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LESSON 2 Describing Populations Guiding Question: What are the important characteristics of populations? • Explain the usefulness of tracking population size. • Define population density. • Describe the three ways populations can be distributed. • Explain what age structure diagrams tell you about a population. Male resplendent quetzal 4.2 LESSON PLAN PREVIEW Inquiry Students model sampling using a “population” of marbles. Real World Students explore local population density. Differentiated Instruction Support students with a Q&A on age structure diagrams. 4.2 RESOURCES Modeling Lab, Using Mark-andRecapture • Lesson 4.2 Worksheets • Lesson 4.2 Assessment • Chapter 4 Overview Presentation GUIDING QUESTION FOCUS Create a class cluster diagram of the phrase Characteristics of Populations. As you cover the lesson, add to and revise this diagram. 104 Lesson 2 Reading Strategy As you read, organize the lesson information using a T-chart. Be sure to note each key term and its definition. Vocabulary population size, population density, population distribution, age structure, age structure diagram, sex ratio Individuals of the same species living in a particular area make up a population. Species can consist of many populations that are geographically isolated from one another. This is the case with the resplendent quetzal (Pharomachrus mocinno), considered one of the world’s most spectacular birds. These birds are characteristic of the Monteverde region. Although it ranges from southernmost Mexico to Panama, the resplendent quetzal lives only in high-elevation tropical forests. Today, the species exists in many separate populations scattered across Central America. In contrast, humans have spread into nearly every corner of the planet. As a result, it is difficult to define a distinct human population on anything less than the global scale. Some would maintain that in the ecological sense of the word, all 6.8 billion of us make up one population. Whether one is considering humans, or quetzals, or golden toads, all populations show characteristics that help population ecologists predict the future of that population. Population size, density, distribution, age structure, and sex ratio all help the ecologist understand how a population may grow or decline. Population Size The overall health of a population can often be monitored by tracking how its size changes. Population size describes the number of individual organisms present in a given population at a given time. Population size may increase, decrease, undergo cyclical change, or remain the same over time. When population size increases or remains steady, it is often a sign of a healthy population. When population size declines quickly, however, it can mean extinction is coming. As late as 1987, scientists reported more than 1500 golden toads in the Monteverde population. In 1988, only 10 toads were reported. In 1989, scientists found only a single toad. By 1990, the species had disappeared. The Decline of the Passenger Pigeon The passen- ger pigeon (Ectopistes migratorius) illustrates the extremes of population size. As with the golden toad, dramatic and rapid changes in population size indicated trouble. Passenger pigeons, shown in Figure 3, were once the most abundant bird in North America. Huge flocks literally darkened the skies. In the early 1800s, an ornithologist (a scientist who studies birds) named Alexander Wilson described a flock of 2 billion individuals that formed a near-solid mass 390 kilometers (240 miles) long, took 5 hours to fly overhead, and sounded like a tornado. Passenger pigeons nested and bred in the forests of the upper Midwest and southern Canada. Once people began cutting down the forests, however, hunters had easy access to the birds. Thousands of pigeons at a time were shot down and shipped to market as food. By the end of the 1800s, the population size was so small that the pigeons could not form the large colonies they needed to breed effectively. In 1914, the last passenger pigeon on Earth died in the Cincinnati Zoo in Ohio. (a) Determining Population Size Do you think Alexander Wilson actually counted 2 billion passenger pigeons in the flock? Of course he didn’t. While simply counting every individual in a population is the most direct way to determine population size, it is almost never possible. In nearly all situations, population size is estimated using sampling techniques. Here’s how sampling works: Instead of counting every individual in a large area, ecologists count the number in a smaller sample area. Then, they use that information to estimate the number of individuals in the larger overall area. If there are 100 oak trees in one square kilometer of a large forest, for instance, it may be reasonable to estimate that there are about 1000 oak trees in ten square kilometers of the same forest. Sampling is particularly helpful for estimating the size of very large populations and populations that are widely spread out over an area. Sometimes it is easier to find signs of organisms rather than the organisms themselves. For example, it can be very difficult to spot organisms that are rare or that simply prefer to remain hidden. For these creatures, such as jaguars, tigers, and tapirs, it is often easier to find and count animal tracks or droppings than the animals themselves. Reading