With more than 750,000 described species, insects are the dominant terrestrial animal life on earth in diversity, numbers, and biomass. Most insects have short generation times, giving the majority of species a tremendous reproductive potential. For example, fruit fly pairs produce 30 generations per year with an average of 40 eggs per pair. With a 1:1 sex ratio, unlimited reproduction and 100% survival for a single year would produce a layer of fruit flies over the earth about 991 million miles deep! However, unlimited reproduction does not occur, nor does every organism born live to reproduce. When the size of a population approaches an environment's carrying capacity, competition for resources will limit reproduction. In addition, predators regulate some prey populations.
Predation involves four steps: search, recognition, capture, and handling. The possibility of co-evolution of predator and prey operates at each of these steps. Predators search the environment for acceptable prey. Predator adaptations to improve foraging success include better visual acuity, development of a search image, and limiting searches to prey-rich habitats. Predators quickly learn prey types and adapt to recognize prey and to avoid inedible species. Predators must be able to capture prey. Adaptations to improve capture efficiency include improved motor skills and appendage modification. Finally, predators must handle prey by efficiently subduing them and detoxifying any defensive compounds. Adaptations promoting handling efficiency include improved foraging appendages to reduce the probability of injury and physiological specialization on otherwise poisonous prey (Krebs and Davies 1993). Predators also improve foraging efficiency by learned avoidance, a behavior in which predators quickly learn to recognize poisonous or distasteful species by remembering adverse reactions from attempted predation events (Brower 1988).
Because life depends on taking life, almost all organisms on earth are potential prey for at least one other species. To escape this predation pressure, natural selection has favored individuals that are more difficult to find, capture, subdue, and consume. Adaptations against predation include coloration, behavior, morphology, phenology, and physiology. Among the most spectacular examples of these different anti-predator defenses are found among the insects that range from species that are nearly invisible against the background (such as the toad bug in figure a), to species that are so heavily armored that most predators cannot subdue them (such as the ironclad beetle in figure b). Other species have spines and many have some form of chemical defense, such as ladybird beetles (figure c), which leave a yellowish, strongly smelling fluid on your hand when you handle them. Often species having chemical defenses are also aposematic, meaning they are brightly colored in order to warn a potential predator.
|At top left (a) is an ambush bug that survives by being cryptic and attacks prey by jumping on their backs. At top right (b) is a darkling beetle called “an iron clad beetle” that most predators cannot consume because of its armor. To the left (c) is a ladybird beetle that is brightly colored (aposematic) and protects itself with chemicals secreted from special joints in its legs.|
Unfortunately for both the species possessing warning coloration and for the potential predator, the recognition of potentially dangerous prey is not innate. Predators must learn to associate bad taste or illness with eating a species that possesses memorable characteristics. Ecologically, the interactions between predators and prey are complex and change through evolutionary time as predators and prey adapt to the environment and each other. Prey species may vary in their ability to create and express chemical defenses and in the costs associated with this ability, which often lead to a reduction in growth and reproductive rate but an advantage in protection when predators are abundant. In contrast, predators vary in their ability to find, capture, subdue, and consume prey. Sometimes when a prey species becomes more toxic, only one predatory species can overcome its defenses. If that predator species specializes on the toxic prey species, the predator selects for the prey to become more toxic, which in turn selects for the predator to be better able to overcome the more toxic prey. This back and forth selection can result in a tight linkage between a predator species and a prey species. When there is a tight association of mutual selection pressures acting on two or more species, the phenomenon is termed "co-evolution."
Among the insects, three predator forms are common. Many predatory insects, including ground beetles, tiger beetles, and ant lion larvae, grasp and kill their prey with their mandibles. A second group of insects, including praying mantids, giant water bugs, and ambush bugs, use enlarged front legs (raptorial legs) to grab and subdue prey. A third form of prey capture most commonly used by aerial predators consists of grasping prey with all the legs while in flight. Insects that use this method include dragonflies, robber flies, and scorpionflies. All these types of insects are generalists, feeding on any appropriately-sized arthropod they happen upon.
