LESSON 6: Cycles and energy-flow – biotic and abiotic interaction


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

The ecosphere could be seen as one large machine controlled by a number of huge as well as several smaller recurring, and sometimes overlapping cycles and systems. Energy needed to run these cycles is usually provided by sunlight. It allows minerals to flow through various living components that are connected to one another in a food web (Supporting Article W). Water is being recycled through the soil, air and biotic components in what is known as the hydrological cycle. The various gasses are constantly being circulated through various ecosystems. Even soil with its minerals form part of a continuous cycle, called the sedimentary cycle. Supporting article DD gives us a short overview on the natural cycles of the ecosystem).

One of the biggest reasons I feel we as humans allow pollution and degradation of our recourse base to continue, is because we do not know and therefore are not able to appreciate the organised functioning of living and non-living components in ecosystems. To this end we will now first have a look at how the environment is forever rejuvenating and maintaining itself by means of energy flow through ecosystems by means of various cycles (Supporting article O).

Energy Flow through the Ecosystem


energy ecosystem 2Here we need to remember that in nature the flow of energy is not recycled – it is supplied by the sun – used in various systems and then it passes out of the ecosystem. Ecologists (scientists who study the relationships of organisms within their living and nonliving environments) view an ecological system as a collection of communities of organisms and the environment in which they live (Supporting article U). It could be small like a pond or huge like a lake – it all depends on the area that the ecologist intends to study. One thing common to all ecosystems, big or small, is that energy flow occurs in only one direction: organisms are always consumed by higher levels of organisms in a food web. As a result, each level of a food web contains less energy than the levels below it.

energy loss ecosystem 1This state of affairs is supported by the Laws of Thermodynamics. The First Law of Thermodynamics states that energy can never be created or destroyed. It is only able to be transformed from one form to another. The Second Law of Thermodynamics further explains that when energy is converted (transformed from one form to another) some energy is lost in the form of heat. Energy transfer is therefore never 100% efficient. By implication, this means that we need to conserve energy at all costs and take care that the life-sustaining natural systems are not overburdened. We will discuss this fact in more detail in our next lesson, but if you would like to get a better understanding of this phenomenon, please read Supporting article CC. In Supporting article S, you will be able to see how this ‘energy economy’ applies to Arctic food web. (By contrast, nutrients can flow in any direction in an ecosystem – Supporting Article BB).

FOOD CHAIN Single path of energy

Energy flow through ecosystems is represented in food chains, food webs and ecological pyramids (Supporting article R). In a food chain one organism feeds on another in a sequence of food (energy) transfers. For example: leaves from a tree (primary producer) are eaten by a grasshopper (primary consumer) which is eaten by a snake (primary carnivore) which in turn is eaten by a rat (secondary carnivore) and finally the rat is eaten by the owl (tertiary carnivore). In an ecosystem there are many different food chains and many of these are cross-linked to form a food web.

Energy flows upwards from the one level of organism to the next in what is known as trophic levels (Supporting article T). Plants form the first trophic level (or T1); the herbivores (e.g. antelope), form the second trophic level (T2); and the carnivores (e.g. lions), form the third trophic level (T3). Top carnivores (such as birds of prey) occupy the forth trophic level (T4). The division into trophic levels is not based on specific species though but rather on the function that species fulfils in the ecosystem-community.

In general, three types of ecological pyramids can be distinguished namely:

Number pyramid: A number pyramid shows the number of organisms in each trophic level without taking into consideration the size of the organisms. This type of food pyramid could therefore over-emphasise the importance of small organisms. In a pyramid of numbers the higher up one moves each consecutive layer or level contains fewer organisms than the level below it.






Biomass Pyramid: Here the total mass of the organisms is indicated in each trophic level. Because the size of all the organisms on a certain trophic level is over-emphasized, it can happen that the mass of level T2 is greater than that of level T1, because the number of individual organisms (productivity) of level T1 is not taken into consideration. Thus an enormous mass of grass is required to support a smaller mass of antelope, which in turn would support a smaller mass of lions.



Energy Pyramid: This kind of pyramid indicates the total amount of energy present in each trophic level. It will therefore also indicate the loss of energy from one trophic level to the next. Here one can clearly see how the energy transfer from one trophic level to the next is accompanied by a decrease due to waste and the conversion of potential energy into kinetic energy and heat energy. Therefore the energy pyramid is more widely used than the others because different ecosystems can be compared. But to compile an energy pyramid involves much more research than required with other types of pyramids.

