Section 1 How Organisms Interact in Communities Objectives Describe coevolution. 12B TAKS 3 Predict how coevolution can affect interactions between species. 7B 11B 12E TAKS 3 Identify the distinguishing features of symbiotic relationships. 11B 12B 12E TAKS 3 Key Terms coevolution predation parasitism secondary compound symbiosis mutualism commensalism Evolution in Communities What are the most important members of an ecosystem? When you try to answer this question, you soon realize that you cannot view an ecosystem s inhabitants as single organisms, but only as members of a web of interactions. Interactions Among Species Some interactions among species are the result of a long evolutionary history in which many of the participants adjust to one another over time. Thus, adaptations appeared in flowering plants that promoted efficient dispersal of their pollen by insects and other animals. In turn, adaptations appeared in pollinators that enabled them to obtain food or other resources from the flowers they pollinate. Natural selection has often led to a close match between the characteristics of the flowers of a plant species and its pollinators, as you can see in Figure 1. Back-and-forth evolutionary adjustments between interacting members of an ecosystem are called coevolution. Predators and Prey Coevolve Predation is the act of one organism killing another for food. Familiar examples of predation include lions eating zebras and snakes eating mice. Less familiar, but no less important, examples occur among arthropods. Spiders are exclusively predators, as are centipedes. In parasitism, one organism feeds on and usually lives on or in another, typically larger, organism. Parasites do not usually kill their prey (known as the host ). Rather, they depend on the host for food and a place to live. The host often serves to transmit the parasite s offspring to new hosts. Many parasites Figure 1 Coevolution. With (such as lice) feed on the host s its long beak and tongue, the hummingbird is able to reach the outside surface. Among the external parasites that may have fed nectar deep within this flower. on you at some time are ticks, mosquitoes, and fleas. More highly specialized parasites like hookworms live entirely within the body of their host.
363 Plant Defenses Against Herbivores As you might expect, animal prey species have ways to escape, avoid, or fight off predators. But predation is also a problem for plants, which live rooted in the ground. The most obvious way that plants protect themselves from herbivores is with thorns, spines, and prickles. But it is even more common for a plant to contain chemical compounds that discourage herbivores. Virtually all plants contain defensive chemicals called secondary compounds. For some plants, secondary compounds are the primary means of defense. As a rule, each group of plants makes its own special kind of defensive chemical. For example, the mustard plant family produces a characteristic group of chemicals known as mustard oils. These oils give pungent aromas and tastes to such plants as mustard, cabbage, radish, and horseradish. The same tastes that we enjoy signal the presence of chemicals that are toxic to many groups of insects. How Herbivores Overcome Plant Defenses Surprisingly, certain herbivores are able to feed on plants that are protected by particular defensive chemicals. For example, the larvae of cabbage butterflies feed almost exclusively on plants of the mustard and caper families. Yet these plants produce mustard oils that are toxic to many groups of insects. How do the butterfly larvae manage to avoid the chemical defenses of the plants? Cabbage butterflies have the ability to break down mustard oils and thus feed on mustards and capers without harm. Real Life TAKS 3 Leaflets three, let it be. Members of the genus Toxicodendron, which includes poison ivy, produce a defensive chemical called urushiol (OO roo shee awl), which causes a severe, itchy rash in some people. Finding Information Do research to discover effective treatments for the rash caused by poison ivy. 0100010110 011101010 0010010001001 1100100100010 0000101001001 1101010100100 0101010010010 Predicting How Predation Would Affect a Plant Species 2C 12E Background TAKS 1, TAKS 3 Grazing is the predation of plants by animals. Some plant species, such as Gilia, respond to grazing by growing new stems. Consider a field in which a large number of these plants are growing and being eaten by herbivores. Analysis 1. Identify the plant that is likely to produce more seeds? 2. Explain how grazing affects this plant species. 3. Evaluate the significance to its environment of the plant s regrowth pattern. 4. Hypothesize how this plant species might be affected if individual plants did not produce new stems in response to grazing. Ungrazed plant Grazed plant Regrowth after grazing
Symbiotic Species In symbiosis (sim bie OH sis), two or more species live together in a close, long-term association. Symbiotic relationships can be beneficial to both organisms or benefit one organism and leave the other harmed or unaffected. Parasitism, mentioned earlier, is one type of symbiotic relationship that is detrimental to the host organism. While it is relatively easy to determine that an organism in a symbiotic relationship is being helped, it can be difficult to determine that an organism is neither harmed nor helped. Figure 2 Mutualism. The small green insects on this plant stem are aphids. They are protected by their ant guards. Figure 3 Commensalism. The clown fish can survive the stings of the sea anemone, which protects it from predators. Mutualism Mutualism is a symbiotic relationship in which both participating species benefit. A well-known instance of mutualism