An ecosystem is defined as all organisms living in a given area along with the abiotic factors with which they interact, a community and its physical environment. These boundaries are usually not discrete. This is the most inclusive level of biological organisation. The structure of an ecosystem involves two processes that cannot be described at lower levels: energy flow and chemical cycling.
Each ecosystem has a trophic structure that represents the different feeding relationships that determine the route of energy flow and the pattern of 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 are:
Primary producers
Autotrophs (usually photosynthetic) are the organisms that support all other trophic levels either directly or indirectly by synthesising sugars and other organic molecules using light energy. Some examples of these include terrestrial plants, aquatic photosynthetic protists, and cyanobacteria. An exception is communities of organisms living around hot water, deep sea vents where producers are chemosynthetic bacteria that oxidise H2S (driven by geothermal energy).
Pond plants
Mountain meadows
Primary consumers
These are herbivores that consume primary producers. Some examples are terrestrial insects, snails, grazing mammals, seed-eating birds, aquatic zooplankton, and some fish.
Secondary consumers
These are the carnivores that eat herbivores. Just some of the many examples of this group include terrestrial spiders, frogs, insects-eating birds, lions, many fish, and sea-stars.
Insect eating snake.
Tertiary consumers
These are the carnivores that eat other carnivores.
Hawk eating a snake
Detritivores
These are the consumers that derive energy from organic wastes and dead organisms some examples include the bacteria and fungi. Also included are scavengers such as cockroaches and bald eagles. This level often forms a major link between primary producers and higher-level consumers, and is important components of the recycling process.
Fungi are important components of the decomposers.
A food chain can be thought of as a transfer of food from trophic level to trophic level (fig. 49.1). Food chains 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. The feeding relationships are usually woven into elaborate food webs within an ecosystem.
Antarctic food web
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 synthesise organic molecules (photosynthesis) which are later broken down to produce ATP (cellular respiration). Energy is obtained by consumers 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 utilise solar energy, an ecosystem's entire energy budget is determined by the photosynthetic activity of the system
The Global Energy Budget
The amount of solar radiation that strikes the earth's surface shows dramatic regional variation that limits the photosynthetic output of ecosystems in different places. The Earth receives an estimated 1022 joules (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 biosphere 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% of the visible light reaching primary producers is converted to chemical energy by photosynthesis; the photosynthetic efficiency also varies with the type of plant, light levels, and other factors. Even with all the variations mentioned above, the primary production of Earth collectively creates about 170 billion tons of organic material each year.
Global net primary productivity (NPP) for the months of June (top) and December (bottom) of 2002 based on space-based measurements taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) with algorithms developed by the NASA Earth Observing team that use a suite of other satellite and surface-based measurements (Source: NASA Earth Observatory).
Primary Productivity
Primary productivity is the rate at which light energy is 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). Where Rs is the 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 larger producers with elaborate nonphotosynthetic structures (such as trees) which support large metabolically active stem and root systems. Net Primary Productivity can be expressed as biomass (expressed as dry weight since water contains no unable 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 synthesised by vegetation, while standing crop biomass is the total biomass of plants present at a given time. Primary productivity varies among ecosystems and their sizes affect each ecosystem's contribution to the Earth's total productivity. Tropical rainforests are very productive and contribute a large proportion to the planet'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.
Factors important in limiting productivity depend 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 in terrestrial systems increases as latitudes approach the equator because availability of water, heat, 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. Exact proportions vary among plants. 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 that will slow down or cease. 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 is usually determined by light intensity, water temperature, and availability of inorganic nutrients. 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 polar seas) are more productive than tropical seas. Freshwater ecosystem productivity also varies from the surface to the depths in relation to light intensity; water temperature is important and seasonal fluctuations in productivity occur in temperate zones; availability of inorganic nutrients is sometimes limiting, but biannual turnovers bring nutrients to the surface waters.
