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AP Biology, Chapter 54
Ecosystems
SUMMARY
INTRODUCTION
TROPHIC RELATIONSHIPS IN ECOSYSTEMS
Introduction
1. Describe the relationship between autotrophs and heterotrophs in an
ecosystem.
a. Producers (autotrophs) convert sunlight into organic chemical energy
b. Heterotrophs utilize that stored energy
i. Primary consumers eat producers
ii. Secondary eat primary, etc.
iii. Most feed at more than one level = a food web
c. Decomposers get all in the end
Trophic relationships determine an ecosystem's routes of energy flow and
chemical cycling
2. Explain how the first and second laws of thermodynamics apply to
ecosystems.
a. 1st: amount of energy is constant
i. Energy is passed in food chains
ii. Either it is stored or released to the environment
b. 2nd: energy is degraded when it is used
i. Most is lost as heat
ii. 90% of the energy is lost at each level
Primary producers include plants, algae, and many species of bacteria
Many primary and higher-order consumers are opportunistic feeders
Decomposition interconnects all trophic levels
3. Explain how decomposition connects all trophic levels in an ecosystem.
a. Organisms are recycled
b. Their matter, released by decomposers, is used by other organisms
ENERGY FLOW IN ECOSYSTEMS
Introduction
An ecosystem's energy budget depends on primary productivity
4. Explain why the amount of energy used in photosynthesis is so much less than
the amount of solar energy that reaches Earth.
a. Most is absorbed, scattered, or reflected by the atmosphere
b. Also required: water, temperature, and nutrients
c. Most that reaches the surface hits bare ground or water
d. Only certain wavelengths are used
5. Define and compare gross primary production and net primary production.
a. Gross is the total light energy converted to chemical energy per time
b. Net is gross minus that used for respiration
6. Compare primary productivity in marine, freshwater, and terrestrial
ecosystems.
a. Highest per m2
i. Algal beds and reefs
ii. Tropical rain forest
iii. Swamp and marsh
b. Highest total
i. Open ocean
ii. Tropical rain forest
iii. Continental shelf
c. Freshwater ecosystems are low
As energy flows through an ecosystem, much is lost at each trophic level
7. Explain why energy is said to flow rather than cycle within ecosystems. Use
the example of insect caterpillars to illustrate energy flow.
a. Energy is lost and degraded at each trophic
b. Of the food energy eaten by caterpillars
i. 50% is lost in feces
ii. 33% is used in cellular respiration
iii. Only 17% is used for growth
8. Distinguish between energy pyramids and biomass pyramids. Explain why
both relationships are in the form of pyramids. Explain the special circumstances
of inverted biomass pyramids.
a. Distinction
i. Energy in terms of net primary productivity
ii. Biomass in terms of the total dry weight of organisms
b. Shape
i. Smaller at higher trophic levels
ii. Both energy and biomass are lost
c. Exceptions
i. Aquatic ecosystems with rapidly reproducing phytoplankton
ii. Low phytoplankton reproduces quickly to feed zooplankton
9. Explain why food pyramids usually have only four or five trophic levels.
a. Starting biomass at the bottom, limits biomass at the top
b. Most energy/biomass is lost at each level
c. Ten times the area is required for each successive level
10. Define the pyramid of numbers.
a. # of individuals at each trophic level
b. Exception: large producers like trees, support many insects
11. Explain why worldwide agriculture could feed more people if all humans
consumed only plant material.
a. Most of the energy is wasted by feeding grain to farm animals
b. Hogs and chickens are more efficient at making meat than cattle
c. Fish are more efficient than either
CYCLING OF CHEMICAL ELEMENTS IN ECOSYSTEMS
Introduction
Biological and ecological processes move nutrients among organic and
inorganic compartments
12. Describe the four nutrient reservoirs and the processes that transfer the
elements between reservoirs.
a. Available organic
i. To and from available inorganic by photosynthesis and
respiration
ii. To unavailable organic by "fossilization"
b. Unavailable organic: to available inorganic by erosion and burning
c. Available inorganic
i. See above for exchange with available organic
ii. To and from unavailable inorganic by weathering and
sedimentation
d. Unavailable inorganic: See above for exchange with available
inorganic
13. Explain why it is difficult to trace elements through biogeochemical cycles.
a. Many process for each material
b. Many are geographically widespread
i. Wind, erosion, etc.
ii. Organisms may come and go
c. Long periods of time may be involved
14. Describe the hydrologic water cycle.
a. Solar evaporation
b. Condensation and precipitation
c. Percolation and runoff
15. Describe the nitrogen cycle and explain the importance of nitrogen fixation to
all living organisms.
a. Cycle
i. N2  NH3 by nitrogen-fixation bacteria in soil and nodules
ii. NH3  NO2, NO3 by nitrifying bacteria
iii. Assimilation into plants then animals
iv. Decomposers release NH3 back into the soil
v. NO3  N2 by denitrifying bacteria
b. Importance: amino acids, nucleic acids, some carbohydrates and lipids
16. Describe the phosphorus cycle and explain how phosphorus is recycled
locally in most ecosystems.
a. Geological cycling
i. Erosion and sedimentation
ii. Rock formation
iii. Tectonic processes including uplift
b. Local cycling
i. Plants assimilate phosphate from the soil
ii. Decomposers release it back into the soil
Decomposition rates largely determine the rates of nutrient cycling
17. Explain how decomposition affects the rate of nutrient cycling in ecosystems.
a. Decomposition varies widely: tundra 50 years; tropical forest a few
years
b. Tropical forest soils have low nutrients because of rapid breakdown
and reassimilation
Field experiments reveal how vegetation regulates chemical cycling: science
as a process
HUMAN IMPACTS ON ECOSYSTEMS
Introduction
The human population is disrupting chemical cycles throughout the biosphere
18. Describe how agricultural practices can interfere with nitrogen cycling.
a. Nitrogen assimilated into crops is removed at harvest
b. Disturbed soil loses nitrogen faster by leaching
19. Explain how "cultural eutrophication" can alter freshwater ecosystems.
a. Eutrophication is the increase in productivity as lakes age
b. Wastes, leached nutrients, silting from human activities accelerate the
process
Toxins can become concentrated in successive trophic levels of food webs
20. Explain why toxic compounds usually have the greatest effect on top-level
carnivores.
a. Fat-soluble substances stick, water-soluble pass in urine
b. Consumers absorb and store the fat-soluble toxins in their food
c. Consuption at successive levels gives biomagnification
Human activities are causing fundamental changes in the composition of the
atmosphere
21. Describe how increased atmospheric concentrations of carbon dioxide could
affect Earth.
a. Carbon cycle
i. Producers fix CO2, consumers release CO2
ii. CO2 is stored as carbonates in the ocean and in sediments
b. CO2 in the atmosphere may double from 1850-2075
c. Greenhouse effect
i. Visible light penetrates the atmosphere and heats the ground
ii. Heat is radiated as infrared
iii. CO2 and other greenhouse gases absorb the infrared
The exploding human population is altering habitats and reducing biodiversity
worldwide
22. Describe how human interference might alter the biosphere.
a. Two problems
i. Exponential growth
ii. Demand for a higher standard of living
b. Exponential pressure on resources we share with ecosystems