Biology II

Chapter 49 Notes

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

· Boundaries are usually not discrete.

· The most inclusive level of biological organization.

· Involves two processes which cannot be described at lower levels: energy flow and chemical cycling.

I.Trophic structure determines an ecosystem’s routes of energyflow and chemical cycling.

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.

The five trophic levels typically recognized are:

Primary Producers= Autotrophs (usually photosynthetic) that support all other trophic levels either directly or indirectly by synthesizing surgars and other organic molecules using light or chemical energy.

Primary consumers= Herbivores that consume primary producers (plants and algae).

Secondary consumers= Carnivores that eat herbivores

Tertiary consumers= Carnivores that eat other carnivores.

Decomposers (detritivores)= Consumers that derive energy from detritus (organic wastes) and dead organisms from other trophic levels.

Food Chain= The pathway along which food is transferred from trophic level to trophic level. (See Campbell, Figure 49.1)

· 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. (See Campbell, Figure 49.2)

A. Producers

The main producers will vary depending on the ecosystem.

· Plants are the main producers in most terrestrial ecosystems.

ð Debris felling from terrestrial plants that reaches streams (directly or through run off) is a major source of organic material.

· Phytoplankton are the most important autotrophs in the limnetic zone of lakes an in the open ocean.

· Multicellular algae and aquatic plants are most important in the shallow, near-shore 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.

ð The main producers are chemoautotrophic bacteria that derive energy from the oxidation of hydrogen sulfide.

B. Consumers

Consumers also vary with the type of ecosystem.

· Primary consumers in terrestrial ecosystems are mostly insects, snails, grazing mammals, and seed-eating and fruit-eating birds and mammals.

· In aquatic systems, the primary consumers are the zooplankton (heterotrophic protests, small invertebrates, numerous larval stages) and some fish.

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

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

C. Decomposers (Detritivores)

Decomposers are a critical component in any ecosystem.

· Are key components of the material recycling.

· Decomposers form a major link between primary producers and higher-level consumers.

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

· Earthworms and a variety of scavengers are also decomposers.

II. An ecosystem’s energy budget depends on primary productivity

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

· Light energy is used by primary producers to synthesize organic molecules (photosynthesis) which are later broken down to produce ATP (cellular respiration).

· 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.

· Since only primary producers can directly utilize solar energy, an ecosystem’s entire energy budget is determined by the photosynthetic activity of the system.

A. The Global Energy Budget

The amount of solar radiation striking the Earth’s surface shows dramatic regional variation which limits the photosynthetic output of ecosystems in different places.

· Earth receives an estimated 10²² joeles (J) of solar radiation each day.

· 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.

· The intensity of solar radiation also varies with latitude resulting in the tropics receiving the most input.

· Only a fraction of the solar radiation which reaches the bioshpere strikes plants 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%-2% of the visible light reaching primary producers is converted to chemical energy by photosynthesis.

ð The photosynthetic efficiency also varies with the type of plant, light levels, and other factors.

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

B. Primary Productivity

Primary productivity= The amount of light energy converted to chemical energy by autotrophs of an ecosystem.

· The total is knows 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 of 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 nonphotsynthetic structures (such as trees) which support large metabolically active stem and root systems.

· 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).

Primary productivity should not be confused with standing crop biomass.

· Primary productivity is the rate at which new biomass is synthesized by vegetation.

· Standing crop biomass is the total biomass of plants present at the given time which may have accumulated over several growing seasons.

Primary productivity varies among ecosystems, and an ecosystem’s and an ecosystem’s size affects its contribution to the Earth’s total productivity. (See Campbell, Figure 49.3)

· Tropical rain forests are very productive and contribute a large proportion to the plant’s overall productivity 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 dpend on the type of ecosystem and temporal changes such as seasons.

1. Generally, precipitation, temperature, and light intensity are factors limiting productivity in terrestrial ecosystems.

· Productivity increases as latitudes approach the equator because availability of water, hear, and light increases in the tropics.

2. 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 production 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.

· The limiting nutrient is one present in insufficient quantities to support further primary production.

· Nitrogen and phosphorus are usually limiting nutrients since they are needed in large quantities but are often present in small or moderate amounts in natural environments.

· Carbon dioxide availability sometimes limits productivity.

3. An aquatic ecosystem’s productivity us usually determined by light intensity, water temperature, and availability of inorganic nutrients.

· Productivity is greates 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.

ð This is a primary reason for the relatively low productivity of open oceans.

· Marin phytoplankton is most productive where upwellings bring nutrient rich waters to the surface.

ð These areas (usually in polar seas) are more productive than tropical seas.

ð Thermal vent communities are also very productive though they are not very wide-spread 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 b biannual turnovers bring nutrients to the surface waters.

