WILLIAM SHEN BLOCK 6 MS VERSTEEG
BIOLOGY AP 12
CHAPTER 55: ECOSYSTEMS
What is an Ecosystem?
An ecosystem is the sum of all living organisms living in an area as large as a forest or as small as a fallen log, in addition to the abiotic factors that affect them.
General Review
Conservation of energy — An ecosystem recieves energy input via sunlight and releases it as heat. Thus the toal amount of energy lost as heat must equal the solar energy that plants recieve and then store in chemicals.
Conservation of mass — Chemicals continuously cycle through the biosphere, moving between ecosystems as inputs and outputs.
Primary production — The amount of light energy converted to chemical energy by autotrophs during a given time period, classified in two ways:
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Gross primary production (GPP) is the total conversion
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Net primary production (NPP) is the GPP minus the energy that is used by the primary producers for their own respiration. NPP is often around one-half of GPP.
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Actual evapotranspiration is the annual amount of water which is transpired by plants and evaporated in a terrestrial ecosystem.
Light limitation — Limited lght is a large factor in the unproductivity (in terms of primary production) of the oceans. Around half of solar radiation is absorbed in the first 15 meters of water
Limiting nutrient — An element that must be added for production to increase. Often this is either nitrogen or phosphorus, which are only found in low concentrations in the photic zone of the ocean.
Eutrophication — The enrichment of an aquatic ecosystem with chemical nutrients (often nitrogen and/or phosphorus). This can lead to algae bloom sand is caused either by human interferance or aging.
Green world hypothesis — Explains why herbivores consume less than one-sixth of the global net primary production by plants. it states that there are many natural limitations in place such as plant spines, chemicals, abiotic factors, competition, predation, and low nutreint concentrations.
Trophic Levels
The autotrophs which ultimately support all others are called primary producers. Herbivores which each plants are classified as primary consumers, while carnivores that eat herbivores are secondary consumers. Finally, carnivores which each other carnivores are tertiary consumers.
Detritivores (or decomposers) are organisms which feed on detritus: the remains of dead organisms or other nonliving organic material. Prokaryotes and fungi are especially important in the breakdown of organic material into inorganic nutrients which primary producers can use.
Notable Experiment
John Ryther and William Dunstan experimented with the limiting nutreint of phytoplankton off the coast of Lon Island, New York. They found that nitrogen has a far greater effect than phosphorus, and thus nitrogen is the limiting nutrient.
Secondary Production
Secondary production is the amount of chemical energy in a consumer's food that is converted into their own biomass. Production efficiency describes this percentage of energy assimilated rather than used for respiration. Trophic efficiency is the percentage of production (generally around 10%) transferred from one trophic level to the next.
Producers often are able to replace their biomass rapidly via reproduction and fast growth, and so they have a very short turnover time. This means that the biomass of primary consumers may outweigh that of the producers, but the pyramid of production remains bottom-heavy.
NUTRIENT CYCLES
Geochemical Cycles and Human Interference
Biogeochemical cycles are nutrient cycles in ecosystems, which involve both biotic and abiotic components. Homan activities now dominate most cycles on Earth, removing nutrients from one part of the bioshpere and adding them to another. Excess nutrients produced during agriculture go beyond the critical load, which is the amount of nitrogen or phosphorus that an ecosystem's plants can handle.
The burning of fossil fuels leads to acid precipitation and increases earth's natural greenhouse affect to an unhealthy extent. Manmade chemicals in the atmosphere are depleting the UV-filtering ozone layer, and toxins all over the biosphere become more concentrated in successive trophic levels as a result of biological magnification.
Water cycle — Of all the water present in the biosphere, 97% lies in the oceans, 2% is found in glaciers and ice caps, and 1% is in lakes, rivers, and groundwater. Very little is found in the atmosphere.
Carbon cycle — Photosynthesis and cellular respiration cycle carbon around the atmosphere. Other major reservoirs include fossil fuels, soils, marine sediments, the ocean, and living organisms. Decomposition deposits carbon in the ground, while volcanoes and human activity release substancial amounts of it.
Terrestrial Nitrogen Cycle — Plants use two inorganic forns of nitrogen with the help of nitrogen-fixing, nitrifying, and denitrifying bacteria. The main reservoir of nitrogen is the atmosphere, which is 80% nitrogen gas.
Phosphorus Cycle — The most biologically important inorganic form of phosphorus is phosphate, used by plants. Phosphorus accumulates in marine sedimentary rocks, soils, and the oceans. weathering of rocks gradually adds phosphate to soil, and is then taken up by producers or leaches into runoff water.
A general model of nutrient cycling is shown above.