Population is defined as any group of organisms of the same species which can interbreed among themselves, and occupy a particular space and function as part of biotic community.

Cryptic animals are those which camouflage perfectly with their environment and are impossible to detect. Eg. Chameleons. This helps them in prey capture or to evade from predators.

Animals respond differently to a biotic condition in their environment. Many animals avoid stress by becoming inactive. Some lower animals suspend a certain phase of their life cycle which is referred to as diapause.

Based on the seasonal distribution of rainfall, the types of tropical forests are
(i) Evergreen rainforest: No dry season.
(ii) Seasonal rainforest: Short dry period in a very wet tropical region.
(iii) Semi evergreen forest: Longer dry season (the upper tree storey consists of deciduous trees, while the lower storey is still evergreen).
(iv) Moist / dry deciduous forest (monsoon): The length of the dry season increases further as rainfall decreases (all trees are deciduous).

(i) Pedosphere: The soil zone is known as pedosphere.
(ii) Soil profile: Soil is formed of many horizontal layers called soil profile.

A change in the speed of locomotion (or frequency of turning) in a motile organism or cell which is made in response to a change in light intensity is called photokinesis. It involves undirected random movement in response to light.

The growth or orientation of an organism in response to light, either towards the source of light (positive phototropism) as seen in sunflower, or away from light (negative phototropism) as in case of the root of plants

The movement of organism in response to light, either towards the source of light as in Moths (positive phototaxis) or away from light (Earthworm (negative phototaxis).

The adaptations seen in animals to overcome extreme winters and summers are:
a) Formation of heat resistant spores
b) Formation of cysts (Entamoeba)
c) Presence of antifreeze proteins (Arctic fishes)
d) Hibernation (Winter sleep)
e) Aestivation (Summer sleep)
f) Migration

(i) Eurythermy can be an evolutionary advantage: adaptations to cold temperatures (cold-eurythemy) are seen as essential for the survival of species during ice ages.
(ii) The ability to survive in a wide range of temperatures increases a species ability to inhabit other areas, an advantage for natural selection.
(iii) Eurythermy is an aspect of thermoregulation in organisms.

(i) The camel is able to use water efficiently and effectively for evaporative cooling through their skin and respiratory system.
(ii) They excrete highly concentrated urine and can also withstand dehydration up to 25% of the body weight.
(iii) The hoofs and hump are also suitable adaptations for survival in this dry sandy environment.

(i) Ecological density: It is the size of a population in relation to the numbers per unit of habitat space. (Available area or volume that can be colonized by a population). Eg. 1000 fish in the volume of water in the pond.
(ii) Crude density: It is the size of a population in relation to the numbers per unit of total space. Eg. 1000 fish in a pond.
(iii) Population density: It is usually expressed as the number of individuals per unit area or volume. Eg. 100 trees per acre.