WHEN THE EARTH WAS FORMED AND THE WATER ON IT WAS CONCENTRATED IN THE SEAS, THE LIFE STARTED FROM THEM ONLY. SO ARTICLE WILL START WITH SEA AS ECOSYSTEM

• Sea as an Ecosystem

About 70% of the earth is covered by the major oceans namely Atlantic, Pacific, Indian, Arctic and Antarctic. Waves, tides currents, temperatures and pressures etc. determine the biological community in each ocean. These factors decide about the bottom sediments and the gases in solution. The food chain of the sea comprises of small autotrophs to large creature like whale. The seas are the largest and thickest of ecosystems since every square meter of sea has so many type of life forms and even the deep sea contains life. The main important characteristics of marine environment is the salinity. The salt concentration of a sea is usually 3.5%, mainly the sodium chloride. This concentration varies with depth and latitudes. It is lesser near the poles and the estuaries. Phytoplanktons like the diatoms, dino flagellates and algae are the primary producers. Some macroscopic seaweeds such as brown and red algae also contribute in the production of food. Herbivores like crustaceans, mollusks fishes and other aquatic animals are the primary consumers. Various species of fish such as Herring etc. are the carnivores. Fishes like cod, shark and whale survive on the smaller carnivores. Bacteria and some fungi are the usual decomposers.

• Estuaries

The portion between the seas and continents is known as the estuary. So the estuary is an area at the mouth of a river where it meets the sea. They have a diverse ecosystem of their own which is different than the both. Salinity and temperature are the most important physical features. As it has an advantage of both conditions so it has such species that are neither there in the sea nor in fresh water. The estuaries are rich in nutrients and generally have three major type of life forms. First is phytoplankton, second is benthic microflora like algae and third is the macroflora like the seaweeds, emergent marsh grasses etc. Generally it is the estuary that has to bear with the manmade  pollution.

• Streams and Rivers

They are the flowing fresh water bodies. Though a very small amount of the total water (0.0001%) flows in the rivers, they are the lifelines for the human civilization. All the early civilizations started on the banks of rivers.    Although the total area of the rivers is much smaller than the seas, they are the most useful ecosystems for the human beings. The rivers are used for water supply, waste disposal, fish production, navigation, electricity production etc. However they have to borrow organic matter from adjacent terrestrial ecosystems, or sometimes from adjacent lakes. The sewage and solid waste discharged into the rivers has polluted almost all the rivers of the world.
The river water differs in quality and quantity depending upon its origin and the area through which it passes. The plankton population is scarce in the river water originating from the glaciers with rapid currents of cold water. In its middle reaches the water absorbs more and more heat from sunlight and due to photosynthesis the biological  activities  become  dominant.  The  sediment  transportation  and  deposition takes place. In the lower reaches in plains the velocity further reduces and both phytoplankton  and  zooplankton  flourish.  Various  species  of  reptiles,  mammals  and birds are fed upon the rivers.

• Lakes and Ponds

These are the stagnant bodies of fresh water (sweet, surface water). The lakes and ponds are comparatively younger in age. The abiotic factors of a pond or lake depends upon location and the surrounding ecosystem. In some rare cases the lakes may have saline water as the Sambhar lake of Rajasthan that is used for manufacturing of common salt.  Generally, the species diversity is low in fresh water system. Distinct stratification and  zoning  are  characteristic  features  of  lakes.  Typically  a  littoral  zone  containing rooted vegetation, a limnetic zone of open water dominated by plankton, and a deep water profundal zone containing heterotrophs. Also the lakes become thermally stratified in winters and summers.

• The terrestrial Formations

Big terrestrial community units are known as biomes. In a given biome the life form is uniform, thus the dominant climax vegetation in the grassland biome is grass. Depending  upon  the  mean  annual  temperature  range  the  terrestrial  ecosystems  has been classified as

1. Arctic  and  Alpine  Tundra
2. Coniferous  Forest
3. Deciduous  Forest
4. Tropical Forest
5. Grassland
6. Desert

There  are  some  other  biomes  like  chaparral,  tropical  savanna,  thorn  shrub  and tropical  monsoon  forest.

• Deserts

These are the biomes with less than 250 mm of annual rainfall. Deserts have about 17% of the total land area on the earth. Lack of rain in the mid latitudes is due to stable high pressure zone. Generally, the deserts lie in “rain shadows’ i.e. where high mountains block off moisture from the seas. The rain fall in deserts is low and the  evaporation  rate  is  high.  Even  the  small  rainfall  is  also  useless  as  the  water quickly penetrates the soil and goes deep inside it. Some of the main deserts of the earth  are  in  south  western  united  states,  Mexico,  coastal  areas  of  Chile,  central western Australia, Asia (Thar, Gobi etc.) The desert may be a cool one like that in Washington and a hot one like that in Arizona.  Generally, there are four distinctive life forms in the deserts:

1. The annuals like cheat grass that avoids drought by growing only when there is sufficient moisture.
2. The desert shrub with numerous branches and small thick leaves. Sometimes with deep roots to absorb moisture and store it before wilting.
3. The succulents, such as cacti that store water in their tissues.
4. Microflora such as mosses, lichens and blue green algae that remain action less in the soil in want of favourable conditions.

The productivity of a desert is a linear function of rainfall. It may be as less as 600 Kg dry matter per hectare to 2000 kg per hectare depending upon the rainfall. Irrigation can increase the production manifolds as the soil is generally fertile. But how long it can sustain it depends upon many factors and the human wisdom. The major irrigation projects like Indira Gandhi Main Canal (600 Km long ) has converted the Thar desert of western Rajasthan into a good productive area in its command.

