Workshop on Predation Thomas Herbert, Ph.D. (revised by Dana Krempels in 2013)

Transcription

1 Workshop on Predation Thomas Herbert, Ph.D. (revised by Dana Krempels in 2013) Pre-Workshop Preparation: and I. Basics of the predator-prey relationship 1. What is the name of the food source for a predator? prey 2. What is the food source for prey? Any organism at a lower trophic level than the prey. 3. Are all prey species herbivores? Give examples of some that are not, and what eats them. Prey: small carnivorous fish Predator: larger carnivorous fish Prey: juvenile carnivores of any species Predator: larger carnivores Prey: Venus flytrap plant Predator: herbivorous insects Prey: Predator: Prey:...ETC. Be creative! Predator: 4. Whenever an animal eats a food item, only about 10% of the energy in that food item is converted into the biomass of that animal. About 90% is lost as entropy as the animal uses the energy in the food for cellular work and homeostasis. Discuss the implications, in terms of energy transfer from one feeding (trophic) level to another, of predators that feed on herbivores, versus predators that feed on secondary (and higher) consumers. The higher the trophic level, the lower the total biomass in that trophic level. For example, there are fewer wolves in a tundra ecosystem than caribou. If animals feed at more than one trophic level, they will have more food available if they feed on a lower trophic level. For example, you can feed more humans if everyone eats plant material than if you feed that plant material to chickens or cattle, and the humans eat those. You lose about 90% of the energy of the producers (plant matter) by feeding it to hervbivores (first trophic level), so there's that much less biomass left to feed the second trophic level (small carnivores) and still less to feed the top carnivores. See below. DO NOT MAKE THE COMMON MISTAKE OF THINKING THAT YOU NEED TO EAT MORE SMALL CARNIVORES THAN HERBIVORES TO GET THE SAME AMOUNT OF ENERGY! NOT TRUE! Protein has the same energy content per unit mass, no matter whether it comes from a mouse or a raccoon (or a Figure 1. Representation of the reduction of biomass with each trophic level. An ecosystem will have more primary producer biomass, than herbivore biomass, more herbivore biomass than small carnivore biomass, and so on up the pyramid of trophic (feeding) levels. cow or a human). We are talking strictly about NUMBERS and BIOMASS in the ecosystem, not calories consumed by each organism.

2 For the next section, be sure to review the information at: 5. When is a human a predator? When--if ever--is a human prey? Humans are predators any time they feed on a whole organism of any kind. Humans can be prey when they are fed upon by other species (e.g., sharks, bears, etc.) What about mosquitoes? Ticks? Fleas? Are they predators or parasites? Sometimes it's hard to draw the line. 6. What the key similarity or difference between the predator-prey relationship and the host-parasite relationship? What about the host-parasitoid relationship? A parasite does not kill its prey outright or eat all of it, as a predator does. A parasitoid kills its host as it metamorphoses, but does not consume it completely. 7. What the key similarity or difference between the predator-prey relationship and a competitive relationship between two species? Two species in competition are vying for the same resource. In predator/prey relationships, the prey species is the resource of the predator. No competition for resources between the predator and prey exists. 8. In terms of evolutionary results, what is the difference between competition between two different species (i.e., interspecific competition), and competition between members of the same species (i.e., intraspecific competition),? Competition between two different species can lead to resource partitioning, in which the two species begin to utilize different resources. This can result from natural selection, in which individuals of the competing populations who do not use the most commonly sought-after resources have a selective advantage (because there's more food!). If there is complete overlap between the two species' niches, one will go extinct if it cannot shift its resource use. When members of the same species compete for resources, some individuals will have traits that make them better suited to compete for those resources, and they will have more energy and all other things being equal leave more offspring. This means that if the situation stays the same, the population's will gradually shift to contain the genes that encode traits that make the organisms better suited to exploit the desired resource. II. Canadian Lynx and Snowshoe Hare: Fact or Myth? A. Background In 1937, MacLulich published a paper analyzing data collected by fur trappers selling pelts to The Hudson Bay Company over a period of nearly 100 years. From these data, a "classic" Lynx vs. Snowshoe Hare population fluctuation phenomenon emerged, as shown below. MacLulich noted that the "boom" and "bust" of hare and lynx population seem to mirror each other, with the lynx peaks and valleys coming slightly after those of the hares. (Figure 2) An overview of this hypothetical interaction can be found at

3 Figure 2. Population density fluctuations of lynx and hare, as calculated from pelts taken by trappers from 1845 through How did the MacLulich and other earlier students of the population fluctuations of lynx and hare explain the population fluctuations? They claimed that as the season progressed, hares became more scarce because the lynx were hunting them so effectively. At a certain critical point of low hare population density, there would no longer be enough hares for all the lynxes to prey upon, and the population of lynxes would also "crash" because lynxes who could not find enough hares would starve. Once the lynxes became scarce, this allowed the snowshoe hare population to rebound, and the cycle continued. 2. What are some other, more recent hypotheses that could be used as alternate explanations? For some ideas, visit Keith (1983) suggested that it was actually the lack of sufficient forage for hares in the winter that caused their decline not hunting by lynxes. He did agree that the lynx population decline could be due to a shortage of hares. Other authors have cited such factors as parasitic infections, bacterial infections and other environmental stresses as being responsible for hare population declines. Even sunspot activity was proposed as one possible driving force of the lynx/hare fluctuations, though this was later discredited. For more references on the hare/lynx cycle, find articles at: ation&wc=on&dc=biological+sciences B. Exercise: Becoming a Predator or Prey Each person in the group should choose to be a Lynx or a Snowshoe Hare. Imagine how you would be living, feeding, foraging, hiding, etc. Picture yourself in the winter boreal forest, and imagine what challenges you would face.

