Community contains a population of species. Three processes competition, predation and Symbiosis help to organise communities. For instance competition among plants, herbivores and carnivores could control the diversity and abundance of species in a community.

Community organisation
Community contains a population of species. Three processes competition, predation and Symbiosis help to organise communities. For instance competition among plants, herbivores and carnivores could control the diversity and abundance of species in a community. Predation could organise the community along feeding lines while symbiosis which includes important interactions like mutualism that link species could help to increase community organisation in a positive way.

Habitat and niche
The habitat of an organism is where it lives.
The niche of an organism refers to the role it plays in the ecosystem. It includes tolerance of physical factors such as temperature, light, soil, moisture, pH and nutrient requirements. It also includes biological aspects such as how it acquires its food, what season of the year it reproduces and how it interacts with other organisms in the community. In short the niche defines a particular species role in the community, and is unique for each species.

Keystone species
The species whose activities determine community structure is called keystone species. Keystone species maybe relatively rare in natural communities and may not be easily recognised.

Dominant species
The species with greater number and more biomass are called dominant species. Dominance is related to the concept of species diversity.

Species interaction
There are two types of interactions of species. If individuals in a species population interact amongst themselves, it is called intraspecific interactions. If individuals of other species population interact, it is called interspecific interactions. At an individual level these relationships can be harmful or beneficial; at a population level they can reduce, stabilise or enhance the rate of population growth.

The effects of these interactions can be positive, negative or neutral. Neutral interactions have no effect on the growth of the population. Positive interactions benefit both the populations and if the relationship is mutually detrimental when the interaction is negative.

The following are the types of biotic interactions
1) Mutualism
Both species benefit.
ex:- bacteria present in termites and stomachs of
ruminant animals help in digestion of cellulose.
2) Commensalism
One species benefit, the other is unaffected.
ex:- An epiphytic plant growing on the trunk of a tree.
3) Competition
Both species are harmed by the interaction.
ex:- if two species eat the same food and there isn't enough for both, both may have access to less food then they would when they were alone.
4) predation
Means plundering. One species is benefited and the other affected/ it is dead.
ex:- tiger killing a deer.
5) Parasitism
Parasitism involves one organism feeding on another and the prey or host is seldom killed out right.
ex:- ticks gain benefit by sucking blood and the host is harmed by losing blood.
6) Amensalism
One species is harmed, the other unaffected.
ex:- a large tree shades a small plant, retarding the growth of the small plant.
7) Neutralism
There is no net benefit or harm to other species.

Biodiversity is the variability among living organisms from all sources, including terrestrial, marine, and other aquatic ecosystems and the ecological complexes of which they are part; this includes diversity within species, between species, and of ecosystems.

The Terrestrial part of the Earth is divisible into enormous regions called biomes, which are characterized by climate, vegetation, animal life and general soil type. Thus a biome is a large community unit characterized by the kinds of plants and animals present. There are three biomes in an ecosystem forests, grasslands and deserts.

Forest ecosystem
The forest biome include a complex assemblage of different kinds of biotic communities. Optimum conditions of temperature and ground moisture are responsible for the growth of trees which contribute to the establishment of forest communities. The forest ecosystems have been classified into three major categories: Coniferous forests, temperate forests and tropical forests.

