Ecology

[Hurriah]

The concept of Ecosystem

An ecosystem is a natural unit composed of living and non-living componenets whose interactions result in a stable , self peretuating system sable in sense that it can adjust to changes within itself;self perpetuating in that it can continue on its own without the necessty for human or other inteference . It is made of commuity of organisms which interact with one another and with non-living constituents of the environment.

It follows that to analayse an ecosystem invovles studying natural communities.Such studies are known as synecology.The community may exist in a fresh water pond, an oakwood, or a rock pool on the sea shore Whatever the situation, it is first necessary to identify the organisms living there. This can be done with the aid of systematic keys.It is then necessary to determine the distribution of the different species quantitatively , and to correlate this with the physical and biotic factors of the environment.This is not always easy and generally involves carrying out analysis on the distribution data.

Biogeochemical cycles

The Carbon cycle

Carbon is a very important element, as it makes up organic matter, which is a part of all life. Carbon follows a certain route on earth, called the carbon cycle. Through following the carbon cycle we can also study energy flows on earth, because most of the chemical energy needed for life is stored in organic compounds as bonds between carbon atoms and other atoms.
The carbon cycle naturally consists of two parts, the terrestrial and the aquatic carbon cycle. The aquatic carbon cycle is concerned with the movements of carbon through marine ecosystems and the terrestrial carbon cycle is concerned with the movement of carbon through terrestrial ecosystems.

The carbon cycle is based on carbon dioxide (CO2), which can be found in air in the gaseous form, and in water in dissolved form. Terrestrial plants use atmospheric carbon dioxide from the atmosphere, to generate oxygen that sustains animal life. Aquatic plants also generate oxygen, but they use carbon dioxide from water.

The process of oxygen generation is called photosynthesis. During photosynthesis, plants and other producers transfer carbon dioxide and water into complex carbohydrates, such as glucose, under the influence of sunlight. Only plants and some bacteria have the ability to conduct this process, because they possess chlorophyll; a pigment molecule in leaves that they can capture solar energy with.

The overall reaction of photosynthesis is:

carbon dioxide + water + solar energy -> glucose + oxygen
6 CO2 + 6 H2O + solar energy -> C6H12O6 + 6 O2

The oxygen that is produced during photosynthesis will sustain non-producing life forms, such as animals, and most micro organisms. Animals are called consumers, because they use the oxygen that is produced by plants. Carbon dioxide is released back into the atmosphere during respiration of consumers, which breaks down glucose and other complex organic compounds and converts the carbon back to carbon dioxide for reuse by producers.

Carbon that is used by producers, consumers and decomposers cycles fairly rapidly through air, water and biota. But carbon can also be stored as biomass in the roots of trees and other organic matter for many decades. This carbon is released back into the atmosphere by decomposition, as was noted before.

Not all organic matter is immediately decomposed. Under certain conditions dead plant matter accumulates faster than it is decomposed within an ecosystem. The remains are locked away in underground deposits. When layers of sediment compress this matter fossil fuels will be formed, after many centuries. Long-term geological processes may expose the carbon in these fuels to air after a long period of time, but usually the carbon within the fossil fuels is released during humane combustion processes.

The combustion of fossil fuels has supplied us with energy for as long as we can remember. But the human population of the world has been expanding and so has our demand for energy. That is why fossil fuels are burned very extensively. This is not without consequences, because we are burning fossil fuels much faster than they develop. Because of our actions fossil fuels have become non-renewable recourses.

Although the combustion of fossil fuels mainly adds carbon dioxide to air, some of it is also released during natural processes, such as volcanic eruptions.

In the aquatic ecosystem carbon dioxide can be stored in rocks and sediments. It will take a long time before this carbon dioxide will be released, through weathering of rocks or geologic processes that bring sediment to the surface of water.

Carbon dioxide that is stored in water will be present as either carbonate or bicarbonate ions. These ions are an important part of natural buffers that prevent the water from becoming too acidic or too basic. When the sun warms up the water carbonate and bicarbonate ions will be returned to the atmosphere as carbon dioxide.

