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Chapter 48
In order to study relationships between organisms, ecologists need to know how groups of organisms change over time. How many individuals are born? How many die? How many organisms live in au area at any given time? To answer these questions, ecologists study populations. A population is a group of organisms that all belong to the same species and that live in a given area.
Exponential Growth: A Baby Boom
Almost any organism provided with ideal conditions for growth and reproduction will experience a rapid Increase in its population. What's more, the larger the population gets, the faster it grows. If nothing stops the population from growing, it will continue to expand faster and faster. The kind of curve this growth pattern produces on a graph is called an exponential growth curve. See Figure 48-2 on page 1034.
As you learned in Chapter 13, Charles Darwin realized that this tendency of populations to grow exponentially (rapidly) presented a puzzle to biologists of his time. Among other things, Darwin calculated that if all the offspring of a single elephant couple were to survive and reproduce, in less than 750 years one pair of elephants alone would produce 19 million offspring!
Obviously, exponential growth does not continue in natural populations for long. Most offspring of plants and animals do not survive long enough to reproduce. The question is... why?
Logistic Growth: A Step Closer to Reality
The population growth history of a particular species is a bit more complicated than simple exponential growth. Most populations go through a number of growth phases, which can be represented on a logistic growth curve. A logistic growth curve is shown on the graph in Figure 48-3.
Let's examine an example of logistic growth. Suppose a few animals are introduced into a new environment. At first their numbers will begin to grow slowly. This initial growth is shown by section A in Figure 48-3. Soon, however, the population will begin to grow very rapidly. Here, in section B of the same graph, the population grows exponentially. The population grows quickly because few animals are dying and a great many are being produced.
Exponential growth does not continue for long. Soon the population reaches point C on the graph. Here the speed at which the population grows begins to slow down. Think about this carefully. Notice we did not say that the size of the population drops. The population is still growing, but it is growing at a slower rate. From here on, the population grows more and more slowly, through section D on the curve. How might we explain what is happening?
A population grows when more organisms are produced in a given period of time than die during the same period. In this situation, an ecologist would say that the population's birthrate is greater than its deathrate. Population growth may slow down because either the birthrate decreases or because the deathrate increases or both.
When the birthrate and deathrate are the same, population growth will stop, or reach zero growth. Remember, when we say that population growth is zero, we mean that the number of organisms in the population remains the same. Look again at the logistic growth curve in Figure 48-3. The portion of the curve labeled E is called the steady state. During the steady state, the average growth rate is zero. However, it is important to note that the steady state is not really all that steady. The population rises and falls somewhat. In fact, in some populations it rises and falls a great deal. But the rises and falls average out around a certain population size.
If we draw a horizontal line through the middle of the steady state region, as in Figure 48-4, that line will tell us how big the population is in the steady state. Ecologists say this line represents the carrying capacity of a particular environment for a particular species.
Once a population reaches the carrying capacity of its environment, certain factors keep the population from growing any further. These factors include a lack of food, overcrowding, and competition among the individuals in the population. If the population does grow larger, either the birthrate will fall or the deathrate will rise. (More individuals will die than will be born and the population will be reduced to the carrying capacity.) If the population falls, either the birthrate will rise or the deathrate will drop. (More individuals will be born than will die and the population will grow once again.)
48-2 Factors That Control Population Growth
Why, you might ask, don't populations grow indefinitely? Recall from the previous chapter that the growth of individuals can be controlled by limiting factors. Similarly, both plant and animal populations can be controlled by several factors. Although controls on natural populations do not keep those populations from changing in size, no single species has ever threatened to overpopulate the entire planet. (That is, not until Homo sapiens came along.) Let us examine the ways in which natural populations are kept between extinction and overpopulation of their environment.
Density - Dependent Limiting Factors
When factors that control population size operate more strongly on large populations than on small ones, they are called density-dependent limiting factors. Density-dependent limiting factors usually operate only when a population is large and crowded. They do not affect small, widely scattered populations much. Density-dependent limiting factors include competition, predation, parasitism, and crowding.
COMPETITION When populations become crowded, both plants and animals compete, or struggle, with one another for food, water, space, sunlight, and other essentials of life. It is easy to see why competition among members of the same species is a density-dependent limiting factor. The more individuals there are, the more of them there are to use up the available food, water, space, and other necessities. The fewer individuals around, the less they compete.
Competition between members of different yet similar species is a major force behind evolutionary change. As you learned in Chapter 14, no two organisms can occupy the same niche in the same place at the same time. When two species compete, both find themselves under pressure from natural selection to change in ways that decrease their competition. This idea is important because it ties ecology and evolution together. It is another example of the way in which all the biological sciences are interrelated when you look at them from an evolutionary point of view.