Prey species have mechanisms to counter predator efficiency at each step of the predation process. Prey can be difficult to locate by way of cryptic or polymorphic forms and often disperse in the environment. Some species have developed potent defense mechanisms, including poisons, armor, and spines. To reduce recognition, some prey species mimic dangerous species.
Usually, warning coloration and associated poisonous characteristics are thought to protect insects from mammalian and avian predators. Berenbaum and Miliczky (1984) demonstrated that warning coloration and poisons protect some insects from predatory praying mantises. They fed one group of milkweed bugs milkweed seeds, which contain high concentrations of cardiac glycosides. They fed a second group of milkweed bugs sunflower seeds. The two groups of bugs appear identical, but those that fed on milkweed are poisonous while the others are not. Berenbaum and Miliczky then fed naïve mantises bugs from either group. When the mantises ate the poisonous bugs, the mantises threw up and quickly learned to avoid the poisonous prey. The mantises also avoided the bugs that had been fed sunflower seeds unless they had not previously encountered a poisonous prey. More recently, an experiment conducted with jumping spiders showed that terrestrial invertebrates also can exhibit associative learning, not only remembering distasteful prey, but also remembering the environment in which they encountered that prey (Skow and Jakob 2006).
So clearly, insect predators can become ill from eating poisonous prey and can learn to avoid prey that look the same. Yet, what keeps insects such as milkweed bugs and monarch caterpillars that feed on milkweed from being able to have exponential population growth? How can milkweed plants survive once insects have specialized on them and are no longer affected by the milkweed defense compounds? Part of the answer is that some insect predators have adapted to feed on prey that are poisonous. Several studies have demonstrated the effects of predators feeding on poisonous prey. For example, Strohmeyer et al. (1998) found that predatory stink bugs grew faster when eating caterpillars that had less plant-derived iridoid glycosides.
Other studies of plant-feeding herbivores have shown that the ability to overcome plant defenses and then use them for self-protection also comes at a cost in terms of growth rate. For example, Kopf et al. (1998) found linkages between the evolutionary history of the plant feeding leaf beetles and their host plants. Ballabeni and Rahier (2000) and Ballabeni et al. (2001) showed trade-offs in performance of beetles feeding on alternative plants. Beetles that acquired more defensive chemicals had slower growth. In addition, predators often force these leaf beetles into choosing nutritionally poorer plants (Ballabeni and Rahier 2000, Ballabeni et al. 2001). Dobler (1996) also showed trade-offs in reproduction associated with food plant use with beetles that either laid eggs or gave birth to live young. In these leaf-feeding beetles, the concentration and distribution of the synthesized chemicals used for the beetle’s defense also depends on genetics of the beetles and the predation pressures they face (Pasteels et al. 1995). Similar trade-offs probably exist for many types of insects besides leaf-feeding beetles. For example, Smith et al. (2001) showed that a fly species (Tipula montana) selected among plants to maximize growth of larvae.
Slowed growth from eating chemical laden prey (in this case caterpillars that fed on plants containing iridoid glycosides that are stored in the haemolymph) also affects predators that eat these prey. In some predators, feeding on prey that have a lot of chemical defenses slows down reproductive rate and growth (Strohmeyer et al. 1998). Experimentally, this has been demonstrated in ladybird beetles that synthesize their own defensive compounds. Holloway et al. (1993) found trade-offs for the production of defensive compounds, which ladybeetles produce themselves. Ladybird beetles that produced more defense compounds grew more slowly and remained smaller. The difference between growth rates and chemical defense was genetically based (Holloway et al. 1993). When individuals of a species vary genetically in the way they use energy for either reproduction or chemical defense, populations that face few predators will adapt to have little chemical defense but high rates of growth and reproduction. Populations that are frequently exposed to predators will have much higher levels of chemical defense.
In this exercise, you will learn about predator-prey relationships and their impact on ecological communities. Competition and predation have been tested repeatedly to determine their influences on a community. The roles of competition and predation in structuring communities are often variable (see reviews by Sih et al. 1985, Chase et al. 2002). Predators can play multiple roles in a community. First, they can restrict prey distribution or reduce prey abundance, which may lead to greater species diversity (Levin 1970). Second, predation can alter the structure of a community by influencing the abundance of species at many different trophic levels. For example, in the absence of predators, herbivores are able to feed and increase in numbers, which results in a decrease of available plant material. However, when predators are in the environment, the number of herbivores decreases and in turn the amount of plant material increases. These phenomena are expressed as the cascading trophic interaction model (Carpenter et al. 1985). Third, predation is a significant selective force leading in some cases to predator-prey co-evolution (Krebs and Davies 1993).