Energy Transfers and Ecological Pyramids
Secondary productivity
There is a limit to the rate at which consumers incorporate organic material (the food they eat) into new biomass, which can be equated to chemical energy. The productivity of each higher trophic level declines, because only a small portion of the energy of each trophic level is transferred to the next trophic level. Not all energy stored in biomass can be converted to productivity at the next trophic level due to loss of organised energy dissipated as heat. Herbivores only consume a small fraction of available plant material. Typically, 2/3 of organic material absorbed by herbivores is used for cellular respiration that degrades compounds into organic waste products and heat. The other 1/3 adds biomass to the trophic level. 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.
Ecological efficiency
Ecological efficiency is the ratio of net productivity at one trophic level compared to net productivity at the level below. It can vary greatly depending on the organisms involved, but is roughly 10%. This means that 90% 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 energy acquired from the level below. Food chains are usually bottom heavy since only 10% of energy is transferred.
2. A biomass pyramid has each tier symbolising the total dry weight of all organisms in an ecosystem's levels at any given time. Biomass represents chemical energy stored in the organic matter of a trophic level. Most narrow sharply from producers at the base to top-level carnivores at the top. Some aquatic systems are inverted since producers can have high turnover rates. They grow rapidly but are consumed rapidly, leaving little standing biomass. Biomass of top-level carnivores is usually small compared to the total biomass of producers and lower-level consumers.
Food chains are limited to 3-5 links due to the multiplicative loss of energy. This also limits the biomass of top-level carnivores that can be supported. Only about 1/1000 of the chemical energy fixed by photosynthesis flows through a food web to a tertiary consumer, only 3 -5 trophic levels can be supported since biomass at the apex is insufficient to support another level. The fact that top-level consumer biomass is concentrated in a small number of individuals can be reflected in a pyramid of numbers where each tier represents the number of organisms at each 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.
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.
Biogeochemical cycles
Biogeochemical cycles are nutrient circuits involving both biotic and abiotic components of ecosystems. Water is an important vehicle for transfer of chemicals and itself moves in a global hydrologic cycle (fig. 49.9). Elements such as carbon, oxygen, sulphur, 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 a more localised 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 (fig. 49.8).
1. Reservoirs are defined by two characteristics:
i) whether they contain organic or inorganic materials;
ii) whether or not the materials are directly available for use by organisms.
Available organic reservoir
This reservoir contains the living organisms and the nutrients are readily available when organisms feed on one another.
Unavailable organic reservoir
This reservoir is comprised of coal, oil, and peat which formed from organisms that died and were buried millions of years ago; these nutrients are unavailable since they cannot be directly assimilated.
Available inorganic reservoir
Included in this reservoir are all elements, ions, and molecules present in the soil or air and those dissolved in water. Organisms can directly assimilate these nutrients from the soil, air, or water.
Unavailable inorganic nutrients
Elements in this reservoir are 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 that 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 that 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 be the covering of detritus by sediments and its eventual fossilisation to oil, coal, or peat. Sedimentary rock formation is the process that 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 Carbon Cycle
During the carbon cycle, autotrophs acquire 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 (fig. 49.10). Carbon cycles in the environment very 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. However carbon 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 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 Carbon cycle
HUMAN INTRUSIONS IN ECOSYSTEM DYNAMICS
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, some regional, and a few are global.
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. Over 75% of the Earth's original forests have been cleared or severely disrupted. While only 15% of the original primary forest and just 1% of the original tallgrass prairie remain in the United States. The tropical rain forests are being cut at a rate of 500,000 km2 per year and will be eliminated in a few decades.
One result of the destruction of natural habitat will be the extinction of many species as their ecosystems disappear. This biodiversity crisis has many aspects that must be considered in order to protect endangered species. Not only is localised protection necessary, but many migratory species are facing habitat destruction in both their northern breeding grounds and their tropical wintering grounds.
Accelerated Eutrophication of Lakes
Lakes undergo a naturally occurring change in chemical composition and character. In young oligotrophic lakes there is low primary productivity because 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. Such lakes reach an equilibrium where nutrient input is balanced by losses due to outflow and sedimentation.
Sewage, factory wastes, livestock runoff, and fertiliser leaching increases inorganic nutrient levels in waters and results in cultural eutrophication. This results in explosive growth of photosynthetic organisms. As the plants die, metabolism of decomposers consume all the oxygen in the water and species die out.
Poisons in Food Chains
A variety of toxic chemicals, including unnatural 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 with other substances or metabolism of microbes. Organisms acquire toxic substances along with nutrients or water, some of which accumulate in their tissues.
Biological magnification
This is the process by which toxins become more concentrated with each link in a food chain. Results from biomass at each trophic level being produced from a much larger biomass ingested from the level below. The 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 (fig. 49.16). This pesticide was 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. Because it is soluble in lipids 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 carnivorous birds that calcium deposition in eggshells was disrupted. Reproductive rates declined dramatically since the weight of nesting birds broke the weakened shells.
The release of radioisotopes by nuclear accidents and the unsafe storage of nuclear wastes present a serious environmental threat. These contaminants can last for many years due to long half-lives and are subject to biological magnification.
Intrusions in the Atmosphere
Human activities have resulted in two very pressing problems with regard to the atmosphere: rising levels of carbon dioxide and degradation of atmospheric ozone.
Carbon dioxide emissions have caused atmospheric CO2 concentrations to increase 11% in 30 years. This increase is due to combustion of fossil fuels and burning of wood removed by deforestation.
Effects of Increased CO2 Levels:
1. Increased productivity by vegetation. C3 plants are more limited than C4 plants by CO2 so spread of C3 species into habitats previously favouring C4 species may have important natural and agricultural implications.
2. Temperature increases with increased CO2 concentration since CO2, though transparent to visible light, absorbs infrared radiation and slows its escape from Earth. This is called the greenhouse effect. Some scientists predict warming near the poles resulting in melting of polar ice and flooding of current coastal areas. A warming trend will probably alter geographical distribution of precipitation that could have major agricultural implications.
Another major environmental problem is the depletion of atmospheric ozone that weakens a protective layer in the stratosphere that absorbs ultraviolet radiation. Much of the ultraviolet radiation that strikes the Earth is absorbed by an ozone layer 17 to 20 miles above the surface. The destruction of the ozone layer is largely due to accumulation of chlorofluorocarbons (CFC) used as aerosol propellants and in refrigeration. The breakdown products of chlorofluorocarbons include chlorine that rises to the stratosphere where it reacts with ozone and reduces it to atmospheric oxygen. 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.
Antarctic ozone hole
Acid deposition
Acidic pollutants can be deposited from the atmosphere to the Earth's surface in wet and dry forms. The common term to describe this process is acid deposition. The term acid precipitation is used to specifically describe wet forms of acid pollution that can be found in rain, sleet, snow, fog, and cloud vapor. An acid can be defined as any substance that when dissolved in water dissociates to yield corrosive hydrogen ions. The acidity of substances dissolved in water is commonly measured in terms of pH (defined as the negative logarithm of the concentration of hydrogen ions). According to this measurement scale solutions with pHs less than 7 are described as being acidic, while a pH greater than 7.0 is considered alkaline. Precipitation normally has a pH between 5.0 to 5.6 because of natural atmospheric reactions involving carbon dioxide. Precipitation is considered to be acidic when its pH falls below 5.6 (which is 25 times more acidic than pure water). Some sites in eastern North America have precipitation with pHs as low as 2.3 or about 1000 times more acidic than natural.
