Biology of Predation

Reading: Smith and Smith, Chapters 15-16

• The term “predation” has come to encompass a range of interspecific interactions where one species obtains its energy and nutrients by consuming another living organism.

– These include

• predation-strict sense– this can include filter and suspension feeding

• parasitism

• parasitoids

• herbivory

Predators, in the strictest sense, are animals that hunt down otheranimals and eat them. In general, predators are larger than their prey,and consume many prey over the course of their lifetimes.

Some predators are specialists to some extent, but many are generalists that will consume all prey of a certain size.This is an alligator lizard, Lacerta vivipara, it is a generalistthough primarily an insectivore.

Not all organismsare strict predators-many unicellular algae are facultative predatorsor mixotrophs.-facultativepredators, such asthese haptophytesphotosynthesize,but also ingest smallprotozoans that they catch via a structurecalled a haptonema.

-A large number of oceanicanimals (and a few protozoans)are filter feeders-they consume prey suspended in the water column. -Filter feeders take preyin a certain range of sizes-no filter can catch preyof every size without beingtremendously inefficient.-Blue mussels Mytillus edulis filteroceanic plankton from moving waves-this can be either plant, animal, or protist.

Parasitoids huntdown animal hostsand use them as foodfor their larvae.-ectoparasitoids laytheir eggs on the outside.-endoparasitoids laytheir eggs inside the host.-koinobionts allow thehost to develop fora while before the larvae kill it and pupate.-idiobionts stun-paralyzeit right away.Either way, the larvaekill the host and disperse

This is an ichneumenoid wasp attackinga sawfly larva.

Parasites encompass a widerange of organisms that liveon or within the host, and consume the host withoutkilling it right away.-ectoparasites, suchas this leech, attack the hostfrom the outside-endoparasites attack the hostfrom within.Microscopic parasites arefrequently called pathogens.

Herbivores areanimals that eatplants.Some, such as theseAsian antelope, aregrazers-they chopaway the leaves of grass and herbaceousplants withoutkilling them.Browsers-selectivelyeat parts of woodyvegetation.-in so doing, theyresemble parasites more than predators

These aphids suckjuices from plants,they are essentiallyplant ectoparasites.

Some herbivores,notably bison, sheep,and elephants, consume the entireplant, and thus resemble predators

Some Possible Defenses Against Predators

• Defensive behavior

• Toxins

• Sheer size

• Armor

• Speed

• Crypsis

• Mimicry

Defensive Behavior• Fighting back-is sometimes, but not usually,

an option. – For herbivores,specialization to the lifestyle

usually precludes being able to fight off a sophisticated carnivore.

• For example; teeth adapted for grinding plant material are useless as weapons.

– Generalist predators usually avoid prey that are able to hold their own, one strategy is to fool the predator into thinking you are tougher than you are-this is called a threat display.

For example, thishognose snake,Heterodon platirhinoshas an impressive threat behavior designedto deter predators.In fact, it is nonvenemous

Threat behaviors scareoff some generalists, like coyotes, but specialists, and predators that have learned to ignore the behavior, catch them anyway.

Vigilance• Vigilance is a very common, and presumably a

very effective, antipredator behavior. – Seeing the predator first confers options to the

potential prey, such as • running away• hiding• clustering into a dense mass• warning one’s relatives

• As we will see, vigilance can be costly, and the presence of predators can affect the growth of prey through fear and caution alone

For example, Belding’s ground squirrel,Spermophilis beldingi,is known for vigilance.Members of a kin group forage together-at least onekeeping watch at any given time (they pay attention to each other’sactivity)When a predator is sighted,the vigilant individual gives an alarm call that potentially gives others time to escapeto their burrows.

• Clustering together can be an effective defense against some predators, because groups of individuals under attack can make it very difficult for a predator to select a single individual as a target.

• Sheer size works well. Elephants and giraffes have essentially no nonhuman predators as adults. – The problem is growing that

large, young individuals can be very vulnerable.

• Large size carries other costs and benefits as well, and it is not an option for many organisms (arthropods).

Toxins• Some animals, such

as the poison arrow frog Dendrobates auratus manufacture powerful toxins.

• This defense is often coupled with bright color, which is thought to warn potential predators of the risk of attacking these animals

• For instance, Milkweeds produce toxic compounds called cardiac glycosides, which can kill mammals.

• Monarch larvae Danaus plexippus have evolved to eat milkweeds and sequester the toxins for protection against predators.

