53
Ecosystems and the Biosphere
Lecture Outline
I.
Energy flow through ecosystems
A. Energy flow is linear as energy cannot be reused by organisms
1. Solar energy is trapped by photosynthesis as chemical energy
2. Chemical energy is then available to do work
B. Energy flow describes who eats whom in ecosystems
1. A food chain describes the sequential passage of energy
2. A food web shows the complexity of interconnected food chains
C. Ecological pyramids illustrate how ecosystems work
1. The primary producers comprise the first trophic level, the herbivores
make up the second trophic level, and the primary consumers the third
2. An ecological pyramid illustrates the trophic levels
a) A pyramid of numbers shows the numbers of organisms in each level
(1) Typical pyramids of numbers show diminishing numbers in
successive levels
(2) Inverted pyramids of numbers are seen among hosts and
parasites, decomposers and others
b) A pyramid of biomass shows the biomass in each trophic level
(1) An inverted pyramid of biomass may be seen in very productive
aquatic ecosystems
c) A pyramid of energy cannot be inverted due to loss of energy between
trophic levels
(1) This loss of energy illustrates the limitation on the number of
trophic levels
D. Ecosystems vary in productivity
1. The gross primary productivity of an ecosystem expresses the total rate of
photosynthesis
2. Net primary productivity is the energy remaining after cellular respiration
3. Both GPP and NPP can be expressed in terms of kilocalories fixed per area
per time, or in terms of grams carbon fixed
4. Rates of productivity are influenced by environmental factors
5. Tropical rain forests are the most productive terrestrial ecosystems
6. Wetlands, coral reefs, and estuaries are the most productive aquatic
ecosystems
7.
8.
II.
The relationship of productivity to biological diversity is complex
a) Ecosystems may be more diverse as productivity increases, but after a
certain point, diversity will decline with increasing productivity
b) This may be important when considering nutrient-enriched
environments, such as those impacted by human application of
fertilizers, and enrichment by animal wastes
Humans consume an increasingly greater percentage of global primary
productivity
Cycles of matter in ecosystems
A. Biogeochemical cycles are cycles of matter between the abiotic and the biotic
components of the environment
1. The carbon, nitrogen, phosphorus, and water cycles are central to life on
Earth
2. The carbon, nitrogen, and water cycles have atmospheric components and
cycle on a global scale
3. Phosphorus has no atmospheric component and cycles on a local scale
B. Carbon dioxide is the pivotal molecule of the carbon cycle
1. Carbon is present as carbon dioxide in the atmosphere, as carbon dioxide,
carbonate, and bicarbonate in aquatic habitats, and in rocks such as
limestone
2. Carbon is fixed by photosynthesis, which incorporates carbon into organic
molecules such as glucose
3. Organic molecules may be used in cellular respiration, or may be passed to
consumers
4. Organic carbon may be stored in the wood of trees, or stored even longer
in fossil fuels
a) Combustion of fossil fuels releases the carbon to the atmosphere
5. The organic carbon of marine organisms may end up in limestone
a) Erosion returns this carbon to the water
6. Human activities have disturbed the global carbon budget
a) Since the beginning of the Industrial Revolution, humans have
contributed an increasing amount of carbon dioxide to the atmosphere
by burning fossil fuels
C. Bacteria are essential to the nitrogen cycle
1. Nitrogen gas in the atmosphere is very abundant, but is such a stable
molecule that bacteria are needed to break it apart
2. Nitrogen fixation is the conversion of gaseous nitrogen to ammonia
a) Most nitrogen fixation is accomplished by nitrogen-fixing bacteria
b) Nitrogenase, which functions only in the absence of oxygen, is present
in some cyanobacteria, some soil bacteria, and Rhizobium, which lives
in root nodules of legumes
3.
Nitrification is the conversion of ammonia to nitrate
a) Nitrifying bacteria convert the nitrogen compounds to produce
metabolic energy
b) Nitrosomonas and Nitrococcus are soil bacteria that convert ammonia to
nitrite
c) Nitrobacter, another soil bacterium, converts nitrite to nitrate
d) Assimilation is the uptake of nitrate or ammonia by plant roots
(1) Animals assimilate nitrogen by consuming plants or other animals
e) Ammonification is the conversion of organic nitrogen compounds into
ammonia
(1) Nitrogen-containing wastes are decomposed by ammonifying
bacteria, which produce ammonia
(2) These bacteria are recyclers of organic nitrogen
f) Denitrification is the conversion of nitrate to gaseous nitrogen, which
represents a net loss of available nitrogen in the system
(1) Denitrifying bacteria are anaerobes
g) Human interaction with the nitrogen cycle can cause water pollution
(1) Production of nitrogenous fertilizers can enrich bodies of water
when agricultural areas are eroded
(2) Nitrogen oxides are produced in automobile exhaust
(3) Nitrogen oxides are also an ingredient of photochemical smog
D. The phosphorus cycle lacks a gaseous component
1. Phosphorus cycles from the land to sediments and back to the land
2. Erosion releases phosphate to the soil where it may be assimilated by plant
roots
3. Phosphate is returned to the soil by decomposers
4. Phosphate may be deposited in oceanic sediments where it remains
unavailable for millions of years
5. Humans affect the natural cycling of phosphorus
a) Fertilizers, runoff containing animal wastes, and sewage introduce
phosphorus into aquatic ecosystems
b) Phosphorus loss from ecosystems is accelerated by clear-cutting
E. Water moves among the ocean, land, and atmosphere in the hydrological
cycle
1. Water may evaporate from land and enter the atmosphere, flow to the
ocean, or enter the groundwater
2. Water in the atmosphere is ultimately precipitated on the land or ocean
III.
Ecosystem regulation from the bottom up and the top down
F. Bottom-up processes are based on food webs; they regulate ecosystem
function by nutrient cycling and other aspects of the abiotic environment
1. Bottom-up processes predominate in certain aquatic ecosystems where
nitrogen or phosphorus is limiting
G. Top-down processes regulate ecosystem function by trophic interactions,
particularly from the highest trophic level
1. Top-down regulation appears to predominate in ecosystems with few
trophic levels and low species richness
IV.
Abiotic factors in ecosystems
H. The sun warms the Earth
1. Solar energy powers biogeochemical cycles
a) All solar energy is ultimately radiated back into space
I. Temperature changes with latitude
1. The sun’s rays strike the equatorial regions vertically, which results in
warmer temperatures
J. Temperature changes with season
1. The tilting of the Earth’s axis causes the amount of solar radiation to vary
during the year
K. The atmosphere contains several gases essential to organisms
1. The atmosphere protects the Earth’s surface from radiation
2. The sun drives global atmospheric circulation
a) Differential warming of the earth results in atmospheric circulation
(1) Heated air at the equator rises, flows away from the equator,
cools, and sinks
3. The atmosphere exhibits complex horizontal movements
a) Horizontal movements are called winds
(1) The Coriolis effect is the deflection of winds by the Earth’s
rotation
(2) Winds are deflected to the right in the Northern Hemisphere and
to the left in the Southern Hemisphere
L. The global ocean covers most of Earth’s surface
1. Four interconnected oceans may be described: the Pacific, Atlantic, Indian
and Arctic Oceans
a) The Pacific Ocean contains more than half of the Earth’s water
2. Surface ocean currents are driven by winds
a) Ocean currents are produced by prevailing winds
b) Circular ocean currents are gyres
c) The continents affect oceanic circulation
(1) The Southern Hemisphere has a circumpolar flow of water
(2) The current along the eastern United States is northward; the
current along the western United States is southward
3. The ocean interacts with the atmosphere
a) The El Niño-Southern Oscillation (ENSO) is a periodic warming of the
ocean in the tropical East Pacific that alters climatic patterns in many
far distant areas
M. Climate profoundly affects organisms
1. Weather is the atmospheric condition at a given place and time
a) Climate is the average weather
2. Air and water movements and surface features affect precipitation patterns
a) Precipitation occurs when air reaches its saturation point
b) Precipitation is highest on the windward coast of continents
c) Temperate continental interiors are drier
3. Some mountains cause rain shadows
a) As air rises over mountains, it cools and precipitation occurs
b) The leeward side of mountains has little precipitation
4. Microclimates are local variations in climate
a) Microclimates may be created by differences in terrain, or by the
organisms themselves
N. Fires are a common disturbance in some areas
1. Fires free nutrients tied up in dry organic material
2. Fires remove plant cover and promote germination of many seeds
3. Fires are a natural event in the African savanna, the California chaparral,
the North American grasslands, and the Ponderosa pine forests
4. Controlled burning is used to burn plant litter before it reaches dangerous
levels
Research and Discussion Topics

Investigate situations in which pyramids of numbers and pyramids of biomass are
inverted. Give examples. Why can pyramids of energy never be inverted?

Incomplete food chains (those with no primary producers) include the abyssal sea,
cave systems, and the interior of a city. What is the energy input to those systems?

Where might you find the least complex food webs? Where would the most
complex food webs occur? In what situations are food chains extremely short?
Where are the longest food chains found?