Chapter 54 – Ecosystems
TJ Biology Olympiad
Cassandra Xia
Trophic structure – feeding systems
Divided into trophic levels- based on main source of nutrition
Primary Producers—autotrophs
- photoautotrophs
 plants
 phytoplankton (algae and bacteria)
 multicellular algae and aquatic plants
- chemoautotrophic bacteria which oxidizes H2S
Autotrophs- need only CO2 as
a carbon source
Heterotrophs- needs at least
one organic nutrient as a
source of carbon for making
other organic compounds
Primary Consumers—herbivores
- eat plants or algae
- Opportunistic – supplement diet with heterotrophic material if available (e.g. with eggs and
hatchlings)
Secondary consumers
- carnivores that eat herbivores
Tertiary consumers
- carnivores that eat carnivores
Photo- derives energy from light
Chemo- derives energy from
chemicals taken from the environment
(animals, fungi, prokaryotes, and
protists are chemoheterotrophs)
Detritivores
- eat detritus (non-living organic material = feces, fallen leaves, dead organisms)
Gross primary productivity- the amount of light energy converted to chemical energy (organic compounds) by
autotrophs of an ecosystem during a time period; the rate at which light energy is converted to chemical energy
in an ecosystem
- not all stored as organic material because some used in cell respiration
Standing crop biomass- total
- measured in g/m2/yr of dry weight or J/ m2/yr
biomass of photoautotrophs
Net primary productivity- what is stored as organic material
present at a given time. Measured
- NPP = GPP – Respiration
in grams/kilograms.
- Available to consumers in the ecosystem
- NPP:GPP ratio smaller for large producers which have to support large stem and root systems
More on primary productivity
- Open ocean has low NPP by area, but contributes a lot to Earth’s NPP due to huge surface area
- Estuaries and reefs have high NPP by area, but contributes little to Earth’s NPP due to small suface area
- Tropical rain forest have high NPP by area, and contributes a lot to Earth’s NPP yay
- Limiting nutrient- the thing that must be added in order for production to increase in a particular area
 usually nitrogen or phosphorus
 sometimes light
Secondary Productivity- the rate at which an ecosystem’s consumers convert the chemical energy of the food
they eat into their own new biomass
- Energy in a consumer is lost in:
o Cell respiration
-
o Feces
o Growth (available energy for the next level)
Carnivores are slightly more efficient at converting energy into biomass because meat is easier to digest.
BUT animals unlike plants use up a lot of energy running around and stuff, so if all humans were to go
vegetarian there would be enough food for everyone to eat
Ecological efficiency- percentage of energy transferred from one trophic level to the next (varies from 5% to
20% but use the “10% rule”)
Ecological pyramids
- Mostly all bottom-heavy
- Types:
o Productivity pyramid
o Pyramid of numbers
o Biomass pyramid
 Turnover time = Standing crop biomass (kg/m2) / Productivity (kg/m2/day)
Water Cycle
- Evaporation over the ocean, evapotranspiration over land
- Clouds rain over ocean and land
- Runoff and groundwater return to ocean (percolation through soil)
Carbon Cycle
- Photosynthesis and respiration have opposing effects
on CO2 levels
- Because of the burning of locked away carbon in the
form of fossil fuels, CO2 levels steadily increasing
and contributing to the Greenhouse Effect
- CO2 levels have seasonal fluctuation:
o Lowest during summer
o Highest during winter
o Why?
