Food Webs. Lesson ENGAGE

Transcription

1 Lesson 2 Food Webs This bird has caught its dinner. Do any other organisms eat this kind of fish? How can one animal be part of two different food chains? 236 ENGAGE LS.25 Compare food chains and food webs (LS-M-C2) SE.48 Determine the ability of an ecosystem to support a population (carrying capacity) by identifying the resources needed by that population (SE-M-A2)

2 Can living things be part of more than one food chain? Materials Purpose Make a model that shows how food chains can overlap. Procedure Use the information in the table to make organism cards. On each card, list an organism and how it gets food. Draw a Sun card. Tape the card to the top of a poster board. Tape cards of organisms that depend on it for food directly below it. Draw arrows to show the flow of energy from the Sun to these organisms. Continue to add cards to the poster. Draw arrows to link each organism to the organisms it directly depends on for food. Draw Conclusions Interpret Data How many food chains does the white-footed mouse belong to? What can you conclude about food chains from this poster? Explore More What might happen if the milkweed was taken off the poster? Make a prediction, and make a plan to test your prediction. Step index cards markers tape poster board Organism Milkweed Oak tree Algae Louisiana Organisms Channel catfish Monarch caterpillar Alligator White-footed mouse Great blue heron How It Gets Food makes own food from sunlight makes own food from sunlight makes own food from sunlight small fish, aquatic plants, algae milkweed leaves fish, frogs, birds, small mammals seeds, berries fish, frogs, small mammals SI.1 Generate testable questions about objects, organisms, and events that can be answered through scientific investigation (SI-M-A1) SI.15 Identify and explain the limitations of models used to represent the natural world (SI-M-A5) Also covers SI.19 Eastern cottonmouth Bullfrog fish, frogs, small mammals insects, small mammals 237 EXPLORE

3 Essential Question How does energy flow between organisms in a food web? LS.25, SE.48 Vocabulary food web, p. 238 competition, p. 242 limiting factor, p. 242 carrying capacity, p. 243 Reading Skill Summarize What are food webs? In most food chains, each type of organism is eaten by more than one type of consumer. This can make organisms a part of more than one food chain. These food chains can be combined to form a food web. A food web is a network of food chains that have some links in common. How do you read a food web? Food webs are just several food chains put together. As with food chains, arrows represent the energy flow from one organism to another. Food Web Summary Technology e-glossary, e-review, and animations online at Read a Diagram Can you name three separate food chains in the food web on these pages? Clue: Follow a single set of arrows. Watch how organisms interact at 238

4 Look at the food web below. It shows several overlapping food chains. For example, the mouse belongs to three separate food chains. The worm is eaten by the mouse. Then the mouse is eaten by a raccoon, a bobcat, or a hawk. A food web tells you a lot about the living things in an ecosystem. What else does the food web on these pages show? A food web can allow you to identify which animals are predators, which are prey, and which are both. Arrows pointing to the hawk show that it is a predator of fish, mice, and small birds. Arrows pointing away from the mouse show that it is prey for hawks, raccoons, and bobcats. Quick Check Summarize Why are different food chains often linked together? Critical Thinking Can an organism be a consumer, an omnivore, a predator, and prey? Give an example that explains your answer. decomposer decomposer 239

5 How can food webs change? What happens when the population of one organism in a food web changes? For example, what would happen if one population decreases in number? When one population in a food web changes, all the other populations in the food web can be affected. When top carnivores are removed from a food web, prey populations are no longer controlled. If resources are available, prey organisms can reproduce without limits. When prey populations increase in number, they need more producers to supply them with food. For example, if bobcats are removed from a forest food web, the populations of birds, mice, and raccoons would increase. These increasing populations would eat more and more producers. Soon there would be less grass and other producers to support these organisms. This process has occurred in kelp forests. Kelp are giant algae masses that grow more than 30 meters long. Some kelp forests have been greatly changed by removing just one organism the sea otter. Kelp-Forest Food Web sea star kelp sea urchin sea otter sea cucumber Read a Diagram crab cabezon Which organisms in this food web would be affected by the removal of the kelp? Clue: Follow the arrows leading from the kelp. gopher rockfish mussel Sea otters make up one population in a kelp forest food web. 240

6 (inset) Dr. David M. Phillips/Visuals Unlimited Sea otters eat several animals in the kelp-forest food web. However, in the 1800s, too many sea otters were hunted for their fur. Without sea otters, fewer sea urchins were eaten. The sea urchin population was soon out of control. Sea urchins destroyed the kelp forests by chewing off the kelp where it was attached to the ocean floor. This caused the kelp to float away. Smaller fish that depended on the kelp could not survive without it, and their populations decreased. The organisms that depended on the small fish began to die off too. Without the sea otter to help control the size of the sea urchin population, the kelp forests almost disappeared. In the 1900s, many countries agreed to stop hunting sea otters. However, the sea otter population is not recovering as quickly as scientists had hoped. Sometimes a single population can grow out of control. For example, a red tide is a sudden explosive growth of single-celled algae in coastal areas. Red tides can occur when nutrient-rich deep water gets brought to the surface after a storm. With so many nutrients in the water, the algae population quickly grows. Toxins produced by the algae can cause the organisms that eat the algae, such as small fish, to die. This population change also reduces the food available for the fish s predators. Quick Check Summarize What occurs when a top carnivore is removed from a food web? Critical Thinking What might happen if a population of producers was removed from a food web? A population of red algae can grow so large that the ocean water appears red. red tide algae 241

7 Why do organisms compete? Life in an ecosystem is a constant struggle. Food, water, space, and other resources are restricted. Organisms struggle to get their share of each resource. This fight for limited resources is called competition. Who competes in an ecosystem? Organisms within a population compete with one another. A fox must compete with other foxes to catch rabbits. Populations also compete. Foxes and hawks, for example, both eat rabbits. Since there is a limited number of rabbits, the two predator populations compete for food. The rabbits must also compete with other herbivore populations for their food. Ultimately, the survival of populations comes down to resources. A limiting factor (LIH muh ting FAK tur) is any resource that restricts the growth of populations. A forest, for example, gets more rainfall and is much warmer in summer than winter. In summer, the forest can support many more populations than in winter. In this case, rainfall and temperature are limiting factors. Nonliving things, or abiotic factors, include water, temperature, weather, soil type, space to grow, shelter, and sunlight. In winter, food is scarce for bison. 242

8 This pond is overcrowded with algae. Living things, or biotic factors, can limit ecosystems. A prairie ecosystem has more producers than a desert ecosystem. As a result, the prairie can support more herbivores, which can then support more carnivores. In this case, the amount of available food is the biotic limiting factor for the desert ecosystem. With more available food, the prairie ecosystem can support larger populations. Together, biotic and abiotic factors determine the carrying capacity (KA ree ing kuh PA suh tee) for each population. The carrying capacity is the greatest number of individuals within a population that an ecosystem can support. For example, a rain forest can support a certain number of jaguars. If the jaguar population increases, their food becomes harder to find. Soon, some of the jaguars die and the population returns to its former level. Overcrowding also limits growth. An algae population in a nutrient-rich pond may seem like it can grow indefinitely. Limiting Factors SI.16, SI.23 Be Careful. Cut out twenty 2.5 centimeter (1 inch) circles. Each circle represents the range that the roots of a plant extend. Measure Create an environment for these plants by making a 20-cm (8-in.) square box on your desk. Toss 8 plants into the environment. If a plant does not touch another plant, it survives. If the plant touches another plant, remove the plant and any plant that it touches. Record your results in a data table. Increase the number of plants that you toss to 10, 12, 14, and so on. Record your results. Which number of plants tossed allows the most plants to survive? Infer How can space be a limiting factor for a population? But the algae will eventually get so thick that they start to use up all the available oxygen in the pond. Without enough oxygen for respiration, the algae and other organisms begin to die off. Quick Check Summarize What determines carrying capacity? Critical Thinking Why is a sudden increase in a predator population usually temporary? Populations cannot grow indefinitely. SI

9 How do populations grow? A population grows when the number of births in that population is higher than the number of deaths. When deaths exceed births, the population gets smaller. For example, there is a population of bluegill sunfish living in a pond. In one year, if 200 bluegills hatch and 150 die, then the population increases by 50. The next year, if 300 bluegills hatch and 400 die, then the population decreases by 100. When a population has access to ample food, living space, and other resources, it will grow very quickly in a pattern called exponential growth. Exponential growth means that the larger a population gets, the faster it grows. As a population grows, competition for living space, food, and other resources increases. Eventually, when the increased competition leads to the death rate and birth rate being equal, the ecosystem s carrying capacity has been reached for that species. The human population is experiencing exponential growth. By the year 2050, Earth s human population could reach 9 billion. As the population grows, people are forced to live closer together. Cities get larger and more people experience the effects of increased population, like overcrowding, high traffic, and increased demand for resources. Quick Check Summarize What is necessary for a population to grow exponentially? Critical Thinking How do we know that Earth s carrying capacity for humans has not been reached? Population Growth Population ,000 8,000 6,000 4,000 2, Exponential Population Growth Carrying carrying capacity Lag lag phase Growth growth phase Time period (in years) Population (billions of people) Increase in Human Population Year Read a Graph What stage of population growth were humans experiencing in the year 1850? Clue: Compare C01-002A the two graphs.

10 Visual Summary A food web is a network of food chains that share common links. Think, Talk, and Write Vocabulary A is a resource that is available in limited supply. Summarize How do producers and consumers obtain energy? Make a Study Guide Make a three-tab book. Tell about the topics on the inside of each tab. Changes to a food web can affect all the organisms in an ecosystem. Biotic and abiotic factors determine the carrying capacity for each population. Summary Critical Thinking How would you know when a population has reached its carrying capacity? Test Prep A food web can be broken down into separate A producers. B decomposers. C food chains. D food pyramids. Test Prep What is the original source of energy for most food webs? A decomposers B omnivores C producers D the Sun Essential Question How does energy flow between organisms in a food web? Math Link Make a Bar Graph Research the population growth of an animal such as the white-tailed deer and make a graph of your data. Plot the years on the x-axis and the number of people on the y-axis. Compare your graph with a classmate s. Art Link Make a Mobile Create a food-web mobile for the classroom. Draw and hang plant and animal cutouts from several food chains in an environment. Connect the food chains with string or crepe paper. -Review Summaries and quizzes online at EVALUATE

11 Fictional Narrative A good fictional story has an interesting beginning, middle, and end; describes a setting that tells when and where the story takes place; has a plot that centers around a problem or a conflict; has characters whose actions move the plot along. It was a cool night in the desert. The kangaroo rat crawled out of his underground burrow. He hopped on his long back legs to some nearby bushes. There he found some seeds on the ground. He was so busy stuffing seeds into his cheek pockets that he did not hear the soft rattling noise coming from behind him. Hello, furry friend, said the rattler. The moonlight shone on the brown diamond shapes along his back. I m very hungry. Are those seeds-s-s-s any good? Stay back, screeched the kangaroo rat when he saw the snake slithering closer. Don t be silly. I won t eat you, I just want some of your seeds-s-s-s, hissed the snake. But he quickly moved to within striking distance. The kangaroo rat tried to hop away, but it was too late. Write About It Fictional Narrative Choose two other organisms that share a predator/prey relationship. Write a fictional narrative in which these two organisms are in conflict. -Journal Research and write about it online at EXTEND LS.28 Explain and give examples of predator/prey relationships (LS-M-C4) ELA.22 Develop grade-appropriate paragraphs and multiparagraph compositions using the various modes (ELA-2-M4)

12 In the energy pyramid, only about 10 percent (%) of the available energy gets used as food energy at each level. This is an inefficient use of energy. You can calculate how much energy is used for food and how much energy is transferred to heat at each level to see exactly how inefficient it is. Suppose that the Sun gave off 95,000 energy units. To calculate how many units of energy are available at the producer level, you must start by finding 10% of 95,000. You can see that 10% of 95,000 is 9,500. This is how much energy would be available at the producer level. Calculating Percents Write the percent as a decimal. 10% = 0.10 Multiply the decimal by the total number. 95,000 x 0.1 = 9,500 Solve It 1. Suppose that there are 9,500 energy units at the producer level. Calculate how many units of available energy each level would receive. How many units of available energy are transferred to heat energy at each level? Hint: To find out how much energy is transferred, you must find 90% of available energy. top carnivores carnivores 2. Suppose that a plant used 6,400 energy units from the Sun. How many units of available energy will a carnivore receive when it eats an herbivore? How many energy units did the plant transfer as heat? herbivores producers M.8 Use the whole number system (e.g., computational fluency, place value, etc.) to solve problems in real-life and other content areas (N-5-M) 247 EXTEND