We have already seen that every sort of mechanism, both inanimate and organic—plant, animal, and steam engine—is an energy dissipating device.  Plants require solar energy; animals require chemical energy in the form of food derived either from plants or other animals; steam engines require the chemical energy of fuel.  It is important to note here that particular kinds of energy-consuming devices can, in general, make use of energy only when it occurs in certain forms.  Thus, a steam engine cannot utilize the energy contained in a waterfall, neither can a horse operate on the energy contained in coal or gasoline.  Certain animals, the herbivores, can utilize only the energy contained in a limited variety of plants; other animals, the carnivores, can utilize only energy occurring in the form of meat.  Most plants can utilize only the energy of light radiation.  All of the energy used by every kind of energy-consuming device on the earth is, as we have pointed out, derived almost without exception, initially from the energy of sunshine.  The energy of sunshine is a vast flow of energy.  The existence of plants and animals is dependent upon a successful competition by each of the different species for a share of this total flow.  A simple illustration will perhaps make this more clear.

Imagine an area of land in a temperate region having the usual array of vegetation peculiar to that area.  Suppose that a block of this land of several square miles in area be completely fenced off in such a manner that no animals at all are allowed within this area.  Under these conditions the grass, in the absence of animals, would become tall and of luxuriant growth.

Now, into this pasture with its luxuriant growth of grass, suppose that we introduce a pair of rabbits, one male and one female, without allowing any other animals within the region.  Suppose, further, that we take a census at regular intervals of the rabbit population within this area.  As we know, rabbits breed rapidly, and in a year’s time one pair of rabbits produce about 12 offspring.  Assuming no rabbits to die in the meantime, and this same rate of multiplication to continue, at the end of the first year the total rabbit population would be 14; at the end of the second year the population would have reached 98; at the end of the third year, it would have reached 686.  One might object to this on the ground that some of the rabbits would have died in the meantime, and this objection is well founded.  Given a situation such as we have assumed here where the food supply is abundant and other conditions are favorable, it is a well-established fact that animals multiply in such a manner that their birth rate exceeds their death rate, and as long as these conditions maintain, the population tends to increase at a compound interest rate.  In the case of the rabbits we are considering, if the births per year were 600 percent and the deaths per year were 200 percent, there would be an expansion of 400 percent.  This, while slightly less spectacular than the case where no deaths occurred, would still result in a very rapid increase in the rabbit population of the area.  Under these conditions, if at the end of a certain time the rabbit population were 100, there would be by the end of the following year 500 rabbits in the area, and by the end of the year after, 2,500 rabbits, etc.

At this rate it is very obvious that it would not take many years for the rabbit population to reach an overwhelming figure.  How long could this rate of growth continue? Is there any upper limit to the number of rabbits that can live in a given pasture area? There very obviously is.  The rabbits eat principally grass and certain other small plants.  For the sake of simplicity, we shall assume that the rabbits eat only grass.  Grass, therefore, being the food, constitutes the energy supply for the rabbit population.  Each rabbit in order to subsist must have a certain number of calories per day, and therefore, must eat a certain minimum amount of grass per day.  In the initial conditions that we have specified, the grass supply far exceeded the needs of the rabbit population.  Under these conditions there were no limitations on the rate of growth of the population.  Finally, however, there would come a time when the number of rabbits would be such that the amount of grass per year required to feed them would just equal the rate at which grass grows.  Under these conditions it is easy to see that if the rabbit population were to get any larger than this, the surplus would starve to death.

Our curve of the growth of the rabbit population, therefore, if plotted as a graph, would at first rise more and more rapidly with time.  After that, the curve would begin to level off, signifying that the food requirements of the rabbit population was approaching the rate of growth of the grass of the region.

When these two things become equal, that is to say, when the rate at which rabbits eat grass is equal to the rate at which grass grows in the region, there will have been reached a state of dynamic equilibrium between rabbits and grass.  If there should be a particularly good growing season, the grass would grow more rapidly, and the rabbit population would increase as a consequence; if this were followed by a drought, the grass would decrease, and the surplus rabbit population would consequently die off.

Now suppose that in this pasture where a state of dynamic equilibrium between rabbits and grass has already been achieved, we introduce a disturbing factor in the form of a pair of coyotes.  Coyotes live on meat, and since we have postulated that rabbits are the only other animals in the area, the coyotes will live upon the rabbits.

