Notesale: Turn your study into money

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One can see an immediate advantage in having a broad diet: there is a much better chance of
finding something to eat, no matter where the individual might find itself
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The energy gain function f ( N 1
) grows with increasing abundance of prey type 1, but there are diminishing returns to this
relationship (Fig
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1)
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This upper limit is set by the limited capacity of the animal to
handle prey
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How would this compare to the energy gain if the forager
generalized by feeding on both prey types 1 and 2? If both prey types were mixed
indiscriminately over the landscape traveled by our hypothetical forager, then the energy gain
by a generalist would be calculated as follows: g ( N 1 , N 2 )= a ( e 1 N 1 + e 2 N 2 ) 1 + a ( h 1 N 1
+ h 2 N 2 ) This equation raises the following question: when does it pay to be a specialist and
when to be a generalist? The answer is to specialize when f ( N 1 ) > g ( N 1 , N 2 ) but act like a
generalist when f ( N 1 ) < g ( N 1 , N 2 )
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Assuming that maximal acquisition of energy can improve the fitness of a forager (improve
reproduction or survival), we might expect something like the optimal strategy to by favored by
natural selection
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Predictions:
o 1 Foragers should rank food types in terms of their energetic profitability (energy
content divided by handling time)
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o 3 The decision to specialize or generalize should depend on the abundance of highly
profitable prey, but not on the abundance of less profitable prey
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By this we mean that the
perfect forager should either always accept alternative prey or never accept them,
depending on whether f ( N 1 ) > e 2 ∕ h 2
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Instead, the
foragers sometimes ate both prey types and sometimes only the more profitable prey, a pattern

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termed “partial preference
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In some cases, particularly with herbivores, foragers need to maintain a balanced intake of
particular nutrients, rather than simply maximizing energy gain in whatever way possible (
many species forage outwards from a central place, whether that place is a den, perch, or
resting site
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The greater energetic cost of
travel to more distant food requires that animals be more selective
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The
rate of consumption of prey type 1 ( f ( N 1 )) by an optimal forager is predicted by the following
multispecies functional response: g ( N 1 , N 2 )= aN 1 1 + ahN 1 + 𝛽 ( N 1 ) ah 2 N 2 1
o where 𝛽 ( N 1 ) is the probability of foraging on the poorer prey, which is a function of
the density of preferred prey, calculated according to the optimal diet choice rule
This equation predicts that there will be a sharp drop in the consumption of the more profitable
prey at the point at which the forager expands its diet to include less profitable types
his model predicts that animals should feed whenever the cropping rate exceeds the average
rate of cropping
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However, a second deer herd that
roamed over a much larger area showed little evidence of being sensitive to the marginal value
of grazing
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Similar patterns
have been recorded in cattle (Laca et al
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1995) and dorcas gazelles ( Gazella
dorcas ) (Ward and Salz 1994)
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e
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This
implies that foragers should concentrate in areas with above-average prey abundance, ignoring
areas with lower levels
aquatic insect Notonecta has been elegantly demonstrated in the laboratory (Sih 1980)
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Sih set up an
experimental arena where individual Notonecta larvae could choose to feed in food-rich or
food-poor patches
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This seems to be a logical way
of reducing the risk of predation, at the cost of reduced food intake

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