Acid deposition is not a recent phenomena. In the 17th century, scientists noted the ill effects that industry and acidic pollution was having on vegetation and people. However, the term acid rain was not coined until two centuries later when Angus Smith published a book called 'Acid Rain' in 1872. In the 1960s, the problems associated with acid deposition became an international problem when fishermen noticed declines in fish numbers and diversity in many lakes throughout North America and Europe.Several processes can result in the formation of acid deposition. Nitrogen oxides (NOx) and sulfur dioxide (SO2) released into the atmosphere from a variety of sources call fall to the ground simply as dry deposition. This dry deposition can then be converted into acids when these deposited chemicals meet water. Most wet acid deposition forms when nitrogen oxides (NOx) and sulfur dioxide (SO2) are converted to nitric acid (HNO3) and sulfuric acid (H2SO4) through oxidation and dissolution. Wet deposition can also form when ammonia gas (NH3) from natural sources is converted into ammonium (NH4).
Distribution of acidic precipitation.
Acid deposition influences the environment in several different ways. In aquatic systems, acid deposition can effect these ecosystems by lowering their pH. However, not all aquatic systems are effected equally. Streams, ponds or lakes that exist on bedrock or sediments rich in calcium and/or magnesium are naturally buffered from the effects of acid deposition. Aquatic systems on neutral or acidic bedrock are normally very sensitive to acid deposition because they lack basic compounds that buffer acidification. In Canada, many of the water bodies found on the granitic Canadian Shield fall in this group. One of the most obvious effects of aquatic acidification is the decline in fish numbers. Originally it was believed that the fish died because of the increasing acidity of the water. However, in the 1970s scientists discovered that acidified lakes also contained high concentrations of toxic heavy metals like mercury, aluminum, and cadmium. The source of these heavy metals was the soil and bedrock surrounding the water body. Normally, these chemicals are found locked in clay particles, minerals and rocks. However, the acidification of terrestrial soils and bedrock can cause these metals to become soluble. Once soluble, these toxic metals are easily leached by infiltrating water into aquatic systems where they accumulate to toxic levels.
In the middle latitudes, many acidified aquatic systems experience a phenomenon known as acid shock. During the winter the acidic deposits can build-up in the snowpack. With the arrival of spring, snowpack begins to melt quickly and the acids are released over a short period of time at concentrations 5 to 10 times more acidic than rainfall. Most adult fish can survive this shock. However, the eggs and small fry of many spring spawning species are extremely sensitive to this acidification.
The severity of the impact of acid deposition on vegetation is greatly dependent on the type of soil the plants grow in. Similar to surface water acidification, many soils have a natural buffering capacity and are able to neutralize acid inputs. In general, soils that have a lot of lime are better at neutralizing acids than those that are made up of siliceous sand or weathered acidic bedrock. In less buffered soils, vegetation is effected by acid deposition because:
- Increasing acidity results in the leaching of several important plant nutrients, including calcium, potassium, and magnesium. Reductions in the availability of these nutrients causes a decline in plant growth rates.
- The heavy metal aluminum becomes more mobile in acidified soils. Aluminum can damage roots and interfere with plant uptake of other nutrients such as magnesium and potassium.
- Reductions in soil pH can cause germination of seeds and the growth of young seedlings to be inhibited.
- Many important soil organisms cannot survive is soils below a pH of about 6.0. The death of these organism can inhibit decomposition and nutrient cycling.
- High concentrations of nitric acid can increase the availability nitrogen and reduce the availability of other nutrients necessary for plant growth. As a result, the plants become over-fertilized by nitrogen (a condition known as nitrogen saturation).
- Acid precipitation can cause direct damage to the foliage on plants especially when the precipitation is in the form of fog or cloud water which is up to ten times more acidic than rainfall.
- Dry deposition of SO2 and NOx has been found to affect the ability of leaves to retain water when they are under water stress.
- Acidic deposition can leach nutrients from the plant tissues weakening their structure.
The combination of these effects can lead to plants that have reduced growth rates, flowering ability and yields. It also makes plants more vulnerable to diseases, insects, droughts and frosts.
