Zoology 3rd Sem# Ecosystem# Definition, components ,energy flow, food chain, food web, ecological pyramids

The term ecosystem was first coined by Sir Arthur George Tansley, 1935.

 Definition: "The ecosystem is the basic  functional unit of organisms and their environment, interacting with each other and within their own components" ...E.P.Odum , 1971.

Components of ecosystem

An ecosystem consists of Biotic & Abiotic components .

Biotic/Living components are Autotroph  and Heterotroph. 

   Autotroph are generally  phytoplanktons,  photosynthetic bacteria(e.g.green sulphur bacteria),   chemosynthetic bacteria (e.g.Nitrosomonas) and  green plants which prepare their own food(potential energy ).

          Heterotrophs consume products of autotrophs or decompose dead organic matters .

On the basis of feeding habit consumers may be 1st order ,2nd order or 3rd order .

(i) 1st order/ primary consumer : They are strictly herbivores and directly feed on the products of producers .e.g. rabbits, deer, rodents, insects ,protozoa, molluscs,etc. 

(ii) 2nd order/ secondary consumers: They feed on 1st order consumers .Elton,1939, treated them as key industry animals. e.g. water beetle, dragonfly, nymph, crow ,cat, fox,tiger,etc.

(iii) 3rd order/Tertiary consumers: These upper graded consumers feed on both of primary and secondary consumers .e.g snake in grassland is tertiary but peacock forms the final/Quaternary consumer . 

Decomposers: Bacteria and fungi break down complex organic substances of cells  of dead organisms into simpler substances , hence , also termed as the reducers /microconsumers .These simple substances are absorbed by microbes to produce secondary metabolites like vitamins ,antibiotics etc. so, decomposers also called as  ectocrines. e.g. Mucor, Rhizopus, Bacillus,Clostridium,etc.

  Abiotic/ non-loving components are the physical factors i.e. light, temperature (45° C is average maximum temperature for life) ,  humidity , atmospheric pressure  (14.7  lbs/inch² at sea leveland gradually decreases with altitude),etc and chemical  factors are inorganic elements e.g. C, H,O, N, P, K, Ca,Mg, etc. & organic  compounds .

Energy flow in ecosystem 

           Energy flow in ecosystem is a one directional  process   and   Raymond  L. Lindeman (1942) first time  documented energy input, flow and its loss in an ecosystem based on gm. cal./cm²/yr (Single Channel Energy Flow model).

                  Ten percent law (Kindemann 1942)  states that only ¹/10 th of energy (10%of total energy ) is transfered from one trophic level to the next trophoc level .

         Energy flow in the ecosystem are of two types :

            (a)Flow of energy through grazing pathway :It starts from green plants and ends in omnivores .The energy content sharply declined in successive trophic levels, as expenditure of energy increases in higher trophic levels.

            (b)Flow of energy through detritus pathway : Here energy content is not lost but supply energy for detrivores from dead bodies of animals and plants .

      Grazing pathway and detritus pathway are interdependent, interlinked and occur simultaneously in an ecosystem forming a  Y-shaped energy flow model (Odum, 1983 ).

     Odum, 1983 , unified the above said two models to make Universal Energy Flow Model which can be easily applied to both of terrestrial and aquatic ecosystems.

              Food Chain :   It  is the linear food-predator relationship exhibited by a series of organisms in an ecosystem .

             Food chain in a pond ecosystem:

Phytoplankton --> Zooplankton--> insects ------>Fishes

        Food chain in a grassland ecosystem:

Grass-->Insects-->Amphibians--->Snakes-->Peacock .

Food chain are of two types :--Grazing and Detritus .

        Grazing food chain :

  A food chain consisting of green plants (as producers), herbivores (as primary consumers ) , carnivores (as secondary consumers )and succesive consumers is termed as grazing food chain .

Autotroph-->Herbivore-->Primary carnivore ----->Secondary carnivore.

 # It starts from autotrophs/ producers and ends in secondary consumers . 

 #The gross production of producers may be oxidised in respiration or the producers may die and decayed or may be etyen by herbivores and stored as carbohydrate, fat, and protein by the expence  of energy supplied during respiration.

# The energy stored in hervivores  used in respiration ; released after decomposition or may be transfered to next trophic level.

# It is larger than a detritus food chain.

#10% of energy of an ecosystem passes stepwise via grazind food chain .

 e.g. food chain of a grassland .

             Detritus food chain

    Dead bodies of animals/ plant parts--> decomposers/detrivores-->predators.

    So, a food chain  in which detritus act as the source of energy is called detritus food chain.

#This food chain is smaller than a grazing food chain.

#90% of energy of an ecosystem passes continuously via detritus food chain .

     Food web:

     A net-work of relationships between various species of  different food chains of an ecosystem are inter -connected and make a food web .

  Difference between food chain and food web :

(i)  # A  Food chain is a straight pathway .

   # A Food web is made up of numerous  interconnected food chains .

(ii)  # In a food chain , a member of higher trophic level feed upon a single type of  organisms of lower trophic level.

 # In a food web , a member of higher trophic level feed upon a number of alternative organisms from interconnected food chains .

(iii)  # A food chain adds to the instability of the ecosystem.

 # A food web adds stability of the ecosystem .

(iv) # Food chain does not add adaptibility and competitiveness .

 # Adaptibility and compititiveness are added by food web among organisms .

            Ecological Pyramids

     Charles Elton,1927, first represented the relationship between the numbers, biomass and energy contents obf different organisms of ecosystems by pyramid-like diagrams , thus termed as Eltonian or Ecological pyramids .

  Types:

(1) Pyramid of Numbers: It plots the relation between the number of member organisms of successive trophic levels.

# Upright....in most of ecosystems like Grassland ecosystem the number of member organisms successively decreasing while going from base to  the apex .

#Partly inverted....e.g. forest ecosystem, where numer of producers are less than the primary consumers number , again the number of secondary consumers are approximately equal to the number of pruducers whereas tertiary consumers are least in number .

#Inverted....In a few tree ecosystem, parasitic food chain , etc . It is totally dependent upon the  decreasing size of the member organisms of successive trophic levels .

A primary producer suports many parasites ,again they support more hyperparasites 

(2) Pyramid of Biomass:It plots the relation between the net dry weight /caloric value of member organisms of successive trophic levels of an ecosystem.

#Upright...........In most of the terrestrial   ecosystems biomass of producer is the largest and successively decresing through each level and least at the top.

#Inverted.... In a Marine ecosystem a producer with a small biomass supports  the next large weighted consumer successively .

(3) Pyramid of energy: 

It represents the otal amount of benergy consumed in each trophic level in an ecosystem.It is always upright and never be inverted as it maintains the 10% law.

     The above said pyramids start with the prodecers keeping them at the base and proceed through different trophic levels .

Important Questions

(1) What do you mean by Ecology?

                                                         1/ 2015

(2) Who coined the term ecosystem ? 

                                                1/2017

(3) State the role of decomposers in forest ecosystem.                            3/2017

(4)Role of biotic components  of ecosystem.                            6/2015,  2017

(5) How is energy flow in ecosystem ?

                                                2/ 2015

(6) Where do inverted pyramid is found?

                                                1/2015,2018

(7)SN on Grazing and detritus food chain .

                                                5/2016,2019

(8) SN on ecological Pyramids.

                                               5/2016