In the previous lessons we saw how vegetation, through the process of photosynthesis gather energy from the sun and then make it available to plants and animals ensuring their existence. We saw how the circulation of all elements in the ecosystem continuously ensure growth to avoid stagnation and death. In this lesson we will focus on control mechanisms in ecological systems that strive to ensure that balance or equilibrium is maintained. The bigger the biodiversity of any ecosystem, the better chance all the components have to survive challenges and remain in a balanced state. Let us now look at examples of different ecosystems in and out of equilibrium.
Imagine walking across a balance beam. You keep your arms out and sway from side to side as you walk across the beam. As long as you keep your weight in the centre you stay balanced and stay on the beam. But if you become out of balance you fall off. For ecosystems to remain sustainable it is very important to stay in balance.
Ecosystems must for example have just the right amount of non-living things like sunlight and water as well as the correct balance of species variety to stay in equilibrium. Too many or too few of a particular species can cause a population to crash. Population crashes can in return be devastating for ecosystems as a whole.
How then do ecosystems remain in equilibrium? Let us start with the composition of ecosystems. As we have seen in the previous lesson, each ecosystem has a delicate food web that starts with producers, or organisms that make their own food like plants and phytoplankton. This is the largest collection of organisms in an ecosystem. Primary consumers or herbivores consume vegetation or producers. These animals, like antelope and rabbits, in a healthy ecosystem are far fewer in number than the vegetation around. This we have noticed in food pyramids of the previous lesson. However, should the number of a rabbit population increase beyond the carrying capacity of the ecosystem, their food will become insufficient and they will die off. This may send a ripple effect throughout the ecosystem.
Secondary consumers like foxes that live from the rabbits will also disappear, and so will the tertiary consumers like lions or eagles that feed on the foxes. There are very few top carnivores in an ecosystem because there is less energy available at the top of the food web due to the loss of energy between the many links in the food chain.
These patterns for populations of organisms apply to all ecosystems. There should always be the most number of producers and the least number of tertiary consumers. Too many or too few at any level of the food web can have catastrophic effects.
One way to ensure survival of species is for individuals to produce an offspring of more than two. But if this rate of population-increase continues without any checks and balances, individuals would forever be multiplying at the expense of the rest of the ecosystem.
To prevent certain species to ‘get out of hand’, as it were, there should exist a state of equilibrium, also known as homeostasis. To make such conditions possible, certain control mechanisms operate as a result of ecosystem feedback. Positive feedback will encourage behaviour to continue in the same direction. For example, a population explosion in a community would be characterized by run-away births. But as soon as resources diminish, the community will be under stress. Negative feedback will inhibit this course of action to continue as it always penalizes an increasing effect. As a result, there will be a reduction of births to bring the system back to a stable state.
Another example where both positive and negative feedback is at work is in tropical forests. The huge biomass is partially maintained by the ability of the forest basin to recycle water. Let us see how this happens in the Congo Forest of Africa
If the humidity that the forest basin receives from the Atlantic Ocean were to flow directly back as if funnelled by the massive Congo river the forest ecosystem would only be a small fraction of its current size. But in fact, the giant trees of the Congolese rainforest rapidly pump humidity back into the sky which then falls down on to the rainforest, thus preventing significant amounts of water from being flushed straight back into the ocean.
Here we have two feedback cycles in operation. A positive feedback promotes large volumes of water to stay within the Congo basin:
The more the rain falls down, the more water there is to pump up into the forest atmosphere, which in turn creates more rainfall. A negative feedback drains water away from the Congo basin: The more rainfall there is, the more water manages to escape down the rivers into the ocean. Thus, these two feedback mechanisms play a major role in determining the exact extent of the Congo ecosystem.
But scientists believe that the present rate of removal of forest cover will significantly weaken this positive feedback mechanism.
They fear that after a critical threshold of deforestation has been reached, the water recycling mechanism of the Congo basin will begin to break down. As the basin is surrounded by grassy savannah landscapes to the north and to the south this may result in extremely rapid advancement of this landscape into the Congo basin and forever change its rich composition of ecosystems.
To summarize, homeostasis is a self-regulating process by which ecosystems tend to maintain stability while adjusting to conditions that are optimal for long-term survival. If homeostasis is successful, life continues; if unsuccessful, disaster or death ensues. The concept of ‘dynamic equilibrium’ is often used to describe the continuous change that occur within certain boundaries. In this way stability is attained and relatively uniform conditions prevail.
For organisms to live within the limits of an ecosystem depends on three factors: the amount of resources available in the ecosystem; the size of the population or community; and the amount of resources each individual within the community is consuming. Vegetation plays the most important role in determining the carrying capacity of a piece of land. Carrying capacity is a measure of the biotic components the environment is able to accommodate.
In theory, as long as the animal community produces offspring enough to survive in the expanding plant community (through seed dispersal, germination and growth), numbers will increase and thrive and compete healthily with other species in the community.
But as limiting factors develop in a community or population, such as food or shelter resources in the case of animals, and in the case of plants, space and sunlight – competition will set in. There exists competition amongst and between species for same resources. This may lead to aggressive interaction. If one species prevails over the other species, it usually means that it is better adapted to the particular habitat than the other. But there may also be ‘peaceful’ cooperation between species. We refer to such arrangements as symbiosis. Other biological control mechanisms include sickness, drought and veldt fires.
Competition between individuals of the same species is actually greater than competition between different species because different species seldom compete for identical resources. We say that interspecies competition is less than intra-specie competition. It is interesting that White rhinos that live from grass can live side by side in the same ecosystem with the black rhino because these feed on leaves.
There are however often overlap for the same food sources resulting in interspecies competition. Wildlife managers therefore distinguish between the grazing (grass) and browsing (leaves) capacity of the land. They need to know what (type of vegetation) and how much of it is consumed by which species in order to maintain the landscape or ecosystem. The threshold of the ecological carrying capacity should not be exceeded by overgrazing that would result in ecosystem deterioration. The specific purpose that a piece of land or habitat is intended to be used for, will determine the impact on the landscape or ecosystem. If a game farm has tourism as its main objective, it will require the maximum number of animals and therefore artificial addition of water and forage. But if the purpose of that piece of land is trophy hunting, it may require the weaker animals to be eliminated to optimize the reproduction of better wildlife stock. A meat producing farmer will yet again have different approach to veldt management and carrying capacity.
As you can see, humans have introduced artificial control mechanisms to maintain ecosystems and to influence the carrying capacity of land. In this way humans control negative and positive feedback artificially.
Although this has led to the elimination or endangerment of many species on earth, in doing so, humankind has overcome, to a large extent, his relationship of dependency on nature. Because of the various artificial control mechanisms that we have put in place, we have created a positive feedback scenario. Civilizations are able to flourish in locations where our numbers far surpass the land’s natural ability to provide. With the help of huge dams and irrigated fertilizer-enriched farmlands, huge cities are able to support their ever-increasing populations. The more people there are in a city, the greater infrastructure required to support this ever-growing number of people. At the same time, nature suffers as dams inhibit the flow of rivers and this impact on associated ecosystems. Also, as fertilizers from agriculture are washed into river systems it causes death to aquatic life.
If we are not able to reduce our impacts on the environment and its resources and adopt a sustainable approach, the current upward trend of ‘more for all’, the upward spiral of positive feedback, will be replaced by a downward spiral, where progress will come to a standstill as a result of ecosystem bankruptcy.
We as humans have failed to keep own numbers in check. In whichever way we would like to look at it, there are some checks in the system that relate to humans as well. In the mid-fourteenth 14th century the Black Plague struck Europe and of the 100 million people then on earth, it is estimated that 70million died. Today we can see the effects of AIDS on the population numbers. Droughts during the early 60’s have created extensive starvation in Ethiopia and surrounding states. These and similar examples could well be regarded as “normal” ecological processes whereby nature tries to keep also human numbers in check.
Mankind is forever trying to circumvent these checks and balances in order to enhance living conditions for the privileged, but in effect we are causing poverty to millions more. The natural ecosystems as we know it will soon no longer be able to support our ever-growing hunger for more instant fossilized energy and material ‘stuff’.
To conclude: Say, we manage to keep our human population numbers in check, would he solar and other environmentally friendly alternatives for energy be sufficient to help nature (our resource base) to recover? Will this be sufficient to support the manufacturing and transport sectors? The final question remains: Do we as the human race have the discipline to adopt a way of life that is in harmony with our natural resource base? Time will tell.