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Lesson 7: Ecology (2.1.1) An ecosystem is the relationship of living organisms and the environment in which they live. Biotic factors include all living things, such as birds, insects, trees, flowers, fungus, and bacteria. Abiotic factors are those components of the ecosystem that are not living such as soil, water, temperature, the amount of light, and weather. THE FLOW OF ENERGY THROUGH AN ECOSYSTEM Matter within an ecosystem is constantly recycled. Elements, chemical compounds, and other sources of matter pass from one state to another through an ecosystem. For example, as a rabbit eats grass, the nutrients contained in the grass are broken down into the chemical components and then rearranged to become tissues in the rabbit and necessary components for life and its survival. When a snake eats the rabbit, the nutrients in the rabbits are broken down into chemical components and rearranged in the snake, just as the rabbit rearranged the nutrients from the grass. Waste products in the animals are released back into the ecosystem and broken down into simpler chemical compounds. Plants growing nearby take up these nutrients and rearrange them back into plant tissues and compounds needed for survival. Thus the energy cycle begins again. FOOD CHAINS A food chain in an ecosystem is made up of producers, consumers, and decomposers. Producers are organisms, such as plants, that use abiotic (not living) factors to obtain, store, and use energy. Consumers cannot make their own food and must eat other organisms in order to get energy. Each time an organism is eaten, energy is passed to the consumer. Deer, foxes, rabbits, raccoons, owls, hawks, snakes, mice, spiders, and insects are all examples of consumers in a forest ecosystem. Decomposers break down dead organisms into simpler chemical compounds. Saprophytes are fungi (such as mushrooms) and bacteria that act as decomposers. In a food chain, energy flows from one organism to another; it is a single step in the energy transfer process of an ecosystem. FOOD WEBS Energy transfer in an ecosystem is not as simple as a food chain. Food webs are used to illustrate competition amongst organisms and the interaction between food chains. Sometimes paths cross directly, and sometimes they do not. A food web shows how the presence of any one species nearly always affects other organisms. Look at the food web in Figure 7.4. What impact would a disease in the rabbit population have on the other organisms in the community? A disease in the rabbit population would probably cause a number of rabbits to die, decreasing their population size. Because snakes and owls eat the rabbits, competition between the wolves, snakes and owls would increase. More of the wolves, snakes and owls would not be able to find food and die of starvation, thus decreasing their overall population size as well. Because the foxes eat snakes, and the snake population has decreased, there would be less food for the foxes. The fox population would also decrease. In effect, a decrease in the rabbit population would most likely cause the populations of its predators to also decrease. Assume the rabbit population decreases as in the above example. What would happen to the grass in this ecosystem? If there are less rabbits, then less grass is being eaten. We would expect to see the amount of grass increase in this ecosystem. As the grass increases, there is more food available for the rabbits to eat. An increase in food supply would mean less rabbits might die of starvation, and their population size would most likely increase. How would an increase in the rabbit population affect the rest of the ecosystem? More rabbits equal more food available to the snakes, owls, wolves, and foxes. More snakes, owls, wolves, and foxes will be able to eat and have the energy needed for survival. Thus their population sizes will increase too. As you can see, population sizes in an ecosystem depend on the other organisms in that ecosystem. Populations fluctuate in size in response to other populations. As the rabbit population decreased, the snake, owl, wolf, and fox populations also decreased. As the rabbit population increased, so did the snake, owl, wolf, and fox. This cycle of fluctuation helps keep population sizes in check and prevent one population of organisms from reaching carrying capacity. If a population of rabbits is too large and over carrying capacity, disease, lack of food, water, and shelter, as well as predation will help to bring the population of rabbits back down to a size the ecosystem can support. ENERGY PYARMIDS A trophic level is the position occupied by an organism in a food chain. Organisms that share a trophic level get their energy from the same source. Producers are found at the base of the energy pyramid and make up the first trophic level of the food chain. Producers capture energy as sunlight and convert it into usable forms. The second trophic level is the primary consumers. These are organisms that eat the producers. The third trophic level in an ecosystem is the secondary consumers. Secondary consumers generally eat the primary consumers. The final trophic level is the tertiary consumers. Tertiary consumers generally eat the secondary consumers. Sometimes an animal will eat organisms from multiple trophic levels. For example, humans are at the top of the food chain and considered tertiary consumers. Humans may eat lettuce (producers), cows (primary consumers), and large fish (secondary consumers). The number of trophic levels is limited by the amount of available energy for organisms to survive and reproduce. Look at the energy pyramid in Figure 7.5. Notice as you move up the pyramid, the size of each level decreases. This is because the amount of available energy decreases as you move up the pyramid. The Law of 10% states that only 10% of the energy from one trophic level is passed to the next trophic level. Because producers make their own food by using the sun’s energy, we can assume the producers have 100% energy. When the mouse eats the grass, only 10% of the energy from the producers is passed on to the mouse. The mouse gets 10% of the energy for each grass it eats. What happens to the other 90% of the energy? The grass uses it for metabolic processes, defense, and reproduction. When the snake eats the mouse, only 10% of the energy from the mouse is passed onto the snake. The snake would get 1% energy for every mouse it eats. Again, the other 90% of the energy is used by the snake for metabolic processes, defense, predation, and reproduction. When the owl eats a snake, it only gets 10% of the energy from the snake. Thus, the owl would only get 0.1% energy for every snake it eats. The amount of energy decreases from a lower trophic level to the next. Therefore, in order to get enough energy to survive, organisms in higher trophic levels must spend more time hunting for prey and eating than in lower trophic levels. The number of trophic levels in an ecosystem is determined by the amount of available energy for predators. Activity Create a food chain and a food web using pictures you cut out of magazines or draw yourself. Show how energy flows through your ecosystem. Disrupt one thing in the food chain and discuss how this would affect your entire ecosystem. **This activity is related to DPI’s “Field Study on School Grounds” Activity. CYCLING OF MATTER Matter is recycled through plants, animals, and nonliving things in biochemical cycles to maintain the health and sustainability of an ecosystem. We will discuss the water cycle, carbon cycle, nitrogen cycle and oxygen cycle to analyze the interdependence of organisms with their environment. THE WATER CYCLE The water cycle circulates fresh water between the atmosphere and the earth. Three quarters of the earth is covered in water but the overwhelming majority of the water is salt water. Most of the earth’s fresh water is unusable because it is found in the form of ice in glaciers. Fresh water is necessary for organisms to carry out essential metabolic processes, and the water cycle helps keep usable fresh water on earth for organisms to use. Precipitation in the form of rain, sleet, snow, hail, and dew falls from the atmosphere and ends up in lakes, rivers, streams, and oceans through the precipitation itself, or runoff. Runoff is the movement of water from higher elevations to lower elevations. Heat energy from the sun causes evaporation which brings water into the atmosphere from bodies of water here on earth. Water is moved through wind and ocean currents as well. Aerobic respiration (C6H12O6 + O2 CO2 + H2O) from many organisms and transpiration (loss of water from leaves) from plants also puts water back into the atmosphere in the form of water vapor. When water vapor cools, it forms clouds, which cool to become saturated and form precipitation. Without the cycle of precipitation, runoff and evaporation, there would not be a supply of fresh water available on earth. Figure 7.6 shows the water cycle. Figure 7.6 The Water Cycle THE CARBON CYCLE The carbon cycle is the cycling of carbon between carbon dioxide and organic molecules. Recall that organic molecules contain carbon, with the exception of carbon dioxide, CO2, which is inorganic. Carbon dioxide accounts for approximately 0.03% of the atmosphere. Plants use carbon dioxide and energy from the sun to perform