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Ecosystems Review Sheet
Courtesy of: Pamela J. Shlachtman, Miami Palmetto High School
Ecosystems - Consist of all the organisms in a community as well as the abiotic factors with which
they interact.
Trophic Levels and Food Webs
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each ecosystem has a trophic structure of different feeding relationships that determines
the route of energy flow and pattern of chemical cycling.
species in an ecosystem are divided into trophic levels based on their main source of
nutrition.
The trophic level that ultimately supports all others is the autotrophic or primary
producers.
All other organisms in the ecosystem are consumers.
detritovores derive their energy from organic wastes.
The pathway along which food/energy is transferred from trophic level to trophic level is
the food chain (usually woven together into complex food webs).
Energy Flow
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The global energy budget
o Much of the solar radiation that reaches the biosphere lands on bare ground and
bodies of water where it is absorbed or reflected.
o Only a small amount actually strikes algae and the leaves of plants.
o Of the light that does reach algae and the leaves of plants, only about 1% is used in
photosynthesis.
Primary Productivity
o The rate at which light energy is converted into chemical energyby autotrophs.
o The total of the productivity is known as gross primary productivity (GPP).
o Net primary productivity (NPP) is equal to the GPP minus the energy used for
respiration and is the storage of chemical energy available to consumers in an
ecosystem.
o Biomass is the dry weight of organic matter comprising a group of organisms in a
particular habitat (doesn't count water because water contains no usable energy).
o Standing crop biomass is the total biomass ofplants present in an area at any given
time.
o Tropical rainforests are among the most productive terrestrial ecosystems and
contribute a large proportion of the planet's overall productivity.
o The factors most important in limiting productivity depend on the type of ecosystem
and on seasonal changes in the environment.
o Inorganic nutrients may also be important in limiting productivity.
o Productivity in the seas is usually greatest in the shallow waters near the continents
and coral reefs where abundant nutrients, light and heat stimulate growth.
Energy Transfers and Ecological Pyramids
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The rate at which an ecosystem's comcumers convert the chemical energy of the
food they eat into their own new biomass is called secondary productivity.
Productivity eventually declines with each transfer of energy through the trophic
levels.
This decline is partially due to the first law of thermodynamics (total energy must
be conserved, but working organisms cannot avoid converting some of it to heat,
which is dissapated from the ecosystem).
Thus, not all of the energy stored as biomass by NPP can be converted into
secondary productivity.
Ecological efficiency is the ratio of NPP at one trophic level to NPP at the level
below (approximately 90% of energy available never transfers to the next level!).
This loss can be represented diagramatically with a pyramid of productivity, in which
the trophic levels are stacked as blocks with primary producers at the bottom.
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Biomass pyramids - each tier isproportional to the standing crop biomass in a trophic
level at any given time.
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Pyramid of numbers - the size of each tier is proportional to the number of
individual organisms present in each trophic level.
Chemical Cycling
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Biogeochemical cycles - various nutrient cycles that involve both biotic and abiotc
components
General Model
o Most nutrients accumulate in four compartments with each compartment defined by
two characteristics:
o whether or not it contains organic or inorganic materials
o whether or not the materials are directly available for use by organisms
o One compartment of organic material is composed of the living organisms
themselves.
The Carbon Cycle
o The reciprocal processes of photosynthesis and respiration provide a link between
the atmosphere and the terrestrial environments.
o Carbon recycles relatively fast because plants have a high demand for CO2.
o Decomposition eventually recycles carbon to the atmosphere.
o The amount of CO2 present in the atmosphere varies with the seasons.
o Burning of fossil fuels adds CO2.
The Nitrogen Cycle
o The atmosphere is almost 80% N2 but plants can't assimilate this form of nitrogen.
o Only certain types of prokaryotes can fix N2 (reduce it from gas to ammonia) in a
process called ammonification which can be used to make amino acids.
o N2 is fixed in terrestrial ecosystems by free living soil bacteria as well as some
symbiotic bacteria in root nodules.
o Small amounts can be fixed by lightening.
o Although plants can use ammonia directly, most ammonia in the soil is used by
aerobic bacteria as an energy source (their activity oxidizes ammonia to nitrate and
then ti nitrite in a process called nitrification).
o Animals can assimilate only inorganic N2 by eating plants and other animals.
o Some bacteria get the oxygen they need from nitrate.
The Phosphorous Cycle
o Cycling does not include movement through the atmosphere.
o Weathering of rocks gradually adds phosphate to the soil.