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General Ecology (BIO 160) Worksheet #13 Dept. of Biological Sciences
Sacramento State
Worksheet 13: Predation and Herbivory 1.
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Population size (N)
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Predation is one of several types of species interactions that takes on different forms. What is the definition of predation and how does it differ from herbivory, parasitism and parasitoidism? (pgs 239, 274) Predation occurs when one organism feeds on another, called the prey. Although this typically kills the prey, it doesn't always do so. For example, herbivores may consume leaves and other plant tissues without killing the plant. Likewise, in parasitism the parasite consumes body fluids without killing the host outright and they can live together for a period of time; fitness is reduced though. Parasitoids are another special case in which the parasitoid doesn't kill the host outright. It occurs in certain wasp and fly species that lay eggs on the host and when the eggs hatch the larvae feed on the host. This can kill the host or at the least reduce its fitness. Because of their intimate consumer‐resource relationship, predators and their prey have strongly influenced one another evolutionarily. What is coevolution and how does it pertain to predators and their prey? Coevolution occurs when each of two interacting species serves as a selective force for the other. Predators exert a strong selective force on prey, such that prey evolve mechanisms to avoid predation. As prey evolve mechanisms of evading predation, predators correspondingly evolve new ways of capturing prey. Answer the following questions about the Lotka‐Volterra predator‐prey model. (pgs 274‐277) Prey
a. Plot (as a graph) the Lotka‐Volterra predator‐prey model below to Predator
show the predicted changes over time of predator and prey. Be sure to label your axes and your predator and prey curves. Timee
b. Explain how the Lotka‐Volterra predator‐prey model produces cycles of predator‐prey population dynamics. As the predator population increases, the model dictates that the predator encounters and consumes ever more prey until the prey population declines. As the prey population declines, the predator encounters and consumes less prey until predator mortality increases and the predator population declines. In turn, encounters of predator and prey decline, resulting in lower prey mortality. The prey population then increases and the cycles continue. c. What are two factors that can cause predator‐prey cycles to persist? Explain how each factor acts to allow persistence. Examples: 1) prey refuges can allow predator‐prey cycles to persist for longer periods of time; 2) spatial heterogeneity can allow predator‐prey cycles to persist longer because prey are more difficult to find; 3) alternative food sources for the predator will cause a decoupling of predator and prey and dampen the oscillations; 4) external density‐dependent factors may affect predators or prey, which would cause the oscillations to dampen; 5) migration of predators or prey into or out of their respective populations will change the nature of the oscillations (e.g., continuous migration of prey into an area would likely dampen the oscillations of both predator and prey). d. What are two assumptions of the Lotka‐Volterra model? Are these assumptions realistic? Examples: 1) the only regulator of prey population size is the predator; 2) the predator will starve in the absence of the prey population (as opposed to switching to another prey); 3) predators can consume unlimited quantities of prey if they are available; 4) there is no habitat in the model that can modify predator‐prey interactions. These assumptions are all unrealistic because they do not take account of the actual conditions in which predator and prey exist in nature. General Ecology (BIO 160) Worksheet #13 The graph shows the population dynamics of a predator and its prey. Both species are protists growing in a flask culture with limited amount of nutrients. (pgs 274‐277) a) Which line (A or B) best represents the prey population? __A___ A
b) Describe two conditions that could allow the predator and prey populations to persist for a longer time period than shown above (i.e., B
for more than one oscillation or cycle). Condition 1: Time