involves ants and aphids, as shown in Figure 2. Aphids are small insects that use their piercing mouthparts to suck fluids from the sugar-conducting vessels of plants. They extract a certain amount of the sucrose and other nutrients from this fluid. However, much of the fluid socalled honeydew runs out in an altered form through their anus. Certain ants have taken advantage of this fact and milk the aphids for the honeydew, which they use as food. The ants, in turn, protect the aphids against insect predators. Thus, both species benefit from the relationship. Commensalism A third form of symbiosis is commensalism, a symbiotic relationship in which one species benefits and the other is neither harmed nor helped. Among the best-known examples of commensalism are the relationships between certain small tropical fishes and sea anemones, marine animals that have stinging tentacles. These fishes, such as the clown fish shown in Figure 3, have the ability to live among and be protected by the tentacles of the sea anemones, even though these tentacles would quickly paralyze other fishes. Section 1 Review 364 Explain why predator-prey coevolution can be described as an arms race. 7B 12B Critical Thinking Applying Information Is the relationship between a plant and its pollinator mutualistic? Why or why not. 11B 12B 12E Critical Thinking Interpreting Interactions In a relationship that is an example of commensalism, would the species that is neither helped nor harmed evolve in response to the other species? Defend your answer. 7B 12B Critical Thinking Illustrating Principles In Japan, native honeybees have an effective defense strategy against giant Japanese hornets. Imported European honeybees, however, are unable to defend themselves. Use this example to illustrate the results of natural selection in adaptation. 7B 12B TAKS Test Prep Which pair of organisms has a mutualistic relationship? 12B A clown fish and C lion and zebra sea anemone B aphid and ant D flea and dog
How Competition Shapes Communities Section 2 Common Use of Scarce Resources and Competition When two species use the same resource, they participate in a biological interaction called competition. Resources for which species compete include food, nesting sites, living space, light, mineral nutrients, and water. Competition occurs for resources in short supply. In Africa, for example, lions and hyenas compete for prey. Fierce rivalry between these species can lead to battles that cause injuries to both sides. But most competitive interactions do not involve fighting. In fact, some competing species never encounter one another. They interact only by means of their effects on the abundance of resources. To understand how competition influences the makeup of communities, you must focus on the day-to-day events within the community. What do organisms eat? Where do they live? The functional role of a particular species in an ecosystem is called its niche (NICH). A niche is how an organism lives the job it performs within the ecosystem. A niche may be described in terms of space utilization, food consumption, temperature range, requirements for moisture or mating, and other factors. A niche is not to be confused with a habitat, the place where an organism lives. A habitat is a location; a niche is a pattern of living. Figure 4 summarizes some aspects of the jaguar s niche in the Central American rain forest. A niche is often described in terms of how the organism affects energy flow within the ecosystem in which it lives. For example, the niche of a deer that eats a shrub is that of a herbivore. The niches of some organisms overlap. If the resources that these organisms share are in short supply, it is likely that there will be competition between the organisms. Objectives Describe the role of competition in shaping the nature of communities. 12D 12E TAKS 3 Distinguish between fundamental and realized niches. 12D Describe how competition affects an ecosystem. 12D TAKS 3 12E Summarize the importance of biodiversity. 7B TAKS 3 Key Terms competition niche fundamental niche realized niche competitive exclusion biodiversity A Jaguar s Niche Figure 4 Each Diet Jaguars feed on mammals, fish, and turtles. Reproduction Jaguars give birth from June to August, during the rainy season. Time of activity Jaguars hunt by day and by night. organism has its own niche. All of the ways that this jaguar interacts with its environment make up its niche. 365
www.scilinks.org Topic: Symbiosis Keyword: HX4171 Size of a Species Niche To gain a better understanding of what a niche is, you can look more closely at a particular species. Imagine a Cape May warbler (a small, insect-eating songbird) flying in a forest and landing to search for dinner in a spruce tree. The niche of this bird is influenced by several variables. These variables include the temperature it prefers, the time of year it nests, what it likes to eat, and where on the tree it finds its food. (The Cape May warbler spends its summers almost exclusively in the northeastern United States and Canada. It nests in midsummer, eats small insects, and searches for food high on spruce trees at the tips of the branches.) The entire range of resource opportunities an organism is potentially able to occupy within an ecosystem is its fundamental niche. Dividing Resources Among Species Now reconsider what the Cape May warbler is doing. It feeds mainly at the very top of the spruce tree even though insects that the warbler could eat are located all over the tree. In other words, Cape May warblers occupy only a portion of their fundamental niche. Why? Closer study reveals that this surprising behavior is part of a larger pattern of niche restriction. In the late 1950s, the ecologist Robert MacArthur, while a graduate student at Yale University, carried out a classic investigation of niche usage, summarized in Figure 5. He studied the feeding habits of five warbler species the Cape May warbler and four of its potential competitors. MacArthur found that all five species fed on insects in the same spruce trees at the same time. As Figure 5 shows, however, each species concentrated on a different part of the tree. Although all five species of warbler had very similar fundamental niches, they did not use the same resources. In effect, Figure 5 Niche restriction Each of these five warbler species feeds on insects in a different portion of the same tree, as indicated by the five colors shown below. Cape May warbler Blackburnian warbler Black-throated warbler Bay-breasted warbler Myrtle warbler
