Resource Acquisition & Allocation Optimal Foraging
Theory
Slide 2
Resource Acquisition & Allocation Foraging tactics and
efficiency Foraging has costs (exposing yourself as a prey item)
and takes time/energy An optimal foraging tactic maximizes the
difference between foraging profits and their costs This should be
under very strong natural selection!
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Resource Acquisition & Allocation Consider the
benefits/costs of chasing prey that are of relatively poor
nutritional value What about the cost/benefit associated with
caloric return (small vs. large; easy vs. hard) Natural routes
should be favored Good locations should be checked These will vary
across the landscape
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Resource Acquisition & Allocation Optimal Foraging Theory
Numerous aspects of OFT can be neatly summarized into a series of
assumptions A) environmental structure is repeatable, with some
statistical expectation of finding a particular resource (such as a
habitat, microhabitat, and/or prey item)
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Resource Acquisition & Allocation Optimal Foraging Theory
b) food items can be arranged in a continuous and unimodal
spectrum, such as size distributions of insects c) similar animal
phenotypes are usually closely equivalent in their harvesting
abilities (e.g intermediates); also similar sized prey are only
slightly less efficient than the optimal sized prey
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Resource Acquisition & Allocation Optimal Foraging Theory
d) the principle of allocation applies, and no one phenotype can be
maximally efficient on all prey types (trade-off in efficiencies)
e) an individuals economic goal is to maximize its intake of food
resources
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Resource Acquisition & Allocation Optimal Foraging Theory
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
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Resource Acquisition & Allocation Optimal Foraging Theory
Search and pursuit efficiencies are largely determined by the
preceding assumptions about foraging morphology Thus MacArthur only
consider 1 and 3 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
Slide 9
Resource Acquisition & Allocation Optimal Foraging Theory
Where to search can largely be the result of previous foraging
attempts Which prey items to select is also relatively
straightforward; however, one does has to decide whether to pursue
it or continue searching for something better
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Resource Acquisition & Allocation Optimal Foraging Theory
Ultimately, the predator will chase again, so then the real
question becomes whether they will find another, better prey item
in the time required to capture and ingest the first prey item
Slide 11
Resource Acquisition & Allocation Optimal Foraging Theory
Many animals spend the majority of the effort searching for prey,
but relatively little capturing and eating small prey items (e.g.
searchers)
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Resource Acquisition & Allocation Optimal Foraging Theory
Conversely, many animals spend little time searching, but a great
deal of time/effort in capturing it Consequently, pursuers should
generally be more selective and more specialized than
searchers
Slide 13
Resource Acquisition & Allocation Optimal Foraging Theory
Currently the currency for which OFT operates is energy gained/time
Incorporating limiting nutrients or predation risk have not been
widely incorporated
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Resource Acquisition & Allocation Optimal Foraging Theory
Carnivorous animals forage in a number of interesting ways sit and
wait vs. actively foraging
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Resource Acquisition & Allocation Optimal Foraging Theory
What conditions are 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
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Resource Acquisition & Allocation Optimal Foraging Theory
For the searchers, prey density and mobility are also important,
but the spatial distribution of prey is paramount
Slide 17
Resource Acquisition & Allocation Optimal Foraging Theory
Even for groups that appear relatively consistent, subtle
differences show why this paradigm has conceptual value
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Resource Acquisition & Allocation Optimal Foraging Theory
There are some general correlates between these foraging modes
Slide 19
Resource Acquisition & Allocation Optimal Foraging
Theory
Slide 20
Herbivores can similarly be viewed Herbivores spend relatively
little energy in finding their prey, but more breaking down the
chemical compounds and absorbing the nutrients
Slide 21
Resource Acquisition & Allocation Optimal Foraging Theory
Because carnivore prey is composed of readily available proteins,
lipids, and carbs (and easily digestible), carnivores can afford to
expend considerable effort in searching for their optimal prey
Slide 22
Resource Acquisition & Allocation Optimal Foraging Theory
Many carnivores have extremely efficient (and elaborate) capturing
aparatii
Slide 23
Resource Acquisition & Allocation Optimal Foraging Theory
Holling estimated the diameter of prey item that should be optimal
for a praying mantid of a particular size
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Resource Acquisition & Allocation Optimal Foraging Theory
He then offered a hungry mantid prey items that varied in size.
They were reluctant to attack small or large prey items
Slide 25
Resource Acquisition & Allocation Optimal Foraging Theory
Because small organisms are disproportionately more abundant than
large ones, most predators encounter and eat many more small items
than large ones, irrespective of their own size (although must
still be energetically profitable) Who (size) should have a larger
diet breadth?
Slide 26
Resource Acquisition & Allocation Physiological Ecology
Environmental physiology is how organisms function within, adapt
and respond to, and exploit their physical environments PEs are
primarily interested in the immediate functional and behavioral
mechanisms by which organisms cope with their abiotic environments
Mutual constraints between physiology and ecology dictate that both
must evolve together
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Resource Acquisition & Allocation Physiological Ecology
Homeostatis: the maintenance of a stable internal state across a
range of environmental conditions Can be achieved by physiological
means and/or behavioral Many factors need to be controlled besides
temperature: humidity, light intensity, and various concentrations
(e.g. pHs, salts)
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Resource Acquisition & Allocation Physiological Ecology
What is the benefit of all of this?
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Resource Acquisition & Allocation Physiological Ecology
Physiological Optima and Tolerance Curves Physiological processes
proceed at different rates under different conditions They
typically look like bell-curves
Resource Acquisition & Allocation Physiological Ecology
Performance curves can sometimes be altered during the lifetime of
an individual, especially as it becomes exposed to different
ambient external conditions Clearly tolerance curves change over
evolutionary time, but little is known about the evolution of
tolerance acclimation
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Resource Acquisition & Allocation Physiological Ecology
Performance or tolerance is often sensitive to two or more
environmental variables For example, temperature and humidity can
impact the performance of many things
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Resource Acquisition & Allocation Energetics A relatively
high % of food passes through the gut unused (80 to 90) Food is
digested and assimilated and some is used for respiration and
metabolic activity The remainder is incorporated into the animal
concerned as secondary productivity (growth or reproduction)
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Resource Acquisition & Allocation Energetics Ingestion =
assimilation + egestion Assimilation = productivity + respiration
Productivity = growth + reproduction The total amount needed per
unit time for maintenance increases with increasing body mass
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Resource Acquisition & Allocation Energetics Metabolic
rates vary on several key aspects
Slide 36
Resource Acquisition & Allocation Energetics Because small
organisms have a very high SA/vol ratio, they have a much higher
metabolic rate (scaled to mass)
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Resource Acquisition & Allocation Energetics Because energy
is required to maintain a constant internal body temperature,
homeotherms have considerably higher metabolic rates, as well as
higher energy needs than poikilotherms (approximating temperature
is that of the environment) of the same body mass Related terms:
endotherm & ecotherm
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Resource Acquisition & Allocation Energetics The vast
majority of animals are ectothermic and all plants are as well Some
of the larger poikilotherms are at times at least partially
endothermic Behavior allows for increased efficiencies
Slide 39
Resource Acquisition & Allocation Energetics Because of the
energy requirements to maintain a constant body temp no matter what
the conditions, endotherms have considerably higher metabolic
rates
Slide 40
Resource Acquisition & Allocation Energetics There is a
distinct lower limit on body size for endotherms (2-3; humm and
shrewniche?)