ECOSYSTEMS
taken from Campbell, N., et al. Biology. 5th ed. Menlo Park, California: Benjamin/Cummings, 1999

OUTLINE
I. Trophic Relationships in Ecosystems
A. Trophic relationships determine an ecosystem's routes of er.ergy flow and chemical cycling
B. Primary producers include plants, algae, and many species of bacteria
C. Many primary and higher-order consumers are opportunistic feeders
D. Decomposition interconnects all trophic levels
II. Energy Flow in Ecosystems
A. An ecosystem's energy budget depends on primary productivity
B. As energy flows througlh an ecosystem, much is lost at each trophic level
III. Cycling of Chemical Elements in Ecosystems
A. Biological and geological processes move nutrients among organic and inorganic compartments
B. Decomposition rtes largely determine the rates of nutrient cycling
C. Field experiments reveal how vegetation regulates chemical cycling: science as a process
IV. Human Impacts on Ecosystems
A. The human population is disrupting chemical cycles throughout the biosphere
B. Toxins can become concentrated in successive trophic levels of food webs
C. Human activities are causing ffindamental changes in the composition of the atmosphere
D. The exploding human population is altering habitats and reducing biodiversity worldwide

OBJECTIVES
After reading this chapter and attending lecture, the student should be able to:
1. Explain the importance of autotrophic organisms with respect to energy flow and nutrient cycling in ecosystems.
2. List and describe the importance of the four consumer levels found in an ecosystem.
3. Explain how gross primary productivity is allocated by the plants in an ecosystem.
4. List the factors that can limit productivity of an ecosystem.
5. Explain why productivity declines at each trophic level.
6. Distinguish between energy pyramids and biomass pyramids.
7. Describe the hydrologic (water) cycle.
8. Describe the carbon cycle, and explain why it is said to result from the reciprocal processes of photosynthesis and cellular respiration.
9. Describe the nitrogen cycle, and explain the importance of nitrogen fixation to all living organisms.
10. Explain how phosphorus is recycled locally in most ecosystems.
11. Explain why the soil in tropical forests contains lower levels of nutrients than soil in temperate forests.
12. Describe how agricultural practices can interfere with nitrogen cycling.
13. Describe how deforestation can affect nutrient cycling within an ecosystem.
14. Describe how the carbon cycle differs in terrestrial and aquatic systems.
15. Explain how "cultural eutrophication" can alter freshwater ecosystems.
16. Explain why toxic compounds usually have the greatest effect on top-level carnivores.
17. Describe how increased atmospheric concentrations of carbon dioxide could affect the Earth.
18. Describe how human interference might alter the biosphere.
 
 

KEY TERMS
 

ecosystem  food chain  biomass  biogeochemical cycle 
trophic structure  foodweb  standing crop biomass  nitrogen fxation 
trophic level  production  limiting nutrient  nitrification 
primary producers  consumption  secondary productivity  ammonification 
primary consumers  decomposition  ecological efficiency  long-term ecological 
secondary consumers  primary productivity  pyramid of productivity  research (LTER) 
tertiary consumers  gross primary  biomass pyramid  biological magnification 
detritivores  productivity  turnover time  denitrification 
detritus  net primary productivity  pyramid of numbers  greenhouse effect 


 

LECTURE NOTES

Ecosystem = All organisms living in a given area along with the abiotic factors with which they interact

The boundaries of ecosystems are not usually discrete.

This is the most inclusive level of biological organization.

Ecosystems involve two processes that cannot be described at lower levels: energy flow and chemical cycling.
´ Energy flows through ecosystems and matter cycles within them.

I. Trophic Relationships in Ecosystems

Each ecosystem has a trophic structure of feeding relationships that determine the paths of energy flow and chemical cycling.
´ Ecologists divide the species in a community or ecosystem into different trophic levels based on their main source of nutrition.

A. Trophic relationships determine an ecosystem's routes of energy flow and chemical cycling
The five trophic levels typically recognized include:
1. Primary producers = Autotrophs (usually photosynthetic) that support all other trophic levels either directly or indirectly by synthesizing sugars and other organic molecules using light or chemical energy
2. Primary consumers = Herbivores that consume primary producers (plants and algae)
3. Secondary consumers = Carnivores that eat herbivores
4. Tertiary consumers = Carnivores that eat other carnivores
5. Detritivores (decomposers) = Consumers that derive energy from detritus (organic wastes) and dead organisms from other trophic levels. Detritivore often form a major link between primary producers and the consumers in an ecosystem.
 

