1.2 Ecosystem

1.2.1 Definition of ecosystem

Arthur Tansley (Fig.1.2.1) was the first person to use the term “ecosystem” in a published work in 1935. Tansley devised the concept to draw attention to the importance of transfers of materials between organisms and their environment. He later refined the term, describing it as “The whole system, ... including not only the organism complex, but also the whole complex of physical factors forming what we call the environment”.

Eugene Odum defined ecosystems as: “Any unit that includes all of the organisms in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles within the system is an ecosystem.”

1.2.2 Structure of ecosystem

All ecosystems include both biotic (living organisms) and abiotic components (nonliving environment) (Fig.1.2.2).

In an ecosystem, nonliving environment provides energy, nutrients, and living space that the living organisms need for reproduction, growth, and maintenance. It includes climate factors (light, temperature and so on), inorganic matter (water, carbon, nitrogen, mineral, and so on), and organic matter (carbohydrate, protein, lipid, and so on).

There are two major groups of organisms: the autotrophs and the heterotrophs. Autotrophs are organisms capable of producing their own food (e.g., simple sugars) from simple inorganic molecules such as carbon dioxide (CO2) and water (H2O). So mainly, autotrophs are producers. All green plants and algae are autotrophs, as are a few special types of bacteria. Heterotrophs are organisms that cannot manufacture their food. They meet their nutritional needs by feeding on other organisms or organic matter. Most species of bacteria, fungi, and all animals including man are heterotrophs.

There are two groups of autotrophs. Photoautotrophs use the sun or some other light sources to manufacture food from simple inorganic molecules. This process is known as photosynthesis. Chemoautotrophs manufacture the food that they need with energy derived from special kinds of chemical reactions (i.e., oxidation process). They are sometimes very important to the movement of nutrients within an ecosystem. There are several groups of soil-inhabiting chemoautotrophic bacteria that make sulfur and nitrogen more readily available to green plants.

Heterotrophs can be divided into three groups: herbivores, carnivores and decomposers. Herbivores are plant eaters. Carnivores are meat eaters. Some organisms, including man, are called omnivores because they eat both plant and animal. Decomposers use dead plant and animal material and wastes as food. They break down organic substances, using the energy and some of the nutrients stored in the “food” and returning the remaining nutrients to the environment. Included in this group are many bacteria and fungi.

1.2.3 Energy flow and nutrient cycling

In ecosystems, both energy and nutrients pass from one organism to another as food. Energy flows indirectly (Fig.1.2.3). It enters the biosphere and it leaves the biosphere. It is not cycled within a system or among systems. Once energy passes through an ecosystem, it is lost to the system forever. Nutrients, on the other hand, can be cycled. They can move out of one system and into another, they are almost never lost from the biosphere as a whole.

1.2.4 Carbon cycle

All living things are made of carbon. Carbon is also a component of the ocean, air, and even rocks. Because the earth is a dynamic place, carbon does not stay still. It is always moving.

Carbon moves between organisms and the atmosphere as a consequence of reciprocal biological processes: photosynthesis and respiration (Fig.1.2.4). Photosynthesis removes CO2 from the atmosphere, while respiration by primary producers and consumers, including decomposers, returns carbon to the atmosphere in the form of CO2. In aquatic ecosystems, CO2 must first dissolve in water before being used by aquatic primary producers. Once dissolved in water, CO2 enters a chemical equilibrium with bicarbonate, HCO3-, and carbonate, CO32-. Carbonate may precipitate out of solution as calcium carbonate and may be buried in ocean sediments.

While some carbon cycles rapidly between organisms and the atmosphere, some remains sequestered in relatively unavailable forms for long periods of time. Carbon in soils, peat, fossil fuels, and carbonate rock would generally take a long time to return to the atmosphere. During modern times, however, fossil fuels have become a major source of atmospheric CO2 as humans have tapped into fossil fuels supplies to provide energy for their systems.

1.2.5 Nitrogen cycle

Nitrogen is important to the structure and functioning of organisms. It forms part of key biomolecules such as amino acids, nucleic acids, and the porphyrin rings of chlorophyll and hemoglobin.

The nitrogen cycle includes a major atmospheric pool in the form of molecular nitrogen, N2. In fact, 78% of the air in our atmosphere is nitrogen. However, only a few organisms can use this atmospheric supply of molecular nitrogen directly. These organisms are called nitrogen fixers (Fig.1.2.5).

Because of the strong triple bonds between the two nitrogen atoms in N2 molecule, nitrogen fixation is an energy demanding process. During nitrogen fixation, N2 is reduced to ammonia, NH3. Nitrogen fixation takes place under aerobic conditions in terrestrial and aquatic environments, where nitrogen-fixing species oxidize sugars to obtain the required energy. Nitrogen fixation also occurs as a physical process associated with the high pressure and energy generated by lightning. Ecologists propose that all of the nitrogen cycling within ecosystems ultimately entered these cycles through nitrogen fixation by organisms or lightning. These are relatively large pools of nitrogen cycled in the biosphere but only a small entryway through nitrogen fixation.