Indian forest types
1) Tropical wet evergreen forests
Wet evergreen forests are found along the western Ghats, Andaman Islands and all along the North Eastern Himalayan regions. It is characterized by tall, straight evergreen trees. The common trees found here are the jackfruit, betel nut palm, jamun, mango and hollock. Orchids also grow in these forests.
2) Tropical semi evergreen forests
Semi evergreen forests are found in western Ghats, andaman and Nicobar Islands and the Eastern Himalayas. Such Forest have a mixture of the wet evergreen trees and the moist deciduous trees.
3) Tropical moist deciduous forests
Moist deciduous forests are found throughout India except in western and the North Western region. The trees are tall, have broad trunks, branching trunks and roots to hold them firmly to the ground. Some of the taller trees shed their leaves in the dry season. The forests are dominated by Sal, teak, mango bamboo and Rosewood.
4) Littoral and swamp forests
These forests are found along the Andaman Nicobar Islands and the delta areas of the Ganga and the Brahmaputra. They have roots that consists of soft tissue so that the plant can breathe in the water.
5) Tropical dry deciduous forests
Dry deciduous forests are found throughout the northern part of the country except in North East. It is also found in Madhya Pradesh, Gujarat, Andhra Pradesh, karnataka and Tamilnadu. Common trees are the Sal, a variety of a acacia and bamboo.
6) Tropical thorn forests
This type is found in areas with black soil: north, West, Central and South India. Spurge, caper and cactus are typical of this region.
7) Tropical dry evergreen forests
Dry evergreens are found along Tamil Nadu, Andhra Pradesh and Karnataka coast.
8) Subtropical broad - leaved forests
Broad leaved forests are found in the Eastern Himalayas and the Western Ghats, along the silent valley. Poonspar, cinnamon, rhododendron, oak, alder, chestnut, birch, cherry trees, variety of orchids, bamboo and creepers are the trees found in this forest.
9) Subtropical Pine forest
Pine forests are found in the steep dry slopes of Shivalik Hills, western and Central Himalayas, kasi, Nagaland and Manipur Hills. The trees predominantly found in these regions are Oak, rhododendron, sal and Amla.
10) Sub tropical dry evergreen forests
These forests are found in the Shivalik Hills and the foothills of Himalayas up to a height of 8000 metres. These forests have evergreen trees with shining shining leaves that have a varnished look.
11) Montane wet temperate forests
These forests are found in the region to the east of Nepal into Arunachal Pradesh, receiving a minimum rainfall of 2000mm.

Desert ecosystem
Deserts are formed in the regions with annual rainfall of less than 25 cm. The climate of this region is characterized by excessive drought, scanty and irregular rainfall. The relative humidity of the atmosphere is always low.

Adaptations in plants
The adaptations in general are of two types, having two distinct objects in view: To enable the plant to obtain water, and to retain it when obtained. The bulk of the vegetation consists of a kind of scrub made up of shrubs and perennial herbs, capable of great drought resistance. There are few trees and these are stunted and generally thorny or prickly, thus protecting themselves against plant feeding animals. Leaves and stem are succulent and have water storing capacity and in some plants even the stem contains chlorophyll for photosynthesis. Root system is well developed and spread over large area. The annuals wherever present germinate, bloom and reproduce only during the short rainy season and not in summer and winter.

Adaptations in animals
The animals are fast runners. They are nocturnal in habit to avoid the sun's heat during day time and conserve water by excreting concentrated urine. Animals and birds usually have long legs to keep the body away from the hard ground. For example Camel can travel long distances by storing water for long period of time and Lizards are mostly insectivorous and can live without drinking water for several days.

Aquatic ecosystems
Water covers about three quarters of the Earth's surface either as fresh water or as saline water or as brackish water (where salt content is intermediate between freshwater and saline water). On the basis of the salt content aquatic ecosystems can be divided into saline and fresh water bodies. Examples of freshwater ecosystems are lakes, ponds, pools, springs, streams, swamps and rivers. Examples of marine ecosystems are Shallow seas and open oceans. Examples of brackish water ecosystems are estuaries, salt marshes, mangrove swamps and forests.

Factors limiting the productivity of
aquatic habitats
Sunlight, transparency, temperature and dissolved oxygen are the factors which are the main limiting factors.
Oxygen is found in water in dissolved form. It enters the aquatic ecosystem through air water interface and by the photosynthetic activities of the aquatic plants. The amount of dissolved oxygen retained in water is also influenced by temperature as oxygen is less soluble in warm water. Warm water also enhances decomposing activity. Therefore, increasing the temperature of a water body increases the rate at which oxygen is depleted from water. Biochemical oxygen demand (BOD, also called biological oxygen demand) is the amount of dissolved oxygen needed (i.e., demanded) by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period. When the dissolved oxygen level falls, many aquatic organisms are likely to die. The surface has maximum oxygen content and beneath the surface of the water the oxygen content decreases. At the bottom the oxygen content decreases further due to utilisation of oxygen by decomposers occurring there.