367 they divided the range of resources among them, each taking a different portion. A different color is used to represent the feeding areas of each of the five warbler species shown in Figure 5. The part of its fundamental niche that a species occupies is called its realized niche. Stated in these terms, the realized niche of the Cape May warbler is only a small portion of its fundamental niche. How does this species of warbler benefit from hunting for food in only a portion of the tree? MacArthur suggested that this feeding pattern reduces competition among the five species of warblers. Because each of the five warbler species uses a different set of resources by occupying a different realized niche, the species are not in competition with one another. MacArthur concluded that natural selection has favored a range of preferences and behaviors among the five species that carve up the available resources. Most ecologists agree with this conclusion. Reading Effectively To better understand the relationship between fundamental and realized niches, draw two circles, one within the other. Label the larger circle Fundamental niche, entire tree. Label the smaller circle Realized niche. 0100010110 011101010 0010010001001 1100100100010 0000101001001 1101010100100 0101010010010 Predicting Changes in a Realized Niche 2C 7B 12D TAKS 1, TAKS 3 Background Two features of a niche that can be readily measured are the location where the species feeds and the size of its preferred prey. The darkest shade in the center of the graph below indicates the prey size and feeding location most frequently selected by one bird species (called Species A). Analysis 1. State the range of lengths of Species A s preferred prey. 2. Identify the maximum height at which Species A feeds. 3. Critical Thinking Predicting Outcomes Species B is introduced into Species A s feeding range. Species B has exactly the same feeding preferences, but it hunts at a slightly different time of day. How might this affect Species A? 4. Interpreting Graphics Species C is now introduced into Species A s feeding range. Species C feeds at the same time of day as Species A, but it prefers prey that are between 10 and 13 mm long. How might this affect Species A? Height above ground (m) 11 10 9 8 7 6 5 4 3 2 1 Prey Length and Location for Species A 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 5. Critical Thinking Predicting Outcomes How would the introduction of a species with exactly the same feeding habits as Species A affect the graph? Prey length (mm) 6. Interpreting Graphics What does the lightest shade at the edge of the contour lines represent?
368 The word niche is from the Latin word nidus, meaning nest. The place an organism occupies in its environment is part of its niche its overall functional role. Competition and Limitations of Resource Use A very clear case of competition was shown by experiments carried out in the early 1960s by Joseph Connell of the University of California. Connell worked with two species of barnacles that grow on the same rocks along the coast of Scotland. Barnacles are marine animals that are related to crabs, lobsters, and shrimp. Young barnacles attach themselves to rocks and remain attached there for the rest of their lives. As you can see in Figure 6, one species, Chthamalus stellatus, lives in shallow water, where it is often exposed to air by receding tides. A second species, Semibalanus balanoides, lives lower down on the rocks, where it is rarely exposed to the atmosphere. When Connell removed Semibalanus from the deeper zone, Chthamalus was easily able to occupy the vacant surfaces. This indicates that it was not intolerance of the deeper environment that prevented Chthamalus from becoming established there. Chthamalus s fundamental niche clearly includes the deeper zone. However, when Semibalanus was reintroduced, it could always outcompete Chthamalus by crowding it off the rocks. In contrast, Semibalanus could not survive when placed in the shallow-water habitats where Chthamalus normally occurs. Semibalanus apparently lacks the adaptations that permit Chthamalus to survive long periods of exposure to air. Connell s experiments show that Chthamalus occupies only a small portion of its fundamental niche. The rest is unavailable because of competition with Semibalanus. As MacArthur suggested, competition can limit how species use resources. Figure 6 Effects of competition on two species of barnacles The realized niche of Chthamalus is smaller than its fundamental niche because of competition from the faster-growing Semibalanus. Chthamalus stellatus Semibalanus balanoides Fundamental niches Realized niches
