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  1. 1. IntroductionIn topic 9d, an ecosystem was defined as a dynamic entity composed of abiological community and its associated abiotic environment. Often thedynamic interactions that occur within an ecosystem are numerous andcomplex. Ecosystems are also always undergoing alterations to theirbiotic and abiotic components. Some of these alterations begin first witha change in the state of one component of the ecosystem which thencascades and sometimes amplifies into other components because ofrelationships.In recent years, the impact of humans has caused a number of dramaticchanges to a variety of ecosystems found on the Earth. Humans use andmodify natural ecosystems through agriculture, forestry, recreation,urbanization, and industry. The most obvious impact of humans onecosystems is the loss of biodiversity. The number of extinctions causedby human domination of ecosystems has been steadily increasing sincethe start of the Industrial Revolution. The frequency of speciesextinctions is correlated to the size of human population on the Earthwhich is directly related to resource consumption, land-use change, andenvironmental degradation. Other human impacts to ecosystems includespecies invasions to new habitats, changes to the abundance anddominance of species in communities, modification of biogeochemicalcycles, modification of hydrologic cycling, pollution, and climaticchange.Major Components of EcosystemsEcosystems are composed of a variety of abiotic and biotic componentsthat function in an interrelated fashion. Some of the more importantcomponents are: soil, atmosphere, radiation from the Sun, water, andliving organisms.Soils are much more complex than simple sediments. They contain amixture of weathered rock fragments, highly altered soil mineralparticles, organic matter, and living organisms. Soils provide nutrients,water, a home, and a structural growing medium for organisms. Thevegetation found growing on top of a soil is closely linked to thiscomponent of an ecosystem through nutrient cycling.The atmosphere provides organisms found within ecosystems withcarbon dioxide for photosynthesis and oxygen for respiration. The
  2. 2. processes of evaporation, transpiration, and precipitation cycle waterbetween the atmosphere and the Earths surface.Solar radiation is used in ecosystems to heat the atmosphere and toevaporate and transpire water into the atmosphere. Sunlight is alsonecessary for photosynthesis. Photosynthesis provides the energy forplant growth and metabolism, and the organic food for other forms of life.Most living tissue is composed of a very high percentage of water, up toand even exceeding 90%. The protoplasm of a very few cells can surviveif their water content drops below 10%, and most are killed if it is lessthan 30-50%. Water is the medium by which mineral nutrients enter andare translocated in plants. It is also necessary for the maintenance of leafturgidity and is required for photosynthetic chemical reactions. Plants andanimals receive their water from the Earths surface and soil. The originalsource of this water is precipitation from the atmosphere.Ecosystems are composed of a variety of living organisms that can beclassified as producers, consumers, or decomposers. Producers orautotrophs, are organisms that can manufacture the organic compoundsthey use as sources of energy and nutrients. Most producers are greenplants that can manufacture their food through the process ofphotosynthesis. Consumers or heterotrophs get their energy andnutrients by feeding directly or indirectly on producers. We candistinguish two main types of consumers. Herbivores are consumers thateat plants for their energy and nutrients. Organisms that feed onherbivores are called carnivores. Carnivores can also consume othercarnivores. Plants and animals supply organic matter to the soil systemthrough shed tissues and death. Consumer organisms that feed on thisorganic matter, or detritus, are known as detritivores or decomposers.The organic matter that is consumed by the detritivores is eventuallyconverted back into inorganic nutrients in the soil. These nutrients canthen be used by plants for the production of organic compounds.The following graphical model describes the major ecosystemcomponents and their interrelationships (Figure 9j-1).
  3. 3. Figure 9j-1: Relationships within an ecosystem.Energy and Matter Flow in EcosystemsMany of the most important relationships between living organisms andthe environment are controlled ultimately by the amount of availableincoming energy received at the Earths surface from the Sun. It is thisenergy which helps to drive biotic systems. The Suns energy allowsplants to convert inorganic chemicals into organic compounds.Only a very small proportion of the sunlight received at the Earthssurface is transformed into biochemical form. Several studies have beencarried out to determine this amount. A study of an Illinois cornfieldreported that 1.6% of the available solar radiation was photosytheticallyutilized by the corn. Other data suggests that even the most efficientecosystems seldom incorporate more than 3% of the available solarinsolation. Most ecosystems fix less than 1% of the sunlight available forphotosynthesis.Living organisms can use energy in basically two forms: radiant orfixed. Radiant energy exists in the form of electromagnetic energy,such as light. Fixed energy is the potential chemical energy found inorganic substances. This energy can be released through respiration.Organisms that can take energy from inorganic sources and fix it intoenergy rich organic molecules are called autotrophs. If this energycomes from light then these organisms are called photosynthetic
  4. 4. autotrophs. In most ecosystems plants are the dominant photosyntheticautotroph.Organisms that require fixed energy found in organic molecules for theirsurvival are called heterotrophs. Heterotrophs who obtain their energyfrom living organisms are called consumers. Consumers can be of twobasic types: Consumer and decomposers. Consumers that consume plantsare know as herbivores. Carnivores are consumers who eat herbivoresor other carnivores. Decomposers or detritivores are heterotrophs thatobtain their energy either from dead organisms or from organiccompounds dispersed in the environment.Once fixed by plants, organic energy can move within the ecosystemthrough the consumption of living or dead organic matter. Upondecomposition the chemicals that were once organized into organiccompounds are returned to their inorganic form and can be taken up byplants once again. Organic energy can also move from one ecosystem toanother by a variety of processes. These processes include: animalmigration, animal harvesting, plant harvesting, plant dispersal of seeds,leaching, and erosion. The following diagram models the various inputsand outputs of energy and matter in a typical ecosystem (Figure 9j-2). Figure 9j-2: Inputs and outputs of energy and matter in a typical ecosystem.