ECOSYSTEM (NOTES)

                   ATS COACHING CLASSES
                   REVISION NOTES 
                   ECOSYSTEM

Ecosystem: 

 An ecosystem may be viewed as a functional unit of nature in which live species interact with one another as well as with the surrounding physical environment.

 • The scale of an ecosystem varies widely, from a little pond to a large forest.

 • Many ecologists believe that the entire biosphere as a large ecosystem as a composite of all local ecosystems. 

For convenience let’s divide this large ecosystem into two categories:

 (i) Terrestrial: e.g. forest, grassland, desert

 (ii) Aquatic: e.g. pond, lake, wetland, river, estuary 

• Man-made ecosystem: aquarium, crop field 

Ecosystem: Structure and Function 

The biotic and abiotic aspects of an ecosystem work together to ensure the flow of energy within the ecosystem's components. 

The interaction of biotic and abiotic components produces a physical structure that is unique to each kind of ecosystem. 

Stratification refers to the vertical distribution of distinct species inhabiting various strata. E.g. Trees, occupy the top vertical stratum or layer of a forest, shrubs the second, and herbs and grasses the bottom. 

The functional components of an ecosystem are as follows: 

• Productivity: In a terrestrial ecosystem, major producers are herbaceous and woody plants. Primary production is defined as the quantity of biomass or organic matter generated per unit area by plants during photosynthesis during a time period. It is measured in either weight (g-2) or energy (kcal m-2). 

Productivity refers to the rate at which biomass is produced. It is measured in g-2y-1 or

 (kcal m-2) yr-1. 

It is divided into two components: 

Gross primary productivity (GPP): The GPP of an ecosystem is the rate of organic matter creation during photosynthesis, 

Net primary productivity (NPP): while the NPP is the leftover biomass after respiration. 

                                            NPP = GPP - R

NPP is the available biomass for heterotrophic consumption. 

Secondary productivity is defined as the pace at which consumers create new organic matter. 

Note: Primary productivity depends on the plant species inhabiting a particular area. It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems. The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 per cent of the surface, the productivity of the oceans are only 55 billion tons and the rest is of land. 

• Decomposition: Decomposition is the breakdown of complex organic matter into inorganic components such as carbon dioxide, water, and nutrients. Detritus is made up of dead plant remnants such as leaves, bark, flowers, and animal remains, this forms the raw material for decomposition. The following stages are involved in decomposition:

 (i)  Fragmentation: Detritivores (e.g. earthworms) feed on detritus and breakdown that increases the surface area of detritus particles for microbial action.

 (ii) Leaching: inorganic nutrients dissolve in water and percolate through the soil and are removed due to leaching action.

 (iii) Catabolism: Decomposers (bacteria, fungi) release enzymes that decompose detritus into simpler inorganic compounds. 

(iv) Humification: Simplified detritus is converted to humus. 

Humus: Humus is a Dark, amorphous substance, it is highly resistant to microbial action and undergoes decomposition very slowly, it is a reservoir of nutrients (due to its colloidal nature).

 (v) Mineralization: Humus is degraded and it releases inorganic substances ( CO2, H2O etc) and nutrients (Ca2+, Mg2+,K+ etc). 

• Energy flow: All living species, directly or indirectly, rely on producers for sustenance. Energy flows unidirectionally from the sun to producers and ultimately to consumers. Plants use photosynthetically active radiation (PAR) to synthesise food. Plants capture only 2 10 per cent of the PAR and this small amount of energy sustains the entire living world. Because animals acquire their nourishment from plants, they are referred to as consumers. 

The green plant in the ecosystem are called producers. In a terrestrial ecosystem, major producers are herbaceous and woody plants. Likewise, producers in an aquatic ecosystem are various species like phytoplankton, algae and higher plants. 

Food Chain: The process of eating and being eaten is referred to as the food chain, and energy flows from producers to consumers.

A GFC (Grazing Food Chain) is represented by: 

       GRASS ⸻⸺❱          GOAT  ⸻⸻ ❱       MAN

  (PRODUCER)                 (PRIMARY CONSUMER)        (SECONDARY CONSUMER)  

The detritus food chain (DFC) begins with decomposing organic materials. It is made up of heterotrophic organisms called decomposers (fungi and bacteria). These are also referred to as saprotrophs (sapro: to decompose). Decomposers release digestive enzymes that convert dead and waste materials into simple, inorganic molecules that they then absorb. 

In an aquatic ecosystem, GFC is the major conduit for energy flow. As against this, in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the GFC. Detritus food chain may be connected with the grazing food chain at some levels: some of the organisms of DFC are prey to the GFC animals, and in a natural ecosystem, some animals like cockroaches, crows, etc., are omnivores.

 The food web is formed by the natural connectivity of the food chain. 

Factors that affect Decomposition: 

• Chemical Composition: When detritus is rich in lignin and chitin, the rate of decomposition is sluggish; when detritus is rich in nitrogen and water-soluble compounds like sugars, the rate of decomposition accelerates. 

 •Climatic conditions: Warm and wet environments promote decomposition, whereas cold temperatures and anaerobiosis hinder it. 

Trophic Level: An organism occupies a certain trophic level in the food chain based on its food supply. At any one moment, each tropic level contains a specific mass of living material known as the standing crop. Producers belong to the first trophic level, herbivores (primary consumer) to the second and carnivores (secondary consumer) to the third. The important point to note is that the amount of energy decreases at successive trophic levels. When any organism dies it is converted to detritus or dead biomass that serves as an energy source for decomposers. Organisms at each trophic level depend on those at the lower trophic level for their energy demands. 

Standing crop: is expressed as the biomass of living organisms or the number of living organisms per unit area. The number of trophic levels in the grazing food chain is restricted because energy transmission follows the 10% law, which states that only 10% of energy is transmitted from the lower trophic level to the higher trophic level. 

GFC allows for the following trophic levels: producer, herbivore, main carnivore, secondary carnivore.

 Ecological Pyramids: The ecological pyramid is a graphical depiction of an ecological parameter (number, biomass, and energy) in various trophic levels of a food chain, with producers at the bottom, herbivores in the middle, and carnivores at the top. It may be upright, inverted, or spindly. 

Ecological pyramids are of following types: 

(i) Pyramid of Number: use the number of persons per unit area at various trophic levels, with the producer at the bottom and several consumers gradually at higher levels. It is mostly upright. 

In the case of a large tree, a number pyramid is usually inverted because the number of insects feeding on that tree outnumbers the number of insects.

 (ii) Pyramid of Biomass: the pyramid indicates the biomass at different trophic levels. Except in aquatic food chains with short-lived plankton, a biomass pyramid is upright. A biomass pyramid in the water is often inverted because fish biomass outnumbers phytoplankton biomass. 

(iii) Pyramid of Energy: the pyramid provides a graphical depiction of the amount of energy trapped by various trophic levels per unit area The energy pyramid is always upright and can never be inverted because as energy travels from one trophic level to the next, some energy is always lost as heat at each stage, such as feeding, digesting, assimilation, and respiration. 

Limitations of ecological pyramid

 Ecological pyramids do not take into account the same species belonging to two or more trophic levels. It assumes a simple food chain, something that almost never exists in nature; it does not accommodate a food web. Moreover, saprophytes are not given any place in ecological pyramids even though they play a vital role in the ecosystem.