Competition Without Division of Resources In nature, shortage is the rule, and species that use the same resources are almost sure to compete with each other. Darwin noted that competition should be most acute between very similar kinds of organisms because they tend to use the same resources in the same way. Can we assume, then, that when very similar species compete, one species will always become extinct locally? In a series of carefully controlled laboratory experiments performed in the 1930s, the Russian biologist G. F. Gause looked into this question. In his experiments, Gause grew two species of Paramecium in the same culture tubes, where they had to compete for the same food (bacteria). Invariably, the smaller of the two species, which was more resistant to bacterial waste products, drove the larger one to extinction, as shown in the first graph in Figure 7. Gause hypothesized that if two species are competing, the species that uses the resource more efficiently will eventually eliminate the other. This elimination of a competing species is referred to as competitive exclusion. When Can Competitors Coexist? Is competitive exclusion the inevitable outcome of competition for limited resources, as Gause suggests? No. When it is possible for two species to avoid competing, they may coexist. In a revealing experiment, Gause challenged Paramecium caudatum the defeated species in his earlier experiments with a third species, P. bursaria. These two species were also expected to compete for the limited bacterial food supply. Gause thought one species would win out, as had happened in his previous experiments. But that s not what happened. As shown in the second graph in Figure 7, both species survived in the culture tubes. Like MacArthur s warblers, the two species of Paramecium divided the food resources. How did they do it? In the upper part of the culture tubes, where oxygen concentration and bacterial density were high, P. caudatum was dominant. It was better able to feed on bacteria than was P. bursaria. But in the lower part of the tubes, the lower oxygen concentration favored the growth of a different potential food yeast. Paramecium bursaria was better able to eat the yeast, so it used this resource more efficiently. The fundamental niche of each species was the whole culture tube, but the realized niche of each species was only a portion of the tube. Because the niches of the two species did not overlap too much, both species were able to survive. Population density (measured by volume) 150 100 50 0 75 50 25 0 Figure 7 Gause s experiments. The outcome of competition depends on the degree of similarity between the fundamental niches of the competing species. Effects of Competition When two species competed for the same resource, one species drove the other to extinction. 4 4 8 12 16 20 24 Days P. caudatum P. aurelia P. bursaria When two species used different resources, both were able to survive. 8 12 16 20 Days
Figure 8 Effect of removing sea stars. When the sea star Pisaster was removed from an ecosystem, the diversity of its prey species decreased. Mussels, the superior competitor, crowded seven other prey species out of the ecosystem. Predation and Competition Many studies of natural ecosystems have demonstrated that predation reduces the effects of competition. A very clear example is provided by the studies of Robert Paine of the University of Washington. Paine examined how sea stars affect the numbers and types of species within marine intertidal communities. Sea stars are fierce predators of marine animals such as clams and mussels. When sea stars were kept out of experimental plots, the number of their prey species fell from 15 to 8. The 7 eliminated species were crowded out by the sea stars chief prey, mussels, shown in Figure 8. Mussels can outcompete other species for space on the rocks. By preying on mussels, sea stars keep the mussel populations too low to drive out other species. Because predation can reduce competition, it can also promote biodiversity, the variety of living organisms present in a community. Biodiversity is a measure of both the number of different species in a community (species richness) and the relative numbers of each of the species (species diversity). Biodiversity and Productivity A key investigation carried out in the early 1990s by David Tilman of the University of Minnesota illustrates the relationship between biodiversity and productivity. Tilman and some co-workers and students tended 207 experimental plots in a Minnesota prairie. Each plot contained a mix of up to 24 native prairie plant species. The biologists monitored the plots, measuring how much growth was occurring. Tilman found that the greater the number of species a plot had, the greater the amount of plant material produced in that plot. Tilman s experiments clearly demonstrated that increased biodiversity leads to greater productivity. In addition to increased productivity, Tilman also found that the plots with greater numbers of species recovered more fully from a major drought. Thus, the biologically diverse plots were also more stable than the plots with fewer species. Section 2 Review 370 Distinguish between niche and habitat. Describe the conclusions reached by Connell and Paine about how competition affects ecosystems. 3F 12D 12E Describe how Tilman s experiments demonstrate the effects of biodiversity on productivity and stability. 3F 7B 12D Critical Thinking Applying Information Can an organism s realized niche be larger than its fundamental niche? Justify your answer. 12D Critical Thinking Evaluating Conclusions A scientist finds no evidence that species in a community are competing and concludes that competition never played a role in the development of this community. Is this conclusion valid? Justify your answer. 3A 12D TAKS Test Prep When two species use the same resource, one species may drive the other to extinction. This phenomenon is called 12D A space utilization. C niche restriction. B competitive exclusion. D resource division.