III. As energy flows through an ecosystem, much is lost at each trophic level

The transfer of energy from one trophic level to another is much less efficient than many realize. The biomass of producers is much greater than that of herbivores and the relationship holds through subsequent consumer levels.

A. Secondary Productivity

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

As an example, consider herbivores which consume only a small fraction of available plant material and they con not digest all of the organic compounds in what they do ingest. (See Campbell, Figure 49.4)

· 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 abailable to the next trophic level.

· Consequently, energy flows through an ecosystem, it does not cycle within ecosystem.

C. 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- the percentage of energy transferred from one trophic level to the next.

· Efficiencies can vary greatly depending on the organisms involved, but usually ranges from 5%-20%.

· This means that 85%-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:

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. (See Campbell, Figure 49.5).

2) A biomass pyramid has each tier symbolizing the total dry weight of all organisms (standing crop biomass) in an ecosystem’s levels. (see Campbell, Figure 49.6).

· Most narrow sharply from producers at the base to top-level carnivores at the top because of low ecological efficiencies.

· Some aquatic systems are inverted since producers can have high turnover rates. They grow rapidly but are consumed rapidly, leaving little standing crop biomass.

3) A pyramid of numbers is comprised of blocks whose size is proportional to the numbers of individuals present at each level. (See Campbell, Figure 49.7)

· 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.

· Only3-5 trophic levels can be supported since biomass at the apex is insufficient to support another 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.

IV. Matter cycles within and between 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.

· 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.

· Elements less mobile in the environment like phosphorus, potassium, calcium and trace elements generally cycle on amore localized scale over the short term. 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. (see Campbell, Figure 49.8)

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.

ð The nutrients are readily available when organisms feed on one another.

· The unavailable organic reservoir is composed of coal, oil, and peat which formed from organisms that died and were buried millions of years ago.

Ø These nutrients are unavailable since they 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.

ð 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.

Ø These nutrients cannot be assimilated until released by weather 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.

V. A combination of biological and geological processes drives chemical cycles.

The cycling of materials through an ecosystem depends on both biological and geological processes.

A. 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.

B. The Carbon Cycle

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.

· Photosynthesis and cellular respiration form a link between the atmosphere and terrestrial environments. (See Campbell, Figure 49.10)

· Carbon cycles fast. 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.

· The ocean may act as a buffer to absorb excess CO₂.

In aquatic environments photosynthesis and respiration are also important by 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; this 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.

C. 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 5-10% 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.

Nitrogen fixation 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.

ð In terrestrial ecosystems some nonsybiotic soil bacteria and some bymbiotic (Rhizobium) bacteria fix nitrogen.

ð Cyanobacteria fix nitrogen in aquatic ecosystems.

· Nitrogen fizing prokaryotes are fulfilling their own metabolic needs, but other organisms benefit since excess ammonia is relased 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⁺).

Ø NH₃ is a gas and can evaporate quickly to the atmosphere.

Ø NH₄⁺ can be used directly by plants.

The nitrogen cycle involves three processes in addition to nitrogen fixation: nitrification, denitrification, and ammonification. (See Campbell, Figure 49.11)

Nitrification is a metabolic process by which certain aerobic soil bacteria use ammonium (NH₄⁺) as an energy source by first oxidizing it to nitrite (NO₂^-) and the to nitrate (NO₃^-).

· While plants can use NH₄⁺ directly, the nitrifying bacteria use most of the available NH₄⁺ as energy source.

· Plants assimilate the NO₃^- released from these bacteria and convert it to organic forms like amino acids and proteins.

· Animals can only assimilate organic nitrogen which they obtain by eating plants and other animals.

Denitrification is a process that returns nitrogen to the atmosphere by converting NO₃^- to N₂.

· Some soil bacteria obtain the oxygen necessary for their metabolism from NO₃^- rather that O_2.

Ammonification is the decomposition of organic nitrogen back into ammonia.

· Carried out by many decomposer anaerobic and aerobic bacteria and fungi.

· Process is especially important since it recycles large amounts of nitrogen to the soil.

Some important aspects of the Nitrogen Cycle to note:

· 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 small 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.

D. 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 occurs in only one important inorganic form, phosphate.

Phosphorus cycles locally as follows: (See Campbell, Figure 49.12)

· 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 and decomposition of detritus.

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 sediments and becomes incorporated into rocks which may eventually be exposed to weathering.

Phosphorus may limit agal 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.

E. Variations in Nutrient-Cycling Time

The rate of decomposition has a great impact on the time table 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 that become anaerobic, 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. 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 for decomposers.

· Relatively little organic material accumulates as litter due to the rapid decomposition.

ð About 10% of the nutrients are in the soil; 75% are present in the woody parts of trees.

