Predation

Question 1

Distinguish between carnivores, herbivores, and omnivores.

Answer 1

• Carnivores— feed on animals
• Herbivores—consume plants
• Omnivores—consume both animals and plants

Question 2

What are true predators? Provide some examples of true predators.

Answer 2

• Kill their prey more or less immediately after attacking them
• Lions, tigers, bears, seed-eating rodents, ants, plankton-consuming whales

Question 3

What are grazers? Give some examples of grazers

Answer 3

• Attack large numbers of prey during lifetime, but remove only part of each prey individual rather than the whole
  o Effect on individual is more difficult to predict, typically harmful in varying degrees
• Sheep, cattle, flies that bite vertebrates, leeches that suck blood

Question 4

How do parasites differ from grazers? What are parasitoids?

Answer 4

• Similar to grazers—consume part of their prey (host)
• Unlike grazers—attacks usually concentrated on one or a very few individuals during the course of their lifetime
• Parasitoids—insects (usually hymenopterans or dipterans) that lay eggs on or in another host, usually another insect, larval develops in host, consuming/killing it before it emerges as an adult (as many as 25% of worlds species)

Question 5

What is predator switching behavior? What is a predator search image?

Answer 5

• Some consumers can switch their preference such that food items are eaten disproportionately often when they are common and disproportionately ignored when they are rare
• A consumer can develop a specific search image for abundant prey while ignoring other items

Question 6

What assumptions are made by the Optimal Foraging Theory?

Answer 6

• Predicting the foraging strategy to be expected under specified conditions, helps to understand what determines a consumer’s actual diet within a wide potential range

Question 7

What are some problems with the assumptions of the Optimal Foraging Theory?

Answer 7

• The pattern of foraging behavior observed has been favored by natural selection in such a way as to enhance the animals fitness
• High fitness is achieved by a high net rate of energy intake, gross energy intake minus the energetic cost of obtaining the food source

Question 8

What is risk-sensitive foraging?

Answer 8

• There may be a strong pressure to avoid predators that forces consumers to forage in a suboptimal manner which may influence fitness more than optimal foraging

Question 9

Give two predictions of the Optimal Foraging Theory.

Answer 9

• Predators should rank prey in order of their energy value per unit of handling time
  o At reduced abundances of preferred prey, optimal diets include progressively lower ranked types
• They should attack only items that increase their rates of energy acquisition

Question 10

What “question” does the Marginal Value Theorem answer?

Answer 10

• When should a forager leave a patch that is depleting?

Question 11

State the Marginal Value Theorem.

Answer 11

• When resources are found in patches, an optimal forager should maximize its overall intake of energy during a bout of foraging
  o A forager should leave all patches at the same rate of energy extraction, the average overall rate for the environment as a whole
  o Once the rate of extraction falls below a certain point, marginal value of the patch, the consumer should move on since it would have the expectation of finding another patch with at least as good of a rate of energy extraction

Question 12

Give three predictions of the Marginal Value Theorem.

Answer 12

• Optimal stay-times should be greater in more productive patches than in less productive ones, and zero if the extraction rate is less than average
• Stay-times should be longer in environments where travel times between patches are longer
• Patches should be abandoned more quickly when the average productivity of the environment is high than when it is low

Question 13

How did Cowie (1977) manipulate “travel time” with his captive bird experiment?

Answer 13

• Used captive birds, getting them to forage for meal worms hidden in plastic cups
• All cups contained same number of prey but traveling time manipulated by covering the cups with cardboard lids that varied in how difficult they were to remove

Question 14

What prediction of the Marginal Value Theorem was supported by Cowie’s time travel bird data?

Answer 14

• Birds remained longer in patches that required longer traveling times, that is, more time to open the containers
• Stay times should be longer in environments where travel times between patches are longer

Question 15

What is the “ideal free distribution” for patch-using consumers?

Answer 15

• There is a tendency for consumers to constantly redistribute themselves until the realized profitabilities of all patches are equal, until they have achieved an ideal free distribution

Question 16

What does a predator’s functional response describe?

Answer 16

• The relationship of an individual predators rate of food consumption to prey density

Question 17

Give two reasons why a predator’s rate of food consumption must eventually reach a plateau in spite of rising prey density.

Answer 17

• As a predator catches more prey, the time spent handling each item cuts into searching time
• Predators must eventually become satiated and cannot feed any faster than they can digest, assimilate and move food through the gut

Question 18

What is a Type 1 functional response? Give an example of organism that might show is a Type 1 functional response.

Answer 18

• The number of prey eaten per predator increases linearly to a maximum and then remains at that maximum irrespective of any further increases in prey densities
• Filter feeders like Daphnia where at the plateau density, they are handling as much food as possible

Question 19

What is a Type 2 functional response? Why does the curve in a Type 2 response gradually decelerate as the plateau is reached?

