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Objectives Identify causes and possible results of interspecific competition. Identify some adaptations of predators and prey. Compare and contrast symbiotic relationships. Key Terms interspecific competition competitive exclusion niche predation symbiotic relationship parasitism mutualism commensalism On your next walk through a patch of woods or park, or even down a city street, look for examples of different species interacting with each other. You might observe birds nesting in trees, bees collecting pollen from flowers, or ferns growing in the shade of larger plants. Populations do not live in isolation instead, they interact with other populations living in the same area. In some cases, populations compete for resources such as food, water, or shelter. Competition Between Species An organism, such as an elephant, cannot survive without other organisms. The elephant is part of a herd of elephants that form a population. The herd is part of a larger community of organisms. Recall from Chapter 34 that a community is a group of species living in the same geographic area. The elephants' community includes gazelles, giraffes, and birds; ants, beetles, fungi, and bacteria in the soil; and grasses and trees. In Concept 35.2 you read that members of a population may compete for limited resources in the environment. This competition within a single species limits the growth of the population. Within a community, interspecific competition (competition between species) takes place when two or more species rely on the same limited resource. For example, in the African savanna community, many species feed on grasses. In times of drought, the grasses may be in short supply, and competition may become especially intense. Competitive Exclusion If two species are so similar in their requirements that the same resource limits both species' growth, one species may succeed over the other. This process is referred to as competitive exclusion. A Russian biologist named G.F. Gause demonstrated this principle in the laboratory. He cultured two different species of Paramecium, P. aurelia and P. caudatum, feeding them a constant amount of food every day. When he grew them in separate containers, both thrived. But when he grew them together, the two species competed for the limited food available. Because P. aurelia could gather food more quickly, P. aurelia survived and reproduced, while P. caudatum starved. Eventually only P. aurelia remained in the culture. Figure 35-14 Two similar species may each thrive in separate locations, but one may exclude the other when they are placed together. The results of an experiment with two Paramecium species demonstrate this principle of competitive exclusion. In nature, the degree to which two species can require the same resources and still coexist depends on other factors in the ecosystem. For example, predators may keep competing populations below the levels at which resources such as food would become limiting factors. Gause's laboratory experiment demonstrated the process of competitive exclusion because he was able to isolate the two species and their common limiting resource (food) in the laboratory. Niches Within a community, each species has a unique living arrangement called its niche. A niche includes an organism's living place (habitat), its food sources, the time of day it is most active, and many other factors specific to that organism's way of life. The local loss of a species due to competitive exclusion is most likely to occur if two species have niches that are very similar. But even when species in a community compete for some of the same resources, their niches are rarely identical. For example, one lizard species in a tropical forest may feed on insects in low shrubs, while a similar lizard species may eat insects high in the trees. Predation Within the same savanna community where grazing animals may compete for grass, other interactions between species are taking place. For instance, a lion chases down an injured zebra, while nearby an egret targets a fish for its meal. These two interactions are examples of predation, an interaction in which one organism eats another. The lion and the egret are examples of predators, the organisms doing the eating. The food species being eaten are the prey. Because eating and avoiding being eaten are so important to survival, it is not surprising that many effective adaptations have evolved in both predators and prey. Predator Adaptations Predators that pursue their prey are generally fast and agile. Many predators have coloring that hides, or camouflages, them in their surroundings. Some predators, such as wolves and killer whales, may team up in packs to capture their prey. Most predators have acute senses that enable them to find prey. Rattlesnakes, for example, locate their prey with heat-sensing organs located between each eye and nostril. Adaptations such as claws, teeth, fangs, and stingers help many predators catch prey. Prey Adaptations Several adaptations help prey avoid being eaten. Some organisms retreat to safe locations; others flee from predators. And, while predators use camouflage to ambush prey, prey may use camouflage to hide from predators. Another type of defensive coloring has the opposite effect—it makes the organism stand out. Such "warning coloration" serves as a caution to predators. Some organisms aren't poisonous or dangerous themselves, but they look like organisms that are. This type of defense is called mimicry. For instance, the hawk moth larva puffs up its head and thorax when disturbed, looking like the head of a small poisonous snake. It even weaves its head back and forth and hisses like a snake. Animals aren't the only prey organisms that have elaborate defense adaptations. Plants cannot run away from predators, but they have other defenses that include poisonous chemicals and structures such as spines and thorns. Symbiotic Relationships A symbiotic relationship (sim bee aht ik) is a close interaction between species in which one of the species lives in or on the other. There are three main types of symbiotic relationships: parasitism, mutualism, and commensalism. Parasitism is a relationship in which one organism, the parasite, obtains its food at the expense of another organism, the host. Usually the parasite is smaller than the host. Both blood-sucking mosquitoes and tapeworms that live and feed in the intestines of larger animals are examples of parasites. The process of natural selection, described in Chapter 14, works on both the parasite and the host. Parasites that can locate and feed on their hosts efficiently are most successful. For example, some aquatic leeches locate their hosts first by detecting movement in the water. Then they confirm their selection by using temperature and chemical cues on the host's skin. Usually the effect of the parasite does not kill the host quickly, which would result in the death of the parasite as well. Natural selection has also produced defensive adaptations that help hosts resist parasites. The immune system of humans and other vertebrates is an example. In mutualism, both organisms benefit from the symbiotic relationship. One example of mutualism occurs inside your own body. Your large intestine is inhabited by millions of bacteria. The bacteria benefit by having a warm, moist home with a constant stream of nourishment, your food. In turn, some intestinal bacteria produce vitamin K. As discussed in Chapter 29, vitamin K is essential for blood clotting. Both you and the bacteria benefit from this relationship. Commensalism is a relationship in which one organism benefits, while the other organism is neither harmed nor helped significantly. For example, a spider crab may place seaweed on its back. The crab benefits by being camouflaged from its predators. The seaweed does not seem to be significantly affected. True commensalism in nature is rare, since most interactions harm one species (parasitism) or help both species (mutualism) to some degree. Concept Check 35.4 1. How did Gause's experiment with Paramecium demonstrate competitive exclusion? 2. Describe two methods predators use to help them capture their prey and two methods prey use to help them avoid being eaten. 3. Define and give an example of each of the three types of symbiotic relationships. Copyright © 2004 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved.