In the previous lessons we saw that vegetation grows continually through the process of photosynthesis and that through this process, the energy plants gather from the sun, is being made available to other creatures, enabling them to maintain themselves (Supporting article A). Accompanying this transfer of energy is the circulation of all elements in the ecosystem to continuously ensure growth and to avoid stagnation and death. In this lesson we will focus on natural control mechanisms in ecological systems that strive to maintain a balanced way of life where all individuals in the system are given the chance for a continued existence. The bigger the biodiversity of any ecosystem, the better the chance all forms of life has to maintain its existence (Supporting article J).
The mechanisms that are employed by nature to maintain a balanced co-existence (whether it exists inside individual bodies or between separate bodies in an ecosystem) adhere to the principles of negative and positive feedback. To maintain a state of equilibrium, also known as homeostasis (Supporting article P), control mechanisms operate. Wikipedia explains the nature of control mechanisms well in the following definition: “All homeostatic control mechanisms have at least three interdependent components for the variable being regulated: The receptor is the sensing component that monitors and responds to changes in the environment. When the receptor senses a stimulus, it sends information to a control center, the component that sets the range at which a variable is maintained. The control center determines an appropriate response to the stimulus. In most homeostatic mechanisms the control center is the brain. The control center then sends signals to an effecter, which can be muscles, organs or other structures that receive signals from the control center. After receiving the signal, a change occurs to correct the deviation by either enhancing it with positive feedback or depressing it with negative feedback” (http://en.wikipedia.org/wiki/Homeostasis)
The presence of negative and positive feedback is of vital importance to sustain ecosystems (Supporting article Q). So before moving on, let’s look a little closer at this concept. Whenever change or transformation occurs in the environmental system, one can identify an input (cause) and an output (effect). In a feedback loop, information (or data) about the result of a transformation is sent back to the input of the system.
• if this data encourages further transformation in the same direction as previously experienced, it is positive feedback resulting in a cumulative effect
• but if the data causes a result in the opposite direction, transformation is inhibited and negative feedback is experienced resulting in a stabilizing effect
In the first case there is movement only in one direction – causing either exponential growth or exponential decline. In the second case, an ecosystem will experience maintenance or a state of equilibrium (Supporting article K and M).
In the natural ecosystem there are complex relationships between animals and plants; predators and prey; bugs and flowers. When any member of that community becomes too numerous, they'll outstrip their food supply. The population of that species will decrease (because of negative feedback), and with that decrease, their food supply will become more plentiful - gradually moving towards a state of positive feedback again. A negative feedback system penalizes an increasing effect and although in nature these systems fluctuate, over time, for a particular ecosystem, the peaks and valleys remain constant (Supporting article C).
It is therefore plain to see that resources within an ecosystem are limited and as population numbers increase, competition for essential life-supporting elements also increases, for example the need for food and shelter, and in the case of plants, space and sunlight. Competition between individuals of the same species is actually greater than competition between different species because different species seldom compete for the identical resources. We say that interspecies competition is less than intra-specie competition.
The most obvious method to secure survival within any specie is for individuals to produce an offspring of more than two. If this situation continues without any checks and balances, populations would forever be increasing. But nature has put certain control mechanisms in place. We will now look at the natural control mechanisms for population numbers. Firstly we will look at the biological control mechanisms amongst organisms themselves. Secondly we will look at the ability of the habitat to sustain organisms making a living there, known as carrying capacity.
The biological control mechanisms are: predation and various forms of cannibalism; various forms of symbiosis (mutualism; commensalism and amensalism) and parasitism. Predation (Supporting article U and V) is when one organism, the predator, kills and devours the other organism, the prey – such as a bird catching a worm. The typical predator is usually larger than the prey but sometimes a number of predators can form a pack and thus kill one prey bigger than themselves, for example a pack of wild dogs hunting down an antelope or ants dragging away an injured or weak insect. Predators see to it that populations are kept healthy because they kill mostly the sick, weak and old individuals that would have died a long grueling death of sickness or starvation in any case. Only the strongest will therefore survive and ensure that the genetic material remains healthy and strong.
Prey (the hunted) again owns certain mechanisms to counteract attacks from predators such as camouflage or speed. A chameleon for example, can change its colour according to its environment. Frogs, butterflies and a host of other animals use this technique to go unnoticed by their predators. Antelope are agile and run fast or have strong horns to defend themselves whilst other species produces millions of offspring (Supporting article W).
Cannibalism (Supporting article R) is not uncommon in the world of nature. There are many examples where adult animals kill and eat their own young. This behavior occurs in approximately 140 different kinds of animals including some spiders, birds, fish, and mammals. Prolicide comes from the Latin word prolis which means descendents. When a lion takes over a new pride, he will often kill the young male lions in order to ensure his dominance. A prolicyder however will not kill his own offspring. When a mature animal eats the young, we call it cronism. Fratricide occurs when the siblings kill one other.
Symbiosis (Supporting article S and T) means to ‘live together’. Many organisms live together under various arrangements, called mutualism, commensalism, amensalism and parasitism.
• Mutualism is where two species of necessity exist together and both derive a benefit. For example herbivores that have mutualistic gut fauna in their stomachs to help them digest plant matter, which is more difficult to digest than animal material.
• In a relationship where only one party benefits, but the other is not harmed, is known as commensalism. Examples are:
o Orchids that live on their tree hosts solely to be closer to the light.
o Certain birds that breed next to breeding crocodiles to protect their own eggs against snakes and leguans.
o Duck-barnacles that attach themselves to the backs of whales and the shells of horseshoe crabs.
• Amensalism is a situation where one individual is definitely inhibited while the other remains unaffected. Often it involves some type of chemical interaction with other organisms where the environment of those organisms are so altered to improve the chances of survival of the unaffected individual. For example a sapling growing under the shadow of a mature tree. The mature tree can begin to rob the sapling of necessary sunlight and, if the mature tree is very large, it can take up rainwater and deplete soil nutrients
• With parasitism one benefits at a remarkable expense of the other (the host). Some parasites derives its nourishment from the host temporarily (like mosquito’s), and some are permanent (like the dodder on lucerne) but they do not kill it. However they could sufficiently weaken the host to fall prey to organisms higher up in the food chain or succumb in adverse climatic conditions.
o Parasites that live on the outside of hosts are called ectoparasites (for example bloodsuckers who live on the tissue of hosts)
o and those that live inside hosts are called endoparasites (like parasites who live on the half-digested food of hosts (roundworms) (Supporting article R).
Suicidal mannerisms is not uncommon in the animal Kingdom. Sometimes a dolphin (even schools of dolphins) or whale will swim ashore and even if people want to put them back to sea, they might come back again and again to die on the beach. This behavior is seen on big scale when the lemmings in Canada from time to time rush over the countryside towards the ocean where they will run over a cliff into the sea by their ten thousands in a massive suicidal trek. Could it be the result of environmental stress that the lemmings instinctively know that their population density is approaching catastrophic proportions for the community at large - that unless their numbers are reduced, the whole species will face extinction?
The second natural control mechanism, namely the carrying capacity of the land, refers (for our purpose) to the size of the population or community that can be supported indefinitely upon the available resources and services of that ecosystem (or piece of land). This is a measure of the biotic components of the environment. In theory, as long as the animal community produce offspring enough to survive in the expanding plant community (through seed dispersal, germination and growth), numbers will increase and thrive. 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.
Before we continue with the issue of carrying capacity, it is worthwhile to again refer to Primary and Secondary production from the previous lesson of ECONATICS, called Dynamics of the Ecosystem (Supporting article D). The total amount of energy assimilated by plants in a given area is called Gross Primary Production, and the rate by which it occurs, is called primary productivity. Biomass is the net primary production of non-living and organic matter over time. When biomass is expressed as the amount of accumulated matter over a given area (for example as (g/m2) and a given time, we call it the standing biomass (Supporting article E).This usually changes with the seasons. The total number of organisms in a certain area is called the biotic community. One can also express and compare the biomass of various species within a community. We call the portion of biomass that is available as food for the subsequent trophic levels, the standing crop. Secondary productivity of an ecosystem is the amount of energy that is consumed by the primary consumers, namely the T2-organisms (herbivores). The digestive systems of various herbivores assimilate different amounts of plant material according to the species’ own metabolism. Part of the energy that is consumed is used for maintenance, part of it for growth, part of it for reproduction and part of it is excreted.
