Ecosystems
Energy and Nutrient Cycling
Outline
I. Ecosystems
A. Definition
B. Energy Flow within Ecosystems
1. Trophic Relationships
2. Energy Budgets
3. Ecological Pyramids
C. Nutrient Cycling
1. Biogeochemical Cycles
2. Nutrient Pollution
Ecosystem
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All of the organisms living in a particular place
as well as the chemical/physical factors with
which they interact.
Range in size from small (like aquarium) to
large (like forest)
E.g. desert ecosystem: organisms that live
there (cacti, snakes, rodents), soil chemistry,
average rainfall, temperatures etc.
Energy Flow
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Energy flows through ecosystems, while matter
cycles within them
Resources critical to human survival and
welfare, ranging from the food we eat to the
oxygen we breathe, are products of ecosystem
processes
Energy Flow
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Energy in ecosystems follows the rules of
thermodynamics
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Ecosystems are open systems where energy
(generally from the sun) is input, is transformed, and
is ultimately released as heat from organisms
Heat lost from organisms is disordered, unusable
energy and therefore has increased entropy of the
system
Energy transfers between organisms are not
completely efficient
Without energy input (sun) ecosystems collapse
Trophic Relationships
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Trophic level: how an organism gets it’s
nutrition (energy)
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Food Web: who eats whom in the ecosystem
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All organisms at a particular trophic level are the
same number of transfer steps away from the energy
input into the system
Interconnected food chains
Energy flows through ecosystems via
trophic levels within the food web
Trophic Levels
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Primary producers: 1st trophic level-the autotrophscapture sun energy (the initial energy source for almost
all of earth’s ecosystems)
Primary consumers: heterotrophs-herbivores
(caterpillars, cows etc), feed on primary producers
Secondary consumers: carnivores (birds, wolves
etc), feed on primary consumers
Tertiary consumers: carnivores that feed on other
carnivores
Decomposers
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aka detritivores -consumers that get their energy from
detritus-nonliving organic material
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Dead organisms, feces, leaf litter, wood
Bacteria, fungi, animals which feed on detritus like earthworms
Decompose organic material in ecosystem and transfer
chemical elements in inorganic forms to reservoirs such
as soil, water, and air
Decomposition by bacteria and fungi accounts for most
conversion of organic materials from all trophic levels to
inorganic compounds usable by primary producers
(thereby closing the loop of nutrient cycling)
Ecosystem Energy Budgets
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Primary production: amount of light energy
converted to chemical energy by autotrophs in
a given time period.
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Sets the “spending limit” for the energy budget of the
ecosystem
Of the visible light that reaches the earth, only about
1% is converted to chemical energy by
photosynthesis
Biomass: amount of biological material in a
group of organisms in an ecosystem (ex. weight
of all trees)
Ecological Pyramids
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A good portion of the energy produced by
primary producers is used to fuel their life
functions-cellular respiration, growth,
reproduction-this is true at every trophic level
General rule: only about 10% of the energy
contained in a particular trophic level is
available for transfer to the next trophic
level
Ecological Pyramids
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The only generalization in an ecosystem that
always holds true: energy decreases with each
successive trophic level
Some generalizations that are often true:
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# of individuals decreases as you increase trophic
level
Size of individuals increases as you increase trophic
level
Nutrient Cycling in Ecosystems
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Within an ecosystem nutrients are cycled
internally, there are leaks (outputs) like erosion
and timber/crop harvest
Must be replaced by nutrient inputs:
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Weathering of rocks
Windblown dust
Precipitation
Decomposition into form available to plants
Nutrient Cycling in Ecosystems
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Nutrients are carried from terrestrial
ecosystems by movement of water and
deposited in aquatic ecosystems and
associated lowlands
If inputs exceed outputs, the nutrient cycle in
the ecosystem becomes stressed or
overloaded resulting in pollution (nutrient levels
exceed capability of the ecosystem to process
them)
Nutrient Cycling in Ecosystems
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Nutrients eroded from agricultural lands,
sewage, industrial wastes drain into streams,
rivers, lakes and estuaries
These pollutants destroy plants and animals
that cannot tolerate their presence or the
changed environmental conditions caused by
them