- In the ocean, it is more complicated:
o The ocean has reactions between limestone
(CaCO3), water, and CO2
o H2O + CO2 <=> H2CO3
H2CO3 + CaCO3 <=> Ca(HCO3)2 <=> Ca2+
+ 2HCO3+
22HCO3 <=> 2H + 2CO3
o As CO2 is used for photosynthesis in marine environments, the first equation shifts to the left.
o Some aquatic autotrophs can use HCO3o The amount of carbon in the ocean is 50 times that
in the atmosphere, the ocean may be an important
carbon sink as carbon dioxide levels increase
Phosphorous Cycle
- Organisms need phosphorus for
o Nucleic acids
o Phospholipids
o ATP
o Bones & teeth
- Weathering of rocks adds phosphate to the soil
-
Plants absorb phosphorus in the inorganic form of phosphate (PO43-)
Consumers get it by eating plants
Decomposers return it to the soil
Also, phosphorus leaches into the water table, draining into the sea
There it becomes incorporated into sedimentary rocks that may be part of a terrestrial ecosystem later in
time
Phosphates in the form of sewage and runoff fertilizer causes eutrophication in aquatic systems
Nitrogen Cycle- this is probably the most quizzed thing
on the whole exam. A question on the nitrogen cycle is
practically guaranteed. It is worth learning.
- Nitrogen is needed to build amino acids and
therefore proteins.
- Nitrogen is the most abundant element in the
Earth’s atmosphere, but it cannot be used by
most organisms if it is in the form N2
- N2 is converted into more usable forms in three
main ways
o By nitrogen fixation- free-living soil
bacteria, cyanobateria, and bacteria in
root nodules of legumes converts N2 to
NH3
o Atmospheric deposition (lightning)converts to NH4+ or NO3- which
dissolves in rain and gets added to the soil
o Industrial production of fertilizer
- Most soil is slightly acidic so NH3 produced by nitrogen fixation picks up an H+ and becomes NH4+
- Plants can absorb NH4+ but they can also absorb NO3- Nitrification by nitrifying bacteria converts NH4+ to NO3- Animals get their nitrogen by eating plants
- Cycle part:
o Denitrification- denitrifying bacteria convert NO3- back to N2
o Ammonification- bacterial and fungal decomposers convert organic nitrogen in plants and
animals to NH4+
Decomposition rates depend on:
 Temperature
 Availability of H2O and O2
 Soil chemistry
 Fires
Fast
Slow
Long Term Ecological Research (LTER) at Hubbard Brook
- Forest logged and herbicide sprayed
- Water runoff increased
- Ca2+ x 4 loss, K+ x 15 loss, NO3- x 60 loss
Tropical rain forests
Temperature forests
Tundra
months-years
4-6-years
50 years
Environmental Hazards
Agriculture and Nitrogen Cycling
- Move nutrients from one area to another
- Nutrients lost through shipping agriculture and erosion
- Fertilizers must be added for “free period” (no need to add nutrients)
Toxins
- Many cannot be degraded by microorganisms
- Some compounds may be converted to toxins
 CH3—Hg
e.g. Hg 
bacteria
-
insoluble
soluble, accumulates in tissues of fish and humans
Chlorinated hydrocarbons (DDT and PCBs) are indicated in endrocrine disruption
Undergo biological magnification- increased concentrations of toxins in successive trophic levels of a
food web caused by eating toxins concentrated in the tissues of lower level organisms
 DDT and DDE (product of partial breakdown) interfere with calcium deposition in egg
shells of eagles, pelicans, and ospreys
Accelerated Eutrophication of Lakes
- Cultural eutrophication is eutrophication caused by humans. Sources of it include
 Sewage
 Factory wastes
 Runoff of animal waste
 Fertilizers
- Algae blooms
- Some algae die
- Decomposing bacteria use up O2
Carbon Dioxide and the Greenhouse Effect
- Greenhouse effect: gases absorb reradiate infrared radiation back at the earth, increasing temperature
(without it, earth’s surface temperature would be -18° C)
- More CO2 is being released by burning fossil fuels and is contributing to global warming
o Melt polar ice
o Alter precipitation patterns (central US drier)
- With more CO2, plants become more productive but the higher levels favor C3 plants (like wheat + soy)
over C4 plants (like corn) causing C3 plants to spread into areas originally covered by C4 plants
Depletion of Atmospheric Ozone
- Chlorofluorocarbons (refrigerants) is a catalyst in the reaction O3 → O2
- Cold temperature over Antarctica facilitate reactions