Ecology is a biological science in which we study the relationships between all living organisms and their environment. The word was used by Ernst Haeckel as early as in 1869. However, the true importance of this science in terms of human survival became obvious only toward the end of the twentieth century. Human ecology is a special branch of general ecology, in which we study the relationships of ourselves to all other forms of life and the earth. Population ecology particularly zeroes in on the effects of population growth dynamics in a limited environment.

Population growth usually follows an S-shaped curve as we plot the numbers of individuals in a population (the Y coordinate) versus time (the X coordinate.) The initial phase of growth is slow because the base population is small. This is followed by a rapid exponential growth as the base population increases. In the final phase, growth slows down and reaches a plateau at zero population growth. The power to increase, or the reproductive power is the driving force behind the increase, and the environmental resistance is the slowing factor. These two ‘forces’ balance out at zero growth. The size of the population at this point of balance is said to be at the carrying capacity of the environment.

It is not realistic to think of the carrying capacity as a single fixed value about the optimal population size in a given environment. The environment is dynamic, that is constantly changing. Consequently, the carrying capacity will fluctuate and change accordingly. It is more realistic to define the carrying capacity of an environment as a range and not as a single value of population size. A good example to illustrate the fluctuating nature of the carrying capacity is to consider the balanced relationship between predator and pray. Such is the relationship between the snowshoe hare and the lynx in the arctic. The numbers of both populations fluctuate within a small range in two yearly cycles where the lynx population lags behind the hare. In harsh conditions, the range of fluctuations in the carrying capacity of ana environment may widen considerably, as in the case of lemmings on the arctic tundra. In their case, the available energy of the system alternates between lemmings and the vegetation upon which they live. As the numbers of the lemmings increases rapidly, the vegetation will become exhausted. This is followed by the massive starvation of lemmings. Their dead bodies will return nutrients to the soil from which then a new lush vegetation will be established. this will be followed by an increase in the number of lemmings and the cycle repeats itself endlessly in every three years.

In the human situation it is clear that the carrying capacity of the earth is relative to our mode of life. For example, if we were to shift down in the food chain from eating mostly meat to eating mostly vegetables the earth’s carrying capacity would increase for us considerably. But where is the carrying capacity of the earth for us? Because of the multitude of variables which may have an effect on the human carrying capacity of the earth, the absolute meaning of this concept remains elusive. First, the practical approach to this issue is to assess the overall impact of humanity on the earth and to see if this impact has an over stressing negative effect or not. If the negative effect increases with time we are close or even over the carrying capacity of the earth. Secondly, it is a good idea to assess the distribution of resources from the point of view of social and economic justice because great inequalities in distribution in a situation of scarcity will much reduce the carrying capacity of the earth for all of us.

It is possible to expand in population size beyond the carrying capacity of the given environment. The reason for this is simple enough. The decline of support is often gradual and it often shows its effects with some delay in time. Such overgrowth beyond the environment’s carrying capacity results in a population crash. Here is an example. In 1944, 29 reindeer were introduced into St. Matthew Island in the North Pacific, close to 60 degrees latitude. The island had a lush vegetation, and the largest predator was the arctic fox. The natural predator of the reindeer, the timber wolf, was absent from the island. Within two decades, the reindeer population increased to close to 6000. At this number the island’s food resources were depleted and in a specially cold and long winter that followed, no recovery was possible. The reindeer, in the absence of their natural predator, grew beyond the carrying capacity of the island. Instead of balancing off at the carrying capacity, there was a population crash reducing the number in the herd to 42 rather sickly individuals, which soon in the following years died off. (D.R. Klein. Journal of Wildlife Management, volume 32. 1968.)

I believe there is a lesson to learn from the above example, applicable to our own situation. We do not exactly know where the range of the earth's carrying capacity is in regard to supporting human lives. Normally, environmental resistance reduces population growth by increasing death rate until a balance is reached. Through medical science and fossil fuel oriented technology in agriculture we managed to neutralize to a considerable extent the effects of environmental resistance. There are many signs, however, which seem to indicate that we are very close to the carrying capacity of the earth with around six billion people, and with our way of living we might even have reached beyond it. (See section on Ecological Perspective, in the essay A Strategy for Change.)

It is something new to us to acquire a sense of ecological realism. I recall when I took my first professional ecology course in 1956 from Charles Elton, ecology was still a rather young science. We were in a lecture room with the seats arranged in tires. The lecture was scheduled for late afternoon in for October and November. It got dark early then, but no lights were turned on except for the small table lamp on the desk where Dr. Elton was. He was a rather short man, perched on top of a lab chair. In the distorting lights and shadows of the lamp, he looked like a leprechaun bewitching us with strange knowledge. There he was playing with a bunch of keys in his right hand as he talked about the fate of a small fly larva in one of the brooks in the Himalayas. When I came out of the class, I looked up onto the heavy gray sky and said with all sincerity of an undergraduate mind: “Dear Lord! Who cares!”

Since then, ecology became a tremendously important science. In the twenty-first century, ecology is a science which spells out for us the conditions of our survival. It is important that we have the proper mindset in ecology that is adjusted to the characteristics of the problems. We often think about things in a way that simplifies a problem by reducing it by one dimension. Our illustrations of a three dimensional world are often two dimensional, and we like to make a snapshot of a dynamic event as if it were static. We look at an exponential growth, but in our minds we try to deal with an arithmetic change. In ecology we study relationships, which are complex issues about the interactions of constantly changing situations. It is easy to yield to the temptation to simplify matters and loose realism.

Here is an example. It is called “The Twenty Ninth Day”, the title of a book by Lester R. Brown, 1978. In the introduction he writes: The French use a riddle to teach schoolchildren the nature of exponential growth. A lily pond, so the riddle goes, contains a single leaf. Each day the number of leaves doubles - two leaves the second day, four the third, eight the fourth, and so on. “If the pond is full on the thirtieth day,” the question goes, “at what point is it half full?” The answer: “On the twenty-ninth day.”

In ecological situations, when the problem is reasonably clear, and the solution obvious, to hesitate means to loose.

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