Once nitrogen is fixed by nitrogen-fixing organisms, it becomes available to other organisms within an ecosystem. Upon the death of an organism, the nitrogen in its tissues can be released by fungi and bacteria involved in the decomposition process. These fungi and bacteria release nitrogen as ammonium, NH4+, a process called ammonification. Ammonium may be converted to nitrate, NO3-, by other bacteria in a process called nitrification. Ammonium and nitrate can be used directly by bacteria, fungi, or plants. The nitrogen in dead organic matter can also be used directly by mycorrhizal fungi, which can be passed on the plants. The nitrogen in bacteria, fungi, and plant biomass may pass on to populations of animal consumers or back to the pool of dead organic matter, where it will be recycled again.

Nitrogen may exit the organic matter pool of an ecosystem through denitrification. Denitrification is an energy-yielding process that occurs under anaerobic conditions and converts nitrate to molecular nitrogen, N2. The molecular nitrogen produced by denitrifying bacteria moves into the atmosphere and can only reenter the organic matter pool through nitrogen fixation.

1.2.6 Phosphorus cycle

Phosphorus is an important element for all forms of life. As phosphate (PO43-),
it makes up an important part of the structural framework that holds DNA and RNA together. Phosphates are also a critical component of ATP as the cellular energy carrier for organisms to use in building proteins or contracting muscles. Like calcium, phosphorus is important to vertebrates; in the human body, 80% of phosphorus is found in teeth and bones.

The phosphorus cycle differs from the other major biogeochemical cycles in which it does not include a gas phase; although small amounts of phosphoric acid (H3PO4) may make their way into the atmosphere. Very little phosphorus circulates in the atmosphere because at earth’s normal temperatures and pressures, phosphorus and its various compounds are not gases. The largest reservoir of phosphorus is in sedimentary rock (Fig.1.2.6).

It is in these rocks where the phosphorus cycle begins. When it rains, phosphates are removed from the rocks (via weathering) and are distributed throughout both soils and water. Plants take up the phosphate ions from the soil. The phosphate then moves from plants to animals when herbivores eat plants and carnivores eat plants or herbivores. The phosphates absorbed by animal tissue through consumption eventually return to the soil through the excretion of urine and feces, as well as from the final decomposition of plants and animals after death.

The same process occurs within the aquatic ecosystem. Phosphorus is not highly soluble, binding tightly to molecules in soil, therefore it mostly reaches waters by traveling with runoff soil particles. Phosphates also enter waterways through fertilizer runoff, sewage seepage, natural mineral deposits, and wastes from other industrial processes. These phosphates tend to settle on ocean floors and lake bottoms. As sediments are stirred up, phosphates may reenter the phosphorus cycle, but they are more commonly made available to aquatic organisms by being exposed through erosion. Water plants take up the waterborne phosphate, which then travels up through successive stages of the aquatic food chain.

1.2.7 Controlling factors

Energy and carbon enter ecosystems through photosynthesis, are incorporated into living tissue, transferred to other organisms that feed on the living and dead plant matter, and eventually released through respiration. Most mineral nutrients, on the other hand, are recycled within ecosystems.

Ecosystems are controlled both by external and internal factors. External factors, also called state factors, control the overall structure of an ecosystem and the way things work within it, but are not themselves influenced by the ecosystem. The most important of these is climate. Climate determines the biome in which the ecosystem is embedded. Rainfall patterns and temperature seasonality determine the amount of water available to the ecosystem and the supply of energy available (by influencing photosynthesis). Parent material, the underlying geological material that gives rise to soils, determines the nature of the soils present, and influences the supply of mineral nutrients. Topography also controls ecosystem processes by affecting things like microclimate, soil development and the movement of water through a system. This may be the difference between the ecosystem present in a wetland situated in a small depression on the landscape, and one present on an adjacent steep hillside.

Other external factors that play an important role in ecosystem functioning include time and potential biota. Ecosystems are dynamic entities—invariably, they are subject to periodic disturbances and are in the process of recovering from some past disturbance. Time plays a role in the development of soil from bare rock and the recovery of a community from disturbance. Similarly, the set of organisms that can potentially be presented in an area can also have a major impact on ecosystems. Ecosystems in similar environments that are located in different parts of the world can end up doing things very differently simply because they have different pools of species present. The introduction of non-native species can cause substantial shifts in ecosystem function.

Unlike external factors, internal factors in ecosystems not only control ecosystem processes, but are also controlled by them. Consequently, they are often subject to feedback loops. While the resource inputs are generally controlled by external processes like climate and parent material, the availability of these resources within the ecosystem is controlled by internal factors like decomposition, root competition or shading. Other factors like disturbance, succession or the types of species present are also internal factors. Human activities are important in almost all ecosystems. Although humans exist and operate within ecosystems, their cumulative effects are large enough to influence external factors like climate.