• Tundras

Tundras are barren grounds with very less biological productivity because of the cold conditions. Rainfall is also low but not limiting factor for the less production as the evaporation is also low because of the low temperature. The number of species are surprisingly higher as they have evolved remarkable adaptations to survive the cold. They lie near the Arctic Ocean and the polar icecaps. Wherever there is enough combined aquatic and terrestrial net production to support thousands of breeding migratory birds and insects during the summer it also supports the permanent resident mammals. The mammals are musk ox, reindeer, polar bears, and marine mammals. Generally, the large land herbivores are migratory as there is not enough production.

• Grasslands

This is a type of terrestrial ecosystem. The major grasslands include the Prairies of Canada and the USA, the Pampas of south America, the Steppes of Europe and Asia and the Veldts of Africa. For rainfalls in between the desert and forest i.e. 250 mm to 750 mm the natural grassland occurs. Tropical grassland may have 1500 mm rainfall in a wet season followed by a long dry spell. In the grassland the dominant plant life are the grasses that range from tall species (1.5 m to 2.4 m) to short ones like 20 cm only. Large herbivores are common features of the grassland. The main animal species are wild horses, asses, and antelope of Eurasia: herds of bison of America: and the antelope of Africa.  The other animal species are larks, the burrowing owl, jackrabbit, badgers, etc.
The abiotic components of this ecosystem are the essential organic nutrients like C, H, N, O and S present in the soil in different forms. The biotic components are (i) producers like the different grasses such as Sp. Brachiaria, Sp. cyndon, etc. (ii) consumers like herbivores and insects termites etc. and the secondary consumers like frogs, snakes, lizards, etc. and the tertiary consumers like hawks etc. (iii) decomposers like fungi, bacteria  and  actinomycetes.

The role of fire is typical in the grasslands. Man misuses the grassland by overgrazing and over ploughing and thus has converted them into deserts. Here is the concern of the environment engineer in the study of all these ecosystems that what we are doing and  for  what.

• Forest

The forests are complete ecosystems as are the seas. A well ordered and often lengthy ecological succession is the characteristic with herbaceous plants often preceding trees. So there is a mixture of vegetation including non forest stages in succession as well as forest variants adapted to special soil and moisture conditions. As moisture is more critical to the trees than to the grass, forests occupy a fairly wide gradient from dry to extremely wet situations. There is high species diversity in the tropical rainforest. There  are  three  major  types  of  forests  in  a  north-south  gradient.  The  northernmost forests  are  characterized  by  evergreen  conifers  like  Spruce  and  Fir.  The  deciduous forests are there in the more southern moist temperate regions. Pines are found in both the forests. The third type is the tropical forest. It ranges from broad leaved evergreen rainforests with good rainfall to tropical deciduous forests that lose their leaves during dry season. The vine (lianas) and the epiphyte (air plants) are special characteristics of tropical forests. Some of the major type of forests are as follows:

1. Taiga  forests: The  taiga  or  boreal  forests  include  the  northern  coniferous forests the cold climates with high altitude at high latitudes. They extend at an east-west band across north America, Europe and Asia, just the 60° north latitude. The rainfall is 100 mm to 350 mm and the average temperature varies from 6o   C to 20o  C throughout the year. Taiga forests have dense vegetation and very close trees like pines, spruces, firs, larches, cedars etc. that obstruct the sunlight. The main animals of this biome are large mammals like deer, wolves, beer etc. and small rodents like rabbits and some migratory birds and carnivores land birds like owls and eagles.   Though there are many distur- bances like fire, storms etc., these forests are rich in lakes, animals, trees etc.
2. Temperate forests: They are found in slightly warmer climates. They occur in the regions of north-central Europe, eastern Asia and eastern United States. The annual rainfall is between 750 mm to1500 mm. The main vegetation is tall deciduous trees like maples, hickories, oaks, beeches, chestnuts etc. These forests are important as the wood produced in them is hardwood suitable as timber for civil works. The animals found are dears, beers, squirrels, snakes, rabbits etc. with plenty of birds and insects.
3. Tropical rainforest: This ecosystem is located in the equatorial regions where the average annual temperature is more than 18° C and annual rainfall exceeds 1400 mm. They occur in south and central America, north eastern Australia, Congo river basin of Africa, Philippines, Hawaii and some parts of Malaysia. They cover about 8% of the earth’s surface and more than 50% of earth’s flora and fauna. Biodiversity is high. As the climate is warm and humid broad leaved evergreen plants are there. The main animals are mon- keys, tropical birds, bats, and so many carnivore animals.
4. Temperate  shrub  forest: These  Mediterranean  shrub  forest  are  with  drier climates and lower (winter)  rainfall followed by drought for rest of the year. The most important biome of this category is chaparral. It is a miniature woodland dominated by dense stands of shrubs that seldom exceed a few meter of height. Chaparrals occur along the Mediterranean coast of Califor- nia,  Chile,  South  Africa  and  South  Australia.  They  are  known  by  broad- leaved evergreen vegetation. The vegetation is very much fire prone. Reptiles and small mammals are generally found there.
5. Tropical savannah: These tropical seasonal forests occur where rainfall is high (1000 mm to 1500 mm) but seasonal. They are warm climate plains with coarse grass and scattered trees. They are located in South and Central America, Africa, Australia, south-east Asia and India. There is not much diversity. This ecosystem has giraffe, elephants, antelopes and kangaroos (Australia).

Man has changed most of the temperate forests and grasslands from their primeval condition though he could not change the basic nature of these ecosystems. Actually, we want to combine the grasslands and the forest to make our habitat. We cut the forest to make the land cultivable to get more and more production and we grow trees by the sides of our residences, offices etc. as we know the importance of trees (discussed on page 126. Still it is a matter of research that which option is better. It is true that only the forest cannot cater our needs without agriculture because of this much population. It is also true that we cannot live without trees as they are very much useful to us.