Major Biological Communities Climate s Effect on Where Species Live If you traveled across the country by car you would notice dramatic changes in the plants and animals outside your window. For example, the drought-tolerant cactuses in the deserts of Arizona do not live in the wetlands of Florida. Why is this? The climate of any physical environment determines what organisms live there. Climate refers to the prevailing weather conditions in any given area. Temperature and Moisture The two most important elements of climate are temperature and moisture. Figure 9 illustrates the different types of ecosystems that occur under particular temperature and moisture conditions. Temperature Most organisms are adapted to live within a particular range of temperatures and will not thrive if temperatures are colder or warmer. The growing season of plants, for example, is primarily influenced by temperature. Moisture All organisms require water. On land, water is sometimes scarce, so patterns of rainfall often determine an area s life-forms. The moisture-holding ability of air increases when it is warmed and decreases when it is cooled. Cold Arctic Section 3 Objectives Recognize the role of climate in determining the nature of a biological community. 12C 13A TAKS 3 Describe how elevation and latitude affect the distribution of biomes. Summarize the key features of the Earth s major biomes. 12C Compare features of plants and animals found in different biomes. 12C 13A TAKS 3 Compare and contrast the major freshwater and marine habitats. 12C Key Terms climate biome littoral zone limnetic zone profundal zone plankton Tundra Decreasing temperature Temperate forest Taiga Temperate grassland Subarctic Desert Temperate Figure 9 Elements of climate. Temperature and moisture help determine ecosystem distribution. For example, the asters and the saxifrage shown are able to produce flowers and seeds in the cold temperatures of the tundra. Hot Tropical Rain forest Savanna Desert Wet Decreasing moisture Dry
372 www.scilinks.org Topic: Biomes Keyword: HX4023 Figure 10 Earth s biomes. Seven major biomes cover most of the Earth s land surface. Because mountainous areas do not belong to any one biome, they are given their own designation. Major Biological Communities If you were to tour the world and look at biological communities on land and in the oceans, you would soon learn a general rule of ecology: very similar communities occur in many different places that have similar climates and geographies. A major biological community that occurs over a large area of land is called a biome. A biome s structure and appearance are similar throughout its geographic distribution. While there are different ways of classifying biomes, the classification system used here recognizes seven of the most widely occurring biomes: (1) tropical rain forest, (2) savanna, (3) taiga, (4) tundra, (5) desert, (6) temperate grassland, and (7) temperate forest (deciduous and evergreen). These biomes differ greatly from one another because they have developed in regions with very different climates. The global distribution of these biomes is shown in Figure 10. Many factors such as soil type and wind play important roles in determining where biomes occur. Two key factors are particularly important: temperature and precipitation. Figure 11 is based on the work of ecologist Robert Whittaker. The graph shows the relationship between temperature and humidity and the biological communities that exist under different conditions. In general, temperature and available moisture decrease as latitude (distance from the equator) increases. They also decrease as elevation (height above sea level) increases. As a result, mountains often show the same sequence of change in ecosystems that is found as one goes north or south from the equator. 60 N Polar ice 30 N Tundra Taiga Mountain zones Temperate forest Tropical forest Equator 30 S Temperate grassland Savanna 60 S Desert or semidesert
Temperature and Moisture in Biomes Figure 11 Conditions in biomes. Different biomes have characteristics of temperature and humidity. Average annual precipitation (cm) 400 300 200 100 HB04TS_C18000039a Taiga Temperate forest Temperate grassland Tropical forest Savanna Tundra Desert -10 0 10 20 30 Average temperature ( C) Terrestrial Biomes Tropical Rain Forests The rainfall in tropical rain forests is generally 200 to 450 cm (80 to 180 in.) per year, with little difference in distribution from season to season. The richest biome in terms of number of species is the tropical rainforest. Tropical rainforests may contain at least half of the Earth s species of terrestrial organisms more than 2 million species. Tropical rain forests have a high primary productivity even though they exist mainly on quite infertile soils. Most of the nutrients are held within the plants; the soil itself contains few nutrients. Savannas The world s great dry grasslands, called savannas, are found in tropical areas that have relatively low annual precipitation or prolonged annual dry seasons. Annual rainfall is generally 90 to 150 cm (35 to 60 in.) in savannas. There is a wider fluctuation in temperature during the year than in the tropical rain forests, and there is seasonal drought. These factors have led to an open landscape with widely spaced trees. Many of the animals are active only during the rainy season. Huge herds of grazing mammals are found on the savannas of East Africa. Tropical rain forest in Puerto Rico Savanna in East Africa
374 Taiga in Manitoba, Canada Tundra in Denali National Park, Alaska Taiga Cold, wet climates promote the growth of coniferous forests. A great ring of northern forests of coniferous trees, primarily spruce and fir, extends across vast areas of Eurasia and North America. This biome, one of the largest on Earth, is called by its Russian name, taiga (TIE guh). Winters in the taiga are long and cold, and most of the precipitation falls in the summer. Many large mammals, including herbivores such as elk, moose, and deer and carnivores such as wolves, bears, lynxes, and wolverines live in the taiga. Tundra Between the taiga and the permanent ice surrounding the North Pole is the open, sometimes boggy biome known as the tundra. This enormous biome covers one-fifth of the Earth s land surface. Annual precipitation in the tundra is very low, usually less than 25 cm (10 in.), and water is unavailable for most of the year because it is frozen. The permafrost, or permanent ice, usually exists within 1 m (about 3 ft) of the surface. Foxes, lemmings, owls, and caribou are among the vertebrate inhabitants. Investigating Factors That Influence the Cooling of Earth s Surface 2A 2B 2C 2D TAKS 1 You can discover how the amount of water in an environment affects the rate at which that environment cools. Materials MBL or CBL system with appropriate software, temperature probes, test tubes, beaker, hot plate, one-holed stoppers, water, sand, test-tube tongs, test-tube rack Procedure 1. Set up an MBL/CBL system to collect and graph data from each temperature probe at 5-second intervals for 240 data points. Calibrate the probe using stored data. 2. Fill one test tube with water. Fill another test tube halfway with sand. 3. Place a temperature probe in the sand, and suspend another temperature probe at the same depth in the water, using one-holed stoppers to hold each temperature probe in place. 4. Place both test tubes in a beaker of hot water. Heat them to a temperature of about 70ºC. Caution: Hot water can burn skin. 5. Using test-tube tongs, remove the test tubes and place them in the test-tube rack. Record the drop in temperature for 20 minutes. Analysis 1. Critical Thinking Analyzing Results Did the two test tubes cool at the same rate? Offer an explanation for your observations. 2. Critical Thinking Predicting Outcomes In which biome tropical rain forest or desert would you expect the air temperature to drop most rapidly? Explain your answer.