An ecosystem's trophic structure determines the routes of energy flow and chemical cycling.
Food chain is the pathway along which food is transferred from trophic level to trophic level, beginning with primary producers.
 


 


 

´ Rarely are unbranched since several different primary consumers may feed on the same plant species and a primary consumer may eat several species of plants
´ Feeding relationships are usually woven into elaborate food webs within an ecosystem
 


 


 

It is important to distinguish between ecosystem structure (trophic levels) and ecosystem processes (production, consumption, decomposition). All organisms carry out each of the ecosystem processes to some extent.
´ Production refers to the rate of incorporation of energy and materials into the bodies of organisms.
´ In this sense, all organisms are producers; however, primary producers are often referred to as "producers" because their production supports all other organisms.
´ Consumption refers to the metabolic use of assimilated organic molecules for organismal growth and reproduction.
´ Decomposition is the breakdown of organic molecules into inorganic molecules.

B. Primary producers include plants, algae, and many species of bacteria
The main primary producers will vary depending on the ecosystem.

Plants are the main producers in most terrestrial ecosystems.
´ Debris falling from terrestrial plants that reaches streams (directly or through runoff) is a major source of organic material.
´ Phytoplankton (algae and bacteria) are the most important autotrophs in the limnetic zone of lakes and in the open ocean.
´ Multicellular algae and aquatic plants are often more important primary producers in the shallow, nearshore areas of freshwater and marine ecosystems.
´ The aphotic zone of the deep sea receives energy and nutrients (dead plankton, detritus) from the overlying photic zone.

Organisms in communities surrounding the hot water vents on the deep-sea floor depend more on chemical energy than solar energy.
´ The main producers are chemoautotrophic bacteria that derive energy from the oxidation of hydrogen sulfide.

C. Many primary and higher-order consumers are opportunistic feeders
Consumers also vary with the type of ecosystem.

Primary consumers in terrestrial ecosystems are mostly insects, snails, plant parasites, grazing mammals, and seed-eating and fruit-eating birds and mammals.
´ Primary consumers are considered opportunistic because they supplement their diet of autotrophs with heterotrophic material if it is available. Many consumers that mainly eat live organisms also scavenge dead organic material.

In aquatic ecosystems, the primary consumers are the zooplankton (heterotrophic protists, small invertebrates, numerous larval stages) and some fish. As with their terrestrial counterparts, aquatic consumers also are opportunistic

Secondary consumers in terrestrial ecosystems are spiders, frogs, insect-eating birds, carnivorous mammals, and animal parasites.

Secondary consumers in aquatic ecosystems are fish and benthic forms such as sea stars and other carnivorous invertebrates.

D. Decomposition interconnects all trophic levels
Organic matter in that composes living organisms in ecosystems is eventually recycled, decomposed, and returned to the abiotic environment in a form that can be used by autotrophs.

The most important decomposers are bacteria and fungi, which digest materials externally and then absorb the products.

Decomposition links all trophic levels.

II. Energy Flow in Ecosystems
Energy for growth, maintenance, and reproduction is required by all organisms; some species also require energy for locomotion.
 

A. An ecosystem's energy budget depends on primary productivity
Light energy is used by most primary producers to synthesize organic molecules (photosynthesis), which are later broken down to produce ATP (cellular respiration).
´ Since only primary producers can directly utilize solar energy, an ecosystem's entire energy budget is determined by the photosynthetic activity of the system.

Consumers obtain energy in the form of organic molecules that were produced at the previous trophic level. Thus, energy flows to higher trophic levels through food webs.

1. The global energy budget
Earth receives an estimated 1022 joules (J) of solar radiation each day.

The amount of solar radiation striking the Earth's surface shows dramatic regional variation that limits the photosynthetic output of ecosystems in different places.
´ The intensity of solar radiation also varies with latitude resulting in the tropics receiving the most input.
´ Most of the solar radiation is reflected, absorbed, or scattered by the atmosphere, clouds, and dust particles in the air; this amount varies over different regions.

Only a fraction of the solar radiation which reaches the biosphere strikes plants, photosynthetic bacteria, and algae (much hits bare ground or is absorbed or reflected by water) and these primary producers can only use some wavelengths for photosynthesis.

Only about 1% to 2% of the visible light reaching photosynthetic organisms is converted to chemical energy by photosynthesis.
´ The photosynthetic effciency also varies with the type of plant, light levels, and other factors.

Even with all the variations mentioned above, primary production of Earth collectively creates about 170 billion tons of organic material each year.

2. Primary productivity
Primary productivity is the amount of light energy converted to chemical energy by autotrophs of an ecosystem.
´ The total is known as gross primary productivity (GPP), which may be determined by measuring the total oxygen produced by photosynthesis.

