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Lesson

The number of humans on Earth remained pretty much the same for thousands of years . What changed this was when humans started practicing agriculture and the number of humans began to increase, that is , the human population started to increase. A population is a group of individuals;

  .....belonging to the same species, living together in the same general area,

 .....making use of the same general resources (ex: food and water),

 and

 .....governed by the same general environmental factors.

An example of a population would be a population of moose:

 

or a population of zebra

POPULATION GROWTH......

may be defined as the increase or decrease in the number of individuals in a population. In general a population will tend to increase in numbers when the available resources (ex: food and water) are greater than that required by the members of the population present at that particular time. As long as resources are available, every population has the tendency to grow.

The size of a population at any time is the result of certain factors that either add to the population size or decrease the population size. These factors are:

*Natality .....birth or the number of individuals in a population that are born in one year.

*Immigration.....the number of individuals that move into a population from outside.

*Mortality......death or the number of individuals in a population that die in one year.

*Emigration......individuals of a population that are leaving the population .

Notice that natality and immigration result in a population increasing in numbers whereas mortality and emigration result in a population decreasing in numbers.

Calculating  Population Growth.....

To calculate changes in a population size, a formula is used:

Population Growth=(births +immigration) – (deaths + emigration)

The following data was collected for a population of 200 Sand hill Cranes at a Banks Island Breeding Site in the Canadian Arctic in the spring of 1991:

             Births: 40             Immigration:0

             Deaths: 55            Emigration: 0

How much did the population grow by?

Population Growth= ( 40 +0) - (55 + 0) = (40 - 55) = - 15

An answer of -15 tells us that there is a decrease in the population of Sand hill Cranes.

What are some factors that may have caused a decrease in the number of cranes?

In ecosystems whereby resources such as food, water and shelter tend to be constant and available, populations don’t change much over long periods of time and thus the population growth will equal zero. Therefore, there will be no great changes in the size of the population. Also, increases in natality are balanced by increases in mortality or increased emigration.

Open vs. Closed Populations

Open population....

a population whereby natality, mortality ,immigration and emigration are affecting the population size. In most ecosystems in nature, populations are considered to be open. For example a population of wolves in an area such as Yellowstone National Park will experience all four of these factors over a certain duration of time. New wolves will be born and also immigrate to the the park as well as older wolves dying and also some wolves moving out of the park and into other areas.

Closed population.....

a population whereby only natality and mortality are affecting the population size. Immigration and emigration do not affect the size of the population. Closed populations occur in a laboratory setting or on a game reserve. The human population is a closed population. There is no immigration of humans from other planets to Earth and neither are humans emigrating from Earth to other planets. The only factors that are changing the size of the human population on Earth are natality and mortality.

For another example, consider a yeast or bacterial culture grown under laboratory conditions. At first there is a large food supply, no toxic build up, and lots of space. Since the culture is isolated, there is no possibility for yeast or bacteria to leave or enter the culture.

Human Population Growth

Human populations are continuing to grow. For the past 200,000 years, the human population has had tremendous growth and has spread over Earth. Beginning in 1650, population increases occurred at a higher rate. New technologies for hunting and farming have enabled this expansion. It took 1800 years to reach a total population of 1 billion, but only 130 years to reach 2 billion, and a mere 45 years to reach 4 billion.

Despite advances in technology, factors influencing population growth will eventually limit expansion of human population. These will involve limitation of physical and biological resources as world populations increased to over six billion in 1999. The 1987 population was estimated at a puny 5 billion!

Not surprisingly, little is known about the global population back beyond a few hundred years. The graph below shows human population growth over the past 1,000 years. Note the effects of worldwide disease ( the Black Death) which caused a sharp dip in population levels around 1500.

 

The population is predicted to peak at around 12 billion, but we should remember that this is just a prediction and it is a very difficult predication to make. For instance, twenty years ago, it was thought that the population would level off at about 8 billion.

If you look closely enough you should notice that the graph above resembles a smooth curve that starts very slowly and then increases rapidly from around 1900(also see text, fig.1.,page 38). The graph below is also an example of a smooth curve:

 

There's just too many of us....

