Ecology is about the interrelationships between organisms (including humans), and their environment, both living (biotic) and non-living (abiotic). Another way to view it is that ecology is how the natural environment works. The integrity and proper functioning of ecological processes, which are many and varied, are essential for the life on earth as we know it. Some of the processes are simple and others are complex, but all may be grouped into five interdependent principles. These are: trophic levels, energy flow, material cycling, limiting factors and constant change.

  Trophic Levels

Trophic or feeding levels are about who eats what. All organisms, whether plants or animals, require food which provides them with chemical materials, or nutrients, required for building and maintaining living cells, and energy required for all metabolic processes, growth and movement. All organisms can be divided into different trophic levels according to the way in which they obtain food. These levels are:

Producers: all green plants, which produce their own food from air, water and sunlight (during photosynthesis) as well as from chemicals absorbed in solution from the soil.

Primary consumers: all herbivorous animals which feed on green plants. Cows and elephants are examples.

Secondary consumers: carnivorous animals that feed on other herbivores. Lions and eagles are examples.

Tertiary consumers: carnivorous animals that feed on other carnivores. Sharks are an example.

Reducers: animals, such a worms or crabs, that feed on any dead organic matter.

Decomposers: fungi and bacteria which break down organic matter into its inorganic components (elements) for re-use by plants.

When animals consume food, energy and chemical materials are transferred from one trophic level to the next. Some energy is lost in each step of the transfer so that for the process to work, a large biomass (total weight) of primary producers is required to support a smaller biomass of primary consumers, and an even smaller biomass of secondary consumers. This can be explained as a food pyramid. The trophic levels can also be compared with the links of a chain. The first link is always a primary producer, and the last link is always a decomposer. The link in between may vary. In reality, and most importantly most food chains are linked to each other to form complex food webs.

  Energy Flow

All life depends on the flow of energy and materials through ecosystems. Energy flows in one direction and diminishes as it is used up by the plants or animals, or is ‘lost’ into space as heat. Energy cannot be recycled.

Nearly all energy available on earth (except for geothermal and nuclear energy) comes from the sun. Some of it may have ‘arrived’ millions of years ago and has been stored in coal or oil; some may have ‘arrived’ more recently and been stored in trees and other vegetation.

A small percentage of the total amount of solar energy reaching the earth is ‘fixed’ by the process of photosynthesis in green plants, which are the foundation of the food pyramid. When a herbivore, e.g. a mouse, digests plant tissue, some of the energy is transferred, and used to power the mouse’s movement, metabolism and growth; and some is stored in the body of the animal, e.g. in liver or fatty acids. Some energy is given off as heat. Indigestible plant tissue passes out of the animal as waste, which is decomposed and reused. Similarly when a carnivore, e.g. an owl, eats the mouse, some of the energy in the mouse is transferred to the owl where it is used for growth movement and metabolism; some is stored; some given off as heat.

Thus between each consecutive step in the food pyramid less energy is transferred. This explains why there is always a greater biomass of plants than herbivores eating them, and a greater biomass of herbivores than carnivores. Those creatures at the ‘top of the food pyramid’ are fewest in number and lowest in biomass. Humans have made themselves an exception to this rule with a resultant impact on all other creatures. Think what this would mean if all humans were vegetarians.

  Material Cycling

Unlike energy, there is no continual input of matter or chemical materials from outer space. All elements on earth are cycled continually between the biotic and abiotic components of ecosystems through the biogeochemical cycles, e.g. water cycle, hydrogen cycle, carbon cycle, nitrogen cycle.

  Limiting Factors

Limiting factors are of two kinds, chemical and physical.

Chemical: Every living organism requires certain kinds and quantities of chemical nutrients (e.g. iron, magnesium, nitrogen). If one of these nutrients is absent, or present in insufficient amounts, even if all the others are available in abundance, the organism will function poorly or die. In some cases the limiting factor may not be the lack of a particular nutrient, but an excess of it, i.e. it occurs in amounts which are poisonous (as when the environment is polluted)

Physical: Examples of physical limiting factors for plants are water, sunlight, fire, temperature, disease, grazing pressure, competition with other species and the activities of people, e.g. destroying certain plant species. Examples of physical limiting factors for animals are water, temperature, the availability of shelter and nesting or breeding sites (including our homes), diseases and parasites, predation and the activities of people, e.g. hunting, violence and habitat destruction.

Thus all plants and animals can only live within a range between too much and too little of their essential requirements. This is called their range of tolerance.

  Constant Change-succession and Evolution

Nothing in nature is static. Even the seemingly ageless mountains are gradually eroded and after millions of years become flat plains. Sediments carried through rivers are continually being deposited in the sea and in the lakes and eventually these, or parts of them, will become dry land. Within any ecosystem, plants and animals are continually developing to maturity, growing old and dying, and being replaced by others. The new organisms may be the same species or they might be the new species colonising the area, thus changing the composition of the biotic community. Unless influenced by people or catastrophic events (such as volcanic eruptions), the biotic community of an ecosystem will change in an orderly and broadly predictable fashion. This process is called ecological succession.

During this process ecosystems develop from a single community to a more complex one. This increased complexity also brings about greater stability, i.e. it is less likely to be affected by outside influences such as the activities of people, invasive plants, or disease. The most robust ecosystems are usually also those with the greatest variety of plants and animals, i.e. the greatest biodiversity.

Also, all organisms change their form and function over the long periods, some to the extent that they no longer resemble their original form. This process is called biological evolution.

The principles and processes described here can contribute to our understanding of many aspects of human activity: agriculture, population growth, energy use and conservation, pollution, health and technology. How well we understand them and the implications of disrupting them will determine the kinds of decisions we make and ultimately, how well we look after our only home.