So food-webs largely define ecosystems, and the trophic levels indicate the position of organisms within the chains or webs. These trophic levels are however in reality not always clear-cut simple units, because organisms often feed at more than one trophic level. For example:
• Some carnivores also eat plants, and some plants are carnivores
• A large carnivore may eat both smaller carnivores and herbivores
• Animals can also eat each other: the bullfrog eats the crayfish, but crayfish also eat young bullfrogs
• The feeding habits of a young animal, and consequently its trophic level, can change as it grows up.

To conclude the first section of our lesson, energy (from the sun and hosted in green plants) is the driving force behind the movement of nutrients and compounds in and through the biotic components of the earth. With every transfer of nutrients from one trophic level to another a large amount of energy is lost. This phenomenon is supported by the second law of thermodynamics which states that energy cannot be used again because it loses its quality when changed from one form to another.

Although this law has huge implications for maintaining life as it is (Supporting article V), for our purposes now it means that in nature, the numbers of the various species in a community will not be allowed to exist (or flourish) at the expense of other species found there.

Secondly, we will now look at the major cycles in the ecosphere. Each of these cycles has an underlying reservoir supporting it, where elements or compounds are stored for various periods of time before they once again take part in the cycle. In the case of the hydrological cycle, the ocean is the main reservoir; the atmosphere is the reservoir for the gaseous cycles, and the crust of the earth, the reservoir for the sedimentary or soil cycles. There are also exchange pools where elements or compounds are only held for a short time. Clouds are exchange pools in the hydrological cycle and organism bodies in the biotic community can serve as exchange pools for various chemicals. Cycles that transport chemicals such as life-supporting nutrients, pass through both the biological and geological world and therefore we can refer to them as biogeochemical cycles (Supporting article P).

As we continue to look at the various cyclic activities in the ecosystem, it is important to take note that because of remarkable population explosions worldwide and consumer demand, human activities are putting ever-increasing pressure on ecosystem cycles (Supporting article B). As a result of various mining activities we have exposed the earth’s buried rocks to the elements, resulting in much quicker weathering and thus an accelerated movement of elements in the global sedimentary cycle. The extraction of coal and oil to be used as energy resources is releasing carbon dioxide into the earth’s atmosphere about seventy times more rapidly than one would expect naturally (Supporting article C). Humans have an enormous capacity to increase the rate of movement of materials in both the sedimentary and biological cycles. With this in mind let’s look at the major cycles.

hydrologic cycle 6dThe water or hydro-logical cycle, in its simplest form consists of water that evaporates from the surface of the oceans and this moisture is carried over the continents by wind where it condenses, form clouds and fall to the earth as some form of precipitation (rain, snow, dew, etc.). Part of the water sinks into the ground whilst about 70% again reaches the atmosphere in the form of vapour as a result of evaporation and transpiration from leaves of plants. The rest eventually reaches the sea as run-off via rivers – and the whole process is continually being repeated.

In the cooler regions of the earth, like the poles, water might be trapped for very long periods in the form of snow or ice. Water might also be temporarily ‘stored’ in lakes, ponds and wetlands. As water runs to the oceans, it carries with it minerals as a result of the weathering of rock – therefore the saltiness of the sea. Organisms also play an important part in the water cycle as up to 90% of their body weight consists of water. Without water many essential body functions in humans and animals will not occur and without water, plants will not be able to take up and transport chemicals; or produce energy in the form of carbohydrates for herbivores; or produce their ‘waste’ products namely carbon dioxide and evaporated water.

water pollution 6We are all aware that water resources, collected in ‘temporary pools’ such as dams and wetlands and even the sea, are being spoiled by various forms of pollution at an extraordinary rate and as environmentalists we are extremely concerned about it. Water from evaporated molecules that returns to the earth in the various forms of precipitation used to be clean from all impurities, but even this is no longer the case. Even this ‘cleansing method’ of nature has been affected by air pollution, resulting in acidic rain. Read more on acid rain and its effects in Supporting article AA.

carbon cycle 6This brings us to the gaseous cycle, where we will look at the carbon cycle as an example. Carbon is extremely important for life on earth as it forms the basic building block of all organic compounds together with hydrogen. Together with calcium and oxygen, carbon also forms the basis of all carbonate rocks. Lastly it forms the main component of fossil fuels. The main reservoirs for carbon dioxide are in the oceans and in rock. It dissolves easily in water and from there it can ‘fall out of solution’ (precipitate) to form sedimentary rock known as limestone. Corrals and algae encourage this reaction and build up limestone reefs in the process (Supporting article H).

carbon cycle 6aGreen plants (on land and in the water) take up carbon dioxide and through the process of photosynthesis converts the carbon in its surroundings into carbohydrates. From the plant, the carbon can move three different ways: it can be released into the air (through the process of respiration); it can stay in the plant until it dies; or it can be eaten by animals. All the carbon in animals ultimately come from plants and here (from the body of the animal) again the carbon can move three different ways: it can be released into the air (respiration); and then be taken up by another plant (photosynthesis); or finally be dissolved in the ocean. Two things can happen to the carbon in a plant or animal when it dies: it can be respired into the air as decomposers assimilate and decompose dead material; or it can be buried in tact and eventually form coal, oil or natural gas (fossil fuels) (Supporting article L and Supporting article N).