The population is predicted to peak at around 12 billion, but we should remember that this is just a prediction and it is a very difficult predication to make. For instance, twenty years ago, it was thought that the population would level off at about 8 billion. If you look closely enough you should notice that the graph above resembles a smooth curve that starts very slowly and then increases rapidly from around 1900(also see text, fig.1.,page 38). The graph below is also an example of a smooth curve:

Population Histograms

Histograms are useful for studying human populations because they help us see trends in population data and they help us make predictions. Histograms known as double histograms allow us to study a population in terms of both sex and age of a population. Histograms often take the shape of a pyramid and allow you to predict changes in the population. The following are three types of histograms you should become familiar with:

1.Young Population.....

This histogram is also referred to as a wide base histogram and it indicates a rapidly growing population as the young mature and breed.

2. Stable Population....

This histogram represents a fairly stable population with few changes occurring within the population

3. Declining Population.....

This histogram represents a decrease in the numbers of individuals in a population. Fewer births are occurring demonstrating a declining population. Notice that the base of the pyramid is narrower than the middle.

Limits to Population Growth

Field mice reproduce every six weeks and have litters with six or more babies. After only six weeks, these babies will be sexually mature and will begin having babies with litters of six or more. In six months, a population of 20 mice could become a population of 5120 mice!!!

Since mice have been around for millions of years, then why aren't we overrun with mice? The answer is that there are limitations that prevent a population from getting out of control such as biotic potential, carrying capacity, and other limiting factors.

Limiting factors refers to any resource that is in short supply and is considered a limiting factor on a population. For example, food, water, territory or the presence of a pollutant are just some of the limiting factors that affect population sizes.

Biotic Potential as a Limiting Factor....

The term biotic potential refers to the highest rate of reproduction possible for a population under ideal conditions, (conditions where birth rate reaches its theoretical maximum). It is the maximum number of offspring that a species could produce if resources were unlimited  .The highest rate of reproduction is common of populations that have recently been introduced into a new environment, or populations that are recovering from some catastrophe such as a flood.  

Factors which affect biotic potential include:

Birth potential ...

which refers to the maximum number of offspring per birth. For example, Whooping crane females lay  two eggs per year and only one chick survives.

 

Capacity for survival ...

which refers to the number of offspring that reach reproductive age. For example, the female codfish lays millions of eggs, but only a few of her offspring will survive and go on to reach sexual maturity in 5 to 6 years.

Procreation...

 which refers to the number of times that a species reproduces each year. For example, moose mate only one time during the fall of the year.

Length of Reproductive life...

 which refers to the age of sexual maturity as well as the number of years that an individual is capable of reproducing. For example, African elephants reach sexual maturity at about age 15 and may continue reproducing until they are 90 years of age.

Carrying Capacity as a Limiting Factor.....

Carrying capacity is defined as the maximum number of individuals (population size) that the environment can support over a relatively long period of time. The carrying capacity of any environment is determined by the limiting factors that exist in that environment. Any environment with fewer resources will have a lower carrying capacity than one that has greater resources for the population under study.

For example, if insects are available in large numbers, the carrying capacity for a bird population that feeds on the insects will be greater than in areas where food resources for the birds is less abundant. Although food resources (energy) are the most common factor to determine carrying capacity, there are other factors that affect the carrying capacity.

Density-dependent and Density-independent factors.....

The growth of populations are regulated by density-dependent and density-independent factors. These factors may affect the population either separately or in combination depending on the species and their specific circumstances.  Density-dependent factors are factors that are related to the size of the population . A population is said to be dense when there are large numbers of organisms in a small area. Examples of density-dependent  factors that mat limit population growth are:

1. Territoriality -

Territoriality is a behavior  in which the members of the population defend a well-bounded physical space, protecting the resources within. This behavior may reduce  competition for food and nesting sites but may increase competition for space. Those that do not find the space do not normally reproduce successfully.

2. Health and Survivorship -

As overcrowding results in less robust individuals, the reduced general health of the individual may decrease the chances of survivability as disease and parasites are much more easily spread.

3. Predators -

 Many predators concentrate on a particular prey when its population density is high, taking a higher than normal number of individuals.

 4. Accumulation of Toxic Metabolic Wastes -

In high density populations the accumulation of toxic metabolic waste may limit population growth even when food is plentiful.

5. Intrinsic Factors -

Intrinsic factors may also play a role in regulating population size. High densities may cause stress syndromes that result in hormