Foraging ecology- Optimal Foraging Theory.pptx

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 Foraging ecology Optimal Foraging Theory

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

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. Non-random, or directional foraging

11. 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.

12. 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).

13. 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).

14. OPTIMAL DECISION RULE 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.

15. Optimal Diet Model 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):   Currency : energy intake per unit time Constrains : actual values of E, H and S

16. 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 d epends on S1 (Search time for Type 1 prey) If : E 2 /h 2  > E 1 /(h 1 +S 1 ), (which would be true if S1 is large), then take both E 2 /h 2  < E 1 /(h 1 +S 1 ), (which would be true if S1 is small), then take only Type 1 prey CLASSICAL MODEL OF PREY CHOICE

17. 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)

18. 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.

19. 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. 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

20. C urve 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 Old patch new patch Time Resources gained

21. 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.

22. '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:        

23. 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

24. 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. 

25. 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

26. 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.

27. Question- aNSWERS 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. What is optimum decision rule? The optimal decision rule (x*) is the strategy for which the currency, energy gain per costs, is the greatest. 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

28. 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 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