photosynthesis and make glucose, C6H12O6. When animals eat plants and other animals, carbon passes into their tissues. Through food chains and food webs, carbon is passed from one organism to the next. Carbon is returned to the earth through respiration, excretion, and decomposition. Sometimes when an organism dies, it is not eaten by other organisms and decomposed. Instead, the organism gets buried deep in the earth and over long periods of time, heat and pressure from the Earth convert the organism into fossil fuels such as coal, oil, and gas. During the burning of these fossil fuels, carbon dioxide is released back into the atmosphere. Figure7.7 The Carbon Cycle NITROGEN CYCLE Nitrogen is the largest quantity of gas found in the atmosphere, accounting for approximately 78% of the atmosphere. Nitrogen is an element found in both amino acids and nucleic acids. However, nitrogen gas from the atmosphere is not in a form that most organisms can use when making these necessary compounds. Through the nitrogen cycle, nitrogen gas is converted into a usable form such as ammonia (NH3), nitrite (NO2-), and nitrate (NO3-). Figure 7.8 shows the nitrogen cycle. Figure 7.8 The Nitrogen Cycle Nitrogen gas is converted into nitrate by some types of bacteria through the process of nitrogen fixation. In nitrogen fixation, nitrogen gas in converted into ammonia by bacteria called nitrogen fixers. Some plants can use ammonia, but most cannot, and must use nitrate. Nitrifying bacteria further convert ammonia into nitrite, and nitrite into nitrate. These nitrogen-fixing bacteria are found in the roots of legumes (pea and bean plants). Nitrogen fixation increases the amount of usable nitrogen in the soil. Plants can use this nitrogen (ammonia and nitrate) to make nucleic acids and proteins. Just as carbon is passed between organisms in the ecosystem through the carbon cycle, nitrogen is also passed from one organism to the next through food chains. As decomposers break down the remains of dead organisms, ammonia is released into the soil and often converted into nitrate by nitrifying bacteria. Other types of bacteria will convert the ammonia from decomposition back into nitrogen gas which is released back into the atmosphere. THE OXYGEN CYCLE In the oxygen cycle, oxygen is recycled from the atmosphere to the earth, and back to the atmosphere. This cycle is mainly initiated by photosynthesis. Photosynthesis is the process in which plants use energy from the sun to make glucose (C6H12O6), their main source of food. In photosynthesis, plants and other photosynthetic organisms, take carbon dioxide from the atmosphere and combine it with water to make glucose. Oxygen is released a waste gas into the atmosphere (CO2 + H2O C6H12O6 + O2). It is thought that early earth’s atmosphere contained no oxygen, but because of the evolution of photosynthetic organisms, oxygen has been released in the atmosphere in large quantities. Today’s atmosphere contains approximately 21% oxygen and is used by a large number of organisms for aerobic cellular respiration. Cellular respiration is the process in which organisms break down glucose (food) in order to gain the energy needed for life. The equation for aerobic (requires oxygen) cellular respiration is C6H12O6 + O2 CO2 + H2O. Oxygen is recycled through the processes of photosynthesis and cellular respiration. To recap, cellular respiration removes oxygen from the atmosphere while photosynthesis returns it. Although the atmosphere has 21% oxygen, the majority of the oxygen is found stored in the earth’s crust and in shells made of calcium carbonate (CaCO3) which are found in the ocean. Oxygen is also recycled through the processes of erosion and decay. Figure 7.9 The Oxygen Cycle Activity Identify each cycle and describe the benefits of each cycle to living things. FIELD TECHNIQUES Scientists often do research on an entire population of organisms or a large forest, for example. It is not always feasible for the scientist to study every individual in a population of organisms. For example, what if a population of insects had 100 thousand individual bees? A scientist would be unable to observe and study every single bee. So, how do scientists study populations of organisms? Sampling techniques are used to take a random sample of organisms to study. A large sample size must still be collected to help support that variation occurred by random chance. A scientist would randomly collect a large sample of bees, mark them, and release the bees back into the wild. By marking the bee, the scientist will know if he had collected this same bee during a previous collection day, and not collect data twice from the same organism. This same concept is