4 Go around the circle of predator and prey and have each lynx or hare answer one of the following questions. As you discuss these questions, think about how your lynx or hare behavior might be more complicated than first imagined. 1. (Lynx) The number of hares is decreasing rapidly because you, your conspecifics, and predators of other species are eating them. Simple predator-prey theory describes a relationship between the predator and prey. But what is the relationship between you and your conspecifics? Intraspecific competition. What is the relationship between you and predators of other species? Interspecific competition. How might intraspecific interactions differ in their evolutionary impact compared with interspecific interactions? Interspecific competition: Competition between two different species can lead to resource partitioning: over evolutionary time, natural selection will drive populations of the two species to utilize different resources. Individuals of the competing populations who do not use the most commonly sought-after resources may have a selective advantage (because there's more food!). If there is complete overlap between the two species' ecological niches, one will go extinct if it cannot shift its resource use. Intraspecific competition: When members of the same species compete for resources, some individuals will have traits that make them better suited to compete for those resources, and they will have more energy and all other things being equal leave more offspring. This means that if the situation stays the same, the population s gene pool will gradually shift to contain the genes that encode traits that make the predators better suited to exploit the desired resource. 2. (Hare) You have to eat, too. Being a member of Order Lagomorpha (rabbits, hares, and pikas), you don't usually eat Lynx. (Though a Knight of the Round Table sometimes makes a tasty snack.) What do you eat in the spring and summer? Fall? Winter? Spring and summer: new growth grass, leaves, twigs Fall: dried grasses, remnants of plants going dormant, seeds, fruit Winter: dried grasses, if you can dig them up; bark, twigs. What type of symbiosis do you share each of the different types of things you eat? If you devour the entire plant, you are a predator (even though you are an herbivore). If you don t kill the entire plant, you are what? A parasite? Check out the definition of parasite and decide. Essentially, you're an herbivore of these food items. If you disperse their seeds in your fecal matter, you re also a seed disperser (mutualism). 3. (Lynx) What will happen to the predator and prey populations if the predator population size is reduced to one individual - you? Prey population will be able to recover without stress of predation, though other factors such as food shortage and pathogens will also affect population size. Predator population will either disappear or migrate to an area where there are other conspecific predators for breeding.

5 4. (Hare) What will happen to the predator and prey populations if the prey population size is reduced to one individual - you? The predators will either have to switch to a new type of prey, or starve. Unless you're a pregnant female, your population is history when you die, unless you migrate to an area where there are others of your species. 5. (Lynx) Recall the data on the relationship between lynx and hare populations as analyzed by MacLulich. What is your relationship to the trappers? (Consider all possibilities.) Predation: The trappers are your predators when they trap you. Competition: They are your competitors when they trap hares. Commensalism? When the trappers kill predators that are not lynxes, they may actually be helping the lynx population by reducing interspecific competition. Side note: If you've avoided the traps because you are particularly smart, and your intelligence is hereditary, you may pass on your smart genes to future generations, and future populations of lynxes might be harder to trap. Natural selection in action! But is commensalism in effect on an individual-organism basis? Not really. 6. (Hare) Now it's your turn to recall the data on lynx and hare populations collected by Hudson Bay Company fur trappers. What is your relationship to the trappers? Again, be sure to consider all possibilities. Predation: The trappers are your predators when they trap you. Commensalism? Trappers benefit the hare population by removing predators, though the trappers are not directly benefitted in this regard by the hare population. As with the lynxes: If you've avoided the traps because you are particularly smart, and your intelligence is hereditary, you may pass on your smart genes to future generations, and future populations of hares might be harder to trap. 7. (Lynx) More recent research suggests that snowshoe hare population numbers fluctuate even when lynxes are excluded from the environment. a. What might be responsible for this "natural" fluctuation in your population numbers? Pathogens, food shortage, competition between hares, migration to areas with better resources, all can occur when population density becomes very high. This can cause a population crash even without predation by lynxes or other predators. b. What are the implications of this phenomenon, in terms of population numbers for lynxes and snowshoe hares over time, even when the populations do not interact? Population density may still fluctuate due to factors other than lynx/hare interactions. 8. (Hare) Would you expect to see the same type of "natural" fluctuations in the lynx population in the absence of snowshoe hare prey? Discuss! Why or why not? The same population-density changes should be true for lynxes, though these solitary predators are not as gregarious as snowshoe hares, and so would be less likely to share pathogens. However, territorial fighting might cause lynx deaths, or cause subordinate lynxes to migrate out of the habitat in search of friendlier hunting grounds.