Any body of standing water generally large enough in area and depth, irrespective of hydrology, ecology and other characteristic is known as the lake.
Types of lakes
on the basis of nutrients status and primary productivity the lakes can be divided into three categories (1) oligotrophic (nutrient poor), (2) Eutrophic ( nutrient rich) lakes, (3) Mesotrophic (medium nutrients ) lakes.

The process of aging of lakes through nutrient enrichment is called eutrophication. Eutrophication can be through natural or through man made processes. Activities such as industrialisation, intensive agriculture, etc result in the rapid addition of nutrients such as nitrogen, phosphates etc from agricultural runoff, sewage drainage and Industrial effluents. This eutrophication which is caused by cultural activities is called cultural eutrophication.

Algae or Phytoplankton are microscopic organisms that can be found in natural coastal waters. They are the major producers of oxygen and food for many other animals to live. some algae and blue green bacteria thrive on the excess nutrients received due to eutrophication and a population explosion covers almost entire surface layer. This excess algal growth is called algal bloom. These blooms are commonly referred to as red or brown tides.This algal bloom covers the surface layer, restricts the penetration of Sunlight, leads to the decrease in availability of oxygen and nutrients resulting the death of aquatic organisms.

Estuary ecosystem
Estuaries are located where river meets the sea. Estuaries are water bodies where the flow of freshwater from river mixes with salt water transported by the tides. Estuaries are the most productive bodies in the world. An estuary is a transitional zone between river and sea representing an ecotone. An estuary is a semi enclosed part of the coastal ocean containing brackishwater that has free connection with the sea on one side and the other side it is connected with the river water and receive freshwater. In India, estuaries are mainly found along the coast of Kerala. Major estuaries occur on the east coast and the estuaries on the west coast are smaller in size. Estuaries have high commercial value. They are vulnerable to climate change.

Mangroves are the characteristic littoral plant formation of tropical and subtropical sheltered coastlines typically on tidal flats, deltas, estuaries, bays etc. These are trees and bushes growing below the high water level of spring tides which exhibit remarkable capacity of salt water tolerance. They require high solar radiation and have the ability to absorb freshwater from saline or brackish water. It produces pneumatophores (blind roots) to overcome respiration problem in the anaerobic soil conditions. Leaves are thick and contain salt secreting glands. Mangroves exhibit viviparity mode of reproduction i.e seeds germinate in the tree itself.

Mangroves in India
The mangroves of Sundarbans are the largest single block of tidal holophytic mangroves of the world. This mangrove forest is famous for Royal Bengal Tiger and crocodiles. The mangroves of bhitarkanika (orissa), second largest in the Indian subcontinent. Mangrove swamps occur in profusion in the intertidal mud flats on both side of the creeks in the Godavari-Krishna deltaic regions of Andhra Pradesh. On the west Coast mangroves occur along the intertidal region in Gujarat, Maharashtra, Goa and Karnataka. Mangroves are also present in the Andaman Nicobar Islands.

Role of mangroves
Mangrove plants help to impede water flow and thereby enhance deposition of sediments in areas, stabilize the coastal shores, provide breeding grounds for fishes. Mangroves moderate monsoonal tidal floods and reduce innundation of coastal lowlands. It prevents coastal soil erosion. It protects coastal land from the affects of Tsunami, Hurricane and floods as themangrove do not get uprooted by storms and tides because of their extensive roots that hold the soil firmly.

