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An ecosystem is a community of organisms interacting within a particular physical environment. Stated another way, an ecosystem is a community plus its abiotic factors, e.g. soil, rain, temperatures, etc. I. The earth
II. Biotic components of an ecosystem
B. Autotrophic organisms - capture energy (e.g., sunlight) and incorporate it into organic compounds; therefore they are also called producers 1. Chemoautotrophs are bacteria that obtain energy from oxidation of inorganic compounds such as ammonia, nitrites, and sulfides; they synthesize carbohydrates and are found in cave communities and ocean depths 2. Photoautotrophs possess chlorophyll and carry on photosynthesis 3. Autotrophs are at the beginning or bottom of a food chain 4. In terrestrial ecosystems, producers are mostly plants; in aquatic ecosystems, dominant producers are algae D. Heterotrophic organisms - need a source of preformed nutrients and consume tissues of other organisms 1. Herbivores are animals that feed directly on green plants (e.g., caterpillars, zooplankton, etc.) 2. Carnivores are animals that eat other animals (e.g., lion, hawks, etc.) 3. Sequences of carnivores that feed in a chain can be labeled primary, secondary and tertiary consumers:
5. Detritivores - animals (e.g., earthworms) that feed on detritus - the decomposing products of organisms. Decomposition returns nutrients back to the soil. Some also recognize decomposers but there is little distinction between them and detritivores I. Energy flow and nutrient cycling A. Ecosystems 1. Ecosystems are dependent upon solar energy flow and finite pools of nutrients 2. Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur make up over 98 percent of body weight of life 3. Plants can make use of inorganic nutrients while animals must take in organic nutrients 4. Primary productivity is the total amount of energy an ecosystem's producers capture within plant material over a length of time a. Soil, climate, and other factors affect gross primary productivity b. Plants must use organic molecules to fuel their own cellular respiration, about 55% c. 55% of gross primary productivity is available to heterotrophs; this is net primary productivity 5. Energy flow in an ecosystem is a consequence of two fundamental laws of thermodynamics: a. First law of thermodynamics - energy can neither be created nor destroyed; it can only be changed from one form of energy to another. b. Second law of thermodynamics - when energy is transformed from one form to another, there is always some loss of energy from the system, usually as low grade heat 6. Therefore, ecosystems are unable to function unless they receive a constant input of energy a. Primary source of energy for ecosystems is sunlight, which photosynthesizers use to produce organic food b. All energy content of organic matter is eventually lost to environment as low grade heat c. Only a small portion of food taken in by heterotrophs becomes available to the next consumer d. Secondary productivity - the portion of energy converted into increased body weight B. Food webs 1. The complex feeding relationships that exist in nature are called food webs 2. A grazing food web begins with leaves, stems and seeds eaten by herbivores and omnivores 3. A detritus food web begins with detritus, followed by decomposers (including bacteria and fungi) 4. Detritus food chains are connected to a grazing food chain when consumers of a grazing food chain feed on the decomposers of the detrital food chain 5. In some ecosystems, less than 1% of energy may move through the grazing food web while over 99% moves through the detritus food web C. Trophic levels 1. A food chain represents passage of energy through populations in a community 2. Trophic level - a feeding level of one or more populations in a food web; those organisms in an ecosystem that are the same number of food chain steps from the energy input into the system: a. First trophic level - primary producers b. Second trophic level - all the primary consumers c. Third trophic level - all the secondary consumers E. Ecological pyramids 1. An ecological pyramid shows the trophic structure of an ecosystem as a graph representing biomass, organism number, or energy content of each trophic level in a food web 2. The base of the pyramid represents the producer trophic level, and from there the consumer trophic level is stacked, with the apex representing the highest consumer trophic level 3. A pyramid of numbers is based on the number of organisms in each trophic level 4. A pyramid of biomass is based on the weight (biomass) of organisms at each trophic level at one time a. Usually a large mass of plants supports a medium mass of herbivores and a small mass of carnivores 5. A pyramid of energy is based on the total amount of energy in each trophic level and is always pyramidal 6. In general, about 10 percent of energy at a particular trophic level is incorporated into the next trophic level a. Thus, 1,000 kg (or kcal in an energy pyramid) of plant material converts to 100 kg of herbivore tissue, which converts to 10 kg of first carnivores, which can support 1 kg of second level carnivores b. This rapid loss of energy is the reason food chains have from three to four links, rarely five c. This rapid loss of energy is also the reason there are few large carnivores
IV. Global biogeochemical cycles
D. Hydrologic (water) cycle 1. In the (hydrologic) cycle, freshwater evaporates and condenses on the earth a. Oceans are the greatest source of evaporated water, but water also evaporates from bodies of freshwater, and from land and plants (transpiration) 2. Evaporation of water from the oceans leaves behind salts 3. Rainfall that permeates the earth forms a water table at the surface of the groundwater 4. An aquifer is an underground storage of freshwater in porous rock, trapped by impervious rock strata 5. Freshwater, which makes up only about 3 percent of the world's supply of water, is called a renewable resource 6. Freshwater can become unavailable when consumption exceeds supply and/or is polluted so it is not usable E. Carbon cycle 1. The exchange pool for the carbon cycle is the atmosphere 2. Photosynthesis removes CO2 from the atmosphere; respiration and combustion add CO2 to the atmosphere 3. CO2 from the air combines with water to produce bicarbonate (HCO3), which is a source of carbon for aquatic producers, primarily algae 4. Similarly, when aquatic organisms respire, the CO2 they release combines with water to form HCO3 5. The amount of bicarbonate in the water is in equilibrium with the amount of CO2 in the air 6. The reservoir for the carbon cycle is largely composed of organic matter, calcium carbonate in shells, and limestone, as well as fossil fuels 7. The carbon cycle involves: a. Short term cycling of carbon through living organisms via photosynthesis, respiration and decay F. Nitrogen cycle
Eutrophication (over enrichment) results from fertilizer runoff; when rampant algae dies off, decomposers use up available oxygen during cellular respiration, and this results in a massive fish kill G. Phosphorus cycle 1. Weathering makes phosphate ions available to plants from the soil 2. Some of this phosphate runs off into aquatic ecosystems where algae incorporate it into organic molecules 3. The phosphate that is not taken up by aquatic phototrophs is incorporated into sediments in the oceans 4. Sediment phosphate becomes available when a geological upheaval exposes sedimentary rocks to weathering 5. The phosphate taken up by producers is incorporated into a variety of organic compounds 6. Animals eat producers and incorporate some of phosphate into teeth, bones, and shells that take long to decompose 7. Death and decay of organisms and decomposition of animal wastes makes phosphate ions available again 8. Because available phosphate is generally taken up quickly, it is usually a limiting nutrient in most ecosystems V. Causes of pollution
VI. Global warming
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