· The rapid cycling time results in the low nutrient content of the soil.

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 in the soil 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.

VI. 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.

Scientists have been studying nutrient cycling in a forest ecosystem under natural conditions and after vegetation is removed. The study began in 1963 at the Hubbard Brook Experiment Forest in New Hampshire.

· The team first determined budgets of six valleys by measuring the inflow and outflow of several key nutrients.

· Rainwater was collected to measure amounts of water to dissolved minerals added to the ecosystem.

· Water and mineral loss were monitored by using concrete weirs across the creek at the bottom of each valley.

ð Found that 60% of the water added by rainfall exists through streams and 40% is lost by plant transpiration and evaporation from the soil.

· Also found that mineral inflow and outflow were nearly balanced and were small compared to minerals being recycled within the forest ecosystem.

ð Only about .3% more Ca++ exited a valley through its creek than was added by rainwater.

ð Net mineral losses were probably replaced by chemical decomposition of bedrock.

· During most years, some net gains of a few mineral nutrients occurred.

After logging an experimental area and preventing reforestation, comparisons were made over three year period.

· Water runoff increased by 30-40% (no trees were left to absorb and transpire water).

· Net losses of minerals were very large:

ð Nitrate loss increased sixty fold (water nitrate levels made the water unsafe for drinking).

ð Calcium loss increased 400%

ð 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.

VII. The human population is disrupting chemical cycles throughout the biosphere.

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 (i.e. acid rain).

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 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.

A. Agricultural effects on Nutrient Cycling

As the human population continues to grow, greater demands for production of food will result in more natural habitats being converted to agricultural use. This will result in more:

· Intrusions into the cycling of nutrients.

· Over-harvesting of natural populations of food-species.

· Introductions of toxic compounds into ecosystems in the form of pesticides.

The time period during which no additional nutrients need to be added to new agricultural ecosystems vary.

• Nutrients reserves in the soil will support crops for sometime after the natural vegetation has been removed.
• These nutrients are not recycled locally since they are removed from the system as crop biomass.
• New farmlands in the tropic can usually be farmed for one or two years.

ð 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 increases 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.

ð Nitrates remaining in the soil are quickly leached out of the system.

• Fertilizers are applied to replace the lost nitrogen.

ð Results in more N₂ being released into the atmosphere by upsetting the balance between denitrification and nitrogen fixation.

• Leaching to the water table and surface runoff carries much of the nitrate in fertilizers into aquatic systems.

Ø Excess algal growth typically results from an over-abundance of nitrates entering an aquatic system.

B. Accelerated Eutrophication of Lakes

Lakes undergo a naturally occurring change in chemical composition and character.

• Oligotrophic lakes have low primary productivity because mineral nutrient levels will not support large phytoplankton populations.
• The lake will gradually mature (become more eutrophic) due to runoff from surrounding land which contains nutrients.
• These additional nutrients are assimilated by phytoplankton and productivity increases; once in the lake the nutrients are continually recycled through the lake’s food webs.
• Lakes reach an equilibrium where nutrient input is balanced by losses due to outflow and sedimentation.

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 photosynthesis organisms.

ð Large algal blooms occur; shallow areas become choked with weeds.

• As these producers die, metabolism of decomposers consumes all the oxygen in the water and many species die.

C. Poisons in Food Chains

A variety of toxic chemicals, including unnaturally synthetics, have been and 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 within other substances or by the metabolism of microbes.

Organisms acquire toxic substances along with nutrients or water, some are metabolized and excreted while others accumulate in their tissues.

Biological magnification = 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. (See Campbell, Figure 49.16)

• 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.
• The concentration is magnified at each trophic level and reached such high concentrations (10 X 10⁶ increase) in top-level carnivores birds that calcium deposition in eggshells was disrupted.
• Reproductive rates declined dramatically since the weight of nesting birds broke the weaked 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.

D. Intrusions in 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.

E. Carbon Dioxide Emissions and the Greenhouse Effect

Carbon dioxide emissions have caused atmospheric CO₂ concentrations to increase 13% 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.

ð 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 increased with increased CO₂ concentration since CO₂ and water vapor absorb infrared radiation and slows its escape from Earth.

Ø Called the greenhouse effect.

Ø Some predict warming near the poles resulting in melting of polar ice and flooding of current costal areas.

Ø A warming trend will probably alter geographical disruption of precipitation which could have major agricultural implications.

• Some predictions about how the climate in each region would be are being made by scientists studying the past.

ð Paleoecologists are studying the records of pollen cores to determine how past temperature changes have affected vegetation.

