Unit D4 - Populations


Unit D4


Usually when we think of some species of plant or animal, we think of an individual of that species, but it would be more correct to think of the whole population, that is of all members of that species alive in one place at any one time. Individuals come and go, but it is the population that perpetuates the species. Old members of a population die, and new ones are born. The individuals making up a population may be totally different in several years from those there today, but the species continues.

One of the characteristics of life is that it reproduces itself. Since nothing lives forever, there is always a turnover of generations. In a stable population, the birth rate equals the death rate, and the total numbers do not change. If the death rate is higher than the birth rate, the population will get smaller and smaller until finally it becomes extinct. If the birth rate is higher than the death rate, the population will grow larger and larger until it finally reaches some limit that causes the births to decline or the deaths to increase.

Populations have the potential to grow very rapidly, in what is called a geometric progression. Suppose a couple (two parents) have four offspring before they die. If those two pairs each have four offspring, there will be eight, then in the next general 16, then 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, etc. Note that four offspring is a very low number for most plants or animals; some species produce thousands or millions. The speed of population growth depends on the number of offspring per generation and the length of time between generations. If the above figures applied to a couple of human beings with 25 years between generations, it would take almost 400 years to reach a hundred thousand. (Many villagers do better than that, but few villages have room for 100,000 people.) If the same figures were for cats breeding at two years of age, the pair of cats would reach 100,000 in a little over 30 years. Suppose a pair of flies manages to lay a thousand eggs in a two-week lifetime and they all survive; in six weeks there would be 250,000,000 flies. Obviously something must happen to limit those populations before they get so large. On the other hand, a green turtle that takes 40 years before being old enough to lay a hundred eggs of which only 2 survive to grow up will just succeed in replacing itself if it is lucky, and just a few adult turtles killed would mean the collapse and disappearance of the population.

Since most plants and animals produce many offspring, it is clear that few of those young survive to adulthood, yet some must succeed if the population is to continue. There are two principal reproduction strategies: producing many vulnerable young in the hope that a few survive; or producing a few young that are very strong and resistant, or that are protected by their parents until they are big enough to defend themselves. It is those that survive to reproduce that count, not the number that die along the way.

Clearly reproduction is the most vulnerable stage for the survival of a population or a species. Those that survive are generally the strongest or best suited to the environment, and the process permits the selection for the best qualities in the population. You can use the same principle to find and maintain the best varieties of crop plants or races of domesticated animals.

Food chains

One of the essential relationships between species is eating and being eaten. All of the food in rural environments starts with plants, which use energy from the sun in order to grow. This plant life, whether in forests, grasslands or food gardens on the land, or mangroves, seagrasses, algae or plankton in the sea, is the starting point and first level of productivity for all rural ecosystems. The amount of food produced by plants will determine how many fish and animals (and people if no food is imported) can live on a rural area. Anything that hurts plants will also affect the animals and reduce the size and productivity of the whole system.

Many animals from caterpillars and snails to cows and dugongs that feed on plants are called herbivores or plant-eaters. Other animals that eat eat animals are called carnivores or meat-eaters. Some animals like rats and men that eat almost anything including plants and animals are omnivores. A bird may eat caterpillars which eat leaves. A plant-eating snail may be eaten by a pig which you then eat for dinner. These are called food chains because the food energy passes along them from one organism to another like links in a chain.

A lot of energy is lost as it goes along a food chain. About nine tenths of an animal's food energy will be used up running around, eating and reproducing, and only about one tenth will go into making its body where it can become the food of the next animal up the food chain. This makes for a structure like a pyramid, with a large amount of plant energy at the bottom supporting ever smaller numbers of animals up the food chain. The animals at the top of the food chain need very large areas to support themselves. It takes much more land to feed people with meat that with plant foods.

These food chains help to keep all the populations in an ecosystem in balance. The world is not overrun with flies because frogs, geckoes, birds, spiders and many other things eat them. (One reason why there is a human population problem is that, apart from the rare crocodile or tiger, nothing eats us). The balance of populations along a food chain can be very complicated, and disturbing it in different places can create different kinds of imbalance with varying environmental effects. For instance, suppose there is a simple food chain with one kind of plant, one kind of plant-eating animal and one animal-eating animal. A reduction in the number of plants means a reduction in all the kinds of animals. Taking away the plant-eating animal will lead to a collapse in the number of animals that feed on it, but an increase in the number or size of the plants, since the animal is no longer there to feed on them and keep them under control. Killing the meat-eating animals at the top of the food chain will allow the plant-eating animals to get more common until they eat up all the plants and then starve to death themselves.

