25.1. The Nature of Ecosystems (p. 428)
A. The Earth (Fig. 25.1)
1. The hydrosphere is the zone of water that covers three-quarters of the earth.
a. Sunlight drives the water cycle.
b. Water evaporates from oceans, rivers and living communities to become
clouds.
c. Water condenses and precipitation cycles through freshwater habitats as it
returns to the ocean.
d. The ability of water to absorb and release great quantities of heat keeps
climate within livable range.
2. The atmosphere is the gaseous layer near earth.
a. The atmosphere is concentrated in the lowest 10 kilometers but extends thinly
out to 1,000 km.
b. Major gases in the atmosphere are nitrogen, oxygen and carbon dioxide.
c. Carbon dioxide is a prime input for photosynthesis.
d. Oxygen is involved in cellular respiration, and in the upper atmosphere
becomes protective ozone (O3).
3. The lithosphere is a rocky substratum that extends about 100 kilometers deep.
a. Weathering of rocks supplies minerals to plants and eventually forms soil.
b. Soil contains decayed organic material (humus) that recycles nutrients to
plants.
4. The biosphere is the thin layer where life is possible between the outer atmosphere
and the lithosphere.
B. Biotic Components of an Ecosystem
1. An ecosystem is the living organisms and the chemical and physical environment.
2. Living things are organized in an ecosystem by how they secure their food:
autotrophs or heterotrophs.
C. Autotrophic Organisms
1. Autotrophs capture energy (e.g., sunlight) and incorporate it into organic compounds;
therefore they are also called producers. (Fig. 25.2)
2. Chemoautotrophs are bacteria that obtain energy from oxidation of inorganic
compounds such as ammonia, nitrites, and sulfides; they synthesize carbohydrates
and are found in cave communities and ocean depths.
3. Photoautotrophs possess chlorophyll and carry on photosynthesis.
4. These organisms are at the beginning of a food chain.
5. In terrestrial ecosystems, producers are mostly plants; in aquatic ecosystems,
dominant producers are algae.
D. Heterotrophic Organisms
1. Heterotrophs need a source of preformed nutrients and consume tissues of other
organisms.
a. Herbivores are animals that feed directly on green plants (e.g., caterpillars,
zooplankton, etc.).
b. Carnivores are animals that eat other animals (e.g., green herons, hawks,
etc.).
c. Sequences of carnivores that feed in a chain can be labeled primary,
secondary and tertiary consumers.
1) The primary consumer is the herbivore.
2) The secondary consumer eats the herbivores.
3) The tertiary consumer feeds on secondary carnivores, etc.
d. Omnivores can feed upon a variety of organisms, including plants and animals (e.g., human).
2. Detritivores are animals (e.g., earthworms) that feed on detritus---the decomposing
products of organisms.
3. Decomposers are nonphotosynthetic bacteria and fungi that extract energy from
dead matter, including animal wastes in the soil.
25.2. Energy Flow and Nutrient Cycling (p. 430) (Fig. 25.3) [transp. 131]
A. Ecosystems
1. Ecosystems are dependent upon solar energy flow and finite pools of nutrients.
2. Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur make up over 98 percent
of body weight of life.
3. Plants can make use of inorganic nutrients while animals must take in organic
nutrients.
4. Primary productivity is the total amount of energy an ecosystem's producers
capture within plant material over a length of time.
a. Soil, climate, and other factors affect gross primary productivity.
b. Plants must use organic molecules to fuel their own cellular respiration, about
55%.
c. 55% of gross primary productivity is available to heterotrophs; this is net
primary productivity.
5. Energy flow in an ecosystem is a consequence of two fundamental laws of
thermodynamics:
a. First law of thermodynamics states energy can neither be created nor
destroyed; it can only be changed from one form of energy to another.
b. Second law of thermodynamics: when energy is transformed from one
form to another, there is always some loss of energy from the system, usually
as low grade heat.
6. Therefore, ecosystems are unable to function unless they receive a constant input of
energy.
a. Primary source of energy for ecosystems is sunlight, which
photosynthesizers use to produce organic food.
b. All energy content of organic matter is eventually lost to environment as low
grade heat.
c. Only a small portion of food taken in by heterotrophs becomes available to the
next consumer.
d. Detritivores do make use of energy in dead organisms and feces before it is
lost to the system.
e. The portion of energy converted into increased body weight or offspring is
secondary productivity.
f. All of the solar energy that enters an ecosystem is eventually lost as heat.
7. Decomposers are saprotrophs, sending out digestive enzymes and retrieving
digestive products.
