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Chapter 54
Ecosystems
Trophic Relationships
• Autotrophs are primary producers of the
ecosystem
• Heterotrophs
– Primary consumers are herbivores
– Secondary consumers eat herbivores
– Tertiary consumers eat carnivores
• Detritivores are decomposers that break
down detritus
The Global Energy Budget
• Earth is bombarded with enough solar
radiation daily that is equivalent to 100 million
atomic bombs the size of Hiroshima
• Only about 1% of visible light is captured for
photosynthesis
Primary Production
• Gross Primary Production (GPP): total
primary production
– Amount of light energy that is converted to
chemical energy by photosynthesis per unit time
• Net Primary Production (NPP): gross primary
production minus the energy used by primary
producers for respiration (R):
– NPP = GPP - R
• NPP
– Measurement of greatest interest to ecologists
because it represents the storage of chemical
energy that will be available for consumers in an
ecosystem
• Primary production is expressed in energy per
unit area per unit time, or by biomass of
vegetation added to the ecosystem per unit
area per unit time
– Expressed as the dry weight of organic matter
Marine Limiting Factor
• It is not light, as we would think
• Limiting factor is actually nutrients
– Nitrogen and Phosphorus
Eutrophication
• Changing of phytoplankton communities in
lakes due to excess nutrients from dumping
• Found that phosphorous was the limiting
nutrient in lakes
– Led to the production of phosphate-free
detergent
Terrestrial Limiting Factors
• Temperature, moisture, and nutrients are the
limiting factors
Secondary Production
• Amount of chemical energy in a consumers’
food that is converted to their own new
biomass during a given time period
Production Efficiency
• Less than 20% of available energy is passed
through trophic levels
• Production efficiency: the fraction of food energy
that is not used for respiration, and is taken in
and used for growth, reproduction
• Pyramid of production: multiplicative loss of
energy from a food chain
• Biomass pyramid: each tier represents the total
dry weight of all organisms in a trophic level
Green World Hypothesis
• Herbivores consume relatively little plant
biomass because they are held in check by a
variety of factors, included predators,
parasites, and disease
– Defenses against herbivores
– Nutrients, not energy supply, limit herbivores
– Abiotic factors limit herbivores
The Nitrogen Cycle
• Atmosphere is 80% nitrogen, but not in a form
that is usable by plants
• Nitrogen Fixation: convert N2 to minerals that
can be used to synthesize nitrogenous organic
compounds such as amino acids
– Only certain prokaryotes can do this
The Phosphorous Cycle
• Weathering of rocks adds phosphorous to the
soil
• Recycles locally among soil, plants, consumers
Nutrient Cycling
• Rate is largely determined by decomposition
Human Impacts
• Changes chemical cycles
– Adding or subtracting nutrients to the cycle
– Agriculture
• Natural store of nutrients in soil is depleted
• Use industrially synthesized fertilizers to combat it
• Human activities have double the globe’s supply of
fixed nitrogen available for producers
– Excess nutrients in the water systems
• Critical Load: amount of added nitrogen that can be
absorbed by plants without damaging the ecosystem
• Cultural Eutrophication: caused by sewage and factory
run off
Human Impacts
• Fossil Fuels
– Acid Precipitation: snow, rain, or fog that has a pH
less than 5.6
• Drifts hundreds of kilometers from source
• Toxins
– Poisons cannot be degraded naturally
• Mercury poisoning from plastic production
– Accumulate in tissues, especially fat
• DDT, PCB
• Biological magnification
– Eagle eggs are brittle
Human Impacts
• Rising CO2
– Leads to increased productivity by producers
• Global Warming
– Climatologists sample bubbles caught in glaciers
to determine what the CO2 content was at the last
ice age
– Look at fossilized pollen to see how plants
adapted to increased temperatures
Human Impacts
• Depletion of the ozone
– Ozone absorbs UV radiation
– Has been thinning since 1975
– CFC’s are a main contributor
• Aerosol cans, refrigeration chemicals