(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.

Characters of biome 
(i) Location, geographical position (Latitude, Longitude)
(ii) Climate and physiochemical environment
(iii) Predominant plant and animal life
(iv) Boundaries between biomes are not always sharply defined. Transition or transient zones are seen as in case of grassland and forest biomes.

(i) Freshwater (Lakes, ponds, and rivers)
(ii) Brackish water (Estuaries and Wetlands)
(iii) Marine (Coral reefs, pelagic zones, and abyssal zones)

Adaptation is a dynamic evolutionary process that fits organisms to their environment and enhancing their evolutionary fitness.
(a) Structural adaptations:
1. The external and internal structures of animals can help them to adapt better to their environment. Some of the most common examples are mammals growing thicker fur to survive freezing climates.
2. Some of the most attractive adaptations in nature occur for reasons of crypsis. (e.g: Camouflage) and mimicry.
3. Cryptic animals are those which camouflage perfectly with their environment and are almost impossible to detect. Certain reptiles and insects such as chameleons and stick insects show this type of adaptation, which helps in prey capture or to evade from predators. Likewise, horse legs are suitable for fast running and adapted for grasslands or plains and similar terrestrial environments.
(b) Behavioural adaptations:
1. Action and behaviour of animals are instinctive or learned. Animals develop certain behavioural traits or adaptations for survival.
2. Fleeing from a predator, hiding during sleep, seeking refuge from climate change or moving to find different food sources are all behavioural adaptations.
3. The two most characteristic forms of behavioural adaptations are migration and courtship.
4. Migration allows the animals to find better resources or evade threat.
5. Courtship is a set of behavioural patterns to find a made to reproduce.
6. Most nocturnal animals remain underground (or) inactive during daytime.
7. This is a modification of their feeding and activity pattern (or) habit (or) behaviour.
(c) Physiological adaptations:
1. The adaptations of organisms that help them to live and survive in their environment with unique niches.
2. Example: Lions have sharp canines to hunt and tear meat of preys and digestive system suitable for digesting raw meat.
3. The two most well-known physiological adaptations are hibernation and aestivation.
4. These are two different types of inactivity where the metabolic rate slows down so much that the animal can survive without eating or drinking.
5. Aquatic medium and terrestrial habitats have their own respective environmental conditions. Hence organisms have to evolve appropriate adaptations to select suitable habitats and niches.