Life on Earth is found only in a thin shell called the biosphere, within a few kilometres of sea level, in the air, the oceans, and sometimes in rocks and soil.
Ecology is the study of how organisms and the environment interact with each other to create a balanced community in terms of energy and nutrients.
Living things can exist together in a number of ways: as prey and predators, as competitors, in a parasitic relationship, as commensals or symbionts.
In 1866, Ernst Haeckel first used 'ecology' to describe the study of living organisms and their interactions with other organisms and with their environment.
In 1893, John Burdon-Sanderson stated that what he called 'oecology' was, along with physiology and morphology, one of the three great divisions of biology.
Many important chemicals move in cycles. Nitrogen cycles in the atmosphere and biosphere: both nitrogen fixation and denitrification are part of the cycle.
Living things may be parasites or parasitized: a parasite may be either an endoparasite or an ectoparasite. In either case it needs to be adapted to its role.
Living things may benefit each other, but it appears common for parasites to evolve into symbionts, given sufficient time, as this favours both organisms.
Animals are found in different habitats, generally those to which they are best adapted, though humans use cultural adaptations to live everywhere.
Animals and plants are usually adapted to their habitat, which includes being adapted to the predators and the other threats that are found there.
Living things do best when the conditions are right, which is one reason why things living in an ecosystem show zonation patterns in their distribution.
An ecosystem involves many living things, and any study must look at them and energy inputs and outputs, as well as material flows in and out of the system.
The form of balance seen in any ecosystem is a dynamic equilibrium, one in which it is normal for some of the levels to fluctuate over longish periods.
It is common to consider an ecosystem as a closed system , even when there are external gains and losses, as these typically balance each other out.
Many important chemicals move in cycles. Phosphorus, carbon, sulfur and key minerals such as iron, sodium, magnesium, potassium and calcium are cycled.
The food chains in the oceans are based on plankton, microscopic plants and animals that are eaten by larger animals, and so on, up the food chain.
A few large organisms in the sea, like the baleen whales, are able to extract plankton and krill from the ocean, rather than waiting for it to be concentrated.
As a diet, flesh is far more nutritious, gram for gram, both in terms of what it contains and the energy delivered. Plants are generally easier to find and eat.
Nitrogen may be artificially 'fixed' by the Haber process, which combines nitrogen and hydrogen gas under high temperature and pressure to make ammonia.
The carrying capacity of an ecosystem will depend on its productivity, the amount of biomass that is developed by photosynthesis and made available.
Food relations in an ecosystem may also be described through a food web or a food pyramid, which reflects biomass at different trophic levels in the system.
A measure of productivity in any ecosystem may be obtained from assessing the biomass and the changes in it, year by year. This must be interpreted with care.
A forest is a great deal more than just a bunch of trees. The trees interact with each other, with the other plants, animals and forest floor inhabitants.
Leaf litter is a key part of the soil and forest floor, since it commonly represents a store of most of the available nutrients available to the community.
Food chains lead to the concentrations of some substances, some of which may be useful, although a number of others such as pollutants may be harmful.
The world can be divided into recognizable biomes, with varying productivity. The desert biome may be harsh, but it is more productive than most people think.
When conditions change, a cline may be observed, with related organisms at different points along the range varying in their genetic make-up.
There is always competition in a habitat for resources, both between members of a single species, and also between species which have similar needs.
Australian ecosystems are generally good at bush fire regeneration, because the plants and animals found in Australia are descended from fire survivors.
Aboriginal Australians made significant use of fire as a management tool, a process called 'firestick farming', to control plant growth to support food animals.
Plant cover may be measured on the Domin scale, which allows a complex situation to be reduced to a simple figure that may be used in calculations.