375 Terrestrial Biomes Found in Texas Deserts Typically, less than 25 cm (10 in.) of precipitation falls annually in the world s desert areas. The scarcity of water is the overriding factor influencing most biological processes in the desert. In desert regions, the vegetation is characteristically sparse. Deserts are most extensive in the interiors of continents. Less than 5 percent of North America is open desert. The amount of water that actually falls on a particular place in a desert can vary greatly, both during a given year and between years. Temperate Grasslands Moderate climates halfway between the equator and the poles promote the growth of rich temperate grasslands called prairies. Temperate grasslands once covered much of the interior of North America. Such grasslands are often highly productive when converted to agriculture. The roots of grasses characteristically penetrate far into the soil, which tends to be deep and fertile. Herds of grazing animals often populate temperate grasslands. In North America, huge herds of bison once inhabited the prairies. Temperate Deciduous Forests Relatively mild climates and plentiful rain promote the growth of forests. Temperate deciduous forests (deciduous trees shed their leaves in the fall) grow in areas with relatively warm summers, cold winters, and annual precipitation that generally ranges from 75 to 250 cm (30 to 100 in.). Temperate deciduous forests cover much of the eastern United States and are home to deer, bears, beavers, raccoons, and other familiar animals. The trees are hardwoods (oak, hickory, and beech). Temperate Evergreen Forests In other temperate areas, drier weather and different soil conditions favor the growth of evergreens. Large portions of the southeastern and western United States have temperate evergreen forests extensive areas where pine forests predominate over deciduous forests. Where conditions are even drier, temperate forests give way to areas of dry shrubs, such as in the chaparral areas of coastal California and in the Mediterranean. Desert, Guadalupe Mountains, Texas Temperate grasslands near Fredericksburg, Texas Temperate deciduous forest, Guadalupe Mountains, Texas Temperate evergreen (pine) forest in East Texas
Organizing Information Make a concept map that describes the zones of a pond or lake as described at right and in Figure 12. For each zone, include the plant and animal life found there. Figure 12 Three lake zones. Each region, or zone, of a lake contains characteristic organisms. Aquatic Communities At a glance, you might at first think that freshwater and marine communities are separate from terrestrial biomes. Yet large amounts of organic and inorganic material continuously enter both bodies of fresh water and ocean habitats from communities on the land. Freshwater Communities Freshwater habitats lakes, ponds, streams, and rivers are very limited in area. Lakes cover only about 1.8 percent of the Earth s surface, and rivers and streams cover about 0.3 percent. All freshwater habitats are strongly connected to terrestrial ones, with freshwater marshes and wetlands constituting intermediate habitats. Many kinds of organisms are restricted to freshwater habitats, including plants, fish, and a variety of arthropods, mollusks, and other invertebrates too small to be seen without a microscope. Ponds and lakes have three zones in which organisms live, as illustrated in Figure 12. The littoral zone is a shallow zone near the shore. Here, aquatic plants live along with various predatory insects, amphibians, and small fish. The limnetic zone refers to the area that is farther away from the shore but close to the surface. It is inhabited by floating algae, zooplankton, and fish. The profundal zone is a deep-water zone that is below the limits of effective light penetration. Numerous bacteria and wormlike organisms that eat debris on the lake s bottom live in this zone. The breakdown of this debris releases large amounts of nutrients. Not all freshwater systems are deep enough to include a profundal zone. Limnetic zone Littoral zone Profundal zone
377 Wetlands Swamps, such as the one shown in Figure 13, as well as marshes, bogs, and other communities that are covered with a layer of water are called wetlands. Wetlands typically are covered with a variety of water-tolerant plants, called hydrophytes ( water plants ). Marsh grasses and cattails are hydrophytes. Wetlands are dynamic communities that support a diverse array of invertebrates, birds, and other animals. Wetlands are among the most productive ecosystems on Earth, exceeded only by tropical rain forests and coral reefs. They also play a key ecological role by providing water storage basins that moderate flooding. Many wetlands are being disrupted by human development of what is sometimes perceived as otherwise useless land, but government efforts are now underway to protect the remaining wetlands. Figure 13 Caddo Lake, Texas. This swampy terrain is typical of the forested wetlands found in the southeastern United States. Watching Out for Wildlife in the Gulf of Mexico TAKS 3 Have you ever seen a dolphin ride the crest of a wave or a sea turtle crawl onto shore to lay its eggs? Such sights are unforgettable experiences for most people who have witnessed them. Researchers at the Center for Marine Life Sciences (CMLS) study the dolphins, sea turtles, and other creatures that inhabit the waters of the Gulf of Mexico. The CMLS is part of the Texas Institute of Oceanography, which is located on the Texas A&M University campus in Galveston. Marine Mammal Research Program An important component of the CMLS, the Marine Mammal Research Program monitors the well-being of cetaceans (dolphins