8. Chemosynthetic bacteria at deep sea vents and caves do not depend directly on solar
energy.
B. Food Webs and Trophic Levels [transp.131]
1. The complex feeding relationships that exist in nature are called food webs. (Fig.
25.5) [transp. 132]
2. A grazing food web begins with leaves, stems and seeds eaten by herbivores and
omnivores.
3. A detritus food web begins with detritus, followed by decomposers (including
bacteria and fungi).
4. Detritus food chains are connected to a grazing food chain when consumers of a
grazing food chain feed on the decomposers of the detrital food chain.
5. In some ecosystems, less than 1% of energy may move through the grazing food web
while over 99% moves through the detritus food web. (Fig. 25.6) [transp. 133]
C. Trophic Levels
1. A food chain represents passage of energy through populations in a community. (Fig
25.5) [transp. 132]
2. A trophic level is a feeding level of one or more populations in a food web; those
organisms in an ecosystem that are the same number of food chain steps from the
energy input into the system:
a. first trophic level---primary producers;
b. second trophic level---all the primary consumers;
c. third trophic level---all the secondary consumers; and
E. Ecological Pyramids
1. An ecological pyramid shows the trophic structure of an ecosystem as a graph
representing biomass, organism number, or energy content of each trophic level in a
food web. (Fig. 25.7)
2. The base of the pyramid represents the producer trophic level, and from there the
consumer trophic level is stacked, with the apex representing the highest consumer
trophic level.
3. A pyramid of numbers is based on the number of organisms in each trophic level.
4. A pyramid of biomass is based on the weight (biomass) of organisms at each trophic
level at one time.
a. Usually a large mass of plants supports a medium mass of herbivores and a
small mass of carnivores.
b. At one point in time at seashores, herbivores can have greater biomass
feeding on algae that reproduce fast but are eaten, producing an inverted
pyramid; over long time periods, biomass is a normal pyramid.
5. A pyramid of energy is based on the total amount of energy in each trophic level and
is always pyramidal.
6. In general, about 10 percent of energy at a particular trophic level is incorporated into
the next trophic level.
a. Thus, 1,000 kg (or kcal in an energy pyramid) of plant material converts to
100 kg of herbivore tissue, which converts to 10 kg of first carnivores, which
can support 1 kg of second level carnivores.
b. This rapid loss of energy is the reason food chains have from three to four
links, rarely five.
c. This rapid loss of energy is also the reason there are few large carnivores.
25.3. Global Biogeochemical Cycles (p. 434)
A. Biogeochemical Cycles
1. Biogeochemical cycles are the circulation pathways of elements (e.g., carbon,
oxygen, hydrogen, nitrogen or mineral elements) through the biotic and abiotic
components of an ecosystem. (Fig. 25.8) [transp. 134]
2. A reservoir is that portion of the earth that acts as a storehouse for the element.
3. An exchange pool is the portion of the environment for which the producers take
their nutrients, such as the hydrosphere, atmosphere, and soil.
4. The biotic community is the pathway (i.e., food chains) by which chemicals move
through an environment.
5. Some cycles are primarily gaseous cycles (carbon and nitrogen); others are
sedimentary cycles, (phosphorus).
B. The Hydrologic (Water) Cycle
1. In the (hydrologic) cycle, freshwater evaporates and condenses on the earth. (Fig.
25.9) [transp. 135]
a. Oceans are the greatest source of evaporated water, but water also evaporates from bodies of freshwater, and from land and plants (transpiration).
2. Evaporation of water from the oceans leaves behind salts.
3. Rainfall that permeates the earth forms a water table at the surface of the
groundwater.
4. An aquifer is an underground storage of freshwater in porous rock, trapped by
impervious rock strata.
5. Freshwater, which makes up only about 3 percent of the world's supply of water, is
called a renewable resource.
6. Freshwater can become unavailable when consumption exceeds supply and/or is
polluted so it is not usable.
C. The Carbon Cycle (Fig. 25.10) [transp. 136]
1. The exchange pool for the carbon cycle is the atmosphere.
2. Photosynthesis removes CO2 from the atmosphere; respiration and combustion add
CO2 to the atmosphere.
3. CO2 from the air combines with water to produce bicarbonate (HCO3), which is a
source of carbon for aquatic producers, primarily algae.
4. Similarly, when aquatic organisms respire, the CO2 they release combines with water
to form HCO3.
5. The amount of HCO3 in the water is in equilibrium with the amount of CO2 in the air.
6. The reservoir for the carbon cycle is largely composed of organic matter, calcium
carbonate in shells, and limestone, as well as fossil fuels.