Net primary productivity (NPP) = GPP - Rs (energy used by producers for respiration)
´ NPP accounts for the organic mass of plants (growth) and represents storage of chemical energy available to consumers.
´ The NPP:GPP ratio is generally smaller for large producers with elaborate nonphotosynthetic structures (such as trees) which support large metabolically active stem and root systems.

Primary produntivity can be expressed as biomass (expressed as dry weight since water contains no usable energy) added to an ecosystem per unit area per unit time (g/m2/yr) or as energy per unit time (J/m2/yr).
´ Primary productivity should not be confused with standing crop biomass.
´ Primary productivity is the rate at which new biomass is synthesized by photosynthetic organisms.
´ Standing crop biomass is the total biomass of photosynthetic autotrophs present at a given time, which may have accumulated over several growing seasons.

Primary productivity varies among ecosystems, and an ecosystem's size affects its contribution to the Earth's total productivity.
 


 

´ Tropical rain forests are very productive and contribute a large proportion to the planet's overall productivily since they cover a large portion of the Earth's surface.
´ Estuaries and coral reefs are also very productive but make only a small contribution to planetary productivity since they do not cover an extensive area.
´ The open ocean has a relatively low productivity but makes the largest contribution to overall productivity of any ecosystem due to its very large size.
´ Deserts and tundra also have low productivity.

Factors important in limiting productivity depend on the type of ecosystem and temporal changes such as seasons.

Generally, precipitation, temperature, and light intensity are factors limiting productivity in terrestrial ecosystems.
´ Productivity increases as latitudes approach the equator because availability of water, heat, and light increases in the tropics.
´ Productivity in terrestrial ecosystems may also be limited by availability of inorganic nutrients.
´ Plants require a variety of nutrients, some in large quantities and some in small quantities.
´ Primary productivity sometimes removes nutrients from the system faster than they can be replenished.
´ If a nutrient is removed in such quantities that sufficient amounts are no longer available, it becomes the limiting nutrient.
´ Adding the limiting nutrient will stimulate the system to resume growth until another nutrient or it becomes limiting (usually nitrogen or phosphorus).
´ Carbon dioxide availability sometimes limits productivity.

An aquatic ecosystem's productivity is usually determined by light intensity, water temperature, and availability of inorganic nutrients.
´ Productivity is greatest in shallow waters near continents and along coral reefs due to abundant nutrients and sunlight.
´ Light intensity and temperature affect primary productivity of phytoplankton in the open oceans; productivity is highest near the surface and decreases with depth.
´ Inorganic nutrients are limiting at the surface of open ocean waters with nitrogen and phosphorus in especially short supply.
* This is a primary reason for the relatively low productivity of open oceans.
´ Marine phytoplankton is most productive where upwellings bring nutrient-rich waters to the surface.
* These areas (usually in Antarctic seas) are more productive than tropical seas.
* Thermal vent communities are also very productive though they are not very widespread and contribute little to marine productivity.
´ Freshwater ecosystem productivity also varies from the surface to the depths in relation to light intensity.
* Availability of inorganic nutrients is sometimes limiting, but biannual turnovers bring nutrients to the surface waters.

B. As energy flows through an ecosystem, much is lost at each trophic level
The transfer of energy from one trophic level to another is not 100%.

The amount of energy available to each trophic level is determined by NPP and the efficiencies with which food energy is converted to biomass in each link of the food chain.

1. Secondary productivity
Secondary productivity is the rate at which consumers convert the chemical energy in the food they eat into their own biomass.

Consider that herbivores consume only a small fraction of available plant material and they cannot digest all of the organic compounds in what they do ingest
 


 


 

´ About 1/6 of the calories is used for growth which adds biomass to the trophic level.
´ The remaining organic material consumed is used for cellular respiration or is passed out of the body as feces.
´ The energy in the feces stays in the system and is consumed by decomposers.
´ The energy used in cellular respiration is lost from the system.
´ Carnivores are more efficient at converting food into biomass but more is used for cellular respiration, further decreasing energy available to the next trophic level.
Consequently, energy flows through an ecosystem, it does not cycle within the ecosystem.

2. Ecological efficiency and ecological pyramids
Ecological efficiency is the ratio of net productivity at one trophic level compared to net productivity at the level below, or the percentage of energy transferred from one trophic level to the next.
´ Efficiencies can vary greatly depending on the organisms involved, but usually range from 5% to 20%.
´ This means that 80% to 95% of the energy available at one trophic level never transfers to the next.