Real ecosystems are more complex than this, since there are many more kinds of organisms and more alternative kinds of food, but even then such imbalances can occur with population explosions or collapses. The crown-of-thorns starfish population explosions on coral reefs are an example well-known in the Pacific Ocean region.

The linkages between species such as the examples given here of food chains and population control mechanisms can play an important part in environmental problems. The following classic example illustrates the unexpected consequence of an environmental action. A village was suffering from a plague of flies. The villagers sprayed an insecticide to control the flies. The dying flies loaded with insecticide moved more slowly, and were thus eaten by the geckoes. The geckoes in turn were affected by the poison and fell from the walls, where they fell easy prey to the village cats. The cats became sick from their poisoned food, and could not run fast enough to catch the rats. The result of trying to get rid of the flies was a population explosion of rats. Stories such as this show how difficult management of the environment can be.

Island and mountain populations

The problems of population numbers and controls are particularly important when the space available is limited, as it is on an island, in a mountain valley or in some similar rural areas. There are special principles which are important to understanding why island populations are the way they are and how they came to be established.

Most oceanic islands, except some continental islands, were created at some point by volcanoes rising from the sea floor. As they approached and then grew above the surface, coral reefs (in the tropics) and other shallow sea life, and then land plants and animals, came to live on them. On some islands this may have happened more than once as the island submerged and reappeared. Once an island is available, a first few pioneering plants and animals get established. Gradually some more competitive ones arrive and push aside the early settlers. An equilibrium is finally established when the number of new arrivals is balanced by the number of extinctions, with the total number of species being related to the size of the island and its distance from other land areas. The populations on any island today are at one point in this series of successive stages, depending on how old the island is. The same processes operate in rural areas on continents, but the result is more complicated because the land is older and larger.

Crossing the sea or going from mountaintop to mountaintop is no easy matter for most organisms. Pioneer species have often developed special ways of crossing the ocean, such as floating seeds and salt-resistance. The coconut and many coastal plants are of this type. For other species transport to a new island or mountain is a rare and random event. Not only must the organism survive the long voyage in storm winds, on a drifting log, or stuck to a migrating bird’s feathers, but for many species both sexes must arrive at the same time in order to reproduce and establish a population.

Island plants and animals may have come originally from a continent or from another island. A chain of islands may serve as stepping stones for the spread of a species. Organisms usually spread in the directions of winds and currents, and of the movements of migrating animals, or they may be transported during storms. For high altitude species, mountaintops are like islands and transport between them is similarly difficult.

For a newly-arrived species, reaching a tiny speck of land in the vast sea is only the first step. New islands may present hostile conditions, while older islands have well-established communities against which the new arrival must compete. However, most islands have relatively few species, so there are often unfilled places waiting for new occupants. Even then, the small populations are still vulnerable to diseases, droughts, hurricanes, volcanic eruptions and other natural disasters. The smaller the island, the smaller the maximum possible population it can support and the easier it is for a calamity to wipe out a species. These continuing processes of the arrival and dying out of species are important contributors to the overall balance of life on an island.

Living on an island can have a number of effects on the evolution of the species concerned. Because most islands have fewer species on them, there is less competition and less danger of being eaten. The plants and animals may then lose their defences and their ability to compete. In the case of continental islands, species present at the time of separation from a big continent may survive long after the parent populations have been driven to extinction by later more highly evolved forms. New Caledonia is an example of such an island, where long isolation without big plant-eating animals, and unusual soils toxic to later arrivals, permitted the survival of many primitive plants.

Sometimes an organism may reach an island and find many different places available with no competition; the species may then evolve rapidly into different forms to fill the available places. Charles Darwin described the classic example of the Galapagos finches. This is one reason why there may be many unusual species on an island. Small isolated island populations descended from single parents may also change slowly just because all the individuals are so closely related genetically; each island population may then develop its own distinct characteristics.

