important cost to consider
is the traveling time between different patches and the nesting site.
Currency: net energy gain per unit time.
Constraints: the travel time and the shape of the curve of
diminishing returns.
As animals forage in patchy systems, they balance resource
intake, traveling time, and foraging time. Resource intake within
a patch diminishes with time, as shown by the solid curve in the
graph to the right.
The curve follows this pattern because resource intake is initially
very fast, but slows as the resource is depleted.
Traveling time is shown by the distance from the leftmost
vertical dotted line to the y-axis. Optimal foraging time is
modeled by connecting this point on the x-axis tangentially to the
resource intake curve.
In order to maximize the currency, one wants the line with the
greatest slope that still touches the curve (the tangent line). the
place that this line touches the curve provides the optimal
decision rule of the amount of time that the animal should spend
in a patch before leaving.

24. 'Marginal value theorem
Exploitation of patches
Prey availability within a patch decreases as a result of the predator's
foraging activity because of:
•Depletion of prey
•Evasive action by prey
•As a result, to maximize the rate of gain of a resource, predators
should follow it
To maximize gain (e.g., energy) per unit time, a predator should leave
at the point (maximum net gain) that gives the greatest gain or food
intake per unit time (steepest slope of the line). The line is not as steep
(which means less gain or intake per unit time) when the predator
leaves too early (or too late).
If travel time increases (e.g., patches further apart), the optimal time to
stay in a patch also increases:

25. ◦ Assumptions of the Marginal Value Theorem:
1 - Each patch type is recognized instantaneously
2 - Travel time between patches is known by the predator
3 - Gain curve is smooth, continuous, & decelerating
4 - Travel time between & searching within a patch have equal energy costs
◦ Predictions of the Marginal Value Theorem:
1 - If travel time & the gain curve are known, then opt can be predicted
2 - If there is more than one patch type in an environment, all should be reduced to the same gain rate

26. Optimal foraging in bees
◦ Worker bees provide another example of the use of marginal value theorem
in modeling optimal foraging behavior. Bees forage from flower to flower
collecting nectar to carry back to the hive
◦ A bee does not experience diminishing returns because of nectar depletion
or any other characteristic of the flowers themselves. The total amount of
nectar foraged increases linearly with time spent in a patch. However,
the weight of the nectar adds a significant cost to the bee's flight between
flowers and its trip back to the hive.
◦ Wolf and Schmid-hempel (1989) showed, by experimentally placing varying
weights on the backs of bees, that the cost of heavy nectar is so great that it
shortens the bees lifespan.
◦ By maximizing energy efficiency, the bees are able to avoid expending too
much energy per trip and are able to live long enough to maximize their
lifetime productivity for their hive.

27. Optimal foraging by hoverflies (Diptera: Syrphidae) and
ladybirds (Coleoptera: Coccinellidae): Mechanism
◦ Coccinellids and syrphids that feed on aphids face the same problem:
an unstable food supply. Their eggs and larvae face cannibalism and/or
starvation if the aphid colony they attack declines in abundance before
they mature. Optimal Foraging Theory predicts that such predators
should lay a few eggs early in the development of an aphid colony.
◦ Studies on two species of coccinellid and one species of syrphid
revealed that they do respond to the quality as well as the abundance of
their prey. By refraining from laying eggs in aphid colonies already
exploited by predators and those that are shortly to decline in
abundance when the aphids disperse, these predators are able to forage
in a way that is consistent with the predictions of optimal foraging
theory.
Hemptinne et al., 1993

28. Conclusion
◦The optimal foraging theory predicts that animal will
forage in a way that will maximize its net yield of
energy. The foraging strategies tend to increase the
expected reward in the next prey visited, by avoiding
patch which have been recently visited, by choosing
more rewarding individual patch.

30. Question-aNSWERS
1. What is phoresy ?
◦ Phoresy is a phenomenon in which an individual is transported by a larger individual of another species. This
relationship benefits the carried species and does not directly affect the carrier. It provides a means of finding
a new host or food source.
2. What is optimum decision rule?
◦ The optimal decision rule (x*) is the strategy for which the currency, energy gain per costs, is the greatest.
3. What is marginal value theorem?
◦ The s is a type of optimality model that is often applied to optimal foraging. This theorem is used to describe
a situation in which an organism searching for food in a patch must decide when it is economically
favorable to leave

31. 4. What is patch departure rule?
◦ The forager has to track changes in patch and habitat quality to save time that could be invested more
effectively on other patches. Hence, the decision mechanism controlling a predator’s departure from a prey
patch is one of the key factors determining its foraging success. When entering a patch, a variable, say ‘w’
representing the time a predator is ‘willing’ to invest in the patch, is set at some initial value. As long as no
prey are captured, ‘w’ declines and when it drops below a critical level the patch is abandoned
5. Explain optimum diet model?
◦ In this model, the predator encounters different prey items and decides whether to eat what it has or search
for a more profitable prey item. The model predicts that foragers should ignore low profitability prey items
when more profitable items are present and abundant.
◦ Profitability is dependent on ecological variables