Loss of energy in a food chain can be represented diagrammatically in several ways:
1 . A pyramid of productivity has trophic levels stacked in blocks proportional in size to the productivity of each level.
´ Usually bottom heavy since ecological efficiencies are low
 


 







 

2. A biomass pyramid has tiers that each symbolize the total dry weight of all organisms (standing crop biomass) in a trophic level
 


 


 

´ Most narrow sharply from producers at the base to top-level carnivores at the apex because of energy transfers between trophic levels are so inefficient.
´ Some aquatic ecosystems are inverted because producers have a short turnover time. They grow rapidly but are consumed rapidly, leaving little standing crop biomass.

3. A pyramid of numbers is comprised of blocks which are proportional in size to the numbers of individuals present at each trophic level.
 


 

´ Biomass of top-level carnivores is usually small compared to the total biomass of producers and lower-level consumers.
´ Only about 1/1000 of the chemical energy fixed by photosynthesis flows through a food web to a tertiary consumer.
´ Only 3 to 5 trophic levels can be supported since the energy in the webs is insufficient to support another trophic level.
´ Predators (top-level consumers) are highly susceptible to extinction when their ecosystem is disturbed due to their small population and wide spacing within the habitat.

For humans, eating meat is a relatively inefficient way of tapping photosynthetic productivityˇeating grains directly as a primary consumer provides far more calories.

III. Cycling of Chemical Elements in Ecosystems
Despite an inexhaustible influx of energy in the form of sunlight, continuation of life depends on recycling of essential chemical elements.
´ These elements are continually cycled between the environment and living organisms as nutrients are absorbed and wastes released.
´ Decomposition of wastes and the remains of dead organisms replenishes the pool of inorganic nutrients available to autotrophs.
Biogeochemical cycles = Nutrient circuits involving both biotic and abiotic components of ecosystems

A. Biological and geological processes move nutrients among organic and inorganic compartments
There are two general categories of biogeochemical cycles:
1. Elements such as carbon, oxygen, sulfur, and nitrogen have gaseous forms, thus, their cycles are global in character and the atmosphere serves as a reservoir.
2. Elements less mobile in the environment like phosphorus, potassium, calcium and trace elements generally cycle on a more localized scale over the short terrn. The soil serves as the main reservoir for these elements.

A general scheme of nutrient cycling includes the four main reservoirs of elements and the processes that transfer elements between reservoirs.
 


 


 

Reservoirs are defined by two characteristics: whether they contain organic or inorganic materials; and whether or not the materials are directly available for use by organisms.
´ The available organic reservoir contains the living organisms and detritus.
* The nutrients are readily available when consumers feed on one another and when detritivores eat nonliving organic matter..
´ The unavailable organic reservoir is comprised of coal, oil, and peat which formed from organisms that died and were buried millions of years ago.
* These nutrients cannot be directly assimilated.
´ The available inorganic reservoir includes all matter (elements and compounds) present in the soil or air and those dissolved in water.
* Organisms can directly assimilate these nutrients from the soil, air, or water.
´ The unavailable inorganic reservoir contains nutrients tied up in limestone and minerals of other rocks.
* These nutrients cannot be assimilated until released by weathering or erosion.

Various processes are involved in the transfer of nutrients between the four reservoirs which form the basis for biogeochemical cycling. The general schemes were determined by adding small amounts of radioactive tracers to systems in order to follow the movement of elements.
´ Weathering and erosion are the primary processes which move nutrients from the unavailable inorganic reservoir to the available inorganic reservoir.
´ Erosion is also important, along with the burning of fossil fuels, in moving nutrients from the unavailable organic reservoir to the available inorganic reservoir.
´ Nutrients are transferred from the available organic reservoir to the unavailable organic reservoir only by the covering of detritus by sediments and its eventual fossilization to oil, coal, or peat.
´ Sedimentary rock formation is the process which moves nutrients from the available inorganic reservoir to the unavailable inorganic reservoir.
´ Nutrients enter the available organic reservoir from the available inorganic reservoir through photosynthesis and assimilation by living organisms.
´ Nutrients are transferred from the available organic reservoir to the available inorganic reservoir by respiration, decomposition, excretion, and leaching.
The cycling of materials through an ecosystem depends on both biological and geological processes.

1. The water cycle
The essential nature of water to living organisms has many facets:
´ It is essential to maintaining homeostasis in every organism.
´ It contributes to the fitness of the environment.
´ Its movement within and between ecosystems transfers other materials in several biogeochemical cycles.

Most of the water cycle occurs between the oceans and the atmosphere.
 