6 III. Predator/Prey Relationships: Graphical Representations Review the Hudson Bay data from Figure 2. Figure 2. Population density fluctuations of lynx and hare, as calculated from pelts taken by trappers from 1845 through Choose any time point in the figure above. With a straight-edge, draw a vertical line intersecting the number of predator and prey living at that time. Do this ten times, drawing ten vertical lines at ten time points in Figure 2. Label each of your lines. The figure should look something like this: 1. Choose any time point in the figure above. With a straight-edge, draw a vertical line intersecting the number of predator and prey living at that time. Plot a point at these two coordinates on Figure 3, and label that point a. Repeat this process ten times, labeling each line alphabetically.

7 2. In the table below, record the numbers of lynx and hares at each point on the vertical lines you ve drawn on Figure 2. For my example, the numbers are (approximately) in thousands: point # lynxes # hares point # lynxes # hares a f b g c h d 8 45 i e j If McLulich s hypothesis is correct, what should you predict about the relative numbers of lynx and hares over time? a. When hare density is low, lynx density should be low. b. When hare density is high, lynx density should be high. c. When lynx density is low, hare density should be high. d. When lynx density is high, hare density should be low. 4. In the space below, plot the number of lynx (y axis) by the number of hares (x axis). LABEL YOUR X AND Y AXES APPROPRIATELY, and give your figure an appropriate title. Figure 3. The relationship of predator abundance as a function of prey abundance. 5. What do the points in Figure 3 tell you about the relationship between the two populations? If there is a relationship between predator and prey numbers, then this figure should show the hypothetical change in predator and prey densities, with predators increasing in number as prey numbers increase, and decreasing together, as well. Does it? Nope. These points are all over the map, and do not form a function.

8 6. Connect the dots. Draw a line from point a to b, b to c, c to d, etc. Do the data allow you to predict population numbers over the course of time? (Do the populations vary predictably with each other as time progresses?) If you draw lines between sequential points in time, here s what you get (at least in this random example): Your lines may vary, depending on which points you picked. But it s pretty clear that there is no relationship between predator and prey numbers over time. This suggests that the populations numbers may not be correlated with each other. (Could you devise a different method to sample the data to see if there is a relationship between lynx and hare population densities?) Time isn't included in Figure 3 axes. But if you try to connect the dots via time progression, you can see that lynx and hare population densities change without respect to time or, apparently, to each other. 7. To clarify your answer to the previous question, redraw the Figure 3, placing point a (from the earlier time) in the same position as where you previously had point e (later time). Place point e where point a used to be. (That is: reverse the positions of points a and e from Figure 3, and plot them in Figure 4, below. (Don t forget to label your axes appropriately every time). Figure 3. The relationship of predator abundance as a function of prey abundance, irrespective of time progression.

9 8. Connect the dots again. Is the function any different? Not appreciably different. Still no function here. 9. What do these plots (Figures 3 and 4) imply about the actual effects of the species we are calling the "predator" and "prey" on each other's population numbers? The numbers of the populations do not fluctuate together in easily predictable ways, especially when time progression is not an axis. 10. What factors besides predation by Lynxes might affect Snowshoe Hare populations? Discuss. Pathogens, overcrowding, intraspecific competition, predation by predators other than lynxes, lack of food, migration to search for new forage, etc. Snowshoe hares are somewhat gregarious, and more likely to interact when crowding occurs. Hence, greater chance for territorial spats (injuries), transmission of pathogens and parasites, etc., that can have an effect on population density. 11. What factors besides a shortage of Snowshoe Hare prey might affect Lynx populations? Discuss. Pathogens, overcrowding, intraspecific competition, competition with predators other than lynxes, Monty Pythonesque rabbits that prey on lynxes, lack of food, migration to search for new forage, etc.

10 Suggested Quiz Questions 1. Define predation. Ans: One population benefits at the expense of another population by killing and eating members of that other population.) 2. Define competition. Ans: Two populations both require the same resource(s), and their interaction is disadvantageous to both populations. 3. What is the predator in today s workshop? Ans. Lynx 4. What is the prey animal in today s workshop? Ans. Snowshoe Hare (or just Hare is okay) 5. What is meant by trophic levels in an ecosystem? Ans. This refers to how many degrees away from primary producers a particular species feeds. Primary producers to primary consumers comprise the first trophic level, (i.e., producers to herbivores), primary consumers (herbivores) to secondary consumers (carnivores) the second level, and so on. 6. Give an example of a top carnivore Ans. Any large predator, including humans, that eat high on the food web, and are not usually eaten by other species. 7. Define symbiosis. Ans. Two populations in an ecosystem engaging in an interaction that affects both populations. 8. What is meant by intraspecific competition? Ans. Competition for resources between members of the same species. 9. McLulich used data from the Hudson Bay Company to estimate population densities of lynxes and snowshoe hares. What did the Hudson Bay Company provide? Ans. Numbers of pelts of each species sold to them by trappers. 10. Give one reason that hare populations might undergo a boom and bust cycle? Ans. Any of the reasons in the workshop, including predation by lynxes. Or you can think up your own.