Answer 19

• Most frequently observed, consumption rate rises with prey density but gradually decelerates until a plateau is reached, consumer has to devote a certain amount of handling time to each food item it consumes
  o As prey density increases, finding prey is easier but handling prey still takes time

Question 20

What is a Type 3 functional response? How might you account for the first portion of the curve in a Type 3 functional response?

Answer 20

• Similar to type 2 at high food densities with the same explanation
• At low food densities, consumption is less than expected because:
  o Difficult to locate prey, best hiding places
  o Low density means they aren’t abundant enough to elicit the formation of a search image
  o ALWAYS involves an alternate prey type, some other prey that predator concentrates on while ignoring the less abundant prey, switches when density of less abundant increases

Question 21

According the Leopold (1933), what are buffer species? What is a predator numerical response?

Answer 21

• An alternate species used by Type 3 animals in times when their preferred prey exist below their threshold of security, stand between the predator on the one hand and a game species on the other
  o Allow populations of preferred species to increase
• Increased food supplies can lead to changes in consumer population size, typically increases in prey density will lead to a positive numerical response

Question 22

Give two possible reasons for a positive predator numerical response.

Answer 22

• Immigration—flock to area to feed

- Population growth—good food supply, fecundity of predators may increase, more energy

Question 23

What are the assumptions of the Lotka-Volterra (L-V) predation model?

Answer 23

• In the absence of predators, prey experience density independent exponential growth
• Predator populations decline exponentially in the absence of prey
• Predators move at random among a prey population that is distributed randomly
• Predator density has no effect on the probability of catching prey, there is no interference
• All responses are instantaneous, there is no lag time for handling/ingesting prey
• The number of prey taken increases linearly with increasing prey density, type 1 response
  o Doubling prey density doubles predation rate

Question 24

What type of predator functional response does the L-V model assume?

Answer 24

• Type 1

Question 25

In terms of its prey, what does the predator isocline represent?

Answer 25

• The number of prey needed for the predators to maintain their populatin

Question 26

In terms of its predator, what does the prey isoclines represent?

Answer 26

• The number of predators at which the prey population will remain constant

Question 27

What happens to the predator population in the regions to the left and right of the isocline and why in the L-V graph?

Answer 27

• Left– predator pop decreases due to a lack of food

- Right—predator pop increases because prey are abundant

Question 28

What happens to the prey population in the regions above and below the isocline and why in the L-V graph?

Answer 28

• Above—prey population decreases because of overwhelming predator pressure
• Below—prey population increases because there are fewer predators to eat them

Question 29

In what direction does the joint population trajectory move on the predator-prey graph in the L-V graph?

Answer 29

• Both predator and prey populations will cycle through time, cycles slightly out of phase
  o Prey pops increase/decrease just ahead of their predator populations

Question 30

What would happen if the reproductive rate of the prey (r) increased and/or the hunting efficiency of predators (c1) decreased in the L-V graph?

Answer 30

• An increase in the value of the prey isocline

Question 31

What does an increase in the value of the prey isocline mean in terms of the prey population in the L-V graph?

Answer 31

• The prey population would be able to bear the burden of a larger predator population

Question 32

What would happen if the predator death rate (d) increased or the predation efficiency (c1) or reproductive efficiency (c2) decreased in the L-V graph?

Answer 32

• The predator isoclines would move to the right

Question 33

What does a shift of the predator isocline to the right mean in terms of the prey population in the L-V graph?

Answer 33

• More prey would be required to support the predator population

Question 34

What would happen if the predator hunting efficiency (c1) increased? What effect would this have on the predator population in the L-V graph?

Answer 34

• Reduction in both isoclines

Question 35

What effect would a reduction of both isoclines have on the prey population in the L-V graph?

Answer 35

• Fewer prey would be needed to sustain a given capture rate, predator isoclines decreases
• Prey population would be less able to support the more efficient predators, prey isoclines decreases

Question 36

State the Volterra Principle.

Answer 36

• If two species are destroyed at the same rate by some outside agency (ex. pesticides), the prey will increase proportionally and the predators will decrease proportionally
  o Ex. if both predator and prey pops reduced by 50%, increase in prey by 25%, decrease in predator pop by 25%

Question 37

How might you account for the results predicted by the Volterra principle?

Answer 37

• The population cross product (HP) determines the birth rate of the predator population while it determines the death rate of the prey population thus reducing both populations by the same amount, actually decreases the predator’s birth rater at the same time reducing the prey’s death rate

Question 38

How does the Volterra Principle relate to the problem of controlling insect pests with broad spectrum insecticides?

