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Chapter 3 How Ecosystems
You could cover the whole world with
asphalt, but sooner or later green grass
would break through.” Ilya Ehrenburg
Energy Flow in Ecosystems
• For most living organisms the sun is the
ultimate source of it’s energy.
• The suns energy is captured by
organisms that can do photosynthesis.
All green plants, some bacteria and a few
• Energy can then be transferred to other
Energy Relationships
• Producers are organisms that can produce
their own food. Also known as autotrophs
• Consumers are organisms that get their energy
from eating other organisms. Also known as
heterotrophs (other-feeders).
• Producers get their energy directly from the
sun. Consumers get their energy indirectly
from the sun.
• Not all organisms use the sun as their energy
• Some bacteria can use the chemical energy
found in sulfide and nitrogen compounds. Also
known as chemotrophs (chemical – eaters).
• Found in deep – ocean ecosystems. Recent
research has shown that these ecosystems may
contain more than 50% of the worlds biomass.
Who eats what?
• Within an ecosystem there are various
• Herbivores – consumers that only eat
• Carnivores – consumers that eat only other
• Omnivores – consumers that eat both
producers and other consumers. The most
versatile organisms in the ecosystem.
• Decomposers – break down dead organisms,
producers and consumers. Return nutrients to
the ecosystem.
Cellular Respiration
• The process that all living things use to obtain
energy from nutrients.
• Done by both autotrophs and heterotrophs.
• Can be aerobic or anaerobic.
• Some organisms are strictly aerobic or
anaerobic. Others can switch back & forth
depending on conditions (facultative
Energy Transfer
• As organisms are eaten or decompose,
energy is transferred.
• This transfer of energy can be tracked
and mapped using food chains, food
webs, and trophic levels.
Food Chains & Food Webs
• Food chain - a sequence that shows the
transfer of energy from one organism to the
next. Usually linear in nature.
• Can also be used to show the movement of
toxins/poisons through the food chain.
• Food webs – more accurate depiction of the
energy transfer. Shows all of the feeding
relationships in the ecosystem. Made up of a
number of interconnected food chains.
Trophic Levels
• Each step in the energy transfer within an
ecosystem is called a trophic level.
• Each time energy is transferred from one
trophic level to the next some is lost.
• Usually figure that only 10% of the energy is
transferred between trophic levels.
• Some decomposer food webs can show higher
efficiencies (20-40%).
Trophic Levels
• Due to the energy loss, usually find fewer
organisms at the higher trophic levels.
• Almost always find the producers to be the
largest number (the most energy available).
• Energy loss also usually limits the number of
trophic levels available.
• Seldom have more than 4 or 5, although some
decomposer webs can have 8-10 trophic levels.
Water Cycle
• Water is physically changed in a cyclic
• Sunlight provides the energy that drives
the water cycle.
• Sunlight causes water to evaporate. As it
cools, it condenses and returns as
Water Cycle (cont.)
• Precipitation can take several pathways:
– Some can land in the oceans.
– Some can land on land.
• Some can run along the surface – surface water.
• Some can soak into the soil and be taken up by
• Some can seep through the soil and collect in
aquifers – groundwater.
Water Cycle (cont.)
• Surface runoff can collect in streams and
rivers that eventually flow into oceans.
• Groundwater can flow into streams and
rivers also.
• Some precipitation can land on trees and
either be evaporated back into the
atmosphere or run down the trunk –
Water Cycle (cont.)
• Water taken up by plants can be
evaporated back by process of
• Water can also be used by domestic uses
as well as industrial uses.
• Development of land can alter the water
Water Cycle
Carbon Cycle
• The carbon cycle involves changing
carbon from inorganic forms to organic
forms and back to inorganic forms.
• Inorganic carbon (CO2) is removed from
the atmosphere during photosynthesis.
• Inorganic carbon is converted into
organic forms (glucose, lipids, proteins,
Carbon Cycle (cont.)
• Organic carbon is converted back into
inorganic form during respiration.
• Burning of fossil fuels releases inorganic
carbon into the atmosphere.
• Deforestation removes large producers
that could help process inorganic carbon.
• Increased CO2 has been linked to global
Carbon Cycle
Carbon Cycle
Nitrogen Cycle
• Nitrogen needed to build proteins.
• 78% of atmosphere made of N2.
• Only a few bacteria can convert N2 into a
useable form. Nitrogen-fixing
bacteria( NH4+)
• Nitrogen-fixing bacteria found free-living
or within roots of certain plants
Nitrogen Cycle (cont.)
• Plants convert NH4+ into amino acids and
• Animals get useable nitrogen (amino
acids) by eating plants or other animals.
• Decomposers (bacteria & fungi)
breakdown wastes & dead organisms,
returning nitrogen to soil & water as
NH4+ .
Nitrogen Cycle (cont.)
• NH4+ can be taken up by plants again and
used again.
• Some NH4+ can be converted into nitrates
(NO3) by nitrifying bacteria. Nitrifying
bacteria require aerobic conditions.
• Nitrates can then be converted into N2 by
denitrifying bacteria. Denitrifying
bacteria require anaerobic conditions.
Nitrogen Cycle
Nitrogen Cycle
• Succession – a regular pattern of changes
over time in the type of species in a
• Process may take hundreds or thousands
of years.
• As species change they alter the
environment, allowing new species to
come in.
Succession (cont.)
• Eventually reach a stable, unchanging
community – climax community.
• When succession occurs where no
ecosystem has existed before – primary
• When succession occurs where a
ecosystem has previously existed –
secondary succession.
Succession (cont.)
• The first plants to colonize an area –
• Eventually taller plants move in and
shade out the pioneers.
• The shrubs and bushes will move in and
eventually trees.
• Climax communities will remain stable
unless there is a disturbance.
• Disturbances can be large scale or small
• Large scale can be forest fires,
earthquakes, floods, volcanic activity, or
• Small scale disturbances can be a tree
falling over.
Disturbance (cont.)
• Some ecosystems need periodic disturbances.
• Jack pine forests need fire to disperse their
• Pin cherry trees need sunlight to warm the soils
for their seeds to sprout.
• Man’s interference w/ natural disturbances can
cause considerable damage to an ecosystem.
Primary Succession
• Succession that starts w/ bare rock.
• Pioneer species are usually lichens &
• Help break up the rock & form soil.
• Allows for grasses & ferns to move in.
• Grasses & ferns “crowd” out the lichens
& mosses.