PREDATION Just about every species serves as food for some other species. In most situations, predators and prey coexist over long periods of time. Like tennis partners who have played together for years, predators and prey have become accustomed to each other's strengths and weaknesses.
Prey, for example, have evolved defenses against predators. Some plants may produce poisonous chemicals. Some animals may have shells, poisonous skins, or camouflage behaviors and colors that help them hide.
At the same time, predators have evolved counter defenses. Some herbivores, such as monarch butterfly caterpillars, have evolved the ability to avoid the effects of certain plant poisons. Carnivores have evolved stronger jaws and teeth, powerful digestive enzymes, or extra-keen eyesight.
If we watch populations of predators and prey over time, we almost always find changes in their numbers. Typically, at some point the prey population grows so large that prey are numerous and easy to find. With such a large and available food supply to feast upon, there may soon be almost as many predators as prey. As you probably know by now, this situation cannot last because each predator needs many prey to satisfy its energy needs.
As predators become numerous, they eat more prey than are born. This means that the prey's deathrate becomes higher than its birthrate and the population decreases in size. But as the prey population drops, predators begin to starve, so the predator population drops too. When only a few predators are left, the prey begin reproducing and surviving in large numbers again, and the whole situation repeats itself.
For many years people did not truly understand (as we do now) that predator-prey relationships are important in controlling natural populations. Travelers and farmers took animals from one part of the world to another, releasing them into the wild. There, without a predator to keep their numbers down, the animals became serious pests. One famous example is the introduction of rabbits by Australians to their island continent a number of years ago. The collection of animals native to Australia is unique to that continent. Thus, rabbits had no natural predators. Within a relatively short time, the Australian rabbit population went into exponential growth and remained that way for a long time. Rabbits infested the countryside and devoured much of the natural vegetation. They have been a serious problem in Australia ever since.
ANALYZING PREDATOR PREY POPULATION MODELS
The relationships between predator and prey are often intertwined, particularly in an environment in which each prey has a single predator and vice versa. Examine the accompanying graph, which shows a computer model of the changes in predator and prey populations over time. After analyzing the graph, answer the following questions.
1. A sudden extended cold spell destroys almost the entire predator population at point F on the graph. What will happen to the prey population? How will the next cycle of prey population growth appear on the graph?
2. A bacterial infection kills off most of the prey at point B on the graph. How will this affect the predator and prey growth curves at point C? At point D?
3. A viral infection kills all of the prey at point D on the graph. What effect will this have on the predator and prey growth curves at point E? What will happen in future years to the predator population? How could ecologists ensure the continued survival of the predators in this ecosystem?
PARASITISM Parasites act like predators in many ways. Parasites live off their hosts, weakening them and causing disease. Like predators, parasites work most effectively it hosts are present in large numbers. Crowding helps parasites travel from one host to another. Stress related to crowding can also reduce a host's resistance to parasites. As a result, parasitism often affects large, concentrated populations more than small, scattered ones. Thus parasitism works as a density-dependent limiting factor on population growth. Note that few parasites kill their hosts--at least not right away. If a parasite kills its host too quickly, the parasite will have no chance to reproduce and spread. It is thus to the parasite's advantage not to be too deadly.
CROWDING AND STRESS Most animals have a built-in behavioral need for a certain amount of space. Both males and females, for example, may need room to hunt for food. They may need a certain amount of space for nesting. Or they may need a territory of a certain size. A number of fishes on coral reefs fit into this latter category. Many of these fishes are extremely territorial. Each male stakes out a territory and chases away all other males of his species. Young fish do not stand a chance of setting up a territory unless an older male dies or is eaten. In such cases, the number of suitable territories regulates population size in a density-dependent manner.
Certain species fight among themselves if they are overcrowded. Too much fighting can cause high levels of stress. This stress disturbs the finely tuned endocrine system you read about in Chapter 42. Large amounts of adrenaline secreted under conditions of stress upset the body's normal balance. Levels of several other hormones also change due to stress. As a result of these hormonal changes, animals fight more and breed less. Often the immune system is weakened as well. Hormonal changes can so upset a female's behavior that she neglects, kills, or even eats her own offspring. Extreme overcrowding among mice can affect the females' endocrine system so that pregnant females miscarry, or lose the fetus they are carrying. All these factors combine to lower birthrate.
