Trophic Levels and Biomes for AP Biology (page 2)

Practice problems for these concepts can be found at: Ecology Review Questions for AP Biology

Trophic Levels

An ecosystem consists of the individuals of the community and the environment in which they exist. Organisms are classified as either producers or consumers.  The autotrophs you should recognize can be one of two types: photosynthetic or chemosynthetic autotrophs. Photoautotrophs (photosynthetic autotrophs) start the earth's food chain by converting the energy of light into the energy of life. Chemoautotrophs (chemosynthetic autotrophs) release energy through the movement of electrons in oxidation reactions.

The consumers of the world are the heterotrophs. They are able to obtain their energy only through consumption of other living things. One type of consumer is a herbivore, which feeds on plants for nourishment. Another consumer, the carnivore, obtains energy and nutrients through the consumption of other animals. A third consumer, the decomposer, or detritivore, obtains its energy through the consumption of dead animals and plants. Fungi, bacteria, earthworms, and vultures are prime examples of detritivores.

Here comes another hierarchy for you to remember. The distribution of energy on the planet can be subdivided into a hierarchy of energy levels called trophic levels. Take a look at the energy pyramid in Figure 18.6. The primary producers make up the first trophic level. The next trophic level consists of the organisms that consume the primary producers: the herbivores. These organisms are known as primary consumers. The primary consumers are consumed by the secondary consumers, or primary carnivores, that are the next trophic level. These primary carnivores are consumed by the secondary carnivores to create the next trophic level. This is an oversimplified yet important basic explanation of how trophic levels work. Usually there are only four or five trophic levels to a food chain because energy is lost from each level as it progresses higher.

The energy pyramid is not the only type of ecological pyramid that you might encounter on the AP Biology exam. Be familiar with a type of pyramid known as a biomass pyramid (Figure 18.7), which represents the cumulative weight of all of the members at a given trophic level. These pyramids tend to vary from one ecosystem to another. Like energy pyramids, the base of the biomass pyramid represents the primary producers and tends to be the largest.

There is also the pyramid of numbers, which is based on the number of individuals at each level of the biomass chain. Each box in this pyramid represents the number of members of that level. The highest consumers in the chain tend to be quite large, resulting in a smaller number of those individuals spread out over an area.

Two more terms to cover before moving onto the biomes are food chains and food webs. A food chain is a hierarchical list of who snacks on who. For example, bugs are eaten by spiders, who are eaten by birds, who are eaten by cats. A food web provides more information than a food chain—it is not so cut and dry. Food webs recognize that, for example, bugs are eaten by more than only spiders. Food webs can be regarded as overlapping food chains that show all the various dietary relationships.

Biogeochemical Cycles

One last topic to briefly cover before we wave goodbye to ecology is that of biogeochemical cycles. These cycles represent the movement of elements, such as nitrogen and carbon, from organisms to the environment and back in a continuous cycle. Do not attempt to become a master of these cycles, but you should understand the basics.

Carbon cycle. Carbon is the building block of organic life. The carbon cycle begins when carbon is released to the atmosphere from volcanoes, aerobic respiration (CO₂), and the burning of fossil fuels (coal). Most of the carbon in the atmosphere is present in the form of CO₂. Plants contribute to the carbon cycle by taking in carbon and using it to perform photosynthetic reactions, and then incorporating it into their sugars. The carbon is ingested by animals, who send the carbon back to the atmosphere when they die.

Nitrogen cycle. Nitrogen is an element vital to plant growth. In the nitrogen cycle (Figure 18.9), plants have nitrogen to consume thanks to the existence of organisms that perform the thankless task of nitrogen fixation—the conversion of N₂ to NH₃ (ammonia). The only source of nitrogen for animals is the plants they consume. When these organisms die, their remains become a source of nitrogen for the remaining members of the environment. Bacteria and fungi (decomposers) chomp at these organisms and break down any nitrogen remains. The NH₃ in the environment is converted by bacteria into NO₃ (nitrate), and this NO₃ is taken up by plants and then eventually by animals to complete the nitrogen cycle. Denitrification is the process by which bacteria themselves use nitrates and release N₂ as a product.

Water cycle. The earth is covered in water. A considerable amount of this water evaporates each day and returns to the clouds. Eventually, this water is returned to the earth in the form of precipitation. This process is termed the water cycle.

Practice problems for these concepts can be found at: Ecology Review Questions for AP Biology