Schematic representations of the aquatic and terrestrial part of the carbon cycle are shown here:

The aquatic carbon cycle

The terrestrial carbon cycle

The Nitrogen cycle

Nitrogen is a part of vital organic compounds in microrganisms, such as amino acids, proteins and DNA. The gaseous form of nitrogen (N2), makes up 78% of the troposphere. One might think this means we always have plenty of nitrogen available, but unfortunately it does not work that way. Nitrogen in the gaseous form cannot be absorbed and used as a nutrient by plants and animals; it must first be converted by nitrifying bacteria, so that it can enter food chains as a part of the nitrogen cycle.

During the conversion of nitrogen cyano bacteria will first convert nitrogen into ammonia and ammonium, during the nitrogen fixation process. Plants can use ammonia as a nitrogen source.

Nitrogen fixation is carried out according to the following reaction:
N2 + 3 H2 -> 2 NH3

After ammonium fixation, the ammonia and ammonium that is formed will be transferred further, during the nitrification process. Aerobic bacteria use oxygen to convert these compounds. Nitrosomonas bacteria first convert nitrogen gas to nitrite (NO2-) and subsequently nitrobacter convert nitrite to nitrate (NO3-), a plant nutrient.

Nitrification is carried out according to the following reactions:

2 NH3 + 3O2 - > 2 NO2 + 2 H+ + 2 H2O
2 NO2- + O2 -> 2 NO3-
Plants absorb ammonium and nitrate during the assimilation process, after which they are converted into nitrogen-containing organic molecules, such as amino acids and DNA.

Animals cannot absorb nitrates directly. They receive their nutrient supplies by consuming plants or plant-consuming animals.

When nitrogen nutrients have served their purpose in plants and animals, specialized decomposing bacteria will start a process called ammonification, to convert them back into ammonia and water-soluble ammonium salts. After the nutrients are converted back into ammonia, anaerobic bacteria will convert them back into nitrogen gas, during a process called denitrification.

Denitrification is carried out according to the following reaction:
NO3- + CH2O + H+ -> ½ N2O + CO2 + 1½ H2O

Finally, nitrogen is released into the atmosphere again. The whole process starts over after release.

A schematic representation of the nitrogen cycle is shown here:

The phosphorous cycle

Phosphorus is an essential nutrient for plants and animals in the form of ions PO43- and HPO42-. It is a part of DNA-molecules, of molecules that store energy (ATP and ADP) and of fats of cell membranes. Phosphorus is also a building block of certain parts of the human and animal body, such as the bones and teeth.

Phosphorus can be found on earth in water, soil and sediments. Unlike the compounds of other matter cycles phosphorus cannot be found in air in the gaseous state. This is because phosphorus is usually liquid at normal temperatures and pressures. It is mainly cycling through water, soil and sediments. In the atmosphere phosphorus can mainly be found as very small dust particles.

Phosphorus moves slowly from deposits on land and in sediments, to living organisms, and than much more slowly back into the soil and water sediment.
Phosphorus is most commonly found in rock formations and ocean sediments as phosphate salts. Phosphate salts that are released from rocks through weathering usually dissolve in soil water and will be absorbed by plants. Because the quantities of phosphorus in soil are generally small, it is often the limiting factor for plant growth. That is why humans often apply phosphate fertilizers on farmland. Phosphates are also limiting factors for plant-growth in marine ecosystems, because they are not very water-soluble. Animals absorb phosphates by eating plants or plant-eating animals.
Phosphorus cycles through plants and animals much faster than it does through rocks and sediments. When animals and plants die, phosphates will return to the soils or oceans again during decay. After that, phosphorus will end up in sediments or rock formations again, remaining there for millions of years. Eventually, phosphorus is released again through weathering and the cycle starts over.

A schematic representation of the phosphorus cycle: