Foraging and Trophic Ecology

What an animal eats, and how it goes about finding its prey, go a long way towards describing its ecology. Optimal foraging theory and modes of foraging began 30 years ago with the publication of three papers (MacArthur and Pianka 1966; Emlen 1966; and Pianka 1966).

Evolutionary innovations of skulls of squamate reptiles (lizards and snakes): streptostyly and mesokinesis. Consequences for foraging mode and ecology.

Modes of Foraging

Sit-and-wait "ambush" predators
Active widely-foraging
Huey and Pianka's ecological correlates, Toft's frogs
Crossovers between trophic levels in foraging modes

Optimal Foraging Theory

Search Time (per item eaten). Search for any and all palatable prey simultaneously
Pursuit Time (per item eaten). Pursue potential prey items one at a time.
Rank foods from most desirable (most calories returned) to least.
As diet expands to include less and less desirable prey types, more prey items are encountered and search time per item falls, but as diet is expanded to include less desirable prey, average handling time (pursuit time) per item eaten must increase.
Where increased pursuit time exceeds the savings in reduced search time marks the optimal diet.
Search time (per item eaten) decreases with prey abundance, but pursuit time (per item eaten) does not. Cannot afford to bypass substandard food items when prey is scarce, but can omit them when food is abundant (can expect to locate and catch a better item in the time it would have taken to handle the first substandard one).
Most robust theorom of OFT: diets should contract in times of prey abundance, expand during periods of prey scarcity.
Cnemidophorus tigris diet.
Do species matched for time of activity (e.g. diurnal x diurnal, or nocturnal x nocturnal) exhibit greater dietary overlap than pairs of species that are active at different times (diurnal x nocturnal)?

Specialists versus Generalists, Dietary Niche Breadth

Tradeoffs: advantages versus disadvantages (benefits and costs)
MacArthur-Levins model:
Phenotypic spectrum of jaw sizes, ant sizes in different environments.
When does a jack of all trades outcompete specialists?
Fine-grained versus coarse-grained utilization of environments.
Electivities, prey resource availability.
Does temporal separation reduce dietary overlap (competition)?

Snakes

Trophic specialists that have specialized on the following prey categories:
birds, eggs, frogs, lizards, mammals, other snakes, and toads.
Most snakes eat quite large prey items
Functional anatomy: cranial kinesis (snake skulls), Frazetta's python
Hinged premaxillary teeth (skink osteoderms). Savitsky, Patchell and Shine
Dentition, Hypodermic needles, Infrared heat sensors, constriction (ambush predators)

Lizards, Frogs, and Salamanders

Whole spectrum ranging from extreme specialists to very generalized diets.
Most insectivorous, but many tadpoles and iguanas are herbivorous.
Some, such as monitor lizards (Varanus) prey primarily on other vertebrates.
Sticky projectile tongues have evolved at least three times, in chameleons, frogs, and some salamanders

Turtles
terrestrial tortoises herbivorous
most aquatic ones carnivorous (some omnivorous)

Crocs
Ambush predators, capture large prey at water's edge.


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Last updated 25 March 1997 by Eric Pianka