Population Summarized Biology Notes

 Learning objectives
 At the end of this unit, you should be able to:
- define population as a group of organisms of one species that live in the same area, at the same time
- state the factors affecting the rate of population growth for a range of living organisms
- define community as all of the populations of different species in an ecosystem
- define ecosystem as a unit containing the community of organisms and their environment, interacting together (e.g. decomposing log and lake)
- identify the phases of a sigmoid population growth curve for a population growing in an environment with limited resources and explain the role of limiting factors to each phase
- describe the increase in human population size in the absence of limiting tactors and social implications of current human survival rate on the environment
- interpret graphs and diagrams of human population growth.

 Introduction
According to the United Nations, by the year 2050, the population of the world will be about 9.1 billion! All these people will need food, water, and a space in which to live and work. The rapid increase in the size of the human population is the result of a reduction in the number of people dying. As a result of improved health care, immunisations and antibiotics, we have overcome many killer diseases, especially those that affect children, such as measles.
In this post, we will look at the social effects of the rapid growth in human population. We will also look at the factors that affect the rate of population growth in animals, such as food supply, predation and disease. Here are some important definitions for this post:

Habitat
The area where organisms live.

Population
A group of organisms of one species that live in the same area, at the same time.

Community
All of the populations of different species in an ecosystem.

Environment
The conditions ot a certain place.

 Niche
The way an organism lives in an ecosystem.

Ecosystem
A unit containing the community of organisms and their environment, interacting together (e.g. decomposing log and lake).

Population
A population is a group of organisms of a single species that live in a given area. For example, all the camel thorn trees in a particular area make up a population of camel thorn trees. All the black rhino in the Etosha National Park make up a population of black rhino. For the majority of species, whether plant or animal, the changes in population size over a period of time tend to rise and fall around a fairly constant level. Figure below shows how a population fluctuates. There are three important factors that affect the rate of population growth:
• food supply
• predation
• disease.

Food supply
A supply of food and water is essential for the animals in a population to survive. An increase in the number of animals means that individuals are competing for the same source of food. This usually means that the stronger and fitter animals survive and the weaker animals die of starvation. If a population is supplied with unlimited supplies of food and is free from disease, it will tend to increase in size. Once there are more individuals than the area can support, lack of food and disease will result in a decrease in population size.

 Predation
Another factor influencing the size of a population is predation. This is a term used to describe the feeding relationship between two organisms. A predator is an animal that feeds on another organism, either plant or animal. We say that impala are predated on by lions. The lion is a predator and the impala are prey. When the prey is plentiful, the number of predators increases. As they increase in numbers, they require more food, so they reduce the number of prey. Once this happens the food supplies diminish and so less food is available to feed the large number of predators. Thus the number of predators, too, will decrease. Figure below shows how the number of predators is affected by the number of prey.

 Disease
The effect of disease on a population shows a similar eftect. As the population size rises, It is easier for disease to be spread from organism to organism. The increased presence of disease within the population will cause a reduction in size until the spread of disease is halted. Diseases are also an important limiting factor for human population. Today, AIDS is one of the leading deadly diseases that decrease human population worldwide and Namibia is no exception.

 Limiting factors
Food supply, disease and predators can be described as limiting factors because they control the size of a population. In the absence of disease and predators, and with adequate food supplies, we say that there are no limiting factors reducing population growth. Food supply, disease and predators are limiting tactors for human population, just as much as they are for other organisms. In recent times, the effects of limiting factors on human population growth have been less and therefore the population has tended to increase at an alarming rate.

Sigmoid curve of population growth
It an organism is introduced into an environment where there are no limiting factors, the population curve will follow a characteristic S-shape. An S-shaped curve is described as a sigmoid curve. By looking at changes in population sizes in other organisms, we can learn quite a lot about our own population. Many experiments on population sizes have been done on organisms like bacteria and yeast, because they reproduce quickly and are easy to grow. Figure below shows a typical sigmoid curve for population growth. When a limiting factor influences population growth, a sigmoid (S-shaped) curve is formed. As you can see on the graph, there are four phases:

 Lag phase:
The population starts with very low numbers asa result of organisms adjusting to the new environment and seeking out food, mates, and breeding areas.

 Log/exponential/geometrical phase:
The population shows a rapid increase in numbers as a result of availability of food. There are no limiting factors. There is a steady doubling in numbers per unit of time, which produces a straightline.

 'Stabilisation'/stationary phase:
The population remains constant; population growth stops; this stage is reached when food supply and other resources are limited. Limiting factors, such as insufficient food, cause the rate of reproduction to slow down and there are more deaths in the population. When the birth rate and death rate are equal, the line of the graph becomes horizontal.

 Death phase:
As tfood is used up, more organisms die than are born, so the number in the population drops.

