Optimality theory; Foraging Strategies
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- Optimality models
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-do animals make the "right" decisions according to our understanding of the factors important to fitness?
-trade offs
-cost-benefit ratios -
T/F
major point of optimality modeling isn't to learn if animals behave 'perfectly', but to see if one has correctly identified the factors that influence behavior. - T
- Foraging decisions
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1.habitat selection:optimal habitat to settle in, with respect to food availability
2.patch selection:what are the optimal foraging sites? how long should the animal forage in a particular patch before moving on? if the resource is renewable, what is the best interval between repeat visits?
3.Diet selection:what food items should be eaten and what should be ignored? -
T/F
Optimal "choice" is the one that is the best (in terms of fitness) from among the available options. Simplist foraging and diet models define fitness in terms of energy intake. - T
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2 categories of foragers
energy maximizers
time minimizers -
energy max: fitness increases in proportion to the toal calories eaten
Time minimizers:extra food is not as important as extra time for non-foraging activities -
T/F
whether energy maximizer or time minimizers, the important pt is to maxmize NET RATES OF ENERGY INTAKE (a cost benefit ration:energy eaten/energy used in foraging) - T
- Optimal diet
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Which of a choice of prey should be eaten and which should be ignored
-Average amount of time needed to locate, capture, and eat the prey - 3 rules of optimal diet
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1.always eat the prey(type 1) with the lowest time per joule gained
2.other types(type 2,3)are eaten ONLY if the animal has a net gain greater than if it ignored type 2 and looked instead for another of type 1. This rule is followed for other prey types down the line.
Predictions:
-as food abundance increases, the optimal diet shrinks and vice versa
-presence of competitors leads to an expanded diet, beacuse competitors reduce prey density. This is opposite to what happens over evolutionary time, when competing species evolve to minimize resourve overlap -
Example of optimal diet
Bluegill sunfish -
bulegill sunfish feeding on Daphnia
-3 prey sizes(small medium large)
-3 dentsities(low, medium, high)
-figure out search and handling times of each prey size
-calculated the optimal "switch points"(in terms of search time)
-Real fish matched predctions
high densitiy:only eat large prey
medium density:eat large and medium
low density:eat all sizes -
Example of optimal diet
Great tits
crows
oyestercatchers -
-select worm sizes from a moving conveyor belt
-crows eating clams
-oystercatchers eating mussles - Criticisms of optimal foraging theory
- -hard t see how animals could be optimal unless they accurately assess prey density; difficult in a complex changing environment. but knowledge comes with experience
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Failed optimality prediction
Adelie penguins -
-adelie penguins eat krill
-how much time spent at depth vs at surface?
-spend about 1/3 of their time on the surface and stay under for a max. 3min.
-The deeper they go, less time they can spend chasing prey(bottom line)
-PREDICTION:they should maximize ratio of bottm time/(surface time+travel Time+bottm time)
-But they do not do this. They spend less time underwater than would be "optimal" under initial assumptions
-maybe improves energy efficiency, or perhaps krill are so abundant in swarms that optimality isn't important to fitness - Things besides prey energy content that could influence the fitness of diet choice and foraging behavior?
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1.nutrients:a diet optimal for energy intake may not provide adequate nutrients
2.Toxic compounds:may have to compromise between toxicity and nergy or nutrition
3.Predation:many species modify foraging if predators are a ris
4.overall physiological condition:a very lean or hungry animal will often modify foraging behavior (with respect to predation exposure) -
Ex of max. effectiveness of foraging by "optimizing" use of space
south american army ant Eciton -
-live in huge colonies that move around the forest floor and make temporary "camps" called bivouacs
-bivouacs are a bit ball of ants with the queen and young inside
-every day a "raid" of ants fans out radially from the bivouac and gathers prey and brought back tot he bivouac
Problem:a riad will essentailly clean out a piece of forest of all available prey. How do colonies avoid wasting time and energy foraging in depleted areas?
Answer:
-every day, raids go out in a different direction(130 from previous day) this min. raid overlap around a bivouac
-when bivouacs move(every week or two) they avoid places where they or other colonies have recently foraged