Lesson: Chapter - 10

Ecology : Ecosystems

The dominant species in a climax community interact with and depend on nonliving (abiotic) factors in that environment. The most important abiotic factors in an environment, and on the SAT II Biology, are the chemical cycles, the availability of sunlight and oxygen, the character of the soil, and the regulation of these various phenomena. These abiotic elements, along with living matter, make up an ecosystem.

Video - Ecological Systems

Chemical Cycles

Inorganic elements such as carbon, nitrogen, and water pass through the environment in various forms. These elements are vital to life: they are consumed, excreted, respired, and otherwise utilized by living things. The passages of these elements between organisms and the abiotic environment are called the chemical cycles.

The Carbon Cycle

The carbon cycle begins when plants use CO2 from the air to produce glucose, which both animals and plants use in respiration and other life processes. Animals consume some of these plants as a source of food. Animals use what they can of the carbon matter and excrete the rest as waste that decays into CO2. Plant and animal respiration releases gaseous CO2. The carbon that plants and animals do use remains in their bodies until death. After death, decay sends the organic compounds back into the Earth and CO2 back into the atmosphere.

Video - Carbon Cycle

The Nitrogen Cycle

Nitrogen is a vital component of amino acids and nucleic acids, which are the fundamental units of proteins and DNA. The nitrogen cycle begins with inert atmospheric nitrogen (N₂), which is generally unusable by living organisms. Nitrogen-fixing bacteria in the soil or on the roots of legumes transform the inert nitrogen into nitrates (NO₃^–) and ammonium (NH₄^–). Plants take up these compounds, synthesize the 20 amino acids found in nature, and transform them into plant proteins; animals, typically only able to synthesize eight of the 20 amino acids, eat the plants and produce protein using the plant’s materials. Plants and animals give off nitrogen waste and death products in the form of ammonia (NH₃). One of two things can happen to the ammonia: (1) nitrifying bacteria transform the ammonia into nitrites (NO₂) and then to nitrates (NO₃^–), which reenter the cycle when they are taken up by plants; ( 2) denitrifying bacteria break down the ammonia to produce inert nitrogen (N₂).

The cycling of water and phosphorus are also important, as these substances are limited and vital to the life processes of most organisms.

Video - Nitrogen Cycle

The Water Cycle

The majority of the Earth’s water resides in the oceans and lakes, which act as water storage depots. This water escapes into the atmosphere through evaporation and condenses into clouds. Precipitation in the form of rain, snow, hail, etc., returns water to the ocean and lakes and also brings water to dry land. Water on land may either return to the oceans and lakes as runoff or penetrate into the soil and seep out as groundwater.

Oxygen, Sunlight, and Competition

Oxygen and sunlight are both vital to most forms of life. The relative abundance or lack of oxygen in a particular geographic or physical locale will create competition among organisms and drive evolution. Oxygen is abundant in the atmosphere and is therefore readily available to terrestrial species. But in order to penetrate aquatic environments, oxygen must be dissolved in water, where it exists in smaller concentration.

Like oxygen, sunlight is necessary to life for most organisms. In terrestrial species, competition for sunlight has pushed evolution of plants, with some plants growing broader leaves and branching to capture more rays. Sunlight cannot travel through water as easily as it can travel through air, so at great ocean depths, light is scarce. At these sorts of depths, autotrophic organisms have to find some way to produce energy that does not use light, such as chemosynthesis.

The Soil

The nature of soil determines which populations can be sustained in a given ecosystem. High acidity inhibits most plant growth but may be ideal for some plants that are better adapted to acidic soil. The texture of the soil and amount of clay it contains affect its ability to retain water, while the presence of minerals and decaying organic matter influences the types of plant life that can be supported.

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