Prey refuges can allow both predator and prey populations to persist for multiple oscillations because prey can effectively “hide” from predators. This gives the prey a chance to reproduce and replace individuals consumed by predators and thus increase their population size. In turn, predators do not as quickly eliminate all of their prey and go extinct. This allows for more predator‐prey cycles to occur. Condition 2: Spatial heterogeneity can allow predator and prey populations to persist for multiple oscillations because prey are not as quickly and easily found by predators. This gives the prey a chance to persist in locations that have not yet been discovered by predators. As above, predators do not as quickly eliminate all of their prey and go extinct, which allows for more predator‐prey cycles to occur. c) Draw a new graph below of the predator and prey species for one of the new conditions you specified in b) above. Be sure to label your axes and your predator and prey curves. Similar to graph in question 3.a. Oscillations will persist for several cycles until the predator goes extinct. Predators must make decisions about how to allocate their time and energy during foraging (e.g., what and where to eat, how long to forage, etc.) that involve trade‐offs between conflicting demands. This area of research is called optimal foraging. Explain the concept of optimal foraging in terms of the costs and benefits involved in predator foraging. (pgs 281‐285) Optimal foraging theory states that natural selection should favor predators that maximize their energy input per amount of energy expended in the act of foraging (i.e., the most efficient predators). Costs associated with foraging include the amount of time spent searching for and handling prey and the caloric energy expended in these activities. Foragers who capture the most prey per amount of energy expended will benefit by maximizing their energy gain and, consequently, their fitness. Mimicry is a defense mechanism used by some prey to escape predation. For the two kinds of mimicry listed below, describe for each how they work and give one example. (pgs 287‐288) Population density
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Dept. of Biological Sciences
Sacramento State
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Müllerian: All unpalatable species take on the same warning pattern or coloration. Mimics benefit because a predator will know that the shared pattern or coloration is dangerous or toxic. Examples are bees and wasps, which all have dark and light colored banding as their warning coloration. Other examples are some butterflies that are toxic to predators and have similar wing color patterns. Batesian: Relies on a palatable species looking like an unpalatable one. The palatable species benefits because predators view the mimic as dangerous or toxic when it is not. An example of a Batesian mimic is the palatable king snake, which mimics the poisonous coral snake. Another example is a harmless mantid that resembles a harmful wasp. General Ecology (BIO 160) Worksheet #13 7.
Dept. of Biological Sciences
Sacramento State
Inducible and constitutive defenses constitute different “strategies” organisms have evolved to protect them from prey. (pg 289) a.
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Compare and contrast inducible versus constitutive defenses. An inducible defense is one that is activated only in response to the action of a predator/herbivore. In other words, the defense is turned on only if the organism detects a threat (i.e., animals) or has been damaged (i.e., plants). What is the advantage of an inducible over a constitutive defense? What is the advantage of a constitutive over an inducible defense? The advantage of an inducible over a constitutive defense is that the organism must only allocate energy to production of the defense in the presence of a predator/herbivore. The advantage of a constitutive defense over an inducible one is that it is always acting to protect the prey. Plants use a variety of means to defend themselves against herbivores. Answer the following questions regarding herbivory. (pgs 292‐294) a.
The book states that herbivores can starve to death on a full stomach of plant tissue. Explain how and why this can happen. Plant tissues are generally low quality food (that is, they have low concentrations of nitrogen and other essential nutrients). So, even though an herbivore may consume a full stomach’s worth of pant tissue, that low quality plant tissue does not contain enough nutrients for the herbivore. b.
Provide an example of a chemical defense and explain how it acts to defend the plant against herbivores. e.g., tannins are produced by many plants, including oaks, grapes, etc. Tannins act as a constitutive defense by continually being produced by the plant. These secondary compounds chemically bind proteins together in the herbivore’s gut, thereby effectively reducing more the already low quality of the plant tissue. So, this defense doesn’t act as a toxin so much as a deferent. c.
What are some of the costs and benefits to the plant in building anti‐herbivore defenses? There is a large metabolic (i.e., maintenance of the biochemical machinery) and caloric (i.e., energy used in production of the secondary compound) cost to a plant to build the secondary chemicals used as anti‐
herbivore defenses. However, the cost is offset by the benefit of these secondary compounds to the increased survival and reproduction – and therefore, persistence – of the plant species.