Previously asked questions
1) There is a concern: over the increase in harmful algal blooms in the seawaters of India. What could be the causative factors for this phenomenon? (2011)
1) Discharge of nutrients from the estuaries.
2) Run-off from the land during the monsoon.
3) Upwelling in the seas.
Select the correct answer from the codes given below:
(a) 1 only
(b) 1 and 2 only
(c) 2 and 3 only
(d) 1, 2 and 3
Solution (d)
2) Which one of the following is the best description of the term "ecosystem"? (2015)
a) A community of organisms interacting
with one another.
b) The part of the earth which is inhabited by
living organisms.
c) A community of organisms together with the
environment in which they live.
d) The flora and fauna of a geographical area.
Solution (c)
3) Biological oxygen demand (BOD) is a standard
criterion for:
a) Measuring oxygen levlels in blood.
b) Computing oxygen levels in forest ecosystems.
c) Pollution assay in aquatic ecosystems.
d) Assessing oxygen levels in high altitude regions.
Solution (c)
4) Among the following States, which one has the most suitable climatic conditions for the cultivation of a large variety of orchids with minimum cost of production, and can develop an export oriented industry in this field?
1) Andhra Pradesh
2) Arunachal Pradesh
3) Madhya Pradesh
4) Uttar Pradesh
solution (2)
5) If a tropical rain forest is removed, it does not regenerate quickly as compared to a tropical deciduous forest. This is because
1) the soil of rain forest is deficient in nutrients.
2) propagules of the trees in a rain forest have
poor viability
3) the rain forest species are slow-growing
4) exotic species invade the fertile soil of rain forest.
Solution (1)
6) Which one of the following terms describes not only the physical space occupied by an organism, but also its functional role in the community of organisms?
2)Ecological niche
4)Home range
Solution (2)
7) If you travel through the Himalayas, you are Likely to see which of the following plants naturally growing there?
Select the correct answer using the code given below
(a)1 and 2 only
(b)3 only
(c)1 and 3 only
(d)1, 2 and 3
Solution (a)
8) Which one of the following regions of India has a combination of mangrove forest, evergreen forest and deciduous forest?
1)North Coastal Andhra Pradesh
2)South-West Bengal
3)Southern Saurashtra
4)Andaman and Nicobar Islands
Solution (4)
9) Consider the following States
1)Arunachal Pradesh
2)Himachal Pradesh
In which of the following states do “Tropical Wet Evergreen Forests” occur?
(a)1 only
(b)2 and 3 only
(c)1 and 3 only
(d)1, 2 and 3
Solution (c)
10) In India, in which one of the following types of forests is teak a dominant tree species?
1)Tropical moist deciduous forest
2)Tropical rain forest
3)Tropical thorn scrub forest
4)Temperate forest with grasslands
Solution (1)
11) What would happen if phytoplankton of an ocean is
completely destroyed for some reason? (2012)
1) The ocean as a carbon sink would be adversely
2) The food chains in the ocean would be
adversely affected.
3) The density of ocean water would drastically
Select the correct answer using the codes given below :
(a) 1 and 2 only
(b) 2 only
(c) 3 only
(d) 1, 2 and 3
Solution (a)
12) Which one of the following is the correct sequence of ecosystems in the order of decreaing productivity?
a) Oceans,lakes,grasslands,mangroves.
b) Mangroves,oceans,grasslands,lakes.
c) Mangroves, grasslands, lakes, oceans.
d) Oceans,mangroves,lakes, grasslands.
Solution (c)
13) The 2004 Tsunami made people realize that mangroves can serve as a reliable safety hedge against coastal calamities. How do mangroves function as a safety hedge? [UPSC 2011 (CS-P)]
1) The mangrove swamps separate the human
settlements from the sea by a wide zone in which
people neither live not venture out
2) The mangroves provide both food and medicines which people are in need of after any natural disaster.
3) The mangrove trees are tall with dense canopies
and serve as an excellent shelter during a cyclone or tsunami
4) The mangrove trees do not get uprooted by
storms and tides because of their extensive roots
Solution (4)

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