One of the greatest challenges facing humans is to develop an understanding of the fundamentals of ecosystem organization. Most people agree that if one part of the environment suffers, the related parts are also affected, but how exactly that happens, they are unsure of. In order to appreciate, protect and nurture the environment and its components, we have to understand how they are related to one another. Today we will see how the environment is forever rejuvenating and maintaining itself by means of energy flow and various cycles. (Supporting article O) shows the importance of a healthy system to regulate the flow of energy through the ecosystem)
The ecosphere could be seen as one large machine driven by a number of huge as well as some 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 the various ecosystems. Even soil with its minerals form part of a continuous cycle, called the sedimentary cycle. (Supporting article DD) offers a short overview on the natural cycles of the ecosystem)
Firstly we will look at energy flow in ecosystems. Here we need to remember that energy is not a cycle – it is supplied by the sun – used in various systems and then 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.
This fact is supported by the Laws of Thermodynamics. The First Law of Thermodynamics states that energy can never be created or destroyed. It only is 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 – if we do not, the life-sustaining natural systems will be overburdened even to the point where they may collapse completely. We will discuss this fact in more detail in the following issue of ECONATICS, 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)
Below is a simple illustration of energy flow through the ecosystem
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 transfers. For example: leaves from a tree (primary producer) – grasshopper (primary consumer) – snake (primary carnivore) – rat (secondary carnivore) – 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), the second (T2); and the carnivores (e.g. lions), 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 fulfill in the ecosystem-community. In general three types of ecological pyramids can be distinguished namely: (graphic representations were taken from (http://www.bcb.uwc.ac.za/sci_ed/grade10/ecology/trophics/troph.htm)
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-emphasize 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 productivity (number of individual organisms) of level T1 is not taken into consideration. Thus an enormous mass of grass is required to support a smaller mass of buck, 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.
The Energy Pyramid
So food-webs largely define ecosystems, and the trophic levels define 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 crayfish and crayfish 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, 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 (Supporting article X). 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 have an underlying reservoir supporting it, where elements or compounds are stored for various periods of time before they once again take part of the cycle. In the case of the hydrological cycle, the ocean is the main reservoir; the atmosphere is the reservoir for the gas 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 is an exchange pools in the hydrological cycle and organisms 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).
Before we look at the various cyclic activities in the ecosystem, it is important to know that because of remarkable population explosions worldwide and consumer demand, human activities are putting ever-increasing pressure on ecosystem cycles (Supporting article B). Because 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 in nature (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.
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 some form of precipitation (rain, snow, dew, etc.). Part of the water sinks into the ground whilst about 70% reaches the atmosphere 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 vary 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 its saltiness. 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 the ‘waste’ products of 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. Read more on acid rain and its effects in Supporting Article Y, Z and AA.
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. 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).
Green 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 when it dies; or it can be eaten by animals. All the carbon in animals comes ultimately from plants and here (in the body of the animal) again the carbon can move three different ways: it can be released into the air (respiration); to be taken up by another plant (photosynthesis); or 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 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 an extraordinary negative impact (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 ‘tropical corral reefs’ essential for the survival of life in the ocean (Supporting article F and E). In essence global warming happens because of an over-abundance of carbon dioxide in the atmosphere allowing more energy from the sun to reach the earth than it allows its energy to escape into space. This is why the burning of fossil fuels so dramatically contributes to global warming (Supporting article D). A South African, disillusioned by the Copenhagen Climate Summit has a warning to governments and decision-makers who puts all their eggs in the ‘fossil fuel basket’ (Supporting Article M).
Sedimentary cycles (Supporting article Q) consist 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, or when animals defecate, the phosphate is again returned to the soil or water where decomposers will break the corpses down to an 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 may also 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 in 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 – especially prevalent at coastal regions at the mouths of rivers. This causes a situation known as ‘eutrohication’ (Supporting article J and K). It is 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. Other interesting cycles that you may like to acquaint yourself with are the oxygen and nitrogen cycles.
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 river Jordan 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 linked to everything else for survival. If a system is not linked with the total environment around it, it becomes isolated and perishes.
