CHAPTER OUTLINE
35.1 The Biotic Components of Ecosystems
An ecosystem possesses both abiotic and biotic components. The abiotic components include the
nonliving aspects. The biotic components are the various populations of organisms that form the
community.
Populations Within an Ecosystem
The populations within 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.
35.2 Energy Flow
The interconnecting paths of energy flow within ecosystems are represented by food webs. 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.
35.3 Global Biogeochemical Cycles
Chemicals circulate through both biotic and geological components of ecosystems, so they are
known as biogeochemical cycles. Chemical cycling may involve a reservoir, an exchange pool,
and the biotic community.
The Water Cycle
During the water (hydrologic) cycle, fresh water is distilled from salt water; it goes
through the processes of evaporation, condensation, and precipitation.
Human Activities
Fresh water is called a renewable resource but it is possible to run out of fresh
water when the available supply is not adequate or has become polluted.
The Phosphorus Cycle
Phosphorus moves from rocks on land to the oceans, where it gets trapped in sediments,
and then moves back onto land following a geological upheaval. The phosphorus cycle is
called a sedimentary cycle because it does not enter the atmosphere.
Human Activities
Humans boost the amount of available phosphate by mining phosphate 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, which
can lead to an algal bloom.
The Nitrogen Cycle
Nitrogen gas makes up about 78% of the atmosphere, but it is unavailable for plants to
use.
Nitrogen Fixation
Nitrogen fixation occurs when nitrogen gas is converted to ammonium ions, a
form plants can use. Some cyanobacteria and free-living bacteria are able to fix
atmospheric nitrogen. Other nitrogen-fixing bacteria live in nodules on the roots
of legumes.
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Nitrification
Plants can also use nitrates as a source of nitrogen. The production of nitrates
during the nitrogen cycle is called nitrification.
Denitrification
Denitrification is the conversion of nitrate back to nitrogen gas.
Human Activities
Human activities nearly double the nitrogen fixation rate when fertilizers are
produced from N2. 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 are the remains of plants and animals
that were subjected to physical processes before decomposition could occur,
transforming them into coal, oil, and natural gas.
Human Activities
Due to human activities, more carbon dioxide is being released into the
atmosphere than is being removed. This is due to the burning of fossil fuels
and the destruction of forests. This increase in carbon dioxide is causing a rise in
temperature called global warming.
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