 


 

´ Solar energy results in evaporation from the oceans.
´ Water vapor rises, cools, and falls as precipitation.
´ Over the oceans, evaporation exceeds precipitation; the excess water vapor is moved onto land by winds.
´ Precipitation exceeds evaporation and transpiration over land; runoff and ground water balance the net flow of water vapor to land.
The water cycle differs from other cycles in that it occurs primarily due to physical processes, not chemical processes.

2. The carbon cycle
In the carbon cycle. photosynthesis and cellular respiration form a link between the atmosphere and terrestrial environments.
´ During the carbon cycle, autotrophs acquire carbon dioxide (CO2) from the atmosphere by diffusion through leaf stomata, incorporating it into their biomass. Some of this becomes a carbon source for consumers, and respiration returns CO2 to the atmosphere.
Carbon recycles relatively quickly. Plants have a high demand for CO2, yet CO2 is present in the atmosphere at a low concentration (0.03%).
´ Carbon loss by photosynthesis is balanced by carbon release during respiration.
Some carbon is diverted from cycling for longer periods of time, as when it accumulates in wood or other durable organic material.
´ Decomposition eventually recycles this carbon to the atmosphere.
´ Can be diverted for millions of years, such as in the formation of coal and petroleum.
The amount of atmospheric CO2 decreases in the Northern Hemisphere in summer due to increased photosynthetic activity.
´ Amounts increase in the winter when respiration exceeds photosynthesis.
Atmospheric CO2 is increased by combustion of fossil fuels by humans, disturbing the balance.

In aquatic environments photosynthesis and respiration are also important but carbon cycling is more complex due to interaction of CO2 with water and limestone.
´ Dissolved CO2 reacts with water to form carbonic acid, which reacts with limestone to form bicarbonates and carbonate ions.
´ As CO2 is used in photosynthesis, bicarbonates convert back to CO2; thus bicarbonates serve as a CO2 reservoir and some aquatic autotrophs can use dissolved bicarbonates directly as a carbon source.
´ The ocean contains about 50 times the amount of carbon (in various inorganic forms) as is available in the atmosphere. The ocean may act as a buffer to absorb excess CO2.

3. The nitrogen cycle
Nitrogen is a key chemical in ecosystems as it is found in all amino acids which comprise the proteins of organisms.

Although the Earth's atmosphere is almost 80% N2, it is unavailable to plants since they cannot assimilate this form.
´ Nitrogen is available to plants in only two forms: ammonium (NH4+) and nitrate (NO3)

Nitrogen enters ecosystems by either atmospheric deposition or nitrogen fixation.
Atmospheric deposition accounts for only 5 to 10% of the usable nitrogen that enters an ecosystem.
´ NH4+ and NO3 are added to the soil by being dissolved in rain or by settling as part of fine dust or other particulates.
´ Some plants (epiphytic bromeliads) in the canopy of tropical rain forests have aerial roots that can take up NH4+ and NO3- from the atmosphere.

Nitrogen fxation is the reduction of atmospheric nitrogen (N2) to ammonia (NH3), which can be used to synthesize nitrogenous organic compounds such as amino acids.
 


 


 

´ Only certain prokaryotes can fix nitrogen.
´ In terrestrial ecosystems some nonsymbiotic soil bacteria and some symbiotic (Rhizobium) bacteria fix nitrogen.
´ Cyanobacteria fix nitrogen in aquatic ecosystems.
´ Nitrogen fixing prokaryotes are fulfilling their own metabolic needs, but other organisms benefit since excess ammonia is released into the soil or water.
´ Industrial fixation in the form of fertilizer makes significant contributions to the nitrogen pool in agricultural regions.
´ The slightly acidic nature of soil results in NH3 being protonated to ammonium (NH4+).
´ NH3 is a gas and can evaporate quickly to the atmosphere.
´ NH4+ can be used directly by plants.

The nitrogen cycle involves three processes in addition to nitrogen fxation: nitrification, denitrification, and ammonification.
1. Nitrification is a metabolic process by which certain aerobic soil bacteria use ammonium (NH4+) as an energy source by fist oxidizing it to nitrite (NO2-) and then to nitrate (NO3-).
´ While plants can use NH4+ directly, the nitrifying bacteria use most of the available NH4+ as an energy source.
´ Plants assimilate the NO3- released from these bacteria and convert it to organic forms, such as amino acids and proteins.
´ Animals can only assimilate organic nitrogen which they obtain by eating plants and other animals.

2. Denitrification occurs when bacteria obtain the oxygen necessary for their metabolism from NO3- rather than O2 under anaerobic conditions. This process returns nitrogen to the atmosphere by converting NO3to N2.

