LESSON 5: Life on Earth (Biotic)


Environmental Education School Curriculum Outcomes mentioned in this lesson: Grade 1; Grade 2; Grade 3; Grade 4; Grade 5; Grade 6; Grade 7; Grade 8; Grade 9; Grade 10; Grade 11; Grade 12

Eco-Assignments that relate to content covered in this lesson: Grade 1; Grade 2; Grade 3; Grade 4; Grade 5; Grade 6; Grade 7; Grade 8; Grade 9; Grade 10; Grade 11; Grade 12

Fossil from Microraptor – 130 million years old

In the previous lesson we have seen the importance of the abiotic or non-living components of the ecosystem to enable a supporting environment for biotic components to survive in. In this lesson we will concentrate on describing this living or biotic part of the environment (Supporting article R). The concept “biotic” actually refers to all material that consists of carbon. It also include organisms that have deceased. Therefore the tremendous diversity in the interconnected chain of life on our planet (Supporting article K) include all prehistoric remains of animals and plants and those that have not yet even been discovered.

We are attracted to the beauty (colours, shapes and abilities) of animals and plants – of vertebrates and invertebrates (Supporting article O). Even the structure and functioning of vital Micro and Macro organisms (Supporting article N) could sometimes be extremely fascinating. Have a look at supporting articles G, and Supporting article F to get a better understanding of this fascinating world of smaller organisms. Remember that micro and macro organisms outnumber by far the more visible kingdoms of plants and animals that most of us are familiar with. Supporting article V describes the important functions of Bacteria.

The variety and number of organisms from the past and present are indeed fascinating. There are different ways to describe or categorise living beings. For our purpose we will explore organisms based on the various roles that they fulfil in the food network. But before we do that,let us have a look at the taxonomy method. Organisms are categorised on the basis of their physical characteristics.

Over the ages many individuals contributed to this process (Supporting article T).  What Carolus Linnaeus, Jean Baptiste de Lamarck and Charles Darwin popularised during their lifetimes (Supporting article U) was the result of thousands of years of speculation, and who knows their views might still again be re-written as new species and interrelationships are being discovered.

Scientists use a ranking system, or taxonomy (Supporting article M) to orderly arrange and connect living things (or what they refer to as life) according to some scheme of resemblance. The basic classes are traditionally referred to as “Kingdoms”. Over the years the number of kingdoms in generally accepted classifications has grown from:
• Two kingdoms – animals and plants, to
• Three kingdoms – animals and plants and protista – microscopic single-celled organisms, to
• Four kingdoms – where distinction is made between microscopic organisms (protista) whose cells do have a distinct nucleus and those who do not have a distinct nucleus.
• Five kingdoms – when animal traits were observed in fungi (multi-cellular saprotrophs) it had to be distinguished from multi-cellular autotroph plantae, and so a fifth kingdom was established.
• Six kingdoms – later some scientists felt that distinctions of kingdoms should be based on genetic characteristics. The six kingdoms were grouped under one of three domains: Domain Bacteria (with kingdom Bacteria under it); Domain Archaea (with kingdom Archea under it) and Domain Eukarya (with kingdoms protista, plantae, fungi and animalia.) under it.

To conclude, in general the “classic” six-kingdom system consists of: Animals; Plants (that has been split into Plantae and Fungi); and single-celled organisms that have been split into Bacteria, Archaea and Protista. But since about 2000, some research no longer support any of the traditional systems. If you would like to have more detailed information of the categories and methods of arranging life into distinguishable groups, you can refer to wikipedia.

Now let us differentiate between organisms in an ecosystem by looking at the function they fulfil in the food network (Supporting article Q). A food network or food web is an interconnecting network of food chains. In a food chain energy is transferred from green plants through a sequence of organisms in which the one organism eats the organism below it in the chain and is eaten by the one above.