Density-Independent Limiting Factors
Not all populations are controlled by density-dependent limiting factors alone. Many species show what are called boom-and-bust growth curves. Their populations grow exponentially for some time and then suddenly crash. After the crash, the population may build right up again or it may stay low for some time.
Thrips, aphids, and other insects that feed on plant buds and leaves can be washed out by a rainstorm. They may also be harmed by long hot periods of dry weather. Frosts, too, can cause sudden drops in insect populations. For these species, storms, cold weather, dry weather, or other natural occurrences can nearly wipe out the population. Such wipeouts can happen regardless of how large the population is at the time. Because population density does not matter in such cases, these natural occurrences are called density-independent limiting factors. As you might expect, the growth of many species is controlled by some combination of density-dependent and density-independent limiting factors.
Human populations, like those of all other animals, tend to increase in size with time. If we examine the size of the human population over the course of history, we see that for a long time it grew slowly. Then, about 500 years ago, the world's human population started growing exponentially. See Figure 48-10 on page 1040.
Today, population growth in the United States and parts of Europe has slowed down. But most of the world's people do not live in these countries. Instead, they live in China, India, and parts of Africa and Latin America--places where populations are still growing very rapidly. This population growth poses a serious threat to global ecology, as we shall see in the next chapter.
48-3 Interactions Within and Between Communities
After populations, the next larger biological units studied by ecologists are communities. A community consists of all the populations of organisms living in a given area. Populations in communities interact with one another in many ways. Plant species, for example, compete for water, nutrients, and sunlight. At the same time, some plants have evolved defenses against herbivores.
Herbivores compete with one another for food and space. There are usually several different herbivore species in any community. Certain of these herbivores may have evolved counter defenses for the protective mechanisms of one or more plant species.
While this is going on, carnivores are hunting the herbivores. Often there are several carnivores in a community, each of which is best at hunting a particular herbivore.
Many of the interactions among organisms we have discussed so far involve predation. But there are several other relationships that play an important role in nature. These relationships between organisms are called symbioses (sym- refers to together; -bios refers to life; symbiosis means living together). Parasitism, which you read about earlier in this chapter, is a symbiosis in which one species benefits and the other is harmed.
There are also many relationships between organisms in which one member benefits and the other is not harmed. This kind of symbiosis is called commensalism (kuh-MEHN-suhlihz-uhm). A good example of commensalism takes place on a coral reel where shrimp live within the stinging tentacles of sea anemones, The shrimp are not affected by the anemone's poison. As a result, the shrimp are protected from predators that cannot tolerate the anemone's stings. Anemones are not harmed by shrimp living on them, but they are not helped either, which is the definition of commensalism.
In still another kind of symbiosis, two species live together in such a way that both species benefit. This kind of symbiosis is called mutualism. Let's return to the coral reef to examine an example of mutualism. Right next to the sea anemone and the shrimp we might find clownfish. Clownfish form a mutualistic symbiosis with sea anemones, Clownfish benefit from living within the stinging tentacles of the sea anemones in the same way shrimp do. However, clownfish also help the anemones by chasing away several species of anemone-eating fish. In this case, both species benefit, which is the definition of mutualism.
Commensal and mutualistic symbioses are everywhere in nature, You may recall that lichens are a mutualistic symbiosis between a blue-green bacteria and a fungus. Many marine animals-such as corals--have symbiotic algae that live inside their tissues. Many land plants can live only with the help of symbiotic fungi on their roots. In fact, practically no organism can live in a world by itself. Each requires other organisms in some way.
Not only do populations and communities interact, ecosystems also interact with one another in many ways. Consider, for example, a pond in the woods. Certainly that pond contains populations of plants and animals that live only in the pond and not in the woods. But where does the water in the pond come from? Most likely from a stream that flows out of the woods. Where does extra water from the pond go? Probably into another stream or a nearby marsh. What about leaves and insects that fall into the pond from surrounding trees? How about raccoons, birds, and other animals that visit the pond from homes in the woods? What about air and rain that blow in from outside, carrying nutrients and possibly pollution into the pond? And don't forget about migrating birds that travel thousands of kilometers to summer in the pond.
Nearly every ecosystem is connected, either directly or indirectly, with other ecosystems. We can see how some of these connections work if we take an imaginary journey-a journey that begins when a farmer puts fertilizer on a corn field. Let's hitch a ride on a single nitrogen atom in that fertilizer and see where our travels take us.
