Organisms belonging to different populations interact for food, shelter, mating or for other necessities. Interaction may be intra specific (interaction within the members of same species) or interspecific (among organisms of different species).
Intraspecific association is observed for all livelihood processes like feeding, territoriality, breeding, and protection.
Interspecific associations or interactions can be:
Neutral: Where different species live together but do not affect each other.
Positive: It is a symbiotic relationship ill which no organism in association is harmed and either one or both may be benefitted. It is of two types - Mutualism and Commensalism.
Negative: One or both of the interacting organisms will be affected as in case of competition, predation, and parasitism.

1. The pectoral fins and dorsal fins act as stabilizers or balancers and the caudal fin helps in changing the direction as a rudder.
2. Arrangement of body muscles in the form of bundles (myotomes) help in locomotion.
3. Stream lined structure helps in the swift movement of the animals in water.
4. Respiration by gills making use of gases dissolved in water.
5. Presence of air-bladders filled with air for buoyancy.
6. Presence of lateral-line system. They function as chemoreceptors which is helpful in echolocating objects in water.
7. Integuments rich in mucous glands are protected by scales.
8. Maintain water and ionic balance in its body with excretory structures.

(i) Grasslands occur in temperate and in the tropical regions.
(ii) They have hot summers, cold winters, and irregular rainfall.
(iii) Often they are characterized by high winds.
(iv) The low irregular rainfall is the factor which makes the difference between a temperate deciduous forest and a temperate grassland.
(v) Herbivores like antelope, bison, wild horse, jack rabbit, ground squirrel and prairie dogs are abundant.
(vi) Predators include coyotes, foxes, hawks and snakes.
(vi) In India, fauna of grasslands includes Elephant, Gaur, Rhino, Antelope.
(vii) Flora of grasslands include purple needle grass, wild oats, foxtail, ryegrass and buffalo grass.

Essential properties of water
(i) Water is one of the main agents in Pedogenesis (soil formation).
(ii) It is the medium for several different ecosystems
(iii) It is present as moisture in the atmosphere and the outer layers of the lithosphere and is uneven in distribution on the earth
(iv) Water is heavier than air and imparts greater buoyancy to the aquatic medium. This enables organism to. float at variable levels.
(v) Water has high heat capacity and latent heat, due to which it can withhold large amounts of heat. Thus, oceans and lakes tend to maintain a relatively constant temperature, and the biosphere is relatively thermostable.
(vi) Water is physically unique because it is less dense as a solid (ice) than as a liquid.
(vii) When water freezes (O°C), it contracts. The maximum density of liquid water occurs at 4°C. Below that, it expands markedly. This enables ice to float on the top of water bodies. Hence, only the surface of water bodies will freeze, while below the surface, water will be in liquid form, sustaining life).
(viii) Water is considered as the Universal solvent. It is the main medium by which chemical constituents are transported from abiotic components to the living components of an ecosystem
(ix) Water has high surface tension. This allows pollen, dust, and even water striders to remain at the surface of a water body even though they are denser than the water.