We can divide biotic components according to their main role namely, producers, consumers or decomposers. The producers (green plants or vegetation) in an ecosystem are those organisms which are able to manufacture organic compounds from inorganic substances. Inorganic elements such as calcium, iron, etc. are being taken up by the roots of plants and synthesised into organic compounds (such as starches, protein, etc.). They absorb energy from the sun through the process of photosynthesis and make this energy available to animals that consume them. Green plants are therefore independent of other organisms to function while consumers are heterotrophic as they rely on autotrophic organisms for their energy needs.

The many roles of green plants are extensive. Apart from their fundamental role of transforming the sun’s energy (by means of photosynthesis) into food (chemical energy) for herbivores, plants influence the formation of organic content in soil as well as the recycling of nutrients. Roots of trees transport nutrients from deep under the surface and eventually returns it  to the surface (in the form of leaves, wood etc.) that fall to the ground where it rots (decompose) and is returned to the soil in the form of nutrients to the upper layers of the soil – available to shallower root systems of shrubs, grasses, etc. Furthermore plants cool the surface as they provide shade. This further enhances the performance of micro-organisms that break down organic matter.  Plant roots bind soil together and prevent erosion caused by floodwater and together with the canopy (leaves, grass cover) it minimises the erosive effect of heavy rains and wind on the soil surface.

Consumers are dependant on producers for their energy needs. They can further be subdivided into primary consumers or herbivores (plant-eaters), secondary consumers or carnivores (meat-eaters) and omnivores (both primary and secondary consumers). Omnivores like humans act as tertiary producers when they for example process their food like milk turned into products like ice=cream or cheese. Those carnivores that kill their prey and eat it are called predators and those who eat what is left by the carnivores are called scavengers.

The third group into which biotic elements can be divided is called decomposers. Among decomposers we get Macro-decomposers which include for example earthworms and wood lice. The Micro-decomposers consist of a large variety of bacteria, fungi and other single-cell micro-organisms. Decomposers live on dead organic material and therefore obtain their energy by consuming corpses of animals and dead plant material. They break down all the organic material (compounds such as proteins) of the bodies of producers and consumers into inorganic materials (elements such as magnesium) that they then restore to the soil or water. These elements (or minerals) could then be taken up by the roots of plants (producers) from where it is transformed again into compounds through the processes associated with photosynthesis.

All components (biotic and a-biotic) are somehow linked together in a dynamic functioning system by means of energy flow. So let us look at food pyramids and the transfer of energy in the food network (Supporting article L). When a herbivore (e.g. rabbit, cow, giraffe) eats, only a fraction of the energy that it gathers from the green plant becomes new body mass. The rest of the energy is used up by the herbivore to carry out its life processes such as movement, digestion and reproduction and some of it is lost as waste. Therefore, when the herbivore is eaten by a carnivore (e.g. lion, leopard, fish eagle), the herbivore passes only a small amount (not more than 10%) of the total energy that it has received from green plants to the carnivore. Of the energy transferred from the herbivore to the carnivore, some energy will again be “wasted”. The carnivore therefore has to eat many herbivores to get enough energy to grow, move and reproduce. For a schematic presentation of this process, you can refer to:  Hugo,M.L. 2004. Environmental Management: An ecological Guide to Sustainable Living in Southern Africa. Par 3.4)

Because of the large amount of energy that is lost at each link, the amount of energy that is transferred gets smaller and smaller. Therefore the further along the food chain you go, the less food (and hence energy) remains available. When there are too many links in a single food chain the animals at the end of the chain would theoretically not get enough food/sustenance to stay alive. Most food chains have therefore no more than four or five links and because of this many animals are part of more than one food chain and eat more than one kind of food in order to meet their food and energy requirements. These overlapping or interconnected chains form a food web. Energy flow in food chains could be graphically represented in food pyramids, but more on that in our next lesson called ‘Dynamics in the Ecosystem’.

From the perspective of human survival on earth, the concept of loss of energy along the food chain, explains why the shorter the food chain, from which humans eat, the more likely they are to be able to provide enough for the growing population. In simplistic terms: if we eat only plant material (such as wheat) the earth can support more people than if we were to depend on meat. This lies at the core of human survival on earth.

Ecologists have an integrated approach towards the natural environment. They are interested in the inter-relationships between different organisms and their habitat as a whole. For example the eating habits of one species has an effect on species that they eat as well as on what their prey feed.

For their purpose ecologists classify the terrain of ecological research under five categories or levels of relatedness (Supporting article P). We will start with the smallest level of relatedness and end with the category most inclusive of all life on earth.

An organism is one single living thing (or species) – for example a single Wolf. Populations are groups of the individual organisms belonging to the same species and living together in the same area, such as a pack of wolves.

Communities are where individuals from various populations co-exist in the same habitat, for example a pack of wolves, a population of rabbits, snakes and a population of birds. This include vegetation from the same species in the same area.

Ecosystems include both living and non-living things in an area. So, added to the plant-populations and animal-populations in an area, ecosystems will include non-living (a-biotic) elements like water, the sun, clouds, soil, rocks and temperature. In an ecosystem organisms can compete with one another for survival. For example both the wolf and the snake could be competing for mice; or both a big tree and a smaller plant growing under it could compete for sunlight.  Plant and animal species belonging to the same ecosystem are all adapted to the specific conditions of that habitat. Each occupies a specific niche that enables the species to survive in that particular ecosystem.

Finally, the ecosphere consists of the parts of the earth where active life is possible. This zone includes the atmosphere, sea and surface water, as well as soil. This means that all ecosystems together form the ecosphere (Supporting article S).

“It is not always possible to make exact divisions between these levels, as all species exhibit mutually interwoven characteristics that make it impossible to study any aspect of ecosystem organisation in isolation. An analytical study on different levels is nevertheless necessary in order to obtain a clear picture of the whole ecosystem.” (Hugo,M.L. 2004. Environmental Management: An ecological Guide to Sustainable Living in Southern Africa. p60)

In terms of their biotic role, one would categorise humans along with other omnivores as consumers. However the impact that human consumption has on the natural environment cannot be compared to the effects animals have (Supporting article H) on their habitat. Humans exert such a major influence on their environment that the very existence of life on earth seems to hinge around human activity:
• Industrialisation has contributed to an astronomic human population growth.
• Industrialisation has been responsible for the burning of fossil fuels and with it a huge increase in CO2 levels, resulting in the greenhouse effect and other forms of pollution.
• Because of deforestation we are not only losing natural habitats and biodiversity, but also loss of oxygen producers and CO2 eliminators.
• In an effort to boost selected mono-cultures (agricultural products) we have manufactured herbicides and pesticides and DDT, weakening many other important biotic species.
Even in Kenya – a major safari destination for almost a century now – population growth is now competing with biodiversity (Supporting article D).

As humans we have the ability to encourage the establishment and maintenance of biotic elements in ecosystems or to destroy ecosystems almost in the blink of an eye (Supporting article A). We are able to peacefully coexist with our pets and there are groups that encourage this healthy coexistence (Green Party of South Africa). However presently most forms of development to advance humankind, are having negative consequences on the natural systems of the world. We need to seriously invest our time and energy to adapt our way of life towards harmonious co-existence with the various elements of nature (Supporting article B and Supporting article E).

It is important to know that what we are talking about is not simply an emotional or theoretical concept of a few extremists. For too long have people acted with apathy and even contempt towards this message because it did not affect their daily lives directly; nor their pockets. The results of global warming (regular floods, sporadic fires, extreme cold, ice caps melting) is starting to get people’s attention at last. Harrison Ford has publicly spoken out on the importance of biodiversity to support life (Supporting article I).

We know that many species may disappear before they are discovered (Supporting article J). The longer we wait to learn to adapt to harmonious interaction with nature (Supporting article C), the harder it will become to turn the downward spiral of environmental decay. If you have more to say about this topic feel free to express yourself on our Twitter Page.