Answer 38

• If you use broad spectrum insecticides, both prey and predators will be killed leading to the proportionate increases in prey and decreases in predators while if you use a selective insecticide, only the pest species will be affected

Question 39

Give three predictions of the L-V predation model with respect to the predator and prey populations.

Answer 39

• Great fluctuations in populations of both predator and prey densities
• Population peaks and lows that alternate regularly
• Cycles that are slightly out of phase with one another, with the predator lagging behind

Question 40

What happened when Gause tested the L-V Model with a protozoan predator and prey? Why?

Answer 40

• The predator drove the prey to extinction and then died off
• The only way to maintain cycles was to constantly introduce new prey

Question 41

Do the well-documented Arctic predator-prey cycles occur because of the predator-prey population interactions predicted by the L-V model (that is, are they predator controlled)?

Answer 41

• No, food shortages during the winter may initiate the decline in hare numbers which then would be followed by predator decline

Question 42

Give two ways in which predators and prey might be limited by their own numbers.

Answer 42

• Intraspecific competition

- Mutual interference

Question 43

How can environmental heterogeneity help to stabilize predator/prey interactions?

Answer 43

• They may provide the prey with refuges where they can avoid predators

Question 44

How can a predator aggregative response help to stabilize predator/prey interactions?

Answer 44

• Prey often distributed unevenly in nature, many predators tend to congregate in patches of high prey density
• Predators that show aggregative response tend to avoid areas with low prey density which provides the prey with a partial refuge from predation

Question 45

How might a Type 3 functional response help to stabilize predator/prey interactions?

Answer 45

• At low densities the prey may partially be immune to predation because the predator may not yet have formed an appropriate search image

Question 46

What did Huffaker’s experiment with orange-eating mites and predatory mites show?

Answer 46

• On oranges alone, both populations became extinct
• When a more complex (heterogeneous) system was created by adding rubber balls and dividing up the oranges into sections with barriers that provided temporary refuges from the predators, localized extinction occurred, periodic recolonization by the better dispersing prey permitted overall stability (cycling)

Question 47

Give two ways in which the relationship between the Australian prickly-pear cactus and its predatory moth (Cactoblastis) appear to be stabilized?

Answer 47

• Interaction is stabilized at low densities by the partial refuge the cactus has as a result of the moths aggregated response
• High death rate of larvae on overloaded plants, self limitation

Question 48

Give some examples of plant structural defenses?

Answer 48

• Spines (cacti), hairs, and tough seed coats

Question 49

What is the strategy of predator satiation and how can it help plants to survive predation?

Answer 49

• A more subtle defense among plants, the timing of their reproduction so that the maximum number of offspring is produced within one short period of time
• Great abundance of prey satiates the predators, allows a percentage of the offspring to escape
• Sometimes plants produce seeds periodically rather than annually
• Longer time between seed crops, less opportunity for seed-dependent predators to maintain a large population size

Question 50

What do digestibility-reducing compounds do?

Answer 50

• Chemical defenses that are stored and released only when plant cells are broken or injured

Question 51

Give an example of a digestibility-reducing compound?

Answer 51

• Tannins in oak leaves

Question 52

Provide some examples of toxic alkaloids produced by plants.

Answer 52

• Nicotine, morphine, caffeine, hallucinogens, terpenes found in conifers

Question 53

Provide some examples of palatability-reducing compounds produced by plants.

Answer 53

• Taste bad, peppermint oils, cloves and many spices

Question 54

Provide some examples of chemical defenses used by animals against predators.

Answer 54

• Stinging cells or organs, sequestering toxins that cause illness or death if consumed or spraying

Question 55

Provide some examples of mechanical defenses used by animals against predators.

Answer 55

• Porcupines have barbed quills, armadillos and turtles have body armor, puffer fish inflate themselves with water and some lizards have protective spines

Question 56

Give three ways in which animals may achieve crypsis.

Answer 56

• Background matching
• Disruptive patterns
• Counter shading

Question 57

How might aposematic colors protect animals from predators?

Answer 57

• Prey species that have chemical defenses may advertise that fact with bright colors arranged in bold patterns so when a predator has an unpleasant experience with one, they will recognize and avoid them.

Question 58

What is Batesian mimicry?

Answer 58

• Harmless animals that mimic venomous/harmful animals

Question 59

What is Mullerian mimicry?

Answer 59

• Several harmful (noxious) forms converge in appearance such that a negative experience with one by a predator will be generalized so that all are avoided

Question 60

What is aggressive mimicry?

Answer 60

• Predators using mimicry to their advantage by resembling their prey
• Ex. females of fireflies imitate flashing patters of other species, attracting males of those species that they promptly kill and eat

Question 61

What is “startle coloration” and how might it help protect a potential prey species from its predator?

Answer 61

• Sometimes prey species may evolve some form of startle coloration that, when revealed, confuse, startle or frighten a potential predator