Crypsis• Crypsis is the

evolutionary modification of an organism’s morphology, color, smell, or behavior, to avoid being detected.

• Predators can also be cryptic to avoid being detected by potential prey.

This is a lappet moth Phyllodesema americana

Mimicry• Mimicry is a widespread

evolutionary adaptation to resemble species that predators are likely to recognize as poisonous.

• Batesean Mimicry-is deceptive, mimic is harmless

• Mullerian Mimicry-mimic is harmful, advantage is that predators are more likely to recognize

potential trouble

Eumenid wasp

Syrphid fly

Defenses Against Herbivores

• Passive defenses-these are always present– Toxins– Spines– Silica-erodes mammal teeth– Tannins-impedes edibility, digestibility, nutrition

• Induced defenses-these are present when the plant is under attack– Toxins

• Tolerate herbivory

Toxins• Toxins are very common

among vascular plants, particularly angiosperms.

• Toxins can provide very good defense against herbivory– drawbacks; toxic

compounds are thought to be expensive to produce

– hervivores, especially insect specialists, may evolve resistance or even sequester them.

This is Jimson weed-Datura stramonium,it produces a toxin deadly to mammals

Tolerate Herbivory• Many plants have evolved to

simply tolerate certain kinds of herbivory. – It is possible that certain

types of plants actually benefit from some grazing.

– Meristems, undifferentiated tissue used to produces new shoots, stay under the ground or in sheltered locations-this allows rapid re-growth and prevents the herbivore from destroying the plant entirely.

Many grassland speciesare notably tolerant of grazing.

Predator-Prey Population Dynamics• Real populations of prey, and their predators, tend to exhibit

cycles in abundance in which the peak in prey abundance precedes a peak in predator abundance.– The following is the classic (Elton, 1928) study of the Canada Lynx

and the snowshoe hare. This figure is based on historical data using the numbers of hare and lynx pelts sold to the Hudson Bay company.

• Note the ten year cycles-originally attributed to sunspots by some.• Note some of the drawbacks in using historical data such as this.

Does this trend reflect economics?• Subsequent studies have supported the notion that lynx predation is

an important factor driving the cycle, partially because fear of lynx predation impairs the foraging efficiency of hares

• The Lotka-Volterra model:– Alfred Lotka and Vito Voterra independently came up

with a simple mathematical model of predator-prey population dynamics.

• For the prey population, it starts with the exponential model of population growth, and invokes a term for prey killed by a predator, thus:

• dR/dt=rR - cRP• where R is the prey population,

• dR/dt is the growth rate of the prey population,

• r is the prey’s intrinsic rate of natural increase,

• c is a constant representing the efficiency of the predator, and

• P is the predator population

• The predator population depends upon the prey population as well. Thus:

• dP/dt=acRP-dP– dP/dt is the growth rate of the predator population

– where P is the population of the predator

– c is a constant reflecting the efficiency of the predator

– R is the prey population

– d is the per capita death rate of the predator population

– a is a constant reflecting the efficiency with which captured prey are converted into new predators.

This model has some interesting properties

• There is an equilibrium point at:

• R*=d/ac P*=r/c– this equilibrium is neutral– this model produces predator-prey oscillations that are

neutrally stable-further “pushing” from the equilibrium produces cycles of greater amplitude.

In each “zone”,you can draw a vectorrepresenting the changein the population ofpredators, and preyrespectively-predator=up-downprey=side-side

• The original model is interesting in that it predicts the predator-prey oscillations frequently observed in nature.– An increase in the birth rate of the prey increases the

equilibrium density of the predator, but not the prey• this prediction is borne out in simple bacteria-bacteriophage

experiments by Bohannan and Lenski.

• It has several weaknesses, however– completely neutral oscillations are not observed in nature-

they are an artifact of the simplicity of the model– model assumes efficiency of prey capture is independent

of prey density-prey cannot be satiated– model assumes no density-dependence on prey

Density-dependence can slow recruitment of prey populations as they reach carrying capacity-This effect tends to dampen predator-prey oscillations and make the equilibrium point stable.

MacArthur and Rozenswig argued that the shape of the prey isoclineshould be a “hill”, because recruitment falls off at low densities nearzero, and at very high densities near carrying capacity.

Some predators tend to compete with each other at high densities. This would change the shape of the predator isocline. -This effect also tends to increase the stability of the system, and make the equilibrium point stable.

• The functional response of a predator describes how its ability to impact the prey population is affected by prey density.– Type I-density has no effect, the rate of prey

consumption is directly proportional to density• A type I functional response is implicit in the simple Lotka-

Volterra model.