used when studying many different living organisms. One way scientist collect random samples of living organisms is through quadrant sampling. To determine species diversity, a square (usually 4 by 4) is constructed and randomly placed throughout the study area. A scientist may throw the square, to make where it lands as random as possible. The scientist would then count all the organisms that are found in that square, for example, the number of different types of grasses. As long a large quadrant sample is collected, the scientist can estimate both the number of different grasses and the percent in which they appear in a forest, without walking through and searching every square meter of ground in that forest. Activity Over time, an ecosystem goes through a series of changes known as ecological succession. There are two types of succession: primary succession and secondary succession. Study the diagrams and describe each type of succession. Make a poster showing what an ecosystem would look like before primary succession, after primary succession, and after secondary succession. Lesson 7 Review: The Flow of Energy A. Define the following terms food chain decomposers biotic saprophyte primary consumer law of 10% biochemical cycle water cycle oxygen cycle precipitation respiration decomposition nitrifying bacteria producer runoff secondary consumer food web tertiary consumer carbon cycle transpiration nitrogen fixation excretion trophic level consumer energy pyramid nitrogen cycle aerobic respiration nitrogen fixers abiotic B. Choose the best answer 1. Which two organisms below share a trophic level? A. elephants and lions C. chipmunks and grass B. cheetahs and giraffes D. wolves and sparrows 2. The owl is a nocturnal hunter of small mammals, insects and other birds. An owl is an example of a/an A. producer B. omnivore C. carnivore d. decomposer 3. Which food would an herbivore always avoid? A. worms B. clover C. pine nuts D. grass 4. Emperor penguins feed on crustaceans, such as krill. They are prey to orca whales and leopard seals. What ecological role does the Emperor penguin play? (Hint: Krill are small animals that feed on tiny plant drifters) A. producer C. secondary consumer B. primary consumer D. top (tertiary) consumer) 5. Which describes a biochemical cycle? A. Matter is recycled through plants, animals, and nonliving things B. Matter is recycled through decomposers, animals, and viruses C. Matter is recycled through plants, bacteria, and fungi D. Matter is recycled through protists, animals, and fungi 6. What process is responsible for breaking down organic matter which is no longer living? A. decomposition C. respiration B. excretion D. photosynthesis 7. What is the main component of organic molecules? A. phosphorous C. nitrogen B. carbon D. carbon dioxide 8. How do plants use carbon? A. to make sugar B. to attract pollinators C. to make proteins and nucleic acids D. to transport water to their leaves 9. How is carbon recycled back into the atmosphere? A. evaporation C. respiration B. burning of fossil fuels D. choices B and C 10. What is the purpose of nitrogen fixing bacteria? A. to convert ammonia into nitrite and nitrate B. to convert nitrate into nitrogen gas C. to convert nitrite into nitrogen gas D. to convert ammonia into nitrogen gas 11. What is the main goal of the water cycle? A. to move salt water between the atmosphere and Earth B. to move dirty water between the atmosphere and Earth C. to move fresh water between the atmosphere and Earth D. to move toxic water between the atmosphere and Earth 12. What process contributes to the oxygen cycle by adding oxygen into the atmosphere? A. cellular respiration C. nitrogen fixation B. photosynthesis D. transpiration 13. What process contributes to the oxygen cycle by removing oxygen from the atmosphere? A. cellular respiration C. nitrogen fixation B. photosynthesis D. transpiration 14. What processes contribute to moving water through the water cycle? A. cellular respiration, transpiration, and nitrification B. photosynthesis, transpiration, and nitrification C. cellular respiration, transpiration, and photosynthesis D. transportation, fermentation, weathering 15. Why must organisms higher on an energy pyramid eat more in order to have enough energy to survive? A. because they are the largest organisms B. because of the law of 10% C. because they get hungry more often D. because they do the most moving C. Complete the following exercises. 1. Organisms that obtain food from dead organisms or waste material are called?____________ 2. If there were no decomposers, what might happen to all the dead organic matter on Earth? 3. Describe a sampling technique used to study a large area.