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CHAPTER 34: NATURE OF ECOSYSTEMS
LECTURE OUTLINE
34.1 The Biotic Components of Ecosystems
An ecosystem possesses both abiotic and biotic components. The abiotic components include
resources and conditions. The biotic components are the various populations of organisms that
form a community.
Populations of an Ecosystem
The populations of an ecosystem are categorized according to their food source:
autotrophs (producers), heterotrophs (consumers such as herbivores, carnivores, and
omnivores), and decomposers.
Energy Flow and Chemical Cycling
Every ecosystem is characterized by two fundamental phenomena: energy flow and
chemical cycling. Energy flows from solar energy to living organisms. Chemicals cycle
between the physical environment and living organisms.
34.2 Energy Flow
The interconnecting paths of energy flow are represented by a food web. A grazing food web
begins with a producer. A detrital food web begins with detritus.
Trophic Levels
Diagrams that show a single path of energy flow are called food chains. A trophic level
is composed of all the organisms that feed at a particular link in a food chain.
Ecological Pyramids
In general, only about 10% of the energy of one trophic level is available to the next
level. The large energy losses that occur between successive trophic levels are
sometimes depicted as an ecological pyramid. Biomass is the number of organisms
multiplied by their weight.
34.3 Global Biogeochemical Cycles
Chemicals cycle through both biotic and geological components. Chemical cycling may involve
a reservoir, an exchange pool, and the biotic community.
The Water Cycle
The reservoir for the water cycle is the ocean. Evaporation and precipitation play
important roles.
Human Activities
Fresh water is called a renewable resource but it is possible to run out of fresh
water if the available supply is not adequate or has become polluted.
The Phosphorus Cycle
Phosphorus moves from rocks on land to the oceans, where it is trapped into sediments,
and then back onto land following a geological upheaval. The phosphorus cycle is called
a sedimentary cycle.
Human Activities
Humans boost the amount of available phosphate by mining ores and using them
to make fertilizers, animal feed supplements, and detergents. Too much
phosphate in a body of water can result in cultural eutrophication.
The Nitrogen Cycle
Nitrogen gas makes up about 78% of the atmosphere, but plants cannot use nitrogen gas.
Nitrogen Fixation
Nitrogen fixation occurs when nitrogen gas is converted to ammonium ions.
Some cyanobacteria and free-living bacteria in the soil can do this. Other
nitrogen-fixing bacteria live in nodules of the roots of legumes.
Nitrification
Plants can also use nitrates as a source of nitrogen. Nitrogen gas can be
converted to nitrate in the atmosphere via cosmic radiation, meteor trails, and
lightning or ammonium ions in the soil can be converted to nitrate by soil
bacteria.
Denitrification
Denitrification is the conversion of nitrate back to nitrogen gas.
Human Activities
Human activities significantly alter the transfer rates in the nitrogen cycle by
producing fertilizers from nitrogen gas. Fertilizer use results in the release of
nitrous oxide, a greenhouse gas.
The Carbon Cycle
Plants take up carbon dioxide and incorporate carbon into food. When all organisms
respire, a portion of the carbon is returned to the atmosphere as carbon dioxide.
Reservoirs Hold Carbon
Living and dead organisms contain organic carbon and serve as one of the
reservoirs for the carbon cycle. Fossil fuels, limestone, and calcium carbonate
shells of marine organisms are carbon reservoirs.
Human Activities
Due to human activities, more carbon dioxide is being released into the
atmosphere than is being removed. This is due to the combustion of fossil fuels
and the destruction of forests. This is causing a rise in the temperature of Earth
called global warming.