Key Concepts
Ecosystem ecology emphasizes energy flow and chemical cycling- Ecosystems and Physical Laws (P. 1195) An ecosystem consists of all the organisms in a community and all the abiotic factors with which they interact. The laws of physics and chemistry apply to ecosystems, particularly in regard to the flow of energy. Energy is conserved, but degraded to heat during ecosystem processes.
- Trophic Relationships (P. 1185) Energy and nutrients pass from primary producers (autotrophs) to primary consumers (herbivores) and then to secondary consumers (carnivores). Energy flows through and ecosystem, entering as light and exiting as heat. Nutrient cycles within an ecosystem.
- Decomposition (PP. 1185-1186) Decomposition connects all trophic levels. Detritivores, mainly bacteria and fungi,
 recycle essential chemical elements by decomposing organic material and returning elements to inorganic reservoirs.
Physical and chemical factors limit primary production in ecosystems- Ecosystem Energy Budgets (PP 1186-1188) The energy assimilated during photosynthesis is a tiny fraction of the solar radiation reaching Earth, but primary production sets the spending limit for the global energy budget. Gross primary production is the total energy assimilated by an ecosystem in a given time period. Net primary production, the energy accumulated in autotroph biomass, equals gross primary production minus the energy used by the primary producers for respiration. Only net primary production is available to consumers.
- Primary Production in Marine and Freshwater Ecosystems (PP 1188-1190) In marine and freshwater ecosystems, light and nutrients limit primary production. Within the photic zone, the factor that most often limits primary production is a nutrient such as nitrogen or iron.
- Primary Production in Terrestrial and Wetland Ecosystems (PP. 1190-1191) In terrestrial and wetland ecosystems, climatic factors such as temperature and moisture affect primary production on a large geographic scale. More locally, a soil nutrient is often the limiting factor in primary production.
Energy transfer between trophic levels is usually less than 20% efficient- Production Efficiency (PP 1191-1193) The amount of energy available to each trophic level is determined by the net primary production and the efficiencies with which food energy is converted to b
 iomass at each link of the food chain. The percentage of energy transferred from one trophic level to the next, called trophic efficiency, is generally 5-20%. Pyramids of production, biomass, and numbers reflect low trophic efficiency. - The Green World Hypothesis (PP. 1193-1194) According to the green world hypothesis, herbivores consume a small percentage of vegetation because predators, disease, competition, nutrient limitations, and other factors keep their population in check.
Biological and geochemical processes move nutrients between organic and inorganic parts of the ecosystem- A General Model of Chemical Cycling (P. 1195) Gaseous forms of carbon, oxygen, sulfur, and nitrogen occur in the atmosphere and cycle globally. Other less mobile elements, including phosphorus, potassium, and calcium, cycle on a more localized scaled, at least over the short term. All elements cycle between organic and inorganic reservoirs.
- Biogeo
 chemical Cycles (P 1195-1198) Water moves in a global cycle driven by solar energy. The carbon cycle primarily reflects the reciprocal processes of photosynthesis and cellular respiration. Nitrogen enters ecosystems through atmospheric deposition and nitrogen fixation by prokaryotes, but most of the nitrogen cycling in natural ecosystems involves local cycles between organisms and soil or water. The phosphorus cycle is relatively localized. - Decomposition and Nutrient Cycling Rates (P. 1198) The proportion of a nutrient in a particular form and its cycling time in that form vary among ecosystems, largely because of differences in the rate of decomposition.
- Vegetation and Nutrient Cycling The Hubbard Brook Experimental Forest (PP. 1198-1199) Nutrient cycling is strongly regulated by vegetation. Long-term ecological research projects monitor ecosystem dynamics over relatively long periods of time. The Hubbard Brook study has shown that logging increases water runoff and can cause huge losses of minerals.
The human population is disrupting chemical cycles throughout the biosphere- Nutrient Enrichment (pp 1200-1201) Agriculture constantly removes nutrients from ecosystems, so large supplements are continually required. Considerable amounts of the nutrients in fertilizer pollute groundwater and surface-water aquatic ecosystems, where they can stimulate excess algal growth (cultural eutrophication).
- Acid Precipitation (pp 1201-1202) Combustion of fossil fuels is the main cause of acid precipitation. North American and European ecosystems downwind from industrial regions have been damaged by rain and snow containing nitric acid and sulfuric acid.
- Toxins in the Environment (pp. 1202-1203) Toxins can become concentrated in successive trophic levels of food webs. The release of toxic wastes has polluted the environment with harmful substances that often persist for long periods of time and become concentrated along the food chain bu biological magnification.
- Atmospheric Carbon Dioxide (pp. 1203-1205) Because of the burning of wood and fossil fuels and other human activities, atmospheric concentration of CO2 has been steadily increasing. The ultimate effects may include significant warming and other climate change.
- Depletion of Atmospheric Ozone (pp. 1205-1206) The ozone layer reduces the penetration of UV radiation through the atmosphere. Human activities, including release of chlorine-containing pollutants, are eroding the ozone layer, with dangerous results.
|
|