Now, what will happen? Since there is an abundance of rabbits the coyotes will have plenty to eat, and while this condition lasts, they will multiply at their most rapid rate.  At the same time, however, because of this, the death rate among the rabbits increases, and the rabbit population declines.  Finally, there comes a time when the rate at which the coyotes require rabbits for food is equal to the rate at which the rabbits grow.  Under this condition the rabbit population will stabilize at a lower figure than formerly, and the coyote population will also stabilize at a different figure.  When this is attained there will then be a state of dynamic equilibrium between coyotes, rabbits, and grass.

We could complicate the picture still further by introducing foxes, owls, field mice and the whole complex array of animals that one normally finds in such localities.  With this more complex picture we would find exactly the same thing; that is, if left alone, each of these different species would tend to come to a stable population.  In the case of each species a stationary population involves an equality between its birth rate and death rate.  Its birth rate is dependent upon its available energy supply; and its death rate is determined in part by age and in part by the rate at which it becomes and energy supply in the form of food for other species.

A disturbance on either side of this equation, a change in the food supply, or a change in the rate at which it is eaten or dies, will disturb this dynamic equilibrium one way or the other.

The principles discussed above are just as valid for the human species as for coyotes or rabbits.

Suppose we consider man in his most primitive state, before he had invented tools and clothing, learned to use fire, or had domesticated plants and animals.  What was his food supply? He must have lived on fruits, grass seeds, nuts, and other such plant products as were available and suitable for human food.  He probably caught and ate small animals such as rabbits, rats, frogs, fish and perhaps insects.  His population in a given area was therefore limited on the upper boundary by the rate at which he could catch these small animals or could gather the plant foods.  On the other side, such large predatory animals as bears, panthers, lions, and saber-toothed tigers were lurking about, and it is entirely probable that our primitive ancestors formed a part of the natural food supply of these animals.  This, as in the case of the coyote-rabbit equilibrium mentioned above, tended to further restrict the human population within a given area.

Now, suppose that this primitive species, man, learned to use such a weapon as a club, what effect would this have toward changing the state of the dynamic equilibrium? In the first place, with a club, a man could probably kill more animals for food than he could have caught using only his hands.  This would tend to increase his food supply, and in so doing, would to that extent curtail the food supply of his predatory competitors.  For example, suppose that with a club a man could kill more rabbits than he could catch with his bare hands; this would increase the human food supply and consequently tend to increase the human population in the given area.  At the same time there would be a decrease in the rabbit population, and a corresponding decrease in the population of other animals depending on rabbits for food.

A club is a weapon of defense as well as a weapon of offense.  With a club, a man would be able to defend himself from beasts of prey, and would accordingly decrease the rate at which he became the prey of other predatory animals.

The result of both of these, the increase of human food supply, and the increase in the expectancy of life of the human being, act in the same direction, namely, to disturb the balance in favor of an increase of human population in the given area.

Now, let our primitive man discover the use of fire.  Fire, by its warming effect, would protect man from the winter cold, and doubtless decrease the number of deaths from freezing and exposure.  This would prolong the average length of life, and consequently increase the population.  Fire also is a powerful medium of defense in that it effectively prevents the depredation by predatory animals.  This also tends to increase the expectancy of life.  The use of fire also would permit man to invade new and colder territories.  Thus, not only would learning to use fire tend to increase the population in areas inhabited by man, but it would enable him to reach a food supply in areas not previously accessible, and, consequently, to still further multiply by inhabiting a larger and larger portion of the earth.

The discovery of the use of fire is of even greater significance in another way.  In this hypothetical development that we have outlined, prior to the use of fire the only part of the total flow of solar energy that had been diverted into the uses of man, prior to the use of fire, was that of the food he ate.  The energy requirements of our primitive ancestors in the form of food was probably not greatly different from that of today, namely, about 2,300 to 2,600 kilogram calories per capita per day.  No other energy was utilized than that of food eaten.  With the discovery of fire, a totally new source of energy was tapped, and use for the first time was made of extraneous energy—energy other than food eaten.

This constituted one of the first steps in a long and tortuous evolution in the learning to convert an ever larger fraction of the total flow of solar energy into uses favorable to the human species.  The results of this learning to direct the flow of solar energy, as we shall see in succeeding lessons, are among the most momentous of the events in the history of life on this planet.


  • Animal Life and Social Growth, Allee.
  • Origin of Species, Darwin.
  • The Biology of Population Growth, Pearl.
  • Elements of Physical Biology, Lotka.