Rain washes us into a stream. That stream flows through a pond and into a river. The river enters an estuary, where we are taken in by the roots of a plant. Later the plant dies, and its decaying remains are washed out into the coastal waters nearby. As we float along, we are eaten by a shrimp that in turn is eaten by a fish. The fish swims out into the open sea, dies. and sinks to the bottom. On the ocean floor the fish decays, and nutrients (including us) are released. Ocean currents carry us for hundreds of kilometers to a place where upwellings carry us back to the ocean surface. There we are taken in by phytoplankton, which are eaten by zooplankton, which are then eaten by fish. This time the fish is eaten by a bird that picks it up and flies to shore with it.
Are you beginning to get the picture? Our journey could continue indefinitely, taking us from one ecosystem to another all over the globe. Herein lies an important lesson for us all: Winds, rivers, and ocean currents tie Earth's ecosystems together in ways that we are just beginning to understand. That is why our growing human population must be ever more careful about how we dispose of wastes. Disposing of something in one ecosystem may just cause it to show up again somewhere else. We shall discuss this matter in more detail in the next chapter.
Gypsy Moths: Nature to the Rescue
In many places around the world, people have unwittingly demonstrated the importance of natural population control mechanisms by introducing into an environment organisms that have no natural predators. Around the turn of the century, a few gypsy moth caterpillars were accidentally introduced into the Northeast. First detected in 1905 in Connecticut, the gypsy moths began chewing their way through the eastern states.
Gypsy moth caterpillars are leaf-eaters and can cause widespread destruction of trees. Without any natural predators, gypsy moths were unaffected by density-dependent limiting factors. The only real check on the gypsy moth population was their breeding cycle: Every 8 to 10 years gypsy moths begin a new breeding cycle, after which they remain to cause problems for about 4 years. Thus gypsy moth populations exhibit a boom-or-bust growth curve characterized by periods in which the gypsy moths present no danger.
The last outbreak of gypsy moths occurred between 1979 and 1983. So in the spring of 1989, scientists prepared themselves for the onset of another breeding cycle and another round of widespread destruction in eastern forests. But something quite unexpected began happening instead. Throughout the Northeast, gypsy moth caterpillars were dying--before they could destroy the trees. Something was killing the gypsy moth caterpillars.
When scientists at the Connecticut Agricultural Experiment Station in New Haven examined the dead caterpillars, they discovered that the animals had been infected by a fungus. It was the fungus that was killing the gypsy moth caterpillars, Researchers now believe that about 80 years ago the fungus was intentionally introduced into the environment by scientists who realized that a similar fungus controlled gypsy moth caterpillars in Japan.
Scientists now believe that the unusually heavy rains in the Northeast in 1988 and 1989 caused the fungus to suddenly thrive. The thriving fungus, in turn, has destroyed the gypsy moths in record numbers. Due to continued heavy rains, scientists expect the fungus to reduce the gypsy moth population even further in subsequent years.
Will the fungus end the cycle completely and eliminate the gypsy moth population from the northeastern states? Scientists cannot be sure, but they suspect it will not. They know that natural controls tend to keep a population in check rather than destroy it completely. But they do hope to use the fungus to help them develop a biological agent that will control the gypsy moths in future years. However. as Dr. Andreadis at the New Haven experimental station points out, biological controls will not eradicate gypsy moths. "The whole idea behind biological weaponry is to create a balance with nature so that the gypsy moth population will perhaps maintain itself at a very low level."
SUMMARIZING THE CONCEPTS
The key concepts in each section of this chapter are listed below to help you review the chapter content. Make sure you understand each concept and its relationship to other concepts and to the theme of this chapter.
48-1 Population Growth
A population is a group of organisms that all belong to the same species and that live in a given area.
Given ideal conditions for growth and reproduction, a population of organisms will grow very rapidly, This rapid growth is shown in an exponential growth curve.
Most populations go through a series of growth phases, which can be represented on a logistic growth curve.
On a typical logistic growth curve, a population of organisms grows slowly at first, then grows rapidly during exponential growth, then slows down before reaching a steady state.
A population with zero growth has reached a steady state. This means the average population over time will not change markedly.
A population usually achieves a steady state when it reaches the carrying capacity of the environment.
48-2 Factors That Control Population Growth
Density-dependent limiting factors--which include predation, competition, parasitism, and crowding--operate strongly on large populations with high density.
Factors that do not depend on population density to control population size are called density-independent limiting factors. Severe storms, dry weather, and other climate conditions that can reduce a species' population are density-independent limiting factors.
48-3 Interactions Within and Between Communities
A community consists of all the populations of organisms living in an area.
Although predation is the primary relationship between organisms in a community, symbioses are other important relationships.
