4 Food Chains and Food Webs

Transcription

1 SECTION 13 4 Food Chains and Food Webs KEY CONCEPT Food chains and food webs model the flow of energy in an ecosystem. Student text pages A food chain is a model that shows a sequence of feeding relationships. As you have read, energy flows through an ecosystem from producers to consumers. A simple way to represent this flow of energy is with a food chain. A food chabt shows the feeding relationships for a single chain of producers and consumers. Grasses are producers. They get Rabbits are consumers. They eat Hawks are also consumers. They eat energy through photosynthesis. producers, such as grasses. other consumers, such as rabbits. Types of Consumers As you can see in the food chain above, not all consumers are alike. Different types of consumers have different food sources. Herbivores, such as the rabbit above, are organisms that eat only plants. Carnivores, such as the hawk above, are organisms that eat only animals. Omnivores are organisms that eat both plants and animals. Most humans are omnivores. Detritivores (dih-try-tuh-volutz) are organisms that eat dead plant and animal matter. Earthworms, for example, are detritivores. Decomposers are detritivores that break down plant and animal matter into simpler compounds. Fungi, for example, are decomposers. Decomposers return nutrients to the ecosystem. 228 McDougal Littell Biology

2 Some organisms eat only one or a few specific types of organisms. For example, a bird called the Florida snail kite eats mostly one particular type of snail. Organisms that have a very selective diet are called specialists. Because specialists eat only one or a few particular organisms, they are very sensitive to changes in the populations of organisms they eat. For example, if the snail population drops, the Florida snail kite does not have another main food source. Other organisms, called generalists, eat a variety of different organisms. For example, the grey wolf eats many different animals, including elk, moose, deer, beavers, and mice. A bird called the Florida snail kite is a specialist. It eats mostly one type of snail, like the organism shown above. Trophic Levels The figure on page 228 shows a food chain of grasses (producers) rabbit (herbivore) hawk (carnivore). You can think of each link in a food chain as a level of feeding, or a trophic level. Energy flows up the food chain from the lowest trophic level to the highest. Producers are the first, or bottom, trophic level. The next trophic level is made of primary consumers herbivores that eat producers. The next trophic level is made of secondary consumers carnivores that eat herbivores. Continuing up the food chain, tertiary consumers are carnivores that eat secondary consumers. Omnivores, such as most humans, can be listed at different trophic levels in different food chains. A person is at the level of primary consumer when eating vegetables. A person is at the level of secondary consumer when eating beef or chicken. Primary means first in order. Secondary means second in order. Tertiary means third in order. At what trophic level are herbivores found? A food web shows a complex network of feeding relationships. A food chain shows a simple sequence of feeding relationships. But most feeding relationships are not very simple. For example, a generalist such as the grey wolf may be a part of several food chains that involve elk, deer, mice, and other organisms. This complex network of feeding relationships and the related flow of energy can be represented by a food web. I Visual Connection See food web in student text, pg. 410 Interactive Reader 229

3 A food web shows the network of feeding relationships between trophic levels in an ecosystem. Food webs can be quite complex, because many organisms feed on a variety of other species. This rabbit is a primary consumer and gets its energy by eating plants. This fox is a secondary consumer and gets its energy by eating rabbits, squirrels, mice, or sparrows. Rabbit Ground squirrel Spider Hawk Toad / - Snake Plants are producers and get their energy from the Sun. ' This hawk may be a secondary consumer, a tertiary consumer, or even at a higher trophic level, depending on what it eats. The stability of a food web depends on producers. Notice that the feeding chains of all organisms can be traced back to producers. In the food web above, a variety of prairie plants are the base of the food web. Also notice that some organisms can be involved with the food web at different trophic levels, depending on what they eat. When the hawk eats a mouse, it is a secondary consumer. But when it eats a snake that ate a beetle that ate plants it is a tertiary consumer. At each link in a food web, some energy is stored within an organism but most energy is lost to the environment as heat.. ntstatir ----"N44 In the food web shown above, at what trophic level does the %P/ feed? 230 McDougal Littell Biology

4 food chain herbivore carnivore omnivore detritivore decomposer specialist generalist trophic level food web Go back and highlight each sentence that has a vocabulary word in bold. 1. What is the difference between a food chain and a food web? 2. What is the difference between a specialist and a generalist? 3. What is the difference between a detritivore and a decomposer? 4. Fill in the chart below to describe your place in the food web. LIST THE LAST THREE WHAT TYPE OF AT WHAT TROPHIC LEVEL TYPES OF FOOD THAT CONSUMER WERE YOU? DID YOU EAT? YOU ATE. Interactive Reader 231

5 13.5 SECTION Cycling of Matter KEY CONCEPT Matter cycles in and out of an ecosystem. Student text pages Water cycles through the environment. Water moves continuously through the water cycle. The water cycle, or the hydrologic cycle (HY-druh-LAHJ-ihk), is the circular pathway of water on Earth from the atmosphere, to the surface, below ground, and back into the atmosphere again. On Earth's surface, living things including you are part of the water cycle. HYDROLOGIC CYCLE The hydrologic cycle is the circular pathway of water on Earth. itnwar"m',' condensation " A YOltiu- During precipitation, water falls to the ground as rain or snow. Water may trickle through the ground in the process of seepage. Liquid water becomes vapor in the process of evaporation. When water evaporates from plants, it's called transpiration. Water vapor becomes liquid water again during condensation.,44;gr ar, Name two ways that water can enter the atmosphere. * ACADEMIC VOCABULARY vapor the gaseous state of a substance 232 McDougal Littell Biology

6 Elements essential* for life also cycle through ecosystems. Oxygen, carbon, nitrogen, hydrogen, phosphorus, and sulfur are some of the elements necessary for life. Like water, these elements also cycle through ecosystems. The movement of a particular chemical through the living and nonliving parts of an ecosystem is called a biogeochemical cycle (By-oh-JEE-oh-KHEM-ih-kuhl). Here you will read about four biogeochemical cycles: the oxygen cycle, the carbon cycle, the nitrogen cycle, and the phosphorus cycle. The Oxygen Cycle Most organisms use oxygen for cellular respiration. Recall from Chapter 4 that plants and other photosynthesizing organisms release oxygen as a waste product. OXYGEN CYCLE WNW In the oxygen cycle, oxygen is produced through photosynthesis. Living organisms take in this oxygen and release it as carbon dioxide through respiration. The Carbon Cycle Carbon is a main component of carbohydrates, proteins, fats, and all of the other molecules that make up living things. Carbon can be found in many different forms as gas in the atmosphere, dissolved in water, in fossil fuels such as oil and coal, in rocks such as limestone, and in the soil. Plants convert carbon dioxide from the air into carbohydrates. Carbohydrates get passed through the living world as one organism eats another. Processes such as respiration and the burning of fossil fuels return carbon to the atmosphere. Carbon dioxide from the atmosphere is used by plants during photosynthesis. Respiration releases carbon dioxide back into the atmosphere. The burning of fossil fuels, such as oil and gas, releases carbon dioxide into the atmosphere as well. Carbon dioxide also returns to the atmosphere as dead organisms decompose. * ACADEMIC VOCABULARY essential necessary, required for Interactive Reader 233

7 The Nitrogen Cycle About 78 percent of the atmosphere is made of nitrogen gas. Organisms need nitrogen to live, but most organisms cannot use nitrogen in a gas form. Instead, most organisms can only use nitrogen when it is in the form of ions such as ammonium (NH 4 ) or nitrate (NO 3-). Certain types of bacteria can turn nitrogen gas into ammonia through a process called nitrogen fixation. Much of the nitrogen cycle happens underground. After nitrogen fixation, other bacteria turn the product, ammonia, into nitrates. Nitrates are used by plants to make amino acids and proteins. Nitrogen moves through the living world as one organism eats another. Some types of bacteria also use nitrates, and release nitrogen gas back into the atmosphere. NITROGEN CYCLE Nitrogen gas in the atmosphere is changed into ammonia by nitrogen-fixing bacteria. Ammonia becomes ammonium, which nitrifying bacteria change into nitrates. Plants use nitrates to make amino acids and proteins. This nitrogen is passed through the food web. Denitrifying bacteria change nitrates back into nitrogen gas, which is released into the atmosphere. 234 McDougal Littell Biology

8 The Phosphorus Cycle The oxygen, carbon, and nitrogen cycles all have some part that involves atmospheric gases. The phosphorus cycle is different. Most of the phosphorus cycle takes place at ground level. Phosphate is released by the slow breakdown of rocks. Plants take up phosphate through their roots. Phosphorus then moves through the food web. When dead organisms are broken down by decomposers, phosphorus is released back into the environment. Underline the main 'te.12:!9' difference between the phosphorus cycle and the other cycles in this section. PHOSPHORUS CYCLE The weathering of rocks releases phosphates into soil and water. Plants take up phosphates, which are then passed through the food web. Phophates are released back into the soil when these organisms die. Some phosphates sink to the botttom of water bodies, where they may become rock over thousands of years. hydrologic cycle biogeochemical cycle nitrogen fixation 1. List four biogeochemical cycles: Go back and highlil each sentence that has a vocabulary word in bold. 2. Nitrogen fixation changes into 3. The hydrologic cycle is the path of what substance? 4. What two main biologcal processes are responsible for the cycling of oxygen? Interactive Reader 235

9 13.6 SECTION Pyramid Models KEY CONCEPT Pyramids model the distribution of energy and matter in an ecosystem. Student text pages An energy pyramid shows the distribution of energy among trophic levels. Producers use energy from sunlight to make food. Herbivores eat plants the producers to get energy. Some of the energy is used by the animals to grow and some is used for cellular respiration. However, most of the energy that is consumed is lost as heat. Carnivores then eat the herbivores. And again, most of the energy is lost as heat. An energy pyramid is a diagram that compares the energy used by producers, primary consumers, and other trophic levels. In other words, an energy pyramid shows how much energy is available at each trophic level. Energy is lost at each trophic level of a food chain. Because of this, a typical energy pyramid has a large base of producers. Each level above gets smaller, because as energy is lost as heat, there is less energy available as food for organisms. The longer the food chain, the more energy is lost between the bottom and top links. An energy pyramid shows the energy flow between trophic levels in an ecosystem. Between each level, up to 90 percent of the energy is lost as heat into the environment. qo..) Where are producers located on the energy pyramid? 236 McDougal Littell Biology

10 Other pyramid models illustrate an ecosystem's biomass and distribution of organisms. An energy pyramid shows energy loss at each trophic level. Pyramid diagrams can also be used to represent other components of an ecosystem. Two other types of pyramid models are a biomass pyramid and a pyramid of numbers. Biomass Pyramids Biomass is a measure of the total amount, or dry mass, of organisms in a given area. A biomass pyramid is a diagram that compares the biomass of different trophic levels within an ecosystem. It shows the mass of producers that are needed to support primary consumers, the mass of primary consumers required to support secondary consumers, and so on. Notice that each trophic level has a smaller biomass than the one below it. The prefix bio- refers to life or living things. Mass is a measurement of the amount of matter. Biomass is a measurement of the amount of living organisms in an area. tertiary consumers 75 g/rni POP* secondary iso op 150 g/m 2 consumers The biomass pyramid shows the total dry mass of organisms found at each trophic level. iteractive Reader 237

11 Pyramids of Numbers A pyramid of numbers gives a count of the numbers of individual organisms at each trophic level in an ecosystem. This type of pyramid gives a good picture of the large numbers of producers that are required to support just a few toplevel consumers. Notice that it takes a huge number of producers to support just a few mountain lions. tertiary consumers secondary consumers DIS.7" rianfr What is represented by a pyramid of numbers? In a pyramid of numbers, each level represents the actual number of organisms at each trophic level. biomass energy pyramid Go back and highlight each sentence that has a vocabulary word in bold. 1. What is represented by an energy pyramid? 2. What is represented by a biomass pyramid? 3. Which trophic level contains the most energy? 4. What happens to most energy at each trophic level? 238 McDougal Littell Biology