and whales) and manatees in the Gulf. Scientists in this program want to learn whether human activities, such as offshore exploration and drilling for oil and gas, are having a negative impact on marine mammals. To answer this question, they conduct surveys to find out how many and what kind of cetaceans are in the Gulf. They have discovered that at least 19 species inhabit Gulf waters. Some, such as bottlenose dolphins, are found mainly in the shallow water of the continental shelf. Others, including sperm whales, typically live in deeper water. Texas Marine Mammal Stranding Network This part of the CMLS is activated whenever a dead marine mammal washes up on a Texas beach. Network scientists search the animal for clues about the cause of death. They look for heavy metals and other chemical traces that may point to pollution as the cause. Coupled with the survey results on cetacean populations, the data collected from strandings enable scientists to keep tabs on the health of the Gulf ecosystem. Sea Turtle Biology Program Researchers in the CMLS s Sea Turtle Biology Program monitor Kemp s Ridley sea turtle Kemp s Ridley sea turtle, a critically endangered species. The goal of this program is to locate the turtles habitats and identify the most serious threats to their survival. The researchers have found that these turtles establish strong home ranges along the eastern Texas and western Louisiana coast. They have also learned that the turtles numbers appear to be rebounding. www.scilinks.org Topic: Texas Gulf Coast Keyword: HXX4017
Threadfin shad, a Texas coastal species Diatoms from surface of the open sea Anglerfish from ocean depths Marine Communities Nearly three-fourths of the Earth s surface is covered by ocean, which consists of three major kinds of marine communities. Shallow Ocean Waters The zone of shallow water is small in area, but compared with other parts of the ocean, it is inhabited by large numbers of species. The seashore between high and low tide, called the intertidal zone, is home to many species of marine invertebrates. Coral reef communities, the world s most diverse, occur in shallow tropical waters. The world s great fisheries are located in the coastal zones of cooler waters, where nutrients washed out from land support huge numbers of fishes. Surface of the Open Sea Drifting freely in the upper waters of the ocean is a diverse community of plankton, composed of bacteria, algae, fish larvae, and many small invertebrate animals. Fishes, whales, and invertebrates such as jellyfishes feed on plankton. And larger fishes and birds, in turn, feed on some of these animals. Photosynthetic plankton (algae such as diatoms and some bacteria) that form the base of this food chain account for about 40 percent of all the photosynthesis that takes place on Earth. Because light penetrates water only to the depth of about 100 m (328 ft), this rich community is confined to the ocean s surface. Ocean Depths In the deepest waters of the sea, the marine community lives in total darkness, in deep cold, and under great pressure. Despite what seem like hostile conditions, the deep ocean supports a diverse community of bizarre invertebrates and fishes. This includes great squids and angler fishes that attract prey with projections from their head that emit light. On the ocean floor, at an average depth of more than 3 km (1.9 mi), researchers have found an unexpected abundance of species, a diversity that rivals the tropical rain forest. Section 3 Review 378 Describe the relationship between climate and location of species. 12C 13A Compare the tolerance to lack of water needed by plants and animals in savannas and tropical rain forests. 12C 13A Critical Thinking Analyzing Information Why can t photosynthesis occur in the deepest parts of the ocean or in a deep lake? Critical Thinking Forming Reasoned Opinions The equator passes across the country of Ecuador. But the climate there can range from hot and humid to cool and dry. What might explain this? TAKS Test Prep In which biome would you most likely find plants that are adapted to infertile soils and fairly constant, plentiful precipitation? A tropical rain forest C temperate grassland B tundra D savanna 12C 13A
379 Key Concepts 1 How Organisms Interact in Communities Species within communities coevolve, making many adjustments to living together. In a predator-prey interaction, prey often evolve ways to escape being eaten. Predators evolve ways to overcome the defenses of the prey. In mutualism and commensalism, species evolve in ways that benefit one or both parties. 2 How Competition Shapes Communities Interactions among species help shape ecosystems. Competition occurs when two species use the same limited resource. An organism s niche is its way of life. An organism may occupy only a part of its fundamental niche, which is called its realized niche. Competition can limit how species use resources. Biodiversity tends to promote stability and productivity. 3 Major Biological Communities Climate largely determines where species live. Temperature and moisture are key factors in determining where biomes occur. The seven major biomes are tropical rain forest, desert, savanna, temperate deciduous forest, temperate grassland, taiga, and tundra. Freshwater communities have three zones of life littoral, limnetic, and profundal. The three major marine communities are shallow ocean waters, open sea surface, and deep-sea waters. Key Terms Section 1 coevolution (362) predation (362) parasitism (362) secondary compound (363) symbiosis (364) mutualism (364) commensalism (364) Section 2 competition (365) niche (365) fundamental niche (366) realized niche (367) competitive exclusion (369) biodiversity (370) Section 3 climate (371) biome (372) littoral zone (376) limnetic zone (376) profundal zone (376) plankton (378)
