Trophic Levels and Biomes for AP Biology (page 2)
Practice problems for these concepts can be found at: Ecology Review Questions for AP Biology
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.
The various geographic regions of the earth that serve as hosts for ecosystems are known as biomes. Read through the following list so that you will be able to sprinkle some biome knowledge into an essay on ecological principles.
- Deserts. The driest land biome of the group, deserts experience a wide range of temperature from day to night and exist on nearly every continent. Deserts that do not receive adequate rainfall will not have any vegetative life. However, plants such as cacti seem to have adjusted to desert life and have done quite nicely in this biome, given enough water. Much of the wildlife found in deserts is nocturnal and conserves energy and water during the heat of the day. This biome shows the greatest daily fluctuation in temperature due to the fact that water moderates temperature.
- Savanna. Savanna grasslands, which contain a spattering of trees, are found throughout South America, Australia, and Africa. Savanna soil tends to be low in nutrients, while temperatures tend to run high. Many of the grazing species of this planet (herbivores) make savannas their home.
- Taiga. This biome, characterized by lengthy cold and wet winters, is found in Canada and has gymnosperms as its prominent plant life. Taigas contain coniferous forests (pine and other needle-bearing trees).
- Temperate deciduous forests. A biome that is found in regions that experience cold winters where plant life is dormant, alternating with warm summers that provide enough moisture to keep large trees alive. Temperate deciduous forests can be seen in the northeastern United States, much of Europe, and eastern Asia.
- Temperate grasslands. Temperate grasslands are found in regions with cold winters. The soil of this biome is considered to be among the most fertile of all. This biome receives less water than tropical savannas.
- Tropical forests. Found all over the planet in South America, Africa, Australia, and Asia, tropical forests come in many shapes and sizes. Near the equator, they can be rainforests, whereas in lowland areas that have dry seasons, they tend to be dry forests. Rainforests consist primarily of tall trees that form a thick cover, which blocks the light from reaching the floor of the forest (where there is little growth). Tropical rainforests are known for their rapid recycling of nutrients and contain the greatest diversity of species.
- Tundras. The tundra biome experiences extremely cold winters during which the ground freezes completely. The upper layer of the ground is able to thaw during the summer months, but the land directly underneath, called the permafrost, remains frozen throughout the year. This keeps plants from forming deep roots in this soil and dictates what type of plant life can survive. The plant life that tends to predominate is short shrubs or grasses that are able to withstand difficult conditions.
- Water biomes. Both freshwater and marine water biomes occupy the majority of the surface of the earth.
The general distribution of biomes on the earth's surface is shown in Figure 18.8.
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