– Type II-prey consumption decreases at high prey densities because predators become satiated or because prey defend themselves as a group.

– Type III-predators become more efficient as prey become more common-this may trail off as they become satiated.

Note that these are not isoclines

Predator efficiencycan have major effectson a predator-preysystem

A-Less efficient predators are only able to reproduce effectively when their prey are near carrying capacity.More stable

C-Very efficient predators can easily drive their prey extinct, and going extinct themselves.Less stable

Predator-Prey Coexistence

• In natural systems, some predators tend to drive their prey locally extinct, others do not.– Some predator-prey systems are stable.

• Predators and prey may attain a stable, oscillating cycle in which predator abundance tracks prey abundance.

• Predators may tend to “regulate” prey populations, keeping them at a stable equilibrium below carrying capacity.

– Some predator-prey systems are unstable.• When driven locally extinct, the predator may

either go locally extinct itself, or it may switch to another prey species. If the predator switches, to another prey, it then permanently drives the prey out of the habitat. The prey can only exist where the predator cannot or does not live.

• If predator and prey both go locally extinct, that may free the habitat to be recolonized by the prey. Ultimately, the predator may show up and the cycle may repeat itself.

– Such a system might be locally unstable, but the metapopulation might be stable.

Stability vs. Instability• Factors that promote

stability – inefficient predators– density-dependence of

either predators or prey– predator switches to

alternative food before prey go entirely extinct

– low time lags in predator response to prey density

– prey refuges

• Factors that promote instability – very efficient predators– inverse density

dependence of predators or prey

– high time lag in predator response to prey density

– simple environments, no refuges

Example, fish and Daphnia• Freshwater fish, such as yellow perch and bluegills, are

incredibly efficient predators of small crustaceans.

• Daphnia sp. are small, filter feeding, freshwater crustaceans with an enormous potential for reproduction, but with no defenses or antipredator behavior.

• When introduced to a lake, freshwater fish will drive vulnerable species such as Daphnia extinct, and then switch to other prey (usually insect larvae).

• Generally, Daphnia only persist in lakes with no fish.

Bluegill Lepornis macrochirus

Yellow perch-Perca flavescens

Water flea-Daphnia pulex

Example, Dieratiella vs. Aphids• The parasitoid wasp, Dieratiella rapae, is an incredibly efficient

predator-many factors make the system locally unstable– Females lay one egg inside each aphid-one female can lay hundreds of

eggs - thus their potential rate of increase is enormous.

– Females are incredibly effective in searching for aphids-they cue in on chemicals the plants emit to lure parasitoids

– There is a time lag between oviposition, and the death of the aphid-the aphid grows to adulthood and then is suddenly eviscerated by the parasitoid.

• Dieratiella quickly drives aphids extinct from a patch of host plants, and then disperses to find other hosts

• Once both species are gone, host plants may be recolonized by aphids

Experiments• In laboratory experiments, predator-prey

systems in simple environments often result in the extinction of the prey, followed by the extinction of the predator.– In an experiment, C.F. Gausse added the

predatory protist, Didinium sp. to an already established colony of Paramecium sp.. The result was the extinction of the prey, followed by the extinction of the predator.

• Refuges– In a second experiment, Gausse added a glass

“sediment” to the cultures. This provided hiding places for Paramecium.

– The result was the extinction of Didinium, followed by a rebound by the prey.

• Extinction-Recolonization– In a third experiment, Gausse repeatedly

inoculated the system above with Didinium.

– The result, was a cycling of predator and prey abundance.

The Huffaker Mite Experiments

• C. B. Huffaker studied a predator prey system involving two species of mites.– The six-spotted mite, Eotetrancyus sexmaculatus

is a common mite that eats oranges.– Typhlodromas occidentalis is a predator of the

six-spotted mite.

• Huffaker sought to create an artificial system that would exhibit the population fluctuations found in real-world systems.

Huffaker workedon an experimental array of oranges,they were covered insuch a way as to enablehim to control the surface area of the system

He also used rubber ballsthe same size as orangesto add areas of unsuitable habitat through which mites might need to pass to get to better areas

• Result-in simple systems, such as a single orange, or an array of oranges clustered together, predators quickly drove their prey to extinction, and went extinct themselves.

• In more complex systems, such as arrays of oranges at random locations, this process took much longer.

• Huffaker was finally able to achieve (temporary) population cycling, by adding Vasaline barriers to predator dispersal, and sticks to serve as launching pads for prey dispersal.