F. Depletion of Atmospheric Ozone

Depletion of atmospheric ozone weakens a protective layer in the stratosphere which 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 cholorfluorocarbons used as aerosol propellants and in refrigeration.
• Breakdown products of chlorofluorocarbons include chlorine which rises to the stratosphere where it reacts with ozone (O₃) and reduces it to atmospheric oxygen (O₂).

Ø The clorine is released in other reactions and reacts with additional ozone molecules.

• Ozone depletion is best documented over Antarctical 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.

VIII. Human activities are altering species distribution and reducing biodiversity.

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.

A. The Introduction of Exotic Species

The introduction of exotic species can cause a variety of problems. Although most transplanted species fail to survive, there are notable exceptions.

• Starlings and English sparrows were imported into the United States from Europe in the 1800s. Both species are now very abundant and have displaced native bird species in many areas.
• While some exotic species have little or no impact on the new communities, other, such as fire ants, have tremendous impact.

ð Fire ants were accidentally introduced into the United States from Brazil in the 1940s (See Campbell, Figure 49.18)

ð Have spread across Southern U.S. and reduced the number of native species dramatically in some areas.

The release of genetically engineered organisms into natural agricultural ecosystems poses some potential ecological problems.

• Genetically engineered plants might escape the plots and become weeds or pests in other areas.
• Genetically engineered plants and animals might expand into new areas and out compete native species.
• Engineered plants might interbreed with wild plants and pass on their “special” traits (such as herbicide resistance) to offspring that could not be controlled by standard means.
• Genetically engineered insect resistance might result in a coevolutionary change in insect pests.

B. Habitat Destruction and the Biodiversity Crisis

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 rain forests are being cut at a rate of 500,000 Km² per year and will be eliminated in a few decades.
• Human activities which disrupt entire systems include: development, logging, war, oil spills, etc.

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

• 73% of the IUCNs designations of extinct, endangered, vulnerable, and rare species are related to destruction of natural habitats.
• Protection of endangered species is made difficult by a variety of problems

ð Minimum viable population sizes and minimal habitat size requirements are not known for many species.

ð Protection for migratory species requires international cooperation and habitat preservation in two areas.

It is difficult to accurately estimate the magnitude of the biodiversity crisis.

• Only a fraction of the Earth’s species has been identified and described by taxonomists.
• Extinction is an obscure, local process that can occur unnoticed because the exact distributions and habitats of many species are unknown.

The absence of clear documentation of the rate of extinction has led some to argue that there no reason to worry at this time. There is, however, evidence that the extinction of some well-known organisms is occurring at a rapid rate.

• 11% of the 9040 known bird species in the world are endangered.

ð Population densities of migratory songbirds in the Mid- Atlantic United States has decreased by 50% in the last 40 years.

• At least 20% of the known species of freshwater fishes in the world have become extinct in historical times or are seriously threatened.

Ø Only 122 of 266 know fish species in lowland Malaysia were observed during a recent search.

• 680 of the 20,000 species of plants known in the U.S. are in danger of becoming extinct by the year 2000.

The above list represents only a few of the many documented examples of extinction and some individuals argue that extinction is a natural phenomenon that has occurred since life first evolved.

· What must be realized is that the rate of extinction is rapidly increasing.

· Species are being lost on a worldwide scale at a rate of about 50 times greater than at any time in the past 100,000 years.

ð Ecological models estimate that human activity in tropical rain forests has increased extinction between 1000 and 10000 times over the normal rate of one in every one million species per year.

The question many ask is “Why should we care about the loss of biodiversity:” Answers to this question range from general to specific:

· Biophilia, the human sense of connection to nature and other forms of life, is centered around aesthetics and ethics.

· Biodiversity is a crucial natural resource and threatened species could provide crops, fibers, and medicines.

ð 25% of the prescriptions dispensed from pharmacies in the United States contain substances derived from plants.

ð Alkaloids isolated from the rose periwinkle of Madagascar in 1970 were found to inhibit cancer cell growth and result in remission for victims of childhood leukemia and Hodgkin’s disease.

ð One of the five other periwinkle species on Madagascar is approaching extinction.

IX. The Sustainable biosphere Initiative is reorienting ecological research

A great deal of basic knowledge is needed before sensible decisions can be made on how to lessen the negative impacts of humans on ecosystems.

· While each individual can play a role, and understanding of the complex interconnections of the biosphere is needed.

· The Sustainable Biosphere Initiative is a new research agenda disgned to obtain this knowledge.

The goal of Sustainable Biosphere Initiative is to acquire the basic ecological information needed for intelligent development, management, and cocservation of the Earth’s resources. This will include studies of:

· Global change including interactions between climate and ecological processes.

· Biological diversity and its role in maintaining ecological processes.

· The ways in which productivity of natural and artificial ecosystems can be sustained.

The nature of ecological research will have to be reoriented but the importance can not be overstated due to the current state of the biosphere.