Light influences growth, pigmentation, migration, and reproduction. The intensity and frequency of light influences metabolic activity, induce gene mutations (UV, X-rays). Light is essential for vision. This is proved by the poorly developed or absence of eyes in cave-dwelling organisms. Diapause is also influenced by light in animals. Gonads of birds become more active with increasing light in summer. Light influences the locomotion and movement of lower animals.

Temperature or degree of hotness and coldness is an essential and variable factor in any environment. It influences all forms of life by affecting many vital activities of organisms like metabolism, behaviour, reproduction, development and even death in the Biosphere.

(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.

(i) The inherent tendency of all animal populations is to increase in number. But it does not increase indefinitely.
(ii) Once the carrying capacity of the environment is reached, population numbers remain static or fluctuate depending on environmental conditions.
(iii) This is regulated by many factors which are:
1. Density independent - Extrinsic factors
2. Density dependent - Intrinsic factors
(iv) Extrinsic factors include the availability of space, shelter, weather, food, etc.
(v) Intrinsic factors include competition, predation, emigration, immigration, and diseases.

S. No.	J shaped curve	S shaped curve
1.	When a population increases rapidly in an exponential fashion and then stops abruptly due to environmental resistance or due to sudden appearance of a limiting factor, they are said to exhibit J-shaped growth form,	Some populations, as in a population of small mammals, increase lowly at first then more rapidly & gradually slow down as environmental resistance increases where by equilibrium is reached and maintained
2.	Many insects show explosive increase in  number during the rainy season followed by their disappearance at the end of season.	Their growth is represented by s shaped growth curve.

S. No. Natality Mortality 1. It refers to the number of births per unit
population per unit time. e.g., per thousand
individuals per year in humans. It refers to the number of deaths per unit population per unit time. e.g., per thousand individuals per year in humans. 2. It adds new members to the population It removes members from the population 3. It increases the size of population. It decreases the size of population 4 Natality adds to population density Mortality reduces density of population 5 It maintains continuity of population It maintains health of population 6 It is high when population size is small and low when population size is large. Mortality is low when population
size is small and high when population size is large

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.

Every living organism responds to its environment. There are various ways by which organisms respond to abiotic conditions. The types of response observed are:
Regulate:
1. Some organisms are able to maintain homeostasis by physiological means which ensures constant body temperature, ionic/ osmotic balance. Birds, mammals and a fewer lower vertebrate and invertebrate species are capable of such regulation.
Conform:
1. Most animals cannot maintain a constant internal environment.
2. Their body temperature changes with the ambient temperature.
3. In aquatic animals like fishes, the osmotic concentration of the body fluids changes with that of the ambient water osmotic concentration. Such animals are called conformers. In case of extreme condition, the inhabitants relocate themselves as in-migration
Migrate:
1. Organisms tend to move away temporarily from a stressful habitat to a new, hospitable area and return when the stressful period is over.
2. Birds migrate from Siberia to Vedanthangal in Tamilnadu to escape from the severe winter periods.
Suspend:
1. In certain conditions, if the organism is unable to migrate, it may avoid the stress by becoming inactive.
2. This is seen commonly in bears going into hibernation during winter.
3. Some snails and fish go into aestivation to avoid summer related problems like heat and desiccation.
4. Some lower animal suspend a certain phase of their life cycle which is to as diapause.