An organism's energy role is determined by how it obtains energy and how it interacts with the other living things in its ecosystem. Each of the organisms in an ecosystem ﬁlls the energy role of producer, consumer, or decomposer.
Plants, algae, and some bacteria can carry out photosynthesis. In this process, the organism uses the sun's energy to turn water and carbon dioxide into sugar molecules. An organism that can make its own food is a producer. Producers are the source of all the food in an ecosystem.
Other organisms cannot make their own food. They depend on producers for food and energy. An organism that obtains energy by feeding on other organisms is a consumer. Consumers are classiﬁed by what they eat. Consumers that eat only plants are called herbivores. Consumers that eat only animals are called carnivores. A consumer that eats both plants and animals is called an omnivore. A scavenger is a carnivore that feeds on the bodies of dead organisms. An organism may play more than one role in an ecosystem.
Organisms that break down wastes and dead organisms and return the raw materials to the environment are called decomposers. As decomposers obtain energy for their own needs, they return simple molecules to the environment to be used again by other organisms.
The movement of energy through an ecosystem can be shown in diagrams called food chains and food webs. A food chain is a series of events in which one organism eats another and obtains energy. The ﬁrst organism in a food chain is always a producer. The second organism, called a ﬁrst-level consumer, eats the producer. The next consumer, called a secondlevel consumer, eats the ﬁrst-level consumer. A food chain shows just one possible path of energy through an ecosystem.
Most producers and consumers are part of many food chains. A more realistic way to show the ﬂow of energy through an ecosystem is a food web. A food web consists of the many overlapping food chains in an ecosystem.
When an organism makes its own food or eats other organisms, it obtains energy. The organism uses most of this energy for its own life processes. Only some of the energy will be available to the next organism in the food web. A diagram called an energy pyramid shows the amount of energy that moves from one feeding level to another in a food web. The most energy is available at the producer level of the pyramid. As you move up the pyramid, each level has less available energy than at the level below. In general, only about 10 percent of the energy at one level of a food web is transferred to the next higher level. For this reason, most food webs have only three or four feeding levels, with few organisms at the highest level in a food web.
Matter is recycled in ecosystems. Matter includes water, oxygen, carbon, nitrogen, and many other substances. Three of the most important cycles of matter are the water cycle, the carbon-oxygen cycle, and the nitrogen cycle.
The water cycle is the continuous process by which water moves from Earth's surface to the atmosphere and back. The processes of evaporation, condensation, and precipitation make up the water cycle. Evaporation is the process by which molecules of liquid water absorb energy and change to the gas state. Water evaporates from Earth's surface and forms water vapor, a gas, in the atmosphere. The process by which a gas changes to a liquid is called condensation. When water vapor in the atmosphere cools, it turns back into tiny droplets of liquid water. As more water vapor condenses, the drops grow larger and heavier. Eventually, the heavy drops fall back to Earth as a form of precipitation-rain, snow, sleet, or hail.
Carbon is the building block for the matter that makes up the bodies of living things. In the ecosystem, the processes by which carbon and oxygen are recycled are linked. Producers, consumers, and decomposers play roles in recycling carbon and oxygen. Producers take in carbon dioxide from the atmosphere during photosynthesis. In this process, the producers use carbon from the carbon dioxide to produce other carbon-containing molecules. These molecules include sugars and starches. Consumers obtain energy from these molecules by breaking them down into simpler molecules. The consumers release water and carbon dioxide as waste products of the process. At the same time, producers release oxygen during photosynthesis. Other organisms take in oxygen from the atmosphere and use it in their life processes.
Like carbon, nitrogen is a necessary building block in the matter that makes up living things. In the nitrogen cycle, nitrogen moves from the air to the soil, into living things, and back into the air. Most organisms cannot use nitrogen gas in the air. Nitrogen gas is called "free" nitrogen because it is not combined with other kinds of atoms. Most organisms can use nitrogen only when it has been "ﬁxed," or combined with other elements to form nitrogen-containing compounds. The process of changing nitrogen gas into a usable form of nitrogen is called nitrogen ﬁxation. Most nitrogen ﬁxation is performed by certain kinds of bacteria. Some of these bacteria live in bumps called nodules on the roots of certain plants. Once the nitrogen has been ﬁxed, it can be used by organisms to build proteins and other complex substances. Decomposers break down these complex compounds. Decomposition returns simple nitrogen compounds to the soil. Certain types of bacteria break down the nitrogen compounds completely. These bacteria release free nitrogen back into the air, and the cycle starts again.
