Stevens, Alison N.P.

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University of Nebraska - LincolnDigitalCommons@University of Nebraska - LincolnPapers in Natural Resources Natural Resources, School of2012Predation, Herbivory, and ParasitismAlison N.P. StevensUniversity of Nebraska-LincolnFollow this and additional works at: https://digitalcommons.unl.edu/natrespapersPart of the Natural Resources and Conservation Commons, Natural Resources Management andPolicy Commons, and the Other Environmental Sciences CommonsThis Article is brought to you for free and open access by the Natural Resources, School of at DigitalCommons@University of Nebraska - Lincoln. Ithas been accepted for inclusion in Papers in Natural Resources by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln.Stevens, Alison N.P., "Predation, Herbivory, and Parasitism" (2012). Papers in Natural Resources. 953.https://digitalcommons.unl.edu/natrespapers/9536/3/20191/6By: Alison N. P. Stevens (Department of Biology, Mount Ida College) © 2010 Nature Education Predation, Herbivory, and Parasitism Predation, herbivory, and parasitism coexist within ecological communities. How do theseinteractions create dynamic, ever-changing biological systems?Individual organisms in a community interact in many different ways. An interaction may benefit bothindividuals, or the interaction may benefit one organism to the detriment of the other. An interaction betweentwo organisms that benefits one to the detriment of the other is an antagonistic interaction. Predation,herbivory, and parasitism are specific types of antagonistic interactions.  PredationIn predation, one organism kills and consumes another. Predation provides energy to prolong the life andpromote the reproduction of the organism that does the killing, the predator, to the detriment of the organismbeing consumed, the prey. Predation influences organisms at two ecological levels. At the level of theindividual, the prey organism has an abrupt decline in fitness, as measured by its lifetime reproductivesuccess, because it will never reproduce again. At the level of the community, predation reduces the numberof individuals in the prey population.   The best-known examples of predation involve carnivorous interactions, in which one animal consumesanother. Think of wolves hunting moose, owls hunting mice, or shrews hunting worms and insects. Lessobvious carnivorous interactions involve many small individuals consuming a larger one. Such group predationis common among social carnivores such as lions, hyenas, and wolves. Group predation also occurs with antsand social spiders. This is, however, only part of the picture. Seed consumption can sometimes constitutepredation. Seeds are considered organisms. Under ideal circumstances, seeds grow to become plants.However, consumption of a seed kills the plant before it can grow, making seed consumption an example ofpredation.   Not all predators are animals. Carnivorous plants, such as the Venus fly trap and the pitcher plant, consumeinsects. Pitcher plants catch their prey in a pool of water containing digestive enzymes, whereas the Venus flytrap captures an insect between the two lobes of a leaf and seals the insect inside with digestive enzymes.These plants absorb nutrients from the insects as they become available during digestion. On a microscopicscale, protozoa and bacteria also consume prey organisms. They play an important role in maintainingCitation: Stevens, A. (2010) Predation, Herbivory, and Parasitism. Nature EducationKnowledge 3(10):366/3/20192/6population sizes in microbial communities, which promotes the diversity of microorganisms and contributes toa stable community structure.  Predation and AdaptationPredation influences the fitness of both predators and prey. Individuals must both feed and avoid being eatento survive and reproduce. Genetically-determined traits that improve an organism’s ability to survive andreproduce will be passed on to its offspring. Traits associated with improved predation for predators andescaping predation for prey tend to be positively selected by natural selection.   Predators exhibit traits such as sharp teeth, claws, and venom that enhance their ability to catch food. Theyalso possess extremely acute sensory organs that help them to find potential prey. Consider the ability ofraptors to spot potential prey from over a kilometer away, the acute sense of smell of moles, the ability of owlsto locate mice by sound, the ability of pit vipers to sense body heat when tracking prey, and the ability of batsand dolphins to echolocate. Predators catch their prey either by pursuing potential prey or by ambushing them.Organisms that give chase are capable of short bursts of speed. Those that lie in wait tend to be camouflagedto avoid detection (Figure 1). Figure 1: Adaptations to predation(A) Cheetahs can sustain bursts of speed while chasing prey. (B) Species that lie in wait for their prey, such as the orchidmantis, are cryptically colored to avoid detection. In a similar manner, prey species exhibit traits that help them avoid detection or capture. Many, such as leafinsects, moths, a variety of frogs and small lizards, and herbivorous mammals, are cryptically colored to makethem more difficult to see. Behaviorally, they freeze after detecting the presence of a predator. This lack ofmovement helps them better blend in with their background and inhibits the ability of the predator to find them.But when predators venture too close, prey will take flight, running or flying to escape. When a chase ensues,prey will typically survive if they stay out of reach until the predator tires. Some species buy extra time bydistracting the predator. Examples include moths that flash brightly colored hindwings, lizards that drop theirtails, and insect larvae that discharge slime. Such actions surprise the predator and give the prey time a fewextra moments to escape.  When a predator chases after potential prey, the predator is running for its dinner. The prey is running for itslife. If the predator fails to capture the prey, it goes hungry, but it will not experience a large decline in fitnessas a result of the interaction. In contrast, if the predator catches the prey, the captured individual loses anyfuture opportunities to reproduce. This “life-dinner principle” sets up an evolutionary arms race between thetwo species (Dawkins & Krebs 1979). In this race, the prey experience strong selective pressure to evolvebetter adaptations to avoid being eaten. At the same time, predators must capture sufficient food to survive6/3/20193/6and reproduce, and they too are subjected to selective pressure for traits that allow them to hunt successfully.Over time, this arms race leads to traits that enable prey to better avoid capture, whereas predators becomebetter able to capture prey. Figure 2: Aposematic colorationBrightly colored animals, such as the red-spotted newt (a) and monarch butterfly (b), warn potential predators againstconsumption. Such organisms contain toxins. In contrast to the examples provided thus far, some prey exhibit bright coloration. Such aposematic colorationhelps prevent predation by signaling to potential predators that the vividly-colored individual is toxic. Toxinsmay be manufactured within the body, as with the red-spotted newt, or they may be acquired passively viaconsumption of toxic plants, as with the monarch butterfly (Figure 2). Figure 3: Batesian mimicryNon-toxic Papilio dardanus swallowtail butterfly females occur in a variety of forms, each of which mimics the physicalappearance of toxic species. Not all species that exhibit vivid coloration are truly toxic. Some have evolved patterns and colors that mimicthose of toxic species. Examples of such Batesian mimicry include the extraordinarily polymorphic Papiliodardanus swallowtail butterfly in southern Africa and Madagascar (Salvato 1997). Females of this species occurin a wide variety of physical appearances, nearly all of which mimic distasteful species of the Danaeus andAmauris genera with which they co-occur (Figure 3).  Herbivory6/3/20194/6Herbivory is the consumption of plant material by animals, and herbivores are animals adapted to eat plants.As in predator-prey interactions, this interaction drives adaptations in both the herbivore and the plant speciesit eats. For example, to reduce the damage done by herbivores, plants have evolved defenses, includingthorns and chemicals. Scientists have identified thousands of plant chemical defense compounds, includingfamiliar compounds such as nicotine and cocaine (Coley & Barone 1996).   To maximize nutrient intake, many herbivores have evolved adaptations that allow them to determine whichplants contain fewer defensive compounds and more high-quality nutrients. Some insects, such as butterflies,have chemical sensors on their feet that allow them to taste the plant before they consume any part of it.Mammalian herbivores often use their keen sense of smell to detect bitter compounds, and they preferentiallyeat younger leaves that contain fewer chemicals.  ParasitismIn parasitism, an individual organism, the parasite, consumes nutrients from another organism, its host,resulting in a decrease in fitness to the host. In extreme cases, parasites can cause disease in the hostorganism; in these situations, we refer to them as pathogens. We divide parasites into two categories:endoparasites, which live inside the body of their hosts, and ectoparasites, which live and feed on the outsideof the body of their host. Examples of endoparasites include flukes, tapeworms, fungi, bacteria, and protozoa.Ectoparasites include ticks and lice, plants, protozoa, bacteria, and fungi. Plants and animals typically act ashosts.  