1. A branch of science concerned with the interrelationship of organisms and their environments.
  2. The totality or pattern of relations between organisms and their environment.
  3. HUMAN ECOLOGY. (Source: Webster's New Collegiate Dictionary)

ECO: It certainly isn't a new word. In Greek ("oikos"), it means the home, the place where we live. And ecology means the science of how all living creatures interact within our home - our environment on this fascinating, complex earth.

Yet, "environment" and "ecology" are extended to encompass the inseparable universe. Mankind was not the first organism to leave the Earth's atmosphere. Mankind, plants and animals are now living in space for extended periods. The Earth, and survival of all species, is reliant upon the harmony of its existence among billions of other planets and space objects. With mankind's entry and enterprises in space since the early 1960s, there is valid and increasing reason to extend environmental and ecological concerns to the farthest regions of our home in space.

No living thing or group of living things exists in isolation. All organisms, both plants and animals need energy and materials from the environment in order to survive and the lives of all kinds of things, or species, affect the lives of others. Ecology is the study of living things (within species and between different species) and between them and their environment. People have always studied living things in their natural environment. Fieldwork for the ecologist involves the collection of information to see what happens to particular species such as population, numbers, diet, form, size and behaviour. Ecologists also study the physical environment such as composition of rocks, soil, air and water. This data can be used to identify patterns and trends.

In every ecosystem energy is trapped and stored by plants - the primary producers. Some of this energy is transferred to the animals that eat the plants. The are the primary consumers. Animals that eat other animals are known as secondary consumers they receive the energy from the plants second hand. Where the secondary consumers are eaten by other predators, the third stage consumers. At each stage, some energy is passed to the next level where is is then stored as plant material or as the flesh of living animals. Some energy is always lost in the transfer from one to the next. Ecologists can use these figures to compare ecosystems and understand how they work.

For Ecologist to understand how energy enters and passes through an ecosystem they must understand he feeding relationships between the organisms in that ecosystem. The transfer of food energy from plants through repeated stages of eating and being eaten is known as a food chain. Food webs are highly interconnected. Here is an example which illustrates the complexity of relationships. When scientists removed all the predatory Pisaster starfish form an area of the North American coastline, there were 15 different species in the food web. Within three months the barnacles on which the starfish feed, had grown to cover three quarters of the area. After a year the 15 species had been reduced to eight. Limpets had disappeared from the area despite the fact that they are the prey of Pisaster. In the absence of the starfish the thriving barnacles had taken up all the rocky surfaces pushing out the algae on which the limpets feed.

All things die eventually. In ecological terms the chemicals to which living things are made are borrowed from the earth and at death they are returned. The dead material and waste matter form the diet of a group of living organisms called decomposers. They include a range of bacteria, fungi and small animals that break down nature's wastes into ever smaller Oceans pieces until all the chemicals are released into the air, the soil and the water making them available to other living things. With out the carbon dioxide that decomposition released, all plant life would die out. Without the oxygen that plants give out, and without the food that they supply, life would grind to a halt and all animals would starve. The decomposers are a vital link in the natural cycle of life and death.

Water is the most common compound on Earth, and all life on this planet depends on it to a greater or lesser extent. Water plays a vital role in the structure of living things (70% of our body weight is made up of water) but its most important quality is that many chemicals will dissolve in it. Plants need water in order to take in dissolved minerals via their roots. Animals rely on water in their lung tissues to absorb oxygen from the atmosphere. However, because it is a solvent, water is very vulnerable to pollution. Many manufactured chemicals including very highly toxic poisons can enter the water cycle at a variety of points and then be carried through the environment. The most serious pollutants are those that do not biodegrade or break down through natural processes. They can be taken up by plants and animals and can accumulate in animals at the top of the food chain. All life on earth is based on the element carbon. It is constantly being passed between different parts of the biosphere in various chemical forms. It is found in the bodies of all living things, in the oceans, in the air, and the earth itself. In the atmosphere, when combined with oxygen, it forms carbon dioxide (CO2). In plants, it becomes carbohydrate, the source of energy for plants and eventually for the animals that eat them. In the ground, and in the bones of shells of animals, carbon is found in the form of chalky calcium carbonate. Plants are the main point of exchange, converting atmospheric carbon dioxide into carbohydrate through photosynthesis. Decomposition eventually returns all the carbon to the atmosphere.

Of all the carbon on Earth, less than 1 per cent is in active circulation in the biosphere. The remainder is locked up an in organic carbon in rocks and as organic carbon in fossil fuels (coal and oil) Growing plants take in carbon from the atmosphere (in the form of CO2) and incorporate this in solid compounds in their structure. In this form, carbon passes into the food chains. Different ecosystems take up carbon at different rates. in a tropical rainforest, where plants grow quickly, carbon is incorporated at a rate that is 100 times greater than in a desert.

Nitrogen is one of the ingredients of protein and DNA. As such, it is an essential element in the structure of all living things. Although gaseous nitrogen makes up 78 per cent of the Earth's atmosphere, plants and animals cannot use it in this form. It is the nitrogen cycle, in which microscopic bacteria transform nitrogen into a variety of compounds that makes nitrogen available to other living things. Bacteria described as 'nitrogen-fixing' can convert nitrogen in the air directly into nitrates in the soil. Nitrates are soluble in water, and plants are able to take them up through their roots. In turn, animals obtain their nitrogen from plants. The protein in waste material such as dung, or dead plants and animals also contains nitrogen. Various bacteria break down the protein and finally convert the nitrogen into nitrates which can be used by other organisms. Some of the nitrates are taken up by plants and some complete the cycle when they are changed back to nitrogen gas by yet another kind of bacteria.

The soils on which plants depend are created by the interaction of living and non-living parts of the environment. Their composition is influenced by five main factors: climate and weathering; geology - the underlying rocks; topography e.g. whether the land slopes or is near a river; the action of living things including humans and time. Soil has six main components: mineral particles, including silt, clay and sand; humus - mainly organic material which forms a thin film around each crumb of soil; nutrient ions, such as calcium and potassium; water; air between the soil particles; and living organisms such as worms and microscopic life. These factors all influence the fertility of the soil. The three main layers are the topsoil, the subsoil and the parent material. New soil is being formed continuously but soil is being eroded almost twice as fast often as a result of human activities such as the destruction of the rainforest and poor agricultural practices.

When ecologists study the distribution of organisms they try to discover the physical and biological factors that influence the presence or absence of particular species. They also look for any historical factors that may have affected were species are found and for patterns that might indicate how the distribution of populations could change in the future. This is especially important in the case of rare or endangered species. Laboratory experiments and observation of the natural world have led to the discovery that most species occupy different ecological niches. It is believed that this is to avoid competition between species when resources are limited. It is believed that this is to avoid competition between species when resources are limited. The ways in which populations of particular species expand and decrease form the subject matter of population dynamics. A close examination of the ways in which populations fluctuate reveals that even in what may seem a very stable natural system, there are dynamic forces that can have dramatic effects and produce wild swings in numbers. The number of living things in any population may go up or down or stay constant depending on what is happening in the habitat. All living things have the capacity to keep on reproducing. If nothing kept their numbers in check, the world would be overrun very quickly with too many plants and animals. These are called "density dependent" factors because their affects changes with the population density. A varying food supply is a prime example. Populations therefore tend to stay at about the same level fluctuating slightly above and below the numbers that a stable environment can support. Populations are also limited by random 'density dependent' factors - natural events such as volcanic eruptions on an island which may destroy certain species regardless of the population size.

Although the world's oceans appear to contain little plant life, sunlight and photosynthesis are the major source for energy for life in the oceans, as they are in all ecosystems. It is just that the phytoplankton, the plants at the base of the ocean food web are microscopically small. These plants also need nutrients and these are unevenly distributed throughout the oceans being carried by the ocean currents that move the waters around the planet.

The features that characterise all deserts are lack of water - less than 10in of rain per year and generally harsh conditions. Nutrients are limited compared with most other ecosystems because there is too little moisture for bacteria and fungi that cause decomposition. Even so, there is a range of organisms which have adapted to such conditions. Obtaining nutrients is a problem for all desert organisms and it is though that most of the plant biomass in deserts exists in the form of underground storage organs, such as roots and tubers.

Tropical rainforests are continuously warm and moist and so fruit and seeds are available all year round. In these stable and relatively constant conditions animal and plant life has been able to diversify more than anywhere else on Earth. Only coral reefs come close in the diversity of their species. The trees in the rainforest are the nutrient store so if they are removed the ecosystem is disrupted.

Ecologically early human beings were much like any other species. They were a natural part of a food web, probably as primary consumers being consumed in their turn by larger and more powerful secondary consumers. With the development of tools, the use of fire and increasing communication skills, humans moved up the trophic pyramid to become hunters - secondary or even tertiary consumers - but their numbers were still limited by the energy available from the trophic level below them. Even with a wide repertoire of skills and tools, hunter-gatherers cannot exceed the carrying capacity of the natural environment. The deliberate cultivation of crops changed everything enabling human beings to increase the productivity of the land and escape the tyranny of the food chain. This single development makes humans ecologically different from all other species. It opened up the possibility of a tremendous population increase and has changed the face of the Earth itself.

Since the industrial Revolution of the 18th and 19th centuries, human impact on the environment has been enormous. The burning of fossil fuels has polluted vast areas and significantly altered the atmosphere. Industrial technology has brought millions of people from rural areas into new towns and cities and advances in mechanisation have dramatically reduced the number of people needed to work the land. The use of fertilisers and pesticides have increased agricultural production and fed the growing human population but they have had dire effects. The consequences of these rapid changes could not be predicted. Now the science of ecology has made it possible to assess how human actions affect the environment and to look for ways to reduce and repair the damage that is being done.

Human beings are now the most influential creatures on the planet. Our activities from energy use and mineral extraction to agriculture, industry and urbanisation, take place on such an enormous scale that the environment is being fundamentally altered. The composition of the atmosphere is changing. Water is being polluted at all stages in the hydrological cycle. Manufactured and often toxic chemicals are being used in pest control in countless industrial processes. Populations of marine animals are being harpooned and netted despite the threat to their continued existence. Valuable resources are locked in consumer goods and materials and very little of this is recycled. The long-term consequences cannot be predicted without a thorough understanding of the complex ways in which the biosphere works. Although ecology is not primarily about solving environment problems, its goal is to deepen our understanding of the relationships between living things and between them and the physical world. Ecologists are already proposing ways of meeting human needs that are sympathetic to the environment and drawing attention to the ecological implications of just about everything that humans do, but to solve the problems people must want to use this knowledge.