Different species of organisms live in different parts of the world. The study of where organisms live is called biogeography. In addition to studying where different species live today, biogeographers also study how these species spread into different parts of the world.
One factor that has affected how species are distributed is the motion of Earth's continents. The very slow motion of the continents is called continental drift. About 225 million years ago, all the continents were part of one large land mass. After millions of years of slow drifting, the continents have moved to their present locations. This has affected the development of species that were carried along with the continents.
The movement of organisms from one place to another is called dispersal. Dispersal can be caused by wind, water, and living things, including humans. Wind disperses seeds, the spores of fungi, and many small, light organisms. Water disperses ﬂoating objects and any organisms that are on them. Seeds are dispersed when an organism eats them and then deposits them somewhere else in its wastes. Water birds can carry algae or ﬁsh eggs from one pond to another. Some seeds have sticky burs that cling to organisms, which carry the seeds to a new place. Humans also disperse other species. As people move from place to place, they take organisms with them-sometimes intentionally and sometimes accidentally. Species that have naturally evolved in an area are referred to as native species. When an organism is carried into a new location by people, it is referred to as an exotic species.
The same species of organisms are not distributed throughout the world. Three factors that limit dispersal of a species are physical barriers, competition, and climate. Physical barriers include water, mountains, deserts, and canyons. Because these barriers are hard for most organisms to cross, they limit the movement of organisms.
When an organism enters a new area, it must compete for resources with the species already there. If the existing species are thriving, they may outcompete the new species. In this case, competition is a barrier to dispersal. However, if the new species is more successful than the existing species, it may displace the native species.
Differences in climate can limit dispersal. Climate is the typical weather pattern in an area over a long period of time. Climate is largely determined by temperature and precipitation. Species are adapted to survive in areas with speciﬁc climate conditions. Places with similar climates tend to have species that occupy similar niches.
A biome is a group of ecosystems with similar climates and organisms. The six major biomes that most ecologists study are the rain forest, desert, grassland, deciduous forest, boreal forest, and tundra. It is mostly the climate conditions-temperature and precipitation-in an area that determine its biome. The climate limits distribution of plants. In turn, the types of plants determine the kinds of animals that live there.
Tropical rain forests are warm and humid and found near the equator. The tall trees form a leafy roof called a canopy. A layer of shorter trees and vines forms an understory. Temperate rain forests are found farther north. They also receive a lot of rain but are cooler than tropical rain forests.
A desert is an area that receives less than 25 centimeters of rain each year. Deserts have large shifts in temperature during the day. Desert organisms are adapted to the lack of rain and to the extreme temperatures.
Most grasslands receive between 25 and 75 centimeters of rain each year and are populated mainly by grasses and other nonwoody plants. Grasslands that are located close to the equator are called savannas. Savannas receive as much as 120 centimeters of rain each year.
The trees found in deciduous forests, called deciduous trees, shed their leaves and grow new ones each year. These forests receive at least 50 centimeters of rain each year. Temperatures vary during the year. Some of the mammals enter a low-energy state called hibernation in the winter.
Boreal forests contain coniferous trees, which produce their seeds in cones and have leaves shaped like needles. Winters are long, very cold, and snowy. Summers are rainy and warm enough to melt all the snow.
The tundra is extremely cold and dry, often with no more precipitation than a desert. Most of the soil is frozen all year long. The frozen soil is called permafrost. Plants include low-growing mosses, grasses, and shrubs.
There are two types of aquatic ecosystems: freshwater ecosystems and saltwater ecosystems. Freshwater ecosystems include streams, rivers, ponds, and lakes. Because water absorbs sunlight, there is only enough light for photosynthesis near the surface. Algae are the most common producers in freshwater ecosystems.
The ocean has different zones. An estuary is found where the fresh water of a river meets the salt water of the ocean. Between the highest high-tide line and the lowest low-tide line is the intertidal zone. Below the low-tide line is the neritic zone, a region of shallow water over the continental shelf. Algae are the producers in most open-ocean food webs. Below the open ocean's surface zone is the deep zone, which is completely dark.
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