Using Key Terms 1. The interaction between a spruce tree and a hemlock tree, both of which require 12D nitrogen from the soil, is an example of a. mutualism. c. succession. b. commensalism. d. competition. 2. The ways in which an organism interacts with its environment make up its 12B 12E a. niche. c. habitat. b. space. d. ecosystem. 3. Which of the following is a transitional zone between tropical rain forest and desert? 12C a. taiga b. savanna c. temperate deciduous forest d. tundra 4. Cold and long winters, very few trees, and little precipitation describe the 12C a. tundra. c. deciduous forest. b. taiga. d. grasslands. 5. For each pair of terms, explain the differences in their meanings. a. parasitism, predation b. mutualism, commensalism c. competition, competitive exclusion d. climate, biome Understanding Key Ideas 6. In predator-prey coevolution, if the prey gains a defense to stop predation, then the predator may evolve 7B 11B 12B a. in a way that enables it to overcome the prey s defense. b. so that it can parasitize the prey. c. secondary compounds. d. into a prey species. 7. The principle of competitive exclusion indicates that 12B 12D a. a niche can be shared by two species if their niches are very similar. b. niche subdivision may occur. c. one species will eliminate a competing species if their niches are very similar. d. competition ends in worldwide elimination of a species. 380 8. Describe the niches of a lion, a zebra, and the grass that grows on the African plain in terms of how each species affects energy flow in the ecosystem. 12A 12E 9. Which abiotic factor is likely not a reason for the desert biome s low primary productivity? a. extreme temperatures 12C b. frequent flooding c. high predation d. strong competition for sunlight 10. When populations of similar species occupy the same area at the same time, these populations often 12B 12E a. share all their resources equally. b. divide their range of resources. c. compete for resources to the death. d. look elsewhere for different resources. 11. Which of the words sets below describes a vulture eating a dead rabbit? 12B 12E a. heterotroph, scavenger b. parasite, predator c. herbivore, mutualism d. competitor, commensalisms 12. Describe how elevation and latitude affect the distribution of biomes. 13. How might information gained from the Marine Mammal Research Program be used to assess the health of Gulf Coast ecosystems? 12B 12E 14. How does the flow of energy through living systems help determine the components of a biological community? (Hint: See Chapter 5, Section 1.) 12A 12E 15. Concept Mapping Make a concept map that shows how the biomes can be classified based on precipitation, temperature, and geographical location. Try to include the following terms in your map: tropical rain forest, savanna, desert, temperate deciduous forest, temperate grassland, taiga, and tundra.
Critical Thinking 16. Justifying Conclusions In Gause s experiments, Paramecium caudatum could coexist with P. bursaria but not with P. aurelia. Predict what would happen if P. aurelia and P. bursaria were grown together, and justify your conclusions. 11B 12D 17. Justifying Conclusions Newly introduced predators often prove devastating to native animals. Explain why prey are often more vulnerable to introduced predators than to native predators. 12B 12D 18. Analyzing Information Explain why the open ocean biome is considered only slightly more productive than the desert biome? 19. Analyzing Data Using the data presented in this chapter, explain why many ecologists refer to the tundra as a frozen desert. 12C Alternative Assessment 20. Finding and Communicating Information Use library or Internet resources to research tropical rain forests. Describe reasons for the decline of tropical rain forests, and discuss actions that some countries have taken to protect tropical rain forests. Prepare a visual presentation that summarizes your findings about the tropical rain forest biome. 12C 21. Summarizing Information Work with a small group of students to develop a map that shows the most prominent terrestrial and aquatic communities within your state. Be certain to include any large swamps or wetlands that connect terrestrial and aquatic communities. 12C 22. Interactive Tutor Unit 7 Ecosystem Dynamics Write a report summarizing how beneficial bacteria are used to eliminate pollutants from waste water. Find out what kinds of factories or plants use beneficial bacteria. 11D TAKS Test Prep Use the chart and your knowledge of science to answer questions 1 3. Increasing primary productivity Comparative Productivity of Ecosystems Desert Open sea Savanna Kind of biome Estuary Tropical rain forest 1. Which of the biological communities named below has the lowest primary productivity? A desert C savanna 12C B open sea D estuary 2. Where would you place a bar representing the primary productivity of the temperate grassland biome, if it could be added to this bar graph? 12C F between desert and open seas G between open seas and savanna H between savanna and estuary J between estuary and tropical rain forest 3. Which statement best explains the similarity of estuaries and tropical rain forests in primary productivity? 12C A Both biomes receive about the same amount of rainfall. B Neither biome is subject to seasonal changes. C Both biomes are rich in species numbers and diversity. D Neither biome can be found outside the tropics. Test When serveral questions refer to the same graph, table, drawing, or text passage, answer the questions you are sure of first. 381