The carrying capacity (denoted by K) of an environment is the maximum population size of a species that the resources of that particular environment can support and sustain indefinitely, without being degraded. When a population grows logistically, it increases until it reaches the carrying capacity, and then it levels off, because the limited resources cannot support any further increase. If the population exceeds K, it declines back due to shortage of resources.

An age pyramid is a diagram that shows the distribution of the individuals of a population in different age groups (pre-reproductive, reproductive and post-reproductive) at a given time. Its shape indicates the growth status of the population: a broad-based (triangular) pyramid indicates a rapidly growing (expanding) population with many young individuals; a bell-shaped pyramid indicates a stable population; and an urn-shaped (narrow-based) pyramid indicates a declining population.

Mutualism is a close, positive interaction between two different species in which both the species are benefited. Examples: (1) lichens, which are a mutualistic association between a fungus and an alga (the fungus provides shelter and absorbs water and minerals, while the alga provides food by photosynthesis); and (2) mycorrhiza, an association between a fungus and the roots of higher plants; and (3) the pollination of flowers by insects, where the plant is pollinated and the insect gets nectar as food.

An ecosystem is a functional unit of nature in which the living organisms (the biotic community) interact among themselves and with their physical (non-living) environment, exchanging energy and matter. Its two basic components are: (1) the abiotic (non-living) components, which include physical factors such as light, temperature, water, air, soil and inorganic nutrients; and (2) the biotic (living) components, which include producers (green plants), consumers (herbivores and carnivores) and decomposers (microbes).

A food chain is a linear sequence of organisms through which food energy passes from one trophic (feeding) level to the next, starting from the producers, for example: grass -> grasshopper -> frog -> snake -> hawk. A food web is a network formed by the interconnection of many food chains in an ecosystem, because most organisms feed on more than one type of food and are eaten by more than one type of predator. A food web gives greater stability to an ecosystem than a single food chain.

The flow of energy in an ecosystem is unidirectional: it starts from the sun, is trapped by producers (green plants) during photosynthesis, and then passes from the producers to herbivores (primary consumers) and then to carnivores (higher consumers) through the food chain. At each transfer, most of the energy is lost as heat in respiration and other activities. According to Lindeman's ten per cent law, only about 10 per cent of the energy present at one trophic level is transferred to and stored at the next higher trophic level; the remaining 90 per cent is lost. This is why food chains usually have only 3 to 5 trophic levels.

only about 10 percent energy transferred per level

An ecological pyramid is a graphical representation of the relationship between the organisms at the various trophic levels of a food chain, with the producers at the base and the top carnivores at the apex. There are three types: (1) the pyramid of numbers (based on the number of individuals at each level); (2) the pyramid of biomass (based on the total dry weight of organisms at each level); and (3) the pyramid of energy (based on the amount of energy at each level). The pyramid of energy is always upright, whereas the pyramids of number and biomass may sometimes be inverted.

Gross primary productivity (GPP) is the total amount of organic matter (or energy) produced by the producers (green plants) during photosynthesis per unit area per unit time. Net primary productivity (NPP) is the amount of organic matter that is left and available to consumers after the producers have used up some of the gross production in their own respiration. Thus NPP = GPP - respiration (R). NPP represents the biomass available for the herbivores and decomposers.

NPP = GPP - respiration

Decomposition is the process by which decomposers (bacteria and fungi) break down the complex organic matter of dead plants and animals (detritus) into simple inorganic substances such as carbon dioxide, water and nutrients, which are released back into the environment. The main steps in decomposition are: fragmentation (breaking of detritus into smaller pieces by detritivores), leaching (water-soluble nutrients go down into the soil), catabolism (enzymatic breakdown into simpler compounds), humification (formation of dark humus) and mineralisation (release of inorganic nutrients).

Nutrient cycling (biogeochemical cycling) is the movement of nutrient elements through the various components (living and non-living) of an ecosystem in a cyclic manner, so that they are used again and again. In the carbon cycle, carbon (as carbon dioxide) is taken from the atmosphere by green plants during photosynthesis and fixed into organic compounds; it passes along the food chain to animals; and it is returned to the atmosphere as carbon dioxide through respiration by plants and animals, decomposition of dead matter by microbes, and the burning of fuels and wood.

Ecological succession is the gradual and orderly process of change in the species composition of a community in an area over time, until a stable community called the climax community is established. Primary succession begins in an area where no living organisms have existed before, such as bare rock, a newly cooled lava flow or a new pond; it takes a very long time as soil has to form first. Secondary succession begins in an area where a community already existed but was destroyed (for example, an abandoned farmland or a burnt forest); it is faster because the soil is already present.

Biodiversity is the variety and variability of all living organisms (plants, animals and microbes) present on the earth, including the ecosystems in which they occur. It exists at three levels: (1) genetic diversity - the variation of genes within a single species (for example, the different varieties of rice or mango); (2) species diversity - the variety of species in a given region; and (3) ecological (ecosystem) diversity - the variety of ecosystems or habitats such as forests, deserts, wetlands and coral reefs in a region.

The latitudinal gradient is a pattern in which species diversity generally decreases as we move away from the equator towards the poles. In other words, tropical regions (near the equator, at low latitudes) have far more species than temperate and polar regions. For example, the tropical rainforests, especially the Amazon rainforest, harbour the greatest biodiversity on earth. This is because tropical regions have a relatively constant, warm climate, more solar energy and have remained undisturbed for a long time, allowing more species to evolve.

We should conserve biodiversity for the following reasons: (1) narrowly utilitarian reasons - humans derive direct economic benefits, such as food, firewood, fibre, medicines and industrial products, from biodiversity; (2) broadly utilitarian reasons - biodiversity provides important ecosystem services, such as the production of oxygen, pollination, and climate regulation; and (3) ethical (moral) reasons - every species has an intrinsic value and a right to exist, and we have a moral duty to protect them and pass on the biological wealth to future generations.

The four major causes of biodiversity loss, together called the Evil Quartet, are: (1) habitat loss and fragmentation - the most important cause, in which natural habitats such as forests are destroyed or broken up (for example, deforestation of tropical rainforests); (2) over-exploitation - excessive hunting, fishing and harvesting of species for human use; (3) alien (invasive) species invasions - the introduction of foreign species that harm the native species; and (4) co-extinctions - when a species becomes extinct, other species associated with it (such as its parasites or partners) also become extinct.

In-situ (on-site) conservation is the conservation of species in their natural habitat, where the whole ecosystem is protected so that the species continue to live and evolve in their natural surroundings; examples include national parks, wildlife sanctuaries, biosphere reserves and sacred groves. Ex-situ (off-site) conservation is the conservation of species outside their natural habitat, in specially protected places, and is used especially for threatened species; examples include zoological parks (zoos), botanical gardens, seed banks, gene banks and cryopreservation.

Biodiversity hotspots are regions with a very high level of species richness and a high degree of endemism (species found nowhere else), which are also under serious threat of habitat loss and destruction. They are given special priority for conservation. Three biodiversity hotspots extend into India: (1) the Western Ghats and Sri Lanka, (2) the Himalaya, and (3) the Indo-Burma region. Protecting such hotspots can greatly reduce the ongoing loss of species.

An endangered species is a species that is in serious danger of becoming extinct in the near future if the factors threatening it continue (for example, the tiger and the Great Indian Bustard). An endemic species is a species that is naturally found only in a particular geographical area or region and nowhere else (for example, the lion-tailed macaque of the Western Ghats). The IUCN (International Union for Conservation of Nature) Red List (Red Data Book) is a document that keeps a record of all endangered and threatened species of plants and animals.

Sacred groves are patches of forest or natural vegetation that are traditionally protected by local communities because of their religious and cultural beliefs, as they are dedicated to a deity or considered holy. In India they are found in regions such as the Khasi and Jaintia Hills of Meghalaya, the Western Ghats of Karnataka and Maharashtra, and the Aravalli Hills. Because cutting even a single tree is often forbidden, these groves are undisturbed and serve as important refuges for many rare, endemic and threatened species of plants and animals, thus helping in in-situ conservation of biodiversity.

Extinction is the complete disappearance of a species from the earth, so that no living individual of that species remains anywhere. It may occur naturally over a long time, but human activities (such as habitat destruction, over-exploitation and pollution) have greatly increased the rate of extinction. The Red Data Book (maintained by the IUCN) is an official document that contains a record of all the species that are endangered, vulnerable, rare or recently extinct, so that special conservation efforts can be directed towards protecting them.