Symbioses include parasitism, commensalism, and mutualism.
All ecosystems are interconnected.
REVIEWING KEY TERMS
Vocabulary terms are important to your understanding of biology. The key terms listed below are those you should be especially familiar with. Review these terms and their meanings. Then use each term in a complete sentence. If you are not sure of a term's meaning, return to the appropriate section and review its definition.
48-1 Population Growth
population
exponential growth curve
logistic growth curve
steady state
carrying capacity
48-2 Factors That Control Population Growth
density-dependent limiting factor
density-independent limiting factor
48-3 Interactions Within and Between Communities
community
symbiosis
commensalism
mutualism
Multiple Choice
Choose the letter of the answer that best completes each statement.
1. During population growth
a. birthrate increases. b. deathrate increases.
c. birthrate decreases. d. birthrate and deathrate decrease.
2. A population that reaches the carrying capacity of its environment is said to have reached
a. logistic growth, c. density dependence.
b. exponential growth, d. a steady state.
3. Density-independent limiting factors include
a. predation, c. crowding.
b. hurricanes, d. parasitism..
4. All of the organisms living in a given area make up a (an)
a. population, c. ecosystem.
b. community, d. steady state.
5. A relationship in which one organism is helped and another organism is neither helped nor hurt is called
a. mutualism, c. symbiosis,
b. parasitism, d. commensalism,
6. A form of symbiosis in which both organisms benefit is called
a. mutualism, c. commensalism.
b. parasitism, d. the carrying capacity
7. A type of symbiosis in which one organism benefits and the other is harmed is called
a. mutualism, c. commensalism,
b. parasitism, d. symbiosis.
8. On a logistic growth curve, the portion of the curve in which the population grows rapidly is called
a. logistic growth, c. exponential growth
b. a steady state, d. the carrying capacity.
True or False
Determine whether each statement is true or false. If it is true. write "true." If it is false, change the underlined word or words to make the statement true.
1. A community is a group of organisms that belong to the same species and that live in a given area.
2. The rapid growth of a population is best shown by a logistic growth curve.
3. A horizontal line drawn through the middle of the steady state region on a growth curve represents the environment's carrying capacity.
4. Predation is a density-independent limiting factor.
5. Crowding is a density-dependent limiting factor.
6. The sea anemone and the clownfish are an example of commensalism.
7. In general parasites kill their hosts.
8. When both organisms in a symbiotic relationship benefit, that relationship is called mutualism.
Word Relationships
In each of the following sets of terms, three of the terms are related. One term does not belong. Determine the characteristic common to three of the terms and then identify the term that does not belong.
1. parasitism, mutualism, predation, commensalism
2. exponential growth, steady state, carrying capacity, symbiosis
3. predation, tornado, crowding, competition
4. parasitism, crowding, hormonal imbalance territoriality
CONCEPT MASTERY
Use your understanding of the concepts developed in the chapter to answer each of the following in a brief paragraph.
1. Explain how a population normally controlled by density-dependent limiting factors might be affected by a density-independent limiting factor.
2. Given ideal conditions, a population will grow exponentially. What limits does nature place on such exponential growth?
3. Why did biologists such as Darwin question the concept of exponential growth?
4. What is the relationship between steady state and carrying capacity?
5. How might the introduction of a toxic waste in a pond affect the carrying capacity of that pond?
6. Why are parasites considered a density-dependent limiting factor?
7. Describe and compare the three main types of symbiosis. What type of symbiosis is formed by the fungus that causes athlete's foot?
CRITICAL AND CREATIVE THINKING
Discuss each of the following in a brief paragraph.
1. Applying concepts Why might a communicable virus that causes a fatal disease be considered a density-dependent limiting factor? What about a virus that is not communicable?
2. Developing formulas Based on the information in this chapter, develop a mathematical formula for population growth rate. Hint. How are the two axes on a growth curve labeled?
3. Interpreting graphs Examine the growth curve shown in the illustration. What does the curve show? How might you interpret this data?
4. Making comparisons Describe the growth curve in a small town made up mainly of senior citizens. Compare this growth curve to a small town made up of newly married couples.
5. Relating concepts Using the concept of carrying capacity, explain bow the growth of both predator and prey are interrelated.
6. Making inferences Would a density-independent limiting factor have more of an effect on population size in a large ecosystem or in a small ecosystem? Using the writing process The union representing predators cannot reach a labor agreement with the union representing prey. Each side wants a large increase in their steady state population, You are the arbitrator hired to mediate the dispute, What is your learned decision?