• The word ‘Environment’ is derived from the French word ‘Environner’ which means to encircle, around or surround. The biologist Jacob Van Uerkal (1864-1944) introduced the term ‘environment’ in Ecology.
• As given by Environment Protection Act 1986, Environment is the sum total of land, water, air, interrelationships among themselves and also with the human beings and other living organisms.
• The environment may be defined as the surroundings or conditions in which an organism lives or operates. The environment broadly includes living and non living components.

Components of the Environment

A. Abiotic Components

• Light: Sunlight provides energy. Green plants utilize sun light for photosynthesis for synthesizing food for themselves as well as all other living organisms.
• Rainfall: Water is essential for all living beings. Majority of biochemical reactions take place in an aqueous medium. Water helps to regulate body temperature. Further, water bodies form the habitat for many aquatic plants and animals.
• Temperature: Temperature is a critical factor of the environment which greatly influences survival of organisms. Organisms can tolerate only a certain range of temperature and humidity.
• Atmosphere: The earth’s atmosphere is made of 21% oxygen, 78% nitrogen and 0.038% carbon dioxide. Rest are inert gases (0.93% Argon, Neon etc).
• Substratum: Organisms may be terrestrial or aquatic. Land is covered by soil and a wide variety of microbes, protozoa, fungi and small animals (invertebrates) thrive in it. Roots of plants pierce through the soil to tap water and nutrients. Terrestrial animals live on land. Aquatic plants, animals and microbes live in fresh water as well as in the sea. Some microbes live even in hot water vents under the sea.

B. Biotic Components

• Green Plants: Prepare food through photosynthesis for all living organisms.
• Animals: Individuals of the same species occur in a particular type of habitat. They also live with other species. One species forms food for another. Micro-organisms and fungi decompose dead plants and animals releasing nutrients locked in bodies of dead organisms for reuse by the growing plants.
•  Environmental Science is the interdisciplinary field and requires the study of the interactions among the physical, chemical and biological components of the Environment with a focus on environmental pollution and degradation. Environment study is a multidisciplinary subject where different aspects are dealt with in a holistic approach.

Ecology

• ‘Ecology may be defined as the scientific study of the relationship of living organisms with each other and with their environment.’
• The term ecology was first coined in 1869 by the German biologist Ernst Haeckel. It has been derived from two Greek words, ‘oikos’, meaning home or estate and ‘logos’ meaning study. The emphasis is on relationships between organisms and the components of the environment namely abiotic (non-living) and biotic (living).
• Ecology is the study of the relationships between living organisms, including humans, and their physical environment; it seeks to understand the vital connections between plants and animals and the world around them. Ecology also provides information about the benefits of ecosystems and how we can use Earth’s resources in ways that leave the environment healthy for future generations.

Levels of Ecological Organization

•  Ecology not only deals with the study of the relationship of individual organisms with their environment, but also with the study of populations, communities, ecosystems, biomes, and biosphere as a whole.

1.  Individuals or organisms: It constitutes the basic unit of study in ecology. At each level, the biological unit has a specific structure and function. At this level the form, physiology, behavior, distribution and adaptations in relation to environmental conditions are studied. 
2. Population: From ecological point of view population refers to group of Ecosystem and Environment individuals of any one kind of organism living together in a particular locality e.g. number of pigeons or sparrows, or dogs or human beings living together in a particular place.
3. Community: In the ecological sense community refers to the association of various populations (various types of living organisms) in a particular locality. This is also referred to as biotic community. There are two types of community:
   • Major Community: It is the smallest ecological unit which is able to sustain itself and is self-regulating. These communities are usually relatively independent of other communities, for example a pond, a forest, a grassland or lake. Long lasting and mature major communities contain only those organisms, which are successfully adapted to the environment and to the other species within the community. A major community is an assemblage of a faunal community or ’zoonenosis’, a floral community or ’phytocenosis’, and a microbial community or ’microbiocenosis’.
   • Minor communities, or merocenoses, which make up major communities, are smaller ecological units that
   are not individually self-sustaining and rely on interactions with other communities. An example of a minor
   community is the collection of organisms, which lives within a piece of deadwood on the forest floor.
4. Ecosystem: An ecosystem is formed when the community (various types of population) and the non-living
   environment interact and function together. Ecosystem is the unit of studies in ecology i.e. living and non living things interact together.
5. Biome: It is a set of ecosystems that share similar characteristics with their abiotic factors adapted to their
   environment. Biomes are land units with a natural boundary that have a mosaic of land that generally represents different ecosystems. It is a large regional unit characterized by an important type of vegetation and associated fauna that is found in a specific climatic zone. The biome includes all associated developing and modified communities occurring within the same climatic region, eg forest biomes, grassland and savanna biomes, desert biome, etc.
6. Biosphere: Biosphere refers to part of earth with living forms and the thin layer of air above its surface that supports life in which ecosystems can operate. It includes soil, the water of earth and the lower regions of the atmosphere. It also includes the regions of the air where birds fly and cloud form and underground regions where worms burrow. It is also the part where solar energy is used to bring about photosynthesis and other chemical and physical changes.