Human Impact on Systems

From http://www.course-notes.org/Biology/Outlines/Chapter_54_Ecosystems

The human population is disrupting chemical cycles throughout the biosphere

·      Human activities and technologies have disrupted the trophic structure, energy flow, and chemical cycling of ecosystems worldwide.

 The human population moves nutrients from one part of the biosphere to another.

·      Human activity intrudes in nutrient cycles.

°      Nutrients from farm soil may run off into streams and lakes, depleting nutrients in one area, causing excesses in another, and disrupting chemical cycles in both places.

°      Humans also add entirely new materials—many toxic—to ecosystems.

·      In agricultural ecosystems, a large amount of nutrients are removed from the area as crop biomass.

°      After a while, the natural store of nutrients can become exhausted.

°      The soil cannot be used to grow crops without nutrient supplementation.

·      Nitrogen is the main nutrient lost through agriculture.

°      Plowing and mixing the soil increase the decomposition rate of organic matter, releasing usable nitrogen that is then removed from the ecosystem when crops are harvested.

·      Recent studies indicate that human activities have approximately doubled the worldwide supply of fixed nitrogen, due to the use of fertilizers, cultivation of legumes, and burning.

°      This may increase the amount of nitrogen oxides in the atmosphere and contribute to atmospheric warming, depletion of ozone, and possibly acid precipitation.

·      The key problem with excess nitrogen seems to be critical load, the amount of added nitrogen that can be absorbed by plants without damaging the ecosystem.

°      Nitrogenous minerals in the soil that exceed the critical load eventually leach into groundwater or run off into freshwater and marine ecosystems, contaminating water supplies, choking waterways, and killing fish.

·      Lakes are classified by nutrient availability as oligotrophic or eutrophic.

°      In an oligotrophic lake, primary productivity is relatively low because the mineral nutrients required by phytoplankton are scarce.

°      Overall productivity is higher in eutrophic lakes.

·      Human intrusion has disrupted freshwater ecosystems by cultural eutrophication.

°      Sewage and factory wastes and runoff of animal wastes from pastures and stockyards have overloaded many freshwater streams and lakes with nitrogen.

°      This results in an explosive increase in the density of photosynthetic organisms, released from nutrient limitation.

°      Shallow areas become choked with weeds and algae.

°      As photosynthetic organisms die and organic materials accumulate at the lake bottom, detritivores use all the available oxygen in the deeper waters.

°      This can eliminate fish species.

 Combustion of fossil fuels is the main cause of acid precipitation.

·      The burning of fossil fuels releases oxides of sulfur and nitrogen that react with water in the atmosphere to produce sulfuric and nitric acids.

·      These acids fall back to earth as acid precipitation—rain, snow, sleet or fog with a pH less than 5.6.

·      Acid precipitation is a regional or global problem, rather than a local one.

°      The tall exhaust stacks built for smelters and generating plans export the problem far downwind.

·      Acid precipitation lowers the pH of soil and water and affects the soil chemistry of terrestrial ecosystems.

°      With decreased pH, calcium and other nutrients leach from the soil.

°      The resulting nutrient deficiencies affect the health of plants and limit their growth.

·      Freshwater ecosystems are very sensitive to acid precipitation.

°      Lakes underlain by granite bedrock have poor buffering capacity because of low bicarbonate levels.

°      Fish populations have declined in many lakes in Norway, Sweden, and Canada as pH levels fall.

§  Lake trout are keystone predators in many Canadian lakes.

§  When they are replaced by acid-tolerant species, the dynamics of food webs in the lakes change dramatically.

·      Environmental regulations and new industrial technologies have led to reduced sulfur dioxide emissions in many developed countries.

°      The water chemistry of many streams and freshwater lakes is slowly improving as a result.

°      Ecologists estimate that it will take another 10 to 20 years for these ecosystems to recover, even if emissions continue to decline.

·      Massive emissions of sulfur dioxide and acid precipitation continue in parts of central and eastern Europe.

 Toxins can become concentrated in successive trophic levels of food webs.

·      Humans introduce many toxic chemicals into ecosystems.

°      These substances are ingested and metabolized by organisms and can accumulate in the fatty tissues of animals.

°      These toxins become more concentrated in successive trophic levels of a food web, a process called biological magnification.

§  Magnification occurs because the biomass at any given trophic level is produced from a much larger biomass ingested from the level below.

