Resource Acquisition & Allocation
Optimal Foraging Theory
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!
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
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)
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
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 individual’s economic goal is to maximize
its intake of food resources
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
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
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
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
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”)
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
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
Resource Acquisition & Allocation
Optimal Foraging Theory
• Carnivorous animals forage in a number of
interesting ways
• ‘sit and wait’ vs. ‘actively foraging’
Resource Acquisition & Allocation
Optimal Foraging Theory
• What conditions are required to support a sitand-wait strategy (1 or more)
• 1) relatively high prey density
• 2) high prey mobility
• 3) low predator energy
requirements
Resource Acquisition & Allocation
Optimal Foraging Theory
• For the ‘searchers’, prey density and mobility
are also important, but the spatial distribution
of prey is paramount
Resource Acquisition & Allocation
Optimal Foraging Theory
• Even for groups that appear relatively
consistent, subtle differences show why this
paradigm has conceptual value
Resource Acquisition & Allocation
Optimal Foraging Theory
• There are some general correlates between
these foraging modes
Resource Acquisition & Allocation
Optimal Foraging Theory
Resource Acquisition & Allocation
Optimal Foraging Theory
• 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
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
Resource Acquisition & Allocation
Optimal Foraging Theory
• Many carnivores have extremely efficient (and
elaborate) capturing aparatii
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
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
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?
Resource Acquisition & Allocation
Physiological Ecology
• Environmental physiology is how organisms
function within, adapt and respond to, and
exploit their physical environments
• PE’s 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
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)
Resource Acquisition & Allocation
Physiological Ecology
• What is the benefit of all of this?
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
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 acclimation
• Clearly tolerance curves change over
evolutionary time, but little is known about
the evolution of tolerance
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
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)
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
Resource Acquisition & Allocation
Energetics
• Metabolic rates vary on several key aspects
Resource Acquisition & Allocation
Energetics
• Because small organisms have a very high
SA/vol ratio, they have a much higher
metabolic rate (scaled to mass)
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
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
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
Resource Acquisition & Allocation
Energetics
• There is a distinct lower limit on body size for
endotherms (2-3; humm and shrew…niche?)
Resource Acquisition & Allocation
Energetics
Resource Acquisition & Allocation
Energetics