In most situations, parasites do not kill their hosts. An exception, however, occurs with parasitoids, which blurthe line between parasitism and predation. The best-known parasitoids include several species of wasp, whichimmobilize — but do not kill — a host by stinging it. The female then carries the host to a burrow, where shelays eggs within the host’s body. After the larvae hatch, they consume the living tissues of the host, eventuallykilling it (Figure 4a). Figure 4: ParasitoidismA parastic wasp stings its prey before laying eggs on or in it (a). The larvae will consume the insect after hatching. Thefruiting bodies of entomogenous fungi extend from the insect it consumed (b). Entomogenous fungi also act as parasitoids; they infect the bodies of insects, either through the mouth whileforaging or by penetrating the outer cuticle of the insect’s body (Ferron 1978, Roy et al. 2006). Spores circulateinside the host, whose body provides the nutrients needed for fungal growth. Eventually, the fungal loadbecomes too great for the host, and the insect dies (Figure 4b). The major distinguishing difference betweenparasitoids and predators is that parasitoids feed on living tissue, whereas the predator kills its prey before, orin the process of, consuming it. 6/3/20195/6Parasite TransmissionFor all parasites, the host exists as an island of habitat. But the island lives for a finite period of time, and theparasites must find a new host before the existing one dies. Transmission to a new host can happen eitherdirectly, or through a vector. In direct transmission, the parasite moves from one host to another of the samespecies without an intermediate organism. In vector transmission, an intermediate organism, the vector,transfers the parasite from one host to the next. Figure 5: Complex life cycle of the Plasmodium parasiteThe life cycle requires both the primary human host and the intermediate Anopheles mosquito host for completion. Many endoparasites have a complex life cycle that involves two hosts, and the parasite must spend time inboth to complete its life cycle. Take, for example, the protozoan parasite Plasmodium, which causes malaria.Plasmodium must spend time in humans and in an Anopheles mosquito to complete its life cycle. The mosquito actsas a vector, transferring Plasmodium from infected humans to uninfected individuals. Additionally, the mosquito6/3/20196/6acts as an intermediate host. When a female mosquito ingests blood containing Plasmodium, some of the redblood cells contain gametes (eggs and sperm). In the mosquito’s gut, the gametes come together to form azygote, the development of which results in sporozoites. It is this life stage that can then go on to infect a newhuman when the mosquito feeds (Figure 5).  SummaryAntagonistic interactions include predation, herbivory, and parasitism. These interactions are complex, withpredators consuming herbivores, plants and animals infected by parasites, and occasional disease outbreaks.Predation, herbivory, and parasitism exist along a continuum of severity in terms of the extent to which theynegatively affect an organism’s fitness.   Herbivorous, predatory, and parasitic interactions among organisms within communities regulate populationsizes by preventing any one population from becoming overly abundant. Each type of interaction becomesmore likely as the population size of the food source increases. Thus these interactions are essential tomaintaining the diversity of organisms that make up an ecological community. Each of these interactions canalter the balance of the food web, and removal of any part of the web can have a drastic impact on thecommunity. References and Recommended ReadingColey, P. D. & Barone J. A. Herbivory and plant defenses in tropical forests. Annual Review of Ecology and Systematics 27, 305-335 (1996).Dawkins, R. & Krebs, J. Arms races between and within species. Proceedings of the Royal Society, London, Series B, Biological Sciences 205, 489-511 (1979).Ferron, P. Biological control of insect pests by entomogenous fungi. Annual Reviews of Entomology 23, 409-442 (1978).Roy, H. E., Steinkraus, D. C., et al. Bizarre interactions and endgames: entomopathic fungi and their arthropod hosts. Annual Review of Entomology51, 331-357 (2006).Salvato, M. Most Spectacular Batesian Mimicry. In University of Florida Book of Insect Records, 1997. 

Predation, Herbivory, and Parasitism

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