 Human Population growth
The world population of humans continues to grow daily. There are already about five billion people in the world. One billion people are added to the total population every ten years. The reason for this growth rate is simple: three people are born for every one that dies. Since 3000 BC, human populations have been kept in check by a combination of diseases, famine and war. Nevertheless, there has still been a steady increase in the growth of the human population. Figure below shows hovw the human population has changed since about 3000 BC. On Figure you can see a sharp increase in the population about 300 years ago. There are two main reasons for this recent growth spurt.

 1.The reduction of disease:
- In many countries, improvements in water supply, sewage treatment, hygienic food handling and general standards of cleanliness have virtually wiped out many diseases, Such as typhoid and dysentery.
- Immunisation against diseases, such as polio, has made this disease very rare.
- Smallpox has been totally eradicated.
- The discovery of antibiotics has now made it possible to treat most diseases caused by bacteria.

 2.An increase in food supply:
- More and more land has been brought under cultivation.
- Agriculture has become more efticient, so that each hectare of land is now producing more than ever before.
- Nitrogen fertilisers and artificial light are used to boost plant growth.
- Predators may be used to control pests during biological control.
- Anaerobic conditions or low temperatures are used to prevent populations of microbes from consuming our food.
- Competitors are eliminated from crops using pesticides.
- Predators are eliminated from farmlands.

Factors affecting population growth

Population growth depends on a number of factors:

- When the birth rate is greater than the death rate, the population grows.
- When the death rate is greater than the birth rate, the population decreases.
- When the birth rate equals the death rate, the population stays the same; when this happens, we say that there is zero population growth, ZPG.
When ZPG is reached, it does not mean that there are no new births. It simply means that the number of people born is equal to the number of people dying. No persons are added to the total number of people.

Social implications of human population growth
Birth rate now exceeds death rate. The human population has increased dramatically because the death rate has been reduced. More and more people are now living long enough to reproduce. If the birth rate doesn't drop by the same amount as the death rate, then the world population will continue to increase. The size of the human population can be brought under control by decreasing the birth rate. In the developed countries, the single largest factor that decreased the birth rate was the introduction of birth control. Considerable efforts are being made to introduce birth control to people in the developing countries, with some success. Many people are, however, suspicious of the methods of birth control or are prevented from using them because of their religious principles. Some people just want to have large families. It seems as though the population will continue to increase for at least another 200 years. If we do not control the overall human birth rate, then famine, war or disease may increase the death rate. We must do our best to stabilise the world population at a level at which everyone has a fair chance of a long, healthy life.

 Interpreting graphs of human population growth
Data on human population growth is presented in a number of ways. A graph, such as the one in Figure titled under 'Human Population growth', is a very clear way of showing how the population has changed over a period of time. It cannot, however, show whether the population will increase or decrease in the future. This can be done by counting the population over many years, or by measuring its birth rate and death rate. But often it is much easier just to count the number of individuals in various age groups, and to draw an age pyramid. An age pyramid shows the relative number of people in different age groups in the population. The size of each box represents the number of individuals of that age. The number of males are shown on one side and the number of females are shown on the other side. Figures below shows two examples of age pyramids. First figure is a bottom-heavy pyramid, because there is a high number of young individuals of reproductive age. This indicates that birth rate is greater than death rate, so this population is increasing. Second figure shows a much more even spread of ages birth rate and death rate are probably about the same. This population will remain approximately the same size. You can see that most of the age groups are of a simlar size. When allowance is made for a few deaths in the younger age groups, there is little overall change in the population size. This pyramid would be typical of developed countries.

Note it, lf an age pyramid is drawn for the human population on Earth, it is bottom-heavy, like first figure above. Age pyramids for most of the world's developing countries are also this shape, showing that their populations are increasing. But an age pyramid for most developed countries looks more like second figure.The human population in many countries in the developed world remains approximately the same.

Birth rates and death rates affect population growth
Both are expressed as percentages, so that:
Birth rate = number of births during the year
divided by original population at start of year x 100 and Death rate = number of deaths during the year divided by original population at start of year x 100.

 Summary
- Population is a group of organisms of a single species that live in a given area.
- Food supply, predators and disease affect the rate of population growth by increasing the death rate or decreasing the birth rate.
- A population grows if birth rate is higher than death rate.
- A sigmoid curve is a S-shaped curve that shows the typical growth of a population and results from the action of one or more limiting factors.
- The four phases of a sigmoid growth curve are the lag phase, log phase, stationary phase and the death phase.
- The human population has increased rapidly due to the absence of limiting factors, such as disease.
- The increase in size of the human population has many social implications, such as providing sufficient water, food, space to live, housing, education and employment.
- An age pyramid is one way of shovwing the age structure of a population.

 The end, posted by Miss Elisabeth N.