3. Ammonification is the decomposition of organic nitrogen back into ammonium.
´ Carried out mainly by decomposer bacteria and fungi
´ Process is especially important because it recycles large amounts of nitrogen to the soil

Some important aspects of the nitrogen cycle to remember include:
´ Prokaryotes serve as vital links at several points in the cycle.
´ Most of the nitrogen cycling involves nitrogenous compounds in the soil and water.
´ While atmospheric nitrogen is plentiful, nitrogen fixation contributes only a small fraction of the nitrogen assimilated by plants; however, many species of plants depend on symbiotic, nitrogen-fixing bacteria in their root nodules as a source of nitrogen in a form that can be assimilated.
´ Denitrification returns only a smal1 amount of N2 to the atmosphere.
´ Most assimilated nitrogen comes from nitrate, which is efficiently recycled from organic forms by ammonification and nitrification.
´ The majority of nitrogen in most ecosystems is recycled locally by decomposition and reassimilation, although nitrogen exchange between the soil and atmosphere are of long-term importance.

4. The phosphorus cycle
Phosphorus is a major component of nucleic acids, phospholipids, ATP, and a
mineral in bones and teeth.

The phosphorus cycle is relatively simple since it does not have a gaseous form and
it occurs in only one important inorganic form, phosphate.

Phosphorus cycles locally as follows:
 


 


 

´ Weathering of rock adds phosphate to the soil.
´ Producers absorb the soil phosphate and incorporate it into molecules.
´ Phosphorus is transferred to consumers in organic form.
´ Phosphorus is added back to the soil by excretion by animals and by decomposition of detritus by decomposers.
´ Phosphorus cycling is localized since humus and soil particles bind phosphate.
* Some leaching does occur and phosphate is lost to the oceans through the water table.
* Weathering of rocks keeps pace so terrestrial systems are not depleted.
* Phosphate that reaches the oceans accumulates in sedhnents and becomes incorporated into rocks which may eventually be exposed to weathering.

Phosphorus may limit algal productivity in aquatic habitats.
´ Production in these habitats is stimulated by the introduction of phosphorus in the form of sewage or runoff from fertilized agricultural areas.

B. Decomposition rates largely determine the rates of nutrient cycling
The rate of decomposition has a great impact on the timetable for nutrient cycling.
´ The rate of decomposition (and thus nutrient cycling) is affected by water availability, oxygen, and temperature.
´ Decomposition of organic material in the tropical forests usually occurs in a few months to a few years.
´ It takes an average of four to six years for decomposition to occur in temperate forests.
´ Decomposition in the tundra may take 50 years.
´ In aquatic ecosystems, where most decomposition occurs in anaerobic bottom muds, decomposition may occur even more slowly than in the tundra.

Soil chemistry and the frequency of fires also influence nutrient cycling times.

Some key nutrients are present in the soil of tropical rain forests in levels much lower than those found in temperate forests. Several conditions influence this paradox:
´ There is rapid decomposition in tropical areas due to warm temperature and abundant water.
´ The large biomass of tropical rain forests creates a high demand for nutrients, which are absorbed as soon as they become available through the action of decomposers.
* About 10% of the nutrients are in the soil; 75% are present in the woody parts of trees.
´ Relatively little organic material accumulates as litter due to the rapid decomposition.
´ The low nutrient content of the soil results from the rapid cycling time.

The soil in temperate forests may contain 50% of all organic material in the ecosystem.
´ The rate of decomposition is slow.
´ The nutrients present in detritus and soil may stay there for long periods before being assimilated.

The sediments of aquatic systems form a nutrient sink and there must be an interchange between the bottom layers of water and the surface for the ecosystem to be productive.
´ The rate of decomposition in the sediments is very slow.
´ Algae and aquatic plants usually assimilate their nutrients directly from the water.

C. Field experiments reveal how vegetation regulates chemical cycling: science as a process
Long-term ecological research (LTER) is being used to examine the dynamics of many natural ecosystems over relatively long periods of time.

Since 1963, scientists have been studying nutrient cycling in a forest ecosystem under natural conditions and after vegetation is removed. The study site is the Hubbard Brook Experiment Forest in New Hampshire.
´ The team first determined mineral budgets of six valleys by measuring inflow and outflow of several key nutrients.
´ Rainwater was collected to measure amounts of water and dissolved minerals added to the ecosystem.
´ Water and mineral loss were monitored by using small concrete dams with a V-shaped spillway across the creek at the bottom of each valley.
´ Scientists found that 60% of the water added by rainfall exits through streams and 40% is lost by plant transpiration and evaporation from soil.
´ They also found that mineral inflow and outflow were nearly balanced and were small compared to minerals being recycled within the forest ecosystem.
* Only about 0.3% more Ca++ exited a valley through its creek than was added by rainwater. Net mineral losses were probably replaced by chemical decomposition of bedrock.
* During most years, some net gains of a few mineral nutrients occurred.
 

