Sections

• Predation Defined
• Prey responses to predation
  • Strategies for predator avoidance:
• Models of Predation
• Graphical model of Predation
• Optimal Yield from populations upon which man preys
• Field Evidence for Predation's Impact
• Terms

Predation Defined

An interaction between two species in which one is harmed and the other helped

Several flavors of interaction fall under the above rubric:

• Herbivory -- plant eaters, algae not usually considered
• Carnivory -- meat eaters (other carnivores or herbivores)
• Cannibalism - eating one's own species
• Parasitoids -- usually insects that lay their eggs on other insects as hosts. The larvae complete development on the host, usually killing the host as a result
• Parasitism -- feeding on another organism's parts without killing the organism (a note: parasitism is very widespread and includes all kingdoms as both parasite and host)

The chart below can be helpful in separating the types (adapted from text)

We will concentrate on predators from now on in this lecture

Prey responses to predation

How important is predation in nature?

• If not important, prey will have no special adaptations for avoiding predation. No risk, then no reward for avoiding it.
• If important, then some type of strategy for predation avoidance will be a common adaptation
  • By this criterion, predation is an evolutionary significant source of mortality, as the strategies below are widespread
  • may be more likely to see anti-predator adaptations rather than predator adaptations because of the Life-Dinner Principle
    • The selective pressure is greater for the prey, which loses its life if it is unsuccessful, than on the predator, which only loses its dinner if it fails
  • however, evolution may respond to small differences in mortality, too small to be ecologically significant
    • to decide on the ecological significance of predation, field experiments are necessary (see below)

Strategies for predator avoidance:

Coloration:

• Aposematic colors: Warn the predator that they (the prey) are distasteful
• Cryptic colors: Hide the prey by blending it with its background
• Mimetic colors: These are attempts by members of one species to resemble another species. There are several types with more than one purpose:
  • Batesian mimicry: The mimic is edible, but looks like a model species that is not palatable due to a toxin in it.
    • one could argue that this is a kind of parasitism, as the mimics are helped and, to the extent that the predators are encouraged to try an occasional prey item, the model suffers a loss of effectiveness of its poison system.
  • Mullerian mimicry: Mimics are all inedible, but are too rare for the predator to learn to avoid them, so they look like one another so that the predator thinks of all of them as a single, poisonous type of prey

    • This is done so that the phenotype to be avoided is reinforced in the mind of the predator
    • one could argue that this is a kind of mutualism among the Mullerian mimics

  • Aggressive mimicry is when predators mimic something seen as desirable by the prey so that the prey are not startled by the presence of the predator
    • Often used by sit-and-wait predators, as it makes them transparent to their prey
      • Many mantids have a flattened body and appendages that are colored like flower petals
      • sit by flowers and catch pollinators that approach the flowers for nectar

Behaviors:

• Catalepsis - prey playing dead so that the predator ignores the prey
• Intimidation display: an attempt to avoid predation by startling the predator long enough to get away or to convince it that the prey will be too costly to attack
  • many large eye-like patches on moth wings are felt to be useful to startle predators

Polymorphism: the presence of more than one morph in the population. Each morph has to be at a higher frequency than would be produced by mutation alone

• can reduce predation by reducing the predator's efficiency.
  • Search images are used by predators to pick out prey from a complex visual environment
    • once a search image is formed, then the searched for morph is taken at a higher rate than its frequency in the population would predict
  • a polymorphic species can prevent a predator from forming a search image that includes all the members of the species
• The formation of search images for polymorphic species can result in evolution of the population
  • the most common morph may suffer so much predation that it is no longer the most common morph
  • then the predator may switch its search image to the new most common morph and start reducing it
  • this kind of selection is called Apostatic Selection

Chemical defenses in prey either make the prey too toxic or smelly or too distasteful to eat

• Toxins can poison the predator, but this often does not save the prey (only the next prey the poisoned predator never eats)
  • This strategy will only work for an individual if it is aposematically colored as the predator must know before it kills the prey that it is toxic
• Fighting chemicals can be used to harm a predator
  • bombardier beetles explosively eject liquids to startle predators into releasing them
  • nasute termites guard the nest and spray attacking insects with disorienting chemicals
• Some chemicals are distasteful or noxious
  • once again, aposematic coloration is needed as an advertisement of the prey's distastefulness

