Energy Flow in an Ecosystem
Energy in an ecosystem is conserved. In other words, energy is neither created nor destroyed but changed from one form into another. As energy moves through organisms, some energy is absorbed by organisms, and some energy is released by those organisms as heat.
Organisms Move Energy and Matter
Living things that can capture matter and energy from abiotic sources are called producers. Producers, such as plants and green algae, use photosynthesis to turn the Sun’s energy, water, and carbon dioxide in the air into complex organic compounds. Other producers, such as the bacteria living near deep-sea vents, can capture energy from other materials. Producers are also called autotrophs, or “self-feeders,” because they make organic compounds from inorganic compounds and do not need to eat food.
Most organisms have to eat other organisms to obtain the energy they need to survive. Organisms that eat other organisms are called consumers, or heterotrophs. Herbivores are consumers that eat only autotrophs. Carnivores are consumers that eat only other heterotrophs. Consumers that eat both autotrophs and heterotrophs are called omnivores.
When an organism dies, small organisms such as fungi, bacteria, and protozoans break down the complex organic molecules in its body for food. These heterotrophs are called decomposers. Decomposers are important parts of every ecosystem. They release the elements stored in dead tissues into the environment, where they can be reused.
Food Chains and Food Webs
Ecologists make models to study how energy and matter flow through an ecosystem. The simplest model of the flow of matter and energy in an ecosystem is called a food chain. A food chain can be constructed by writing the names of the organisms in an ecosystem according to the role each plays and drawing arrows between them. The arrows show that one organism is eaten by the next. For example, a food chain from a desert ecosystem could be shown as the following:
seeds ↝ kangaroo rats ↝ rattlesnakes
Each step in a food chain is called a trophic level In the food chain shown, seeds make up the first trophic level, and the next trophic level contains the kangaroo rats that eat the seeds. Organisms that eat plants are called first-order heterotrophs, or primary consumers. In this example, the final trophic level contains the rattlesnakes that eat the kangaroo rats. Organisms that eat first-order heterotrophs are called second-order heterotrophs, or secondary consumers.
A food chain can illustrate and help explain one feeding pathway, but it does not show the complex feeding relationships in a real ecosystem. Biological communities have many species at each trophic level. Organisms may also eat more than one kind of food. These relationships can be drawn as a series of food chains, or they can be connected to form a food web. A food web is a model that shows all the possible feeding relationships at each trophic level.
Food chains and food webs show ecologists “who eats whom” in a community. Another type of model, an energy pyramid, helps ecologists see the relationships among trophic levels. Each layer of an energy pyramid represents a different trophic level. Autotrophs make up the base of the pyramid, and higher trophic levels are stacked above them.
One type of energy pyramid shows how energy moves through an ecosystem. Energy is lost as it moves through heterotrophs. Each trophic level contains less energy than the level below it. This happens because a heterotroph uses most of the energy it gets from food as fuel for life processes. Only about 10 percent of the energy a heterotroph takes in is stored in new body tissues. Most of the energy is lost as heat.
Another way to show the stored energy in each trophic level is with a pyramid of biomass. Biomass is the total mass of living tissue in a trophic level.
A pyramid of numbers can show how many organisms are in each trophic level. Typically population sizes get smaller in higher trophic levels. There are more autotrophs than herbivores; and there are more herbivores than carnivores.
Cycles of Matter
Have you ever recycled a plastic bottle and wondered what happens after you dropped it off? After you send the bottle to the recycling center, it is melted down and reused in a new product. The plastic may become part of an automobile, a cell phone, or a fleece jacket, but it is still the same plastic that made up the original bottle.
This process is similar to the movement of matter through an ecosystem. Water molecules and elements such as nitrogen and carbon constantly move among the ocean, the atmosphere, and terrestrial ecosystems. Energy is converted to heat as it passes through a community, but matter is recycled and reused over and over again. The recycling systems of an ecosystem are called biogeochemical cycles.
Oxygen is an element that is important to many life processes. Carbon and oxygen make up molecules essential for life, including carbon dioxide and simple sugar.
In the oxygen cycle, green plants and algae convert carbon dioxide and water into carbohydrates and release oxygen gas into the air during a process called photosynthesis. The carbohydrates are used as a source of energy for all organisms in the food web. Carbon dioxide is recycled as organisms take in the oxygen from photosynthesis and release it as carbon dioxide through cellular respiration.
Oxygen also participates in parts of the carbon, nitrogen, and phosphorus cycles by combining with these elements and cycling with them through the biosphere. When oxygen cycles with these other nutrients, it is indirectly transferred through an ecosystem.
The Carbon Cycle
Carbon is the main building block of molecules in living things. It constantly moves among the ocean, the atmosphere, and Earth’s ecosystems. Every carbon atom in a living thing was, at some point in the past, part of a carbon dioxide molecule in the air. Autotrophs use carbon dioxide from the air to make the complex organic molecules found in their tissues. Heterotrophs that eat autotrophs break down the autotroph’s molecules. Some of the molecules are broken down for energy, and the carbon in these molecules then returns to the air as carbon dioxide. Heterotrophs use other molecules to form their tissues. When a heterotroph dies and decays, some of the carbon in its tissues returns to the air as carbon dioxide. The carbon dioxide given off by all living things is reused by autotrophs.
The Water Cycle
The water cycle starts with evaporation. Evaporation is the process by which heat from the Sun turns liquid water on Earth’s surface into water vapor. The water vapor moves into the atmosphere, where it cools and turns into liquid droplets that form around dust particles in the air. The process by which water vapor cools and becomes a liquid is called condensation. The liquid droplets produce clouds, and as more water vapor condenses, more water droplets form. When the drops get heavy enough, they fall back to Earth as precipitation, which takes the form of rain, snow, sleet, or hail. Some water falls into lakes and oceans. Water that falls on land may flow to the ocean in streams and rivers or soak deep into the ground to form groundwater.
Water also moves through organisms. Plants pull water out of the ground through their roots and lose it by evaporation from their leaves during transpiration. Animals drink water and lose it when they urinate, sweat, and breathe.
The Nitrogen Cycle
Nitrogen makes up most of the air you breathe, but plants and animals cannot use it directly from the atmosphere. Most of the nitrogen in organisms comes from bacteria that convert atmospheric nitrogen into usable forms in a process called nitrogen fixation.
Plants absorb nitrogen compounds made by bacteria and use the nitrogen to build proteins and other molecules. When herbivores eat plants, they reuse the nitrogen in their own tissues. The nitrogen is reused every time one animal eats another. At each step, some nitrogen leaves an animal through that animal’s urine. Animal wastes return nitrogen compounds to the soil, where plants and bacteria reuse them. Dead and decomposing organisms also leave nitrogen compounds in the soil, and soil bacteria convert these compounds into nitrogen gas, which returns to the air.
Fertilizers with nitrogen help plants grow and thrive. However, nitrogen also dissolves in water and can be washed away. If excess nitrogen enters aquatic (water) ecosystems, algae may grow uncontrollably. As the algae dies, decomposition consumes oxygen in the water. As the oxygen is depleted, marine animals and other organisms may suffocate and die. This kind of nutrient pollution is called eutrophication.
The Phosphorous Cycle
Most of Earth’s phosphorous is found in sedimentary rock. When it rains, tiny bits of phosphates are dissolved from the rocks and spread through soil and water. Plants take up the phosphates from the soil. The phosphates then move from plants to animals when herbivores eat plants and carnivores eat herbivores. The phosphates absorbed by animal tissue eventually return to the soil through body wastes or from the decomposition of dead organisms. Phosphorus is not highly soluble. It binds tightly to particles in soil and reaches water by traveling with eroded soil. Phosphates also enter waterways through fertilizer runoff, sewage seepage, natural mineral deposits, and wastes from other industrial processes. These phosphates settle on ocean floors and lake bottoms.
In lakes, excessive phosphorus is a pollutant. Phosphate stimulates the growth of algae, similar to nitrogen. As the excess algae dies, decomposition consumes the oxygen in the water, causing a lack of oxygen for other organisms. The result can be eutrophication.