In 1966, after logging an experimental area and preventing reforestation, comparisons were made over a three-year period.
´ Water runoff increased by 30 to 40% (no plants were left to absorb and transpire water).
´ Net losses of minerals were very large:
* Nitrate loss increased 60-fold (water nitrate levels made the water unsafe for drinking).
* Calcium loss increased 400%
* Potassium loss increased 1500%.

The study demonstrated the importance of plants in retaining nutrients within an ecosystem and the effects of human intrusion into a system.
´ None of the watersheds was undisturbed by human activity even when the study began. Acid precipitation has leached most of the Ca2+ from forest soil, resulting in increased levels of Ca2+ in stream water. By the 1990s, the forest plants stopped adding new growth, apparently due to the lack of Ca2+.
 

IV. Human Impacts on Ecosystems
The ever increasing human population has intruded into the dynamics of most ecosystems through human activities or technology.
´ Some natural systems are totally destroyed while others have had major components (trophic structure, energy flow, chemical cycling) disrupted.
´ Most effects are local or regional, while others are global in scale (e.g., acid rain).

A. The human population is disrupting chemical cycles throughout the biosphere
Human activity often removes nutrients from one part of the biosphere and adds them to another.
´ May deplete one area of key nutrients while creating an excess in another area
´ These occurrences disrupt the natural equilibrium of chemical cycles in both areas.

Farming exhausts the natural store of nutrients as crop biomass is removed from an area, this greatly reduces the amount of nutrients recycled. Supplements must then be added in the form of fertilizer.
´ Nutrients in crops soon appear in human and livestock wastes, and then turn up in lakes and streams through sewage discharge and field run-off.
´ Once in aquatic systems, these nutrients may stimulate excessive algal growth which degrades the system.
´ Consequently, disruptions can flow from one system to another.

1. Agricultural effects on nutrient cycling
As the human population has continued to grow, greater demands for production of food has resulted in natural habitats being converted to agricultural use. This has resulted in:
´ Intrusions into the cycling of nutrients
´ Overharvesting of natural populations of food species
´ Introductions of toxic compounds into ecosystems in the form of pesticides

After natural vegetation is cleared from an area, the time period during which no additional nutrients need to be added to new agricultural ecosystems varies greatly.
´ Nutrient reserves in the soil will support crops for some time after the natural vegetation has been removed.

´ These nutrients are not recycled locally since they are removed from the system as crop biomass.
´ Some new farmlands in the tropics can be farmed for only one or two years.
* Remember, in the tropical rain forests only about 10% of the nutrients are in the soil.
´ In temperate areas, crops may be grown for many years due to the nutrients present in the soil.
´ When nutrients are added, it is normally in the form of industrially synthesized fertilizers.

The nitrogen cycle of an area is greatly impacted by agriculture.
´ Breaking up and mixing the soil increase the rate of decomposition of organic matter.
´ This releases usable nitrogen, which is taken up by the crop and exported from the system at harvest.
´ Nitrates remaining in the soil are quickly leached out of the system.
´ Fertilizers are applied to replace the lost nitrogen.
* Human activities have approximately doubled the Earth's supply of fixed nitrogen.
* Excess nitrogen in fertilizers leaches into the water table.
* Increased nitrogen fixation is also associated with a greater release of nitrogen compounds into the air by denitrifiers.
> Nitrogen oxides can contribute to atmospheric warming, to the depletion of atmospheric ozone, or to acid precipitation.
* Excess algal and bacterial growth typically results from an overabundance of nitrogen entering surface waters.

2. Accelerated eutrophication of lakes
Lakes are classified on a scale of increasing nutrient availability as oligotrophic, mesotrophic, or eutrophic.
´ Oligotrophic lakes have low primary productivity because mineral nutrient levels will not support large phytoplankton populations.
´ In other lakes, basic and watershed characteristics cause the addition of more nutrients that are captured by the primary producers and continuously recycled through the lake's food webs.
´ Overall productivity is higher in mesotrophic lakes and highest in eutrophic ones.

Sewage, factory wastes, livestock runoff, and fertilizer leaching increases inorganic nutrient levels in waters and results in cultural eutrophication.
´ This enrichment often results in explosive growth of photosynthetic organisms.
´ Large algal blooms occur; shallow areas become choked with weeds.
´ As these producers die, metabolism of detritivores consumes all the oxygen in the water and many species die.

B. Toxins can become concentrated in successive levels of food webs
A variety of toxic chemicals, including unnatural synthetics, are dumped into ecosystems.
´ Many cannot be degraded by microbes and persist for years or decades.
´ Some are harmless when released but are converted to toxic poisons by reactions with other substances or by the metabolism of microbes (e.g., conversion of mercury to methyl mercury).

Organisms acquire toxic substances along with nutrients or water, some are metabolized and excreted while others accumulate in their tissues (e.g., DDT, PCBs).

Biological magnif cation = Process by toxins become more concentrated with each successive trophic level of a food web; results from biomass at each trophic level being produced from a much larger biomass ingested from the level below.
´ Top level carnivores are usually most severely affected by toxic compounds released into the environment.

The pesticide DDT is a well known example of biological magnification.
 


 


 

´ It is used to control mosquitoes and agricultural pests.
´ DDT persists in the environment and is transported by water to areas away from the point of application.
´ It is lipid-soluble and collects in fatty tissues of animals.
´ One of the first signs that DDT was a serious environmental problem was the decline in bird populations that feed at the top of food chains.
* Reproductive rates declined dramatically because DDT interfered with the deposition of calcium in eggshells and the weight of nesting birds broke the weakened shells.
´ DDT use was banned in the United States in 1971 and the affected bird populations have recovered.
´ The use of DDT still continues in other parts of the world.

C. Human activities are causing fundamental changes in the composition of the atmosphere
Human activities have resulted in the release of many gaseous waste products into the atmosphere.

One problem directly related to nutrient cycling is the rising levels of carbon dioxide.

1. Carbon dioxide emissions and the greenhouse effect
Carbon dioxide emissions have caused atmospheric CO2 concentrations to increase 14% since 1958. This increase is due to combustion of fossil fuels and burning of wood removed by deforestation.

Some effects of increased carbon dioxide levels might appear to be beneficial while others are definitely detrimental.
´ Increased productivity by vegetation would occur with increased CO2.
´ C3 plants are more limited than C4 plants by CO2, so spread of C3 species into habitats previously favoring C4 species may have important natural and agricultural implications.
´ Temperature increases with increased CO2 concentration since CO2 and water vapor absorb infrared radiation and slows its escape from Earth.
´ Called the greenhouse effect
´ A number of studies predict a doubling of CO2 by the end of the 21st century and an associated average temperature increase of about 2░C above that in l990.
´ Scientists are predicting a variety of scenarios based on the global warming trend.
´ Some predict warming near the poles will result in melting of polar ice and flooding of current coastal areas.
´ A warming trend would probably alter geographical distribution of precipitation, which could have major agricultural implications.
´ Ecologists are studying the records of pollen cores to determine how past temperature changes have affected vegetation.

2. Depletion of atmospheric ozone
Depletion of atmospheric ozone weakens a protective layer in the stratosphere that absorbs ultraviolet radiation.
´ Much of the ultraviolet radiation is absorbed by an ozone layer 17 to 25 km above the Earth's surface.
´ Destruction of the ozone layer is largely due to accumulation of chlorofluorocarbons used as aerosol propellants and in refrigeration.
´ Breakdown products of chlorofluorocarbons include chlorine which rises to the stratosphere where it reacts with ozone (03) and reduces it t o atmospheric oxygen (02).
* The chlorine is released in other reactions and reacts with additional ozone molecules.
´ Ozone depletion is best documented over Antarctica but levels in the middle latitudes have decreased 2% 10% in the last 20 years.
 
 
 
 

Ozone depletion could have serious consequences.
´ Increases are expected in lethal and nonlethal forms of skin cancer and cataracts among humans.
´ Unpredictable effects on crops and natural communities (especially phytoplankton) are expected.

D. The exploding human population is altering habitats and reducing biodiversity worldwide
The growth of human populations, human activities, and our technological capabilities have disrupted the trophic structure, energy flow, and chemical cycling of ecosystems in most areas of the world.

Some effects are local while others affect the biosphere's distribution and diversity of organisms.

The destruction of natural systems due to human encroachment has resulted in only a small proportion of natural, undisturbed habitat remaining in existence.
´ Only 15% of the original primary forest and just 1% of original tallgrass prairie remain in the United States.
´ Tropical rainforests are being cut at a rate of 500,000 km2 per year and will be eliminated in a couple of decades.
´ Human activities that disrupt entire systems include development, logging, war, and oil spills.

One result of the destruction of natural habitat will be the loss of biodiversity.