D. Humans Alter Transfer Rates
1. Transfer rate is amount of nutrient moving from one part of the environment to
another in a time period.
2. Transfer rates between land and atmosphere and oceans and atmosphere due to
respiration are about even.
3. Because humans burn fossil fuels and forests, there is more CO2 entering the
atmosphere than is removed.
4. The oceans are apparently taking up much excess carbon dioxide.
E. The Nitrogen Cycle (Fig. 25.11) [transp. 137]
1. Nitrogen gas (N2) comprises about 78 percent of the atmosphere, yet nitrogen
deficiency often limits plant growth.
2. In the nitrogen cycle, plants cannot incorporate N2 into organic compounds and
therefore depend on various types of bacteria to make nitrogen available to them.
3. Nitrogen Gas Becomes Fixed
a. Nitrogen fixation is the process whereby N2 is reduced and added to
organic compounds.
b. Some cyanobacteria in water and free-living bacteria in soil are able to
reduce N2 to ammonium (NH4+).
c. Other nitrogen-fixing bacteria, living in nodules on the roots of legumes (Fig.
25.12), make reduced nitrogen and organic compounds available to the host
plant.
d. Plants cannot fix atmospheric nitrogen but take up both NH4+ and nitrate
(NO3- from the soil.
e. After plants take up NO3-, it is enzymatically reduced to NH4+ used to
synthesize amino and nucleic acids.
4. Nitrogen Gas Becomes Nitrates
a. Nitrification is the production of NO3 -.
b. Nitrogen gas is converted to NO3-n the atmosphere when cosmic radiation,
meteor trails, and lightning provide the high energy for nitrogen to react with
oxygen.
c. Nitrifying bacteria convert NH4+ to NO3-.
d. Ammonium in the soil is converted to NO3- by nitrifying bacteria in the soil in a
two-step process:
1) First, nitrite-producing bacteria convert NH4+ to nitrite (NO2-).
2) Then, nitrate-producing bacteria convert NO2- to NO3-.
e. Denitrification is conversion of NO3- to nitrous oxide (N2O) and N2.
f. There are denitrifying bacteria in both aquatic and terrestrial ecosystems.
g. Denitrification counterbalances nitrogen fixation, but not completely; more
nitrogen fixation occurs.
h. Humans contribute much to the nitrogen cycle when they convert N2 to
ammonium and urea in fertilizers.
i. Eutrophication (over enrichment) results from fertilizer runoff; when rampant
algae dies off, decomposers use up available oxygen during cellular
respiration, and this results in a massive fish kill.
F. The Phosphorus Cycle (Fig. 25.13) [transp. 138]
1. In phosphorus cycle, weathering makes phosphate ions (PO4 and HPO4-2) available
to plants from the soil.
2. Some of this phosphate runs off into aquatic ecosystems where algae incorporate it
into organic molecules.
3. The phosphate that is not taken up by aquatic phototrophs is incorporated into
sediments in the oceans.
4. Sediment phosphate becomes available when a geological upheaval exposes
sedimentary rocks to weathering.
5. The phosphate taken up by producers is incorporated into a variety of organic
compounds.
6. Animals eat producers and incorporate some of phosphate into teeth, bones, and
shells that take long to decompose.
7. Death and decay of organisms and decomposition of animal wastes makes phosphate
ions available again.
8. Because available phosphate is generally taken up quickly, it is usually a limiting
nutrient in most ecosystems.
G. Humans Influence the Phosphorus Cycle
1. Humans boost the supply of phosphate by mining phosphate ores for fertilizer
production, detergents, etc.
2. Phosphate ore is slightly radioactive and, therefore, mining phosphate poses a health
threat to all organisms.
3. Only a portion of land that has been mined has been properly reclaimed; the rest is
subject to severe erosion.
4. Runoff of animal wastes from livestock feedlots and commercial fertilizers from
cropland as well as discharge of untreated and treated municipal sewage can all add
excess phosphate to nearby waters.
H. The Cause of Pollution
1. Human activities impact biogeochemical cycles and ecosystems.
2. Pumping water from aquifers is not a normal part of the water cycle.
3. Burning fossil fuels and trees is increasing the amount of carbon dioxide in the
atmosphere.
4. Phosphorus cycle is affected if we produce detergents; nitrogen cycle is affected
when we produce fertilizers.
5. Human activities affect the transfer rates by moving one element from one component
of the ecosystem to another at a rate greater than natural transfer rates.
6. Pollution can be defined as a change in transfer rate that can lead directly or
indirectly to a degradation of human health or a degradation of plant and animal life.