Masting - the production of lots of young in some years, few in others

• Prey populations are kept low by the non-mast years, and more of the mast year young survive than would be the case if the predator population was larger due to no non-mast years
• if masting produces great excess of propagules one year and few or none in non-mast years, the mast years may offer so many propagules that the predators eat their fill but there is little effect on repoductive success
  • if, for example, 50% of the young will die from starvation when very young with no predation, then it does not matter to the overall success if most starving young are eaten by predators.
  • periodical cicadas are believed to reproduce in their odd manner as a means of masting

Models of Predation

We will use an approach that builds on the logistic that we have worked with previously

First, we assume that the prey population will grow exponentially except that the predator is there to eat some of the prey

• The number of prey consumed depends on the number of prey present, the number of predators present, and a fudge term called a, the "searching efficiency", which can not be determined theoretically but must be measured for each combination of predator and prey.

If we set N as the number of prey, C as the number of predators, and use other, familiar terms, then the rate of change of prey is:

Read this equation as: The rate of change in the number of prey present (dN/dt) is a function of the birth of new prey (rN) minus the death of others due to predation (a'CN). The death rate is assumed to depend on the number of prey, the number of predators (C) and the fudge factor (a').

The rate of change in the predator population is:

Read this as: The change in the number of predators present is a function of the birth of new predators minus the death of others due to some constant mortality rate (old age? mishap? parasitism?). The birth rate is assumed to depend on the number of prey (the resource the predator uses to make new offspring), the number of predators (more predators, more predator offspring), a' (the ability to catch prey), and a second fudge factor called f, which is the efficiency with which predators turn their food (prey) into offspring.

Taking the same tack as before, we solve these equations for conditions where no growth is taking place (hence some sort of equilibrium) by substituting 0 for dN/dt and for dC/dt:

Prey zero isocline:

So, prey numbers do not grow when the number of predators is equal to the ratio of the prey's intrinsic rate of increase (a property of the prey) and a', the searching efficiency of the prey-predator combination (the graph of this is a straight line parallel to the prey axis - usually the x axis). When the number of predators is below this line, the prey increase, when the predator number is above the line, the prey decrease.

Predator zero isocline

So, predator numbers do not grow when the number of prey is equal to the ratio of the predator's intrinsic death rate (a property of the predator) and the product of f and a', (this is also a straight line but is parallel to the predator axis, not the prey axis - usually the y axis). When the number of prey is to the left of this line, the predators decrease (not enough prey per predator). When the number of prey is to the right of this line, there are sufficient prey per predator and the number of predators increases.

This situation leads to an Equilibrium (where the isoclines cross).

• Notice that there are no other equilibrium points on the axes except for 0,0 (the origin) where neither are present
  • With no prey present, the predators decline to 0
  • With no predators present, the prey population takes off to infinity (exponential growth, as we mentioned at the beginning of the modeling effort in this chapter)
• These equations are neither stable nor unstable (called neutrally stable), as the system will oscillate in what is called a predator-prey cycle
  1. prey increase until there is enough for predators to start to increase
  2. predators increase until they eat enough prey to cause a decline in the prey population
  3. prey start to decline in number until predators can't find enough to eat and the predator population declines
  4. prey begin to increase, and we are back at step 1
• Some evidence for these under special circumstances
  • Lynx and hare

The predictions of the model can be modified by changing the shape of the isoclines to include:

• Prey self-limitation
  • this is done my making the prey zero isocline (the horizontal line) curve down as the number of prey increase so that, even if there are no predators present, the prey will decline past the point where the isocline crosses the x axis
  • the crossing point is K, the carrying capacity of the environment
• Interference between predators when they are numerous and prey are scarce
  • this is done by bending the predator zero isocline, the vertical line (q/fa'), to the right
  • think about what this means
  • the level of prey that results in a decline in predators now increases as the number of predators increases, which makes sense
• Under these situations, stable coexistence is possible as the cycles diminish in their amplitude as time goes on.

Graphical model of Predation

Now that you are used to a graphical analysis of species interactions, Stiling presents you with a completely graphical analysis of predation first proposed by Michael Rosenzweig and Robert MacArthur

• note that the lines are still ZERO ISOCLINES, where the predator or prey populations are neither growing larger nor decreasing
• to analyze the graphs, first make sure you
  • 1. understand why the graphs are the shape they are
  • 2. understand what happens when the populations are not on the lines (the book has arrows to tell you, but be sure you can explain why the arrows are pointed the way they are)

The lowest level of the prey zero isocline (where it hits the X-axis near 0) is the smallest population of prey that can successfully find mates. It can be as small as 1 prey. The upper bound, K, is the carrying capacity from the logistic equation.

The prey isocline is humped because, at low prey size, it takes more predators to reduce prey growth to zero as the number of prey increases. So, the prey zero isocline first goes up. As you get toward K for the prey, the number of predators needed to halt prey population growth is less and less, as each prey is more and more food limited. Food limitation means lower reproductive success, so fewer predators can keep the population in check. Thus, the prey isocline goes down as it approaches K. Thus the hump.

The predator isocline is not a smooth curve or a line, rather Stiliing has it as a box (it does not have to be a box, but that's the way he wants it, ok). The upper edge of the box represents the carrying capacity of the predator species - but note that this carrying capacity is not due to the food the predator can get (prey population size), but to some other environmental limitation. The left edge of the box is set by the smallest population of prey that will support the predator population.

• Notice that all of the graphs have an intersection between the two isoclines. So there is an equilibrium possible that allows both predator and prey to coexist in the system. The question: Is that equilibrium stable, neutral, or unstable?
  • Now, notice that the analysis in Fig. 10.5 allows prediction of the neutrally stable equilibrium found in the Nicholson-Bailey equations above when the prey and predator isoclines are at right angles. The prey zero isocline is a curve, so it is not the curve that is at a right angle (that makes no sense), but the right angle is formed by the slope of the curve at the point of intersection with the prey zero isocline and the predator zero isocline.
  • When the slope of the prey zero isocline is less than 1, the oscillations are dampened until there is no predator-prey oscillation in the system, which is a stable equilibrium.
  • When the slope of the prey isocline is greater than 1, the oscillations increase until one of the species, predator or prey, falls to 0. If it is the predator that is lost, the prey goes to its K. If it is the prey that is lost, the predator soon follows. This is an unstable equilibrium.

Two variations

• Refugia for the prey make it impossible for the predators to drive the prey from the system There is a portion of the prey population that can't be eaten and these can always repopulate the portion that does get eaten.
• Paradox of Enrichment
  • the prey isocline is extended by a change in K (that is a result of the enrichment of the prey's environment so that the prey's carrying capacity is increased)
  • this changes the shape of the curve or moves it. If this change in shape means that the intersection of predator and prey isoclines are switched from a point at which the prey isocline's slope is negative to one where it is positive (look at the graph), then the stability of the system is changed from a stable to an unstable equilibrium, where either predator or both predator and prey are lost from the system

  The paradox is that, by enriching the environment, you destabilize the system! Not all good acts have the intended outcome.

Optimal Yield from populations exploited by man

Optimal yield is the maximal number of prey that can be taken without diminishing the prey population

• based on the idea that the prey can produce more young than can be sustained in the environment
  • these are "extra" organisms in the sense that they would not be successful due to resource limitation if not taken as prey
• rather than have some other source of mortality kill the prey, we harvest them
  • assumes that the population is near K, so that there will be resource-based deaths if we do not harvest
  • also assumes that we can measure K and that it will not fluctuate from year-to-year
• For many fisheries, we know so little about how to set K that optimal yield models have not worked well
• For many mammals, we know enough to make reasonable models and so deer harvest can be rather well predicted.

Field Evidence for Predation's Impact

Introductions or exclusions

• Many good studies have been the result of an introduction of a predator into a new environment
  • Dingo and Red Kangaroo
    • no dingo, plenty of kangaroo -- dingo present, no kangaroo
  • Dingo and Feral Pigs
    • dingo eat young pigs, not adults, so, where dingos are present the structure of pig populations is missing the younger classes
  • Lampreys and Lake Trout
    • when lampreys were introduced, several populations of native lake trout disappeared

Natural systems

• Wolves and moose on Isle Royale

Terms

Herbivory, Carnivory, Cannibalism, Parasitoids, Parasitism, Life-Dinner Principle, Aposematic colors, Cryptic colors, Mimetic colors, Batesian mimicry, mimic, model, Mullerian mimicry, Aggressive mimicry, sit-and-wait predators, Catalepsis, Intimidation display, Polymorphism, Search images, Apostatic Selection, Chemical defenses, Toxins, Fighting chemicals, Masting, a, searching efficiency, Prey zero isocline, Predator zero isocline, predator-prey cycle, Lynx and hare, Prey self-limitation, Interference between predators, (unstable, neutrally stable, stable coexistence), Optimal yield, Dingo, Lampreys, Isle Royale

Last updated on September 19, 2006