In nature these fossil fuels can be released through volcanoes or pushed to the surface by forces inside the earth. But when we extract and burn fossil fuels, huge excesses of carbon dioxide are being released into the atmosphere and this is having a compounding negative impact on the natural environment (Supporting article C). As more carbon dioxide is being released into the atmosphere more carbon also enters the oceans affecting amongst others, coral reefs (Supporting article G). Just as the disappearing forests are crucial for the recycling of carbon on land, so are the disappearing ‘coral reefs’ essential for the survival of life in the ocean (Supporting article F and Supporting article E). In essence global warming happens because of an over-abundance of carbon dioxide in the atmosphere allowing more energy (heat) from the sun to reach the earth than it allows its energy (heat) to escape into space.

The burning of fossil fuels so dramatically contributes to hot-house effect (Supporting article D). A South African, disillusioned by the Copenhagen Climate Summit has a warning to governments and decision-makers who put all their eggs in the ‘fossil fuel basket’ (Supporting Article M).

phosphorous cycle 6When we consider sedimentary cycles (Supporting article Q), we need to keep in mind that it consists of two phases: the salt solution; and the rock phase.  The phosphorous cycle is an important example of a sedimentary cycle. Because of its weight, heavy phosphor molecules never rise into the air. It is always part of an organism, dissolved in water or in the form of rock. When rock containing phosphorous elements is exposed to water – especially if the water has some acid in it – the rock is weathered and goes into solution with the water it has been exposed to. Plants rooted in this rock or soil, take this mineral up and use it for example to constitute their cell membranes. Animals feeding on this plant will use phosphate for example as a vital component in bones, teeth and shells. And when the animal or plant eventually dies and defecate, the phosphate is again returned to the soil or water as decomposers break the corpses down to a consumable form for other living plants.

This cycle occurs over and over again until the phosphorous settles on the ocean floor. Here it becomes part of the sedimentary rocks being formed over millions of years and when the rock is ultimately  brought to the surface (for some reason, e.g. crust movement may rise the surface above sea level), it will be exposed to the elements and to weathering. Then the whole process is repeated again. In nature, marine birds play an important role in returning phosphor from the ocean to the land. Phosphor enters their systems through the bones of fish they eat and the places where they defecate are known as guano.Humans mine the phosphate in guano, or from areas that were once covered by the sea, to be used as fertilizer (Supporting article A). Eventually this then leads to a situation where an overabundance of phosphate concentrations is released into the natural water system in the form of sewerage and run-off from cultivated land. This is especially prevalent at coastal regions at the mouths of rivers.

Phosphor-‘infected’ river

It causes a situation known as ‘eutrohication’ (Supporting article J and Supporting article K) where a shortage of oxygen in the water is experienced because of the increased activity of algae thriving on phosphate. The rest of the natural aquatic life in the water body is no longer able to survive in the oxygen-deprived water and dies.

The effectiveness of cycles in nature has been greatly affected by overproduction and the various forms of pollution. When too much stress is put on a natural system it will eventually stop to function. When stagnation occurs, death follows inevitably.  The Dead Sea is an example. Here water flows from the Golan highlands along the Jordan River into the Dead Sea, which has no outlet. The salts washed out of the soils over thousands of years accumulate in the inland lake. No life exists in the Dead Sea: It is an ecosystem that has stopped functioning because there was an unnatural concentration of one element. In nature nothing can survive in isolation (Supporting article I). Everything is of necessity actively linked to everything else for survival. If a system is not actively linked with the total environment around it, it becomes isolated and perishes.

It is clear that natural systems all work in harmony to sustain life on earth as we know it. It also becomes more clear  that we as humans spoil the quality of our natural resources by short-circuiting the natural rhythm of nature when we for instance,

  • mass-produce materials (like plastic and nuclear waste) that cannot be recycled naturally
  • continually increase our dependence on fossil fuels and burden the gaseous cycle evermore to cleanse water vapour from impurities
  • over-saturate rivers and estuaries with organic waste material to such an extent that marshes and reeds are no longer effective water cleansers.

It is for time for us to re-think our relationship with nature. Unless we find and implement cleaner methods of production, agriculture, waste disposal, transport… the list goes on, our resource base will continue to deteriorate. Let us know how you feel about these issues and what should be done by raising your voice on our Twitter Page.