The result of these processes is a large number of distinctive and often unique species of plants and animals on islands. On some of the larger, more isolated islands, almost all the species may be unique to that island and found nowhere else on earth.

Pest control

Weeds, damaging insects, moulds, diseases and other pests are often species that are able to multiply their populations rapidly. The principles concerning populations are therefore important in understanding how to control pests.

Natural systems are made up of many kinds of species mixed together, with each balancing and controlling the other. Man-made systems, however, such as agricultural fields or forest plantations, are usually large areas with just a single species. The pests which feed on our crops appreciate our providing them with so much food, and their populations multiply rapidly. A pigsty is an ideal nursery for flies, and the tender shoots in a vegetable garden could not be better for snails.

Some pests have natural enemies that keep their populations under control most of the time. When the pest is numerous, there is more food for the enemy, which will multiply and reduce the pest population to normal levels. Unfortunately, the pests have often reached new rural areas or islands while their enemies have not, so pest problems can be more frequent and more severe where there are introduced species.

Controlling pests is a kind of population management. It is almost impossible to eliminate a pest population entirely; destroying pests gets more expensive as their numbers get smaller and more scattered. However, there are several ways to reduce pest populations to manageable levels. The simplest is to change the conditions so the pest has less food or has a harder time spreading. This means doing things like cleaning the pigsty to control flies, or spacing a crop with other resistant or repellant crops so that it is harder for a pest to spread.

A second method is to use biological controls, which means encouraging a natural enemy or finding and introducing a new enemy for the pest. Unfortunately it is hard to find an enemy that will not also attack other useful plants or animals, and in too many cases something introduced as a biological control has proven to be as much of a problem as the pest it was brought in to control. It is also possible to interfere with pest reproduction by releasing sterile males that keep the females from laying fertile eggs, or spreading sex attractant chemicals that confuse the males and keep them from finding the females. Biological controls are not always easy to use and they require a great deal of research, so you should not try to introduce your own biological controls unless you use traditional controls that have already proven their worth locally.

If there are no alternatives available, then chemical controls may be necessary. This means using some kind of poison (a pesticide, insecticide, herbicide, fungicide, etc.) that kills the pest. Pesticides cost money, although they may be cheaper than the labour involved in other control methods. They have some other severe disadvantages:

1. A pesticide almost never kills all of the pests; a few will survive to come back when the pesticide is gone, so the poison may need to be applied again and again.

2. The pests that survive may develop a resistance to the pesticide, so that it may take more and more poison to kill them, and eventually the poison may no longer work at all.

3. The pesticide may kill more things than just the pest, such as the pest's natural enemies. If the enemies or biological controls have disappeared, then the pest may come back in even larger numbers than before. Other pests may also become a problem because the controls limiting their populations may be affected too.

4. Most pesticides are dangerous poisons for people too. They must be stored, handled and used with great care, following exactly the directions for their application, and using as little as possible. If pesticides are misused or an accident occurs, they can easily contaminate food or the environment, which can be particularly serious in rural areas. If people are exposed to pesticides or become contaminated while using them, they may become sick or die. Rural people and medical personnel may not recognize the symptoms of pesticide poisoning and think it is something else.

It is therefore best to keep chemical pesticides as a last resort in case everything else fails, especially since rural areas are very vulnerable to pollution by chemicals.

Since pest control is basically population management, it is important to understand the population factors for each pest, such as its life cycle and reproductive strategy, in order to know when control may be easiest and most effective. Just as natural populations can be upset by interfering at a sensitive time such as reproduction, so can pest be managed more easily by attacking them when they are the most vulnerable.


What determines whether a population is stable, increasing or decreasing?

What factors influence the speed of population growth?

What are different ways that a species can ensure that some of their young survive to be adults? Can you think of some examples in animals or plants that you know?

Can you think of some examples of food chains in your local environment?

How big a garden does it take to feed you and your family for a whole year?

How do food chains help to control populations?

Are their special or unique species in your rural area?

Do you have a problem with introduced species?

Why does something become a pest?

What are some ways of controlling pest populations?

Which methods of pest control do you think are best? Why?

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Last updated 1 January 2008