Based on their specific preference of vegetation, herbivores can be divided into three groups namely: Grazers (grass eaters) such as buffalo, white rhinoceros, hippopotamus and zebra; Mixed feeders (grass and leaf eaters) such as elephants that live on grass, fruit, pods and leaves of trees and shrubs; and Browsers (leaf eaters) like black rhinoceros and giraffe which lives primarily on leaves of trees and shrubs.
It is interesting to note that the White rhino (grazer) and the Black rhino (browser) survive on different types of vegetation and could therefore live in the same ecosystem side by side without competing. There are however often overlap for the same food sources resulting in interspecies competition. Wildlife managers therefore distinguish between the grazing and browsing capacity of the land. They need to know what and how much of it is consumed by which species in order to maintain the ecosystem (Supporting article G). The threshold of the ecological carrying capacity should not exceeded by overgrazing and resulting ecosystem deterioration. The specific purpose that a piece of land is intended to be used, will determine the intensity of the use. If a game farm has tourism as its main goal 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 will 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. The term carrying capacity is therefore much more than an objective measurable quantity (Supporting article B).
As we can see, humans have introduced ‘artificial’ methods to control ecosystems by influencing the carrying capacity of land. For example on a certain game farm one may find that the biomass of a certain species of grass that serves as food for a certain species of antelope, is becoming smaller. Steps could be taken to keep the antelope numbers down, for example by allowing hunting only in certain seasons. If the area is only used from time to time and given opportunity to recover, its carrying capacity can be much higher as compared to sustained use over long periods.
In the natural world negative and positive feedback does not occur artificially. Here the growth of species populations exhibits one of two patterns. In one pattern, the population grows rapidly as long as food and habitat are abundant (positive feedback). As resources become scarce again, the population decreases as birth rates slow and death from competition for food increases (negative feedback). Other biological control mechanisms for controlling population numbers could also include sickness, drought and veldt fires.
But humankind has overcome, to a large extent, this relationship with nature which has lead to the elimination or endangerment of many species on earth (Supporting article I and O). Because of the various artificial control mechanisms that we have put in place, we have created a positive feedback scenario. Civilizations flourish in locations where our numbers far surpass the land's ability to provide (Supporting article H). With the help of huge dams (inhibiting the flow of rivers) and fertilizer-enriched farmlands (causing eutrophication), huge cities are able to support their ever-increasing populations. The more people there are in a city, the more money will be generated, and the greater the ability our infrastructure (artificial system) has to support an ever growing number of people.
Positive feedback systems have a perpetual cycle, causing increasing effects to further increase. It either leads to an indefinite expansion or explosion (running away into infinity) or it may lead to the opposite: a total blocking of activities (running away toward zero). Industrial expansion, capital invested at compound interest, inflation, proliferation of cancer cells... these are all examples of positive feedback systems - systems that reinforces an increasing effect, forever gaining momentum. All we are waiting for now, is for the moment when the upward trend will be replaced by a downward spiral, where minus leads to another minus and events will come to a standstill, resulting in ecosystem bankruptcy and economic depression.
Humans have failed to keep their own numbers in check (Supporting article L and N). In whichever way we would like to look at it, there are some checks in the system that relates to humans as well. In the mid 14th century the Black Death struck Europe and of the 100 million people it is estimated that 70million died. Currently we can see the effects of AIDS on the population numbers of Africa. 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 more and more people (Supporting article F), but in effect is causing poverty to millions more – worse for us – the ecosystems as we know it will soon no longer be able to support our ever-growing hunger for more. In the absence of checks and balances the theoretical SRO (Standing Room Only) condition (1 sq. meter per person) in our cities will be reached very soon!
