LESSON 6: Cycles and energy-flow

Figure 1: Energy required to run cycles in the ecosystem is provided by the sun.

One of the biggest reasons why we as humans allow pollution and degradation of our recourse base to continue, might be because we are not able to comprehend and therefore appreciate the delicate interdependent functioning of living and non-living components in ecosystems. In order to appreciate, protect and nurture the environment and its components, we have to understand how components are related to one another. To this end we will now have a look at how the environment is forever rejuvenating and maintaining itself by means of energy flow, and various cycles.

The ecosphere could be seen as one large machine driven by 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. Water is being recycled through the soil, air and biotic components in what is known as the hydrological cycle. Gasses like O2 and CO2 are constantly being circulated through ecosystems. Even soil with its minerals form part of a continuous cycle, called the sedimentary cycle.

Figure 2: Energy is supplied by the sun – used within the ecosystem – and passes out of it. mostly in the form of heat.

We will now first see how energy flows through the ecosystem and then we will look at major cycles that support the ecosphere.

With energy-flow, one needs to remember that energy is not a cycle – it is supplied in one form or another by the sun – used in various systems and then passed out of the ecosystem, often as heat. Common to all ecosystems, big or small, is that energy flow occurs in only one direction. The sun always radiates and supplies light to the earth, never the opposite. Also, organisms are always consumed by those positioned on higher levels on a food web. As a result, each level of a food web contains less energy than the levels below it. Nutrients can flow in any direction in an ecosystem.

Energy flow through ecosystems is represented in food chains, food webs and ecological pyramids. 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 small snake (primary carnivore) which in turn is eaten by a cat (secondary carnivore) and finally the cat 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.

Figure 3: The division of trophic levels is based on the function that a species ill in the ecosystem-community.

In a ecological pyramid energy flows upwards from the one level of organism to the next in what is known as trophic levels. Plants form the first, or bottom trophic level (or T1). Herbivores for example antelope, form the second trophic level (T2), and carnivores like lions, form the third trophic level (T3). Top carnivores such as birds of prey occupy the fourth trophic level (T4). The division into trophic levels is not based on specific species, but rather on the function that species fulfill in the ecosystem-community. In general, three types of ecological pyramids can be used.

A number pyramid shows the number of organisms in each trophic level without taking into consideration the size of the organisms.

Figure 4: Different food pyramids emphasizes different aspects of an ecosystem.

This type of ecological pyramid could therefore over-emphasize the importance of small organisms. In a biomass pyramid the total mass of the organisms is indicated in each trophic level. An energy pyramid indicates the total amount of energy present in each trophic level. It will therefore also reflect 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. The energy pyramid is more widely used to compare different ecosystems. But to compile an energy pyramid involves much more research as required with the other two types of pyramids.

So, food-webs mainly define the composition of ecosystems, whilst trophic levels indicate the position of organisms within the food-chain or food-web. 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.

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.

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 held for a short period of time. Clouds are exchange pools in the hydrological cycle and the bodies of organisms are exchange pools for various chemicals in the biotic community. Cycles that transport chemicals such as life-supporting nutrients, pass through both the biological and geological world and therefore we can refer to these as biogeochemical cycles.

Figure 5: Due to activities such as mining we have accelerated the movement of sediments through the natural system.

Before 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. 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 recourses is releasing carbon dioxide into the earth’s atmosphere about seventy times more rapidly than one would expect naturally. Humans have an enormous capacity to increase the rate of movement of materials in both the sedimentary and biological cycles. Keep this in mind as we look at the major cycles.

The water or hydrological 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, forms clouds and can later fall to the earth as rain or snow. Part of the water sinks into the ground whilst about 70% again reaches the atmosphere in the form of vapor 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.

Figure 6: Clean water is vital to maintain all forms of life.

In the cooler regions of the earth, like the north and south 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.

We 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 collected 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 as even this ‘cleansing method’ of nature has been affected by air pollution, resulting in acidic rain.

Figure 7: Carbon forms the basic building block for all organic compounds

This 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. The main reservoirs for carbon dioxide are 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.

Green plants on land and in 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 possible 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. From the body of the animal the carbon can also move three possible ways: it can be released into the air through respiration; and then be taken up by another plant through photosynthesis; or finally it can 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 intact and eventually form coal, oil or natural gas.

Figure 8: An effective phosphorous cycle is important as phosphate is a vital component in bones, teeth and shells.

These are natural fossil fuel resources and can be pushed to the surface by forces inside the earth and released through volcanoes. But when we artificially expose fossil resources to the environment by extraction and by burning 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. In essence global warming happens because of an over-abundance of carbon dioxide in the atmosphere allowing more heat from the sun to reach the earth than it allows its heat to escape back into space. As more carbon dioxide is being released into the atmosphere more carbon also enters the oceans causing amongst others the bleaching of coral reefs. Just as the disappearing forests are crucial for the recycling of carbon on land, so are the disappearing ‘tropical coral reefs’ essential for the survival of food chains in the ocean.

The phosphorous cycle is an important example of a sedimentary cycle. When we consider sedimentary cycles, we need to keep in mind that it consists of two phases: the salt solution; and the rock phase. Because of its weight, heavy phosphor molecules never rise into the air. It is always part of an organism, dissolved in water or exist 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 defecate or dies, the phosphate is again returned to the soil or water as decomposers break the corpses down to a consumable form for other plants.

Figure 9: The accelerated release of phosphor-rich fertilizer and sewage has burdened natural water systems causing eutrophication.

This cycle occurs over and over again until the phosphorous settles on the ocean floor. Here it becomes part of the sedimentary rocks that are being formed over millions of years. When the rock is ultimately brought to the surface, for example as result of crust movement, it may rise the surface above sea level, where it will be exposed to the elements and to weathering. 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. Eventually this can lead to a situation where an overabundance of phosphate concentrations is released into the natural water system due to run-off from cultivated land. This is especially prevalent at coastal regions at the mouths of rivers. It causes a situation known as ‘eutrophication’ 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.

Figure 10: If a system is not actively linked with the total environment around it, it stops to function.

The effectiveness of cycles in nature has been greatly affected by overproduction and the various forms of pollution. When too much pressure is exerted 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 River Jordan into the Dead Sea, which has no outlet. The salts washed out of the soils over thousands of years accumulate in this inland lake. No life exists in the Dead Sea: It is an ecosystem that has stopped functioning because there has been such a build-up of dissolved substances, that it could no longer support natural life. In nature nothing can survive in isolation. Everything is of necessity actively linked to everything else for the sake of survival. If a system is not actively linked with the total environment around it, it perishes.

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

  • Figure 11: Until we have found a safe method to recycle or neutralize nuclear waste, we are jeopardizing the lives of future generations.

    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 vapor 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 time for us to re-think our relationship with nature. Unless we find and implement sustainable methods of production, agriculture, waste disposal, transport… the list goes on, our resource base will continue to deteriorate.