(i) When alien species (those species which are introduced from elsewhere into a particular region) are introduced deliberately into an ecosystem for some purpose, some of then may become invasive and cause damage and decline or even extinction of the native species in the ecosystem.
(ii) The introduction of Nile perch into Lake Victoria led to the extinction of more than 200 species of cichlid fish in that lake.
(iii) Introduction of African catfish Clarias gariepinus for aquaculture poses a threat to the indigenous catfishes in Indian rivers.
(iv) Carrot grass (Parthenium) and Lantana introduced into our country have become invasive and caused environmental damage; they pose a threat to the native species of plants in our forests.
(v) Illegal introduction of African catfish, Clarias gariepinus for aquaculture purposes poses a threat to the indigenous catfishes in our rivers.

Tropical (torred) zone, temperarate zone and polar (frigid) zone.

S = CA², where
S = species richness
A = area
C = V-intercept
1. Alexander Von Humboldt observed that within a region, species richness increased with increasing explored area, but only upto a limit.
2. The relation between species richness and area for a wide variety of taxa like angiosperms, birds, fishes, etc. turns out to be a rectangular hyperbola.

Heat loss or heat gain is a function of surface area.
Since small animals (like humming birds) have a larger surface area relative to their volume, they tend to lose body heat very fast, when it is cold outside.
They spend more energy to generate body heat through metabolism; considering the cost and benefit of maintaining   a constant internal temperature, small animals (humming birds) are not found in polar regions.

The cactus plants grow in desert regions where they adapt to store water in the roots, stems or fruits. So, they need very less water to survive.If she waters it daily, it will gradually decay due to excess of water.
Kangaroo rats, camels and desert lizards are animals that survive without water. This is because, the kangaroo rat in North American deserts is capable of meeting all its water requirements through its internal fat oxidation (in which water is a by product). It also has the ability to concentrate its urine so that minimal volume of water is used to remove excretory products.

(i) The sequential inter-linking of organisms involving the transfer of food energy from the producers, through a series of organisms with repeated eating and being eaten is referred to as the food chain.
(ii) It is defined as a structural and functional unit of the biosphere, compressing living organisms and their non living environment that interact by means of food chains and chemical cycles resulting in energy flow, biotic diversity and material cycling to form a stable, self-supporting system.
(iii) Genesh's father wanted to enhance his son's interest in environmental science.

1. Sun is the ultimate source of energy for all ecosystems on the earth.
2. Only about 50% of the incident solar radiation is photosynthetically active radiation (PAR).
3. Plants capture only 2-10% of the PAR and all organisms are dependent for their food on producers, either directly or indirectly. 
4. The flow of energy from the sun to producers and then to consumers is unidirectional and is keeping with the first law of thermodynamics.
5. Ecosystems need a continuous supply of energy for the synthesis of molecules needed and to counteract the tendency towards increasing disorderliness, i.e. energy flow follows the second law of thermodynamics too.

Co-extinction is a phenomenon in which when a species becomes extinct, the plant and animal species associated with it, in an obligatory manner, also become extinct.
(i) In case of a co-evolved plant and its pollinator, if one of them becomes extinct, invariably the other one becomes extinct.
(ii) If a host fish becomes extinct, the unique parasites depending on it would also become extinct.

Effect of Oxygen : The decomposition of detritus is an energy requiring process. Most of the decomposers (bacteria and fungi) are aerobic organisms. They require Oxygen for their cellular activities while acting on dead organic matter.
Effect of Chemical : Composition Decomposition rate is slower, if detritus is rich in lignin and chitin. It is quicker if detritus is rich in nitrogen and water soluble substances like sugars.

(a) Healthy ecosystems are the base for a wide range of economic, environmental and aesthetic goods and services. The products of ecosystem processes are named as ecosystem services. For examples, healthy forest ecosystem purifies air and water, minimize droughts and floods, cycle nutrients, generates fertile soils, provides wildlife habitat, maintains biodiversity, pollinate crops, provid storage site for carbon and also provide aesthetic,cultural and spiritual values. Though value of such services of biodiversity is difficult to determine, it seems reasonable to think that biodiversity should carry a healthy price tag.
(b) Based on the nature of the habitat - whether it is water (or very wet areas) or it is on very dry areas- succession of plants is called hydrarch or xerarch, respectively.
Differences: Hydrarch succession takes place in wetter areas and the successional series progress from hydric to mesic conditions. As against this, xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions.
Similarity: Both hydrarch and xerarch successions lead to medium water conditions (mesic), neither too dry (xeric) nor too wet (hydric).

(i) Site Y has the lowest content of dissolved oxygen due to increased bacterial population. It is caused by sewage run-off from a pig farm.
(ii) There are very few fisher at size Z due to low oxygen Concentration and choked lake (by algae and weeds)
(iii) Algae and weeds will thrive after site Z.

(i) A = Dissolved Oxygen
(ii) B=Biochemical oxygen demand(BOD)

(i) Relationship between (1) and (2) is of predator-prey relationship.
(ii) Relationship between (3) and (4) is producer and (3) is primary consumer.

Acclimatization. The gradual physiological adjustment to shoe changing environmental conditions is called acclimatization.
Ectotherms are the cold-blooded animals who can not regulate their body temperature.
Endotherms are the warm-blooded animals who are capable of regulating their body temperatures.

(i) Lichens.
(ii) Mosses and ferns are very few as they need humid conditions in forests and forests are disappearing.
(iii) Fungi are able to sustain themselves as a large population because of their wider adaptability to the changing environment conditions.
(iv) Most advanced group is of angiosperms and most primitive group is of fungi.