Ecosystem Structure And Function

Ecosystem Structure And Function-Structure

Important structural features of an ecosystem are :

Species composition Different ecosystems have different kinds of species composition so have different physical appearance (called physiognomy).

e.g. a tropical rain forest is dominated by broad-leaved evergreen trees so giving the appearance of a tall plant canopy. A desert has very scanty flora and fauna so shows a low and discontinuous herb layer. Vegetation is less and separated by extensive bare patches of soil.

Stratification is the way in which plants of different species are arranged in different vertical layers in order to make full use of the available physical and physiological requirements. Data on stratification are obtained by using a bisect. Vertical projection is constructed and stature of the species may be plotted to find out distribution pattern of stem, roots etc. of different species.

Trophic (food) structure of an ecosystem is indicated by food relationships of different trophic levels of a food chain. It can also be described in terms of the amount of living material (called standing crop) present in different trophic levels. Standing crop is the number or biomass of organisms per unit area. The amount of nutrients, such as nitrogen, phosphorus and calcium present in the soil at any given time, is called as standing state. The standing states of nutrients different from one ecosystem to another, or with season even in the same ecosystem.

Ecosystem Structure And Function

The key functional aspects of the ecosystem are : Productivity and energy flow; nutrient cycling and development and stabilisation.

PRODUCTIVITY

The rate of biomass production is called productivity. The productivity of an ecosystem refers to the rate of production i.e. the amount of organic matter accumulated in any unit time.

Productivity is of the following three types (Odum, 1983) :

• Primary productivity It is associated with the producers which are autotrophic, most of which are photosynthetic, and toa much lesser extent the chemosynthetic microorganisms. These are the green plants, higher macrophytes as well as lowerforms, the phytoplanktons and some photosynthetic bacteria. ‘‘The rate at which radiant energy is stored by photosynthetic and chemosynthetic activity of producers is called primary productivity.’’ It is generally expressed in terms of gm–2 year–1, or k calm–2 year–1. Hence, the productivity of different ecosystems can be easily compared.

Primary productivity is further distinguished as :

Net primary productivity (Net productvity) = Gross productivity – Respiration rate

The net primary productivity results in the accumulation of plant biomass, which serves as the food of herbivores and decomposers.

• Gross primary productivity (PG) It is the total rate of photosynthesis including the organic matter used up in respiration during the measurement period. This is also sometimes referred to as total (gross) photosynthesis or total assimilation. It depends on the chlorophyll content. The rate of Gross primary productivity are estimated in terms of either chlorophyll contents as, Chl/g dry weight/unit area, or photosynthetic number i.e. amount of CO2 fixed/g Chl/hour.
• Net primary productivity (PN) It is the rate of storage of organic matter in plant tissues in excess of the respiratory utilisation by plants during the measurement period. This is thus the rate of increase of biomass and is also known as apparent photosynthesis or net assimilation. Thus, net primary productivity refer to balance between gross photosynthesis and respiration and other plant losses as death etc.

• Secondary productivity : It refers to the consumers or heterotrophs. These are the rates of energy storage at consumers level. Since consumers only utilise food materials (already produced) in their respiration, simply converting the food matter to different tissues by an overall process, secondary productivity is not divided into ‘gross’ and ‘net’ amounts.
• Net productivity: It refers to the rate of storage of organic matter not used by the heterotrophs (consumers) i.eequivalent to net primary production minus consumption by the heterotrophs during the unit period, as a reason or year etc. It is thus the rate of increase of biomass of the primary producers which has been left over by the consumers. Net productivity is generally expressed as production of cal g/m2/day which may then be consolidated on month, season or year basis.

DECOMPOSITION

Decomposition is the process by which complex organic compounds are broken into simpler and inorganic substances that can be reutilized by the plants for their growth. It also provides energy and nutrients to the decomposers which include bacteria and fungi.

RATE OF DECOMPOSITION

Different organic compounds are decomposed at different rates e.g. compounds like carbohydrates, fats and proteins are decomposed rapidly, while compounds like cellulose, lignin, chitin, hair and bones are decomposed very slowly.

MECHANISM OF DECOMPOSITION

Decomposition is a complex process of enzymatic reaction and involves the step-wise degradation of detritus (dead organic matter formed of excreta of animals and dead bodies of plants and animals).

It involves following processes.

• Fragmentation of detritusIt can be done by detritivorous invertebrates which are microscopic organisms. This increases the surface area of detritus particles for the microbial action. These detritivores also add certain growth substances which stimulate the microbial growth. Some of these detritivores are coprophagic (kopros = dung) and cause breakdown of faecal pellets of animals. It is estimated that one gram of soil may contain one billion bacteria, 5 million members of Actinomycetes,500,000 protozoans and 200,000 moulds of different types.
• Catabolism In this, decomposers release extracellular enzyme in their surroundings to breakdown detritus into simple organic compounds and inorganic substances. Specific decomposers perform specific chemical actions on specific complex compounds through specific enzymes e.g. Pseudomonas bacteria decompose the proteins into ammonia and simple nitrogen compounds. But the percolating water may carry downward certain soluble substances like sugars from the fragmented detritus, and is called leaching.
• Humification It is process by which simplified detritus is changed into dark coloured amorphous substance called humus. It is highly resistant to microbial action so undergoes low decomposition. Humus acts as a reservoir of nutrients. It is dark coloured amorphous organic matter rich in lignin and cellulose.
• Mineralisation It involves the release of inorganic substances (e.g. water, CO₂, etc.) and other nutrients (NH₄⁺, Ca⁺⁺, Mg⁺⁺, K⁺, etc.) from organic matter in the soil.

Fig. : Process involved in decomposition of detritus

FACTORS AFFECTING DECOMPOSITION

Decomposition is affected by two categories of factors :

• Climatic factors
• Chemical quality of detritus

Climatic factors includes temperature and Soil moisture.

• Temperature: It is most important factor affecting the decomposition rate e.g. in tropical areas with high temperature (> 25°C) and moist conditions, the detritus decomposes very rapidly. Conversely, decomposition rate is very low in regions of high altitude or latitude characterized by low temperature(less than 10°C) though has sufficient soil moisture. In these areas, decomposition is completed in several years.
• Soil moisture: Rate of decomposition is low in soil with low moisture e.g. in tropical deserts, there is low decomposition rate even in high temperature. It is so because these have low soil moisture.

Chemical quality of detritus It is determined by relative proportions of water-soluble substances like sugars, polyphenols, lignin and nitrogen. Presence of lignin and chitin decrease the decomposition rate while the presence of nitrogen-containing substances increase the rate of decomposition.