§  Thus, top-level carnivores tend to be the organisms most severely affected by toxic compounds in the environment.

°      Many toxins cannot be degraded by microbes and persist in the environment for years or decades.

°      Other chemicals may be converted to more toxic products by reaction with other substances or by the metabolism of microbes.

§  For example, mercury was routinely expelled into rivers and oceans in an insoluble form.

§  Bacteria in the bottom mud converted it to methyl mercury, an extremely toxic soluble compound that accumulated in the tissues of organisms, including humans who fished in contaminated waters.

 Human activities may be causing climate change by increasing atmospheric carbon dioxide.

·      Since the Industrial Revolution, the concentration of CO2 in the atmosphere has increased greatly as a result of burning fossil fuels and wood removed by deforestation.

°      The average CO2 concentration in the environment was 274 ppm before 1850.

°      Measurements in 1958 read 316 ppm and have increased to 370 ppm today.

·      If CO2 emissions continue to increase at the present rate, the atmospheric concentration of this gas will be double what it was at the start of the Industrial Revolution by the year 2075.

·      Increased productivity by vegetation is one consequence of increasing CO2 levels.

·      Because C3 plants are more limited than C4 plants by CO2 availability, one effect of increasing CO2 levels may be the spread of C3 species into terrestrial habitats previously favoring C4 plants.

°      For example, corn may be replaced on farms by wheat and soybeans.

·      To assess the effect of rising levels of atmospheric CO2 on temperate forests, scientists at Duke University began the Forest-Atmosphere Carbon Transfer and Storage (FACTS-1) experiment.

°      The FACTS-1 study is testing how elevated CO2 influences tree growth, carbon concentration in soils, insect populations, soil moisture, understory plant growth, and other factors over a ten-year period.

·      Rising atmospheric CO2 levels may have an impact on Earth’s heat budget.

°      When light energy hits the Earth, much of it is reflected off the surface.

§  CO2 causes the Earth to retain some of the energy that would ordinarily escape the atmosphere.

à  This phenomenon is called the greenhouse effect.

à  If it were not for this effect, the average air temperature on Earth would be −18°C.

à  A number of studies predict that by the end of the 21st century, atmospheric CO2 concentration will have doubled and average global temperature will rise by 2°C.

·      An increase of only 1.3°C would make the world warmer than at any time in the past 100,000 years.

°      If increased temperatures caused the polar ice caps to melt, sea levels would rise by an estimated 100 m, flooding coastal areas 150 km inland from current coastlines.

°      A warming trend would also alter geographic distribution of precipitation, making major U.S. agricultural areas much drier.

·      Scientists continue to construct models to predict how increasing levels of CO2 in the atmosphere will affect Earth.

·      Global warming is a problem of uncertain consequences and no certain solutions.

·      Stabilizing CO2 emissions will require concerted international effort and the acceptance of dramatic changes in personal lifestyles and industrial processes.

·      Many ecologists think that this effort suffered a major setback in 2001, when the United States pulled out of the Kyoto Protocol, a 1997 pledge by industrialized nations to reduce their CO2 output by 5% over a ten-year period.

 Human activities are depleting atmospheric ozone.

·      Life on earth is protected from the damaging affects of ultraviolet radiation (UV) by a layer of O3, or ozone, that is present in the lower stratosphere.

·      Studies suggest that the ozone layer has been gradually “thinning” since 1975.

·      The destruction of ozone probably results from the accumulation of CFCs, or chlorofluorocarbons—chemicals used in refrigeration, as propellant in aerosol cans, and for certain manufacturing processes.

°      The breakdown products from these chemicals rise to the stratosphere, where the chlorine they contain reacts with ozone to reduce it to O2.

§  Subsequent reactions liberate the chlorine, allowing it to react with other ozone molecules in a catalytic chain reaction.

§  At middle latitudes, ozone levels have decreased by 2–10% during the past 20 years.

°      The result of a reduction in the ozone layer may be increased levels of UV radiation that reach the surface of the Earth.

§  Some scientists expect increases in skin cancer and cataracts, as well as unpredictable effects on crops and natural communities.

§  Even if all chlorofluorocarbons were banned globally today, chlorine molecules already present in the atmosphere will continue to reduce ozone levels for at least a century.

·      The impact of human activity on the ozone layer is one more example of how much we are able to disrupt ecosystems and the entire biosphere.

 

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