Checkpoint When is sampling necessary? (b) FIGURE 3 Population Size (a) The passenger pigeon was once North America’s most numerous bird. (b) However, habitat destruction and hunting drove the species to extinction within a few decades. ANSWERS Reading Checkpoint Sampling is necessary when you cannot directly count the individuals in a population. Usually, the population is very numerous and/or very spread out. Population Ecology 105 What Do you think? Some scientists say that extinction is our greatest environmental problem. Do you think we should be concerned about species extinction? What consequences do you think the loss of golden toads and harlequin frogs has had on the wildlife and people of the Monteverde region? ANSWERS What Do You Think? Answers will vary. Check that students’ opinions include how the loss of the harlequin frog and golden toad has affected the Monteverde region. Reading Checkpoint Population density is the number of individuals within a population per unit area; population distribution describes how the organisms in an area are arranged. Population Density A population’s density is a measure of how crowded it is. The huge flocks and breeding colonies of passenger pigeons showed high population density. Population density describes the number of individuals within a population per unit area. For instance, the 1500 golden toads counted in 1987 were found within 4 square kilometers (988 acres). So their population density was 1500 toads/4 square kilometers, or 375 toads/square kilometer. In general, larger organisms, like lions and other big cats, have lower population densities because they require more resources, and thus more room, to survive. Different Densities High population density can make it easier for organisms to group together and find mates. However, it can also lead to conflict as individuals compete for resources. Overcrowded organisms may also become more vulnerable to the predators that feed on them, and close contact among individuals can increase the transmission of infectious disease. In contrast, at low population densities, organisms benefit from more space and resources but may find it harder to locate mates and companions. Density and the Harlequin Frog Overcrowding is thought to have doomed the harlequin frog (Atelopus varius), which disappeared from the Monteverde Reserve at the same time as the golden toad. The harlequin frog, shown in Figure 4, lived in very specific locations called “splash zones.” Splash zones are areas alongside rivers and streams that receive spray from waterfalls and rapids. In the 1980s and 1990s, the Monteverde region of Costa Rica got warmer and drier. Water flow decreased and many streams dried up. Soon, there were only a few small splash zones left. Scientists recorded frog population densities up to 4.4 times higher than normal around the remaining splash zones. Such overcrowding likely made the frogs vulnerable to disease transmission, predator attack, and assault from parasitic flies. From their field research, researchers concluded that these factors led to the harlequin frog’s disappearance from Monteverde. A very small population is thought to remain outside the reserve, but scientists worry for its future. Figure 4 Splashing No Longer Harlequin frogs like this one are no longer found in the Monteverde Reserve. Scientists think that unusually high population densities, brought about by changes in climate, contributed to their decline. 106 Lesson 2 Population Distribution Populations can be distributed randomly, uniformly, or in clumps. Another factor that contributed to the disappearance of harlequin frogs is how individuals were arranged within the Monteverde ecosystem. Population distribution, sometimes called population dispersion, describes how organisms are arranged within an area. Ecologists define three distribution types: random, uniform, and clumped, as shown in Figure 5. In a random distribution, individual organisms are arranged within a space in no particular pattern. This type of distribution can occur when the resources an organism needs are found throughout an area and other organisms do not strongly influence where members of a population settle. ▶ Random Distribution A uniform distribution is one in which individual organisms are evenly spaced throughout an area. This can occur when individuals hold territories or otherwise compete for space. In a desert, where there is little water, each plant needs a certain amount of room for its roots to gather adequate moisture. As a result, there tends to be roughly the same amount of space between individual plants. (a) Random ▶ Uniform Distribution In a clumped distribution, individual organisms arrange themselves according to the availability of the resources they need to survive. This is the most common pattern in nature. For example, desert animals may live in patches around isolated sources of water. During their mating season, golden toads were found clumped at seasonal breeding pools. The harlequin frogs were clumped around splash zones. Humans, too, show clumped distribution; most people live and work near large urban centers. Distributions can depend on the scale at which you’re looking. For example, ants show a clumped distribution at a large scale; populations live together in colonies. Within the colony, however, individuals may be distributed more evenly. (b) Uniform ▶ Clumped Distribution Reading Checkpoint hat is the difference between popuW lation density and distribution? (c) Clumped Figure 5 Population Distribution There are three fundamental population distribution patterns seen in nature: (a) random, (b) uniform, and (c) clumped. Population Ecology 107 How do changes in population size relate to environmental conditions? Application As a class, discuss how each of the populations pictured in Figure 6 will likely grow. Then, have small groups of students talk about how the different types of population growth might affect the environment in which each population lives. Make sure students discuss present and future effects. Then, ask a few volunteers to summarize their group’s discussion for the class. ANSWERS Lesson 2 Assessment 1. A population that stays the same size or grows is likely healthy. A rapidly decreasing population size can mean that the population is going extinct. 2. 40 flamingos in a 10-m2 area 3. In random distribution, individuals are arranged in no particular pattern. A uniform distribution is one in which individuals are evenly spaced. In clumped distribution, organisms are gathered into groups; this is the most common distribution in nature. 4. The “mostly young” population. It has many pre-reproductive individuals that can have offspring in the near future. 5. Population ecologists might be interested in what caused mass extinctions so that they can recognize extinction when it is happening today and possibly avoid future extinctions caused by human activity. Figure 6 Age Structure Diagrams A population’s age structure diagram shows relative frequencies of males and females in different age groups. Populations with more young, prereproductive individuals tend to grow quickly. Populations with more older, post-reproductive individuals tend to decline. 108 Lesson 2 Age Structure and Sex Ratios Age structure diagrams show the number of males and females in different age groups within a population. Different populations have different mixes of ages. Likewise, populations can vary in the proportion of males and females present. These differences can affect how a population grows. Age Structure Populations almost always include individuals of different ages. Age structure, or age distribution, describes the relative numbers of organisms of each age within a population. Age pyramids, or age structure diagrams, are visual tools scientists use to show the age structure of populations. As shown in Figure 6, the width of each horizontal bar in the diagram represents the relative size of each age group. Individuals capable of having offspring make up the reproductive group. Young individuals who have not yet reached the age where they can have offspring are called pre-reproductive. Older individuals past the age of having offspring are called post-reproductive. Age structure diagrams can be effective tools when predicting population growth. A population with an even age distribution will likely remain stable as births keep pace with deaths. A population made up mostly of individuals past reproductive age will tend to decline over time. In contrast, a population of mostly reproductive or pre-reproductive individuals is likely to increase over time. Populations that are “bottom heavy” are capable of rapid growth. In this respect, the wide base of an age pyramid is like an oversized engine in a sports car—the bigger the engine, the faster it accelerates. Age Structure Diagrams Pre-reproductive age Male Female Reproductive age Male Post-reproductive age Female Male Female Age BIG QUESTION Mostly young (growing) Mix of young and old (stable) Mostly old (declining) Sex Ratios A population’s sex ratio is its proportion of males to females. Notice that age structure diagrams also give information about sex ratio by providing the relative numbers of males and females in each age group. Sex ratio is an important characteristic for populations of organisms that reproduce sexually and have distinct male and female individuals. For example, in monogamous species (in which each sex takes a single mate), a 50:50 sex ratio is often ideal. If there are too many males or females, called an unbalanced sex ratio, many individuals would be left without a mate—making it much harder for any given individual to reproduce and pass on genes. Figure 7 Sex Ratio The relative number of males and females determines a population’s sex ratio. In this South African population of springboks, the ratio of males to females is about 50:50. 2 1. Relate Cause and Effect How is a population’s size related to its well-being? 2. Calculate Which population of flamingos is more dense: 15 flamingos in a 5-square-meter area, or 40 flamingos in a 10-square-meter area? 3. Apply Concepts Describe the three patterns of population distribution. Which of these is the most common distribution in nature? 4. Infer Which of the populations shown in Figure 6 do you think is most likely to get bigger in size? Explain your reasoning. 5. Explore the BIGQUESTION Paleontologists use fossils to study past life on Earth. Paleontologists have documented several times in Earth’s history when massive numbers of species have declined and then disappeared from the fossil record all at once. Why do you think population ecologists studying living organisms might be interested in these mass extinction events? Population Ecology 109