During the exercise, you will learn about some of the many factors that constrain predator ability. Intrinsic factors include limited time available for prey searching, limited stomach size, and time needed for digestion. Extrinsic factors include competition with other predators and environmental disturbances. Predator-prey relationships often change when an environment is disturbed, such as by flooding, fire, pesticide use, or farming. If few predators are in the environment, herbivores can reproduce rapidly reaching high local densities. This effect may be countered by a numerical increase of predators moving to the area. When there are many predators and abundant prey available, the predators capture as many prey as quickly as possible. Predation under these conditions is termed "resource competition" or "scramble competition" (Birch 1957). As predation causes prey to become more limited, competing predators may interact directly, leading to injuries or limited predatory success. Such competition is termed "interference competition" or "contest competition" (Birch 1957). When the environment becomes more stable, competition may return the community to an equilibrium of predator-prey species.
The exercise can be performed in a laboratory or outside. The space chosen for the exercise should allow all students access to the "environment." For a lab of 24 students, we use a 4 x 8 foot (1.2 x 2.4 meter) table with access on all sides. We use carpet samples to create a heterogeneous environment. In advanced courses, if students are investigating different scenarios in smaller groups, smaller tables can be used.
In this exercise, students use a variety of feeding appendages (see Figure 1, below) as they search the environment grabbing prey items and placing them into their stomachs. For our simulations, we place equal numbers (n = 200) of three kinds of candy (M&Ms, candy corn, and Skittles) onto shag carpet samples in a 2 x 3 m area. Candy simulates foraging for edible prey and provides a handy snack at the end of lab; however, three or four small items such as beans, buttons, plastic insects, etc., could also be used. Likewise, instead of using shag carpet to simulate a heterogeneous environment, the environment could consist of a grassy lawn (here, wrapped candy would be a must), a tabletop, or a cardboard box filled with packing material.
Figure 1. The pairs of forks represent grasping legs such as with dragonflies, pairs of teaspoons represent raptorial legs such as with mantises, and pairs of knives represent mandibles such as for beetles.
When the students arrive, they are shown pictures of various insect predators, and the roles of these predators in shaping a community are discussed. Assign feeding appendages based on these predator types and provide small paper cups that represent stomachs. Feeding appendages consist of pairs of plastic forks, knives, and teaspoons that are rubber-banded together to form a chopstick-like apparatus (Figure 1). The pairs of knives represent mandibles, pairs of teaspoons represent raptorial legs, and pairs of forks represent grasping legs. An equal number of each predator type is randomly assigned among the students.
To simulate learned avoidance, at the start of foraging, the student predators do not know which prey species is inedible. Foraging takes place for 30 seconds prior to the announcement of the kind of inedible prey. Then, students having inedible species in their stomachs must dump all items back in to the environment to simulate sickness prior to resuming foraging. With limited edible prey available, the students experience increasing competition and the foraging exercise is stopped after 45 seconds.
The rules of foraging are simple. The predator must only use its feeding appendage to capture prey. The predator must stop foraging when the stomach (cup) is full or when time expires. If any inedible prey are in the stomach at the end of the 45 seconds, all of that predator's prey are returned to the environment. The stomachs must be held upright at all times (no shoveling). After the end of feeding, the predators must digest their prey, represented by the time it takes to determine the number and kind of prey species in their stomachs. To speed this process, students sort their prey by kind and use balances to weigh each type. Prior to the exercise, the average weight per prey individual is determined. After foraging, the number of prey of each type eaten is determined by weighing and then calculating the number of individuals. A running tally for each predator type can be displayed on a board for later discussion.
Based on the number captured, the number of prey remaining in the environment per species is calculated (200 - # eaten = # surviving) and prey are allowed to reproduce according to the following formula. The poisonous species produces one copy of itself for each member that remains in the environment (doubles the number in the environment). The species that will remain palatable reproduces at a rate of three individuals for each that remains in the environment. The third species reproduces at an intermediate rate of two individuals for each that remains in the population.
In the second generation of the exercise, a second prey type becomes poisonous and cannot be eaten by any predator. Often, predation rates cannot keep pace with reproductive rates of some prey species. To avoid saturating the environment with prey (and to simultaneously maintain reasonable costs), we imposed a limit to reproduction by prey. Typically, we add no more than 300 prey per generation. This cap is explained ecologically as the role of intraspecific competition for resources (or environmental carrying capacity for a species). In the absence of predation, herbivorous species will increase in number until their food becomes limiting; then, only those members of the species that are able to acquire sufficient nutrients will be able to reproduce. In our exercise, each prey species is assumed to feed on different resources and thus is unaffected by interspecific competition.
The predator numbers also change. Foraging success for each predator type is calculated and the reproductive success and thus reproduction is determined as the ratio of each predator type's success divided by the total prey consumed by all predators (e.g., amount eaten by P1/ amount eaten by (P1 + P2 + P3). The ratio of predators is adjusted by changing foraging morphologies of unsuccessful predators into those of successful predators at the appropriate frequencies.
A predator type is randomly chosen to adaptively detoxify the poisonous prey species. That predator type is able to eat all prey types except the prey type that becomes poisonous in the next round. The other predator types must selectively avoid both the poisonous prey type from the previous round and the new poisonous prey type as they struggle to overcome competition and reproduce.
The exercise is continued until stability is reached or all but one predator type or prey type has become extinct. A short discussion is conducted as the prey are replaced in the environment. Predictions about the community's behavior are made and the exercise is repeated as above again randomly assigning poisonous prey and predator adaptations.
After the exercise is complete, the instructor can generate figures using the supplied Excel® predpreyblank.xls (29k) spreadsheet, and these can be printed and distributed to the class, or students can plot the changes in predator and prey species on the supplied Worksheet (*.doc 31k) or (*.pdf 23k). The students can generate a formal laboratory report, answer questions for further thought, and speculate on the community dynamics between the insect predators and their prey.
Questions may include interpretation of the outcomes between generations. Almost certainly, the outcomes will differ even if the same prey types become poisonous and the same predator types adapt. The differences may result from the fact that the predators will be experienced and are more efficient in the second trial, and thus the outcome is affected. Additionally, predators may gamble by guessing prey types to initially avoid. Occasionally, predator or prey types are forced to extinction in one or both trials. Students should explain the results and speculate on predator-prey interactions. For introductory ecology classes, students could be assigned primary literature or instructed to find an article that reports on predator-prey interactions.
The outcomes of this laboratory activity are assessed by grading answers to discussion questions that are submitted at the end of the laboratory period. In addition, the concepts and terms covered in this exercise are tested on class exams. In particular, students are responsible for all bold terms in their introduction. These terms should be defined by the students during their preparation for exams. The discussion questions are answered by students during the laboratory period. We have provided sample answers in the Notes to Instructors section.
What are the steps for successful predation to occur?
4 pts. 1 point for each stage (search, recognition, capture, and handling).
When a predator consumes a chemically defended prey item and becomes ill afterwards, the predator will tend to avoid this type of prey in future encounters. What term describes this predator behavior? What features of prey items might allow the predator to more easily avoid such prey?
4 pts. Learned avoidance. Prey that are aposematic trigger easier predator recognition and aid in this process.
2 pts. Provide term learned avoidance but do not describe features of prey items OR vice versa.
When a potential prey first becomes toxic, what happens initially to the number of individuals of this population?
4 pts. The prey numbers increase because no predators can consume them. Competition for resources is not a factor initially.
2 pts. For saying that prey numbers increase because of protection from predators but students argue that competition is important.
When no predators can consume a particular prey species, will the numbers of this species increase indefinitely?
4 pts. No. Carrying capacity related to resources will constrain population and predators will evolve to utilize the new resource.
2 pts. No but the students do not describe factors associated with carrying capacity.
When a species becomes established outside of its native range, often no predators exist that consume it. Other than the application of pesticides, what strategies relating to this exercise might scientists use to control these pests?
4 pts. Introduce predators from the native range of these pests as classical biological control.
2 pts. Introduce a predator but don't discuss that the predator is from the pest's native range
Questions 6-11 allow advanced students to be assessed for greater subject knowledge. It also allows for instructor-led discussions of these topics. Here we provide sample answers and evaluation criteria. In the Comments section we provide discussion of ways to improve student understanding.
What happens when insect herbivores reproduce more quickly than can be controlled by predators? What other factors control populations?
4 pts. Limitations for food control prey numbers limiting survival and affecting reproduction. Alternatively if the herbivores greatly over shoot the carrying capacity, they are extirpated because of lack of food.
2 pts. Students discuss competition but do not mention food limitations affects on reproduction.
What would happen if the environment were affected by a chance event, such as a tornado, hurricane, or severe drought? Which of the "species" in your exercise would be most likely to survive and why?
4 pts. Environmental change often disrupts food webs in ecosystems. Generalist species are better equipped to survive such change because they have a broader pool of resources that can be utilized.
2 pts. Students recognize effects of environmental disruption but incorrectly answer that specialists will survive better.
Was there evidence of predator specialization when a predator type adapted to feed on the poisonous prey type?
4 pts. Answers will vary based on the outcome of student trials. Examine student answer for correct interpretation based on the experiment. Alternatively, present a graph of data and have students interpret the likelihood of specialization. Assign partial credit based on student’s demonstrated understanding.
Are predators successful based on only the quantity of prey consumed? Are there other factors that need to be considered?
4 pts. Students should recognize that prey vary by mass, and in real systems by quality of nutrients. Prey also have the ability to fight back and predator handling efficiency may result in changes to net energy acquired.
2 pts. Students recognize differences in prey quality but fail to specifically discuss energetic considerations of predator prey interactions.
Did all predator types remain in the environment or was your community eventually reduced to two species? In your experience with the game, which species remained and which were eliminated? Why?
4 pts. Answers will vary based on the outcome of student trials. Students should recognize that prey defenses provide a temporary advantage and that predators shift to consume common prey. Often when a single predator can detoxify a given prey, that predator will "specialize" focusing just on that prey item.
2 pts. Students cannot explain why certain predators or prey will be extirpated.
What would happen if the food quality varied and the prey were not able to assimilate large quantities of defensive compounds? If potential prey species varied in the amount of toxins they possessed and if the prey reproduced faster if they were less toxic, what would happen to the predator-prey relationships?
4 pts. This question is very open-ended. Predators might be unable to learn to recognize protected prey if they were variable in chemical defenses. In the absence of strong predator pressures, the prey would remain non-toxic because of their increased reproductive capacity.
2 pts. Students recognize either that predators will have a more difficult time learning or that non-toxic prey have a reproductive advantage in the absence of a large number of predators but do not include both parts in their answer.
Have the students report their findings in a report. You could have students, working in groups of four, create their own grading rubric. This should be facilitated by suggesting that they include a minimum set of three evaluation criteria (i.e. Did the Student Identify the Problem, What are the Conclusions and Implications). These criteria should be based on a scale of 1-6 (1: Emerging knowledge of the subject, 6: Mastery of the subject). The students can use their grading rubric to evaluate their own and a peer's report.
Further discussion of assessment and evaluation is contained in the TIEE site:
Charlene D'Avanzo. July 2000. Evaluation of Course Reforms: A Primer On What It Is and Why You Should Do It. (http://tiee.ecoed.net/teach/essays/evaluation.html)
A term paper can be assigned that requires students to investigate biological control success and failure. Assign the students the following problem:
"Based on the laboratory activity, you have seen the effects of predation in limiting population growth of insect herbivores. This principle has become the backbone of efforts to introduce predaceous species to limit population growth of pest species. Visit the ESCAPE website (www.unk.edu/escape/) and read about the impacts of exotic species and explore the issues surrounding these species.
Based on your in class exercise and your experience with the ESCAPE website, write a 3-4 page typed double-spaced paper to address the following questions. What are the reasons that exotic species sometimes become a problem in the area where they have been introduced? Based on the results of the laboratory exercise, what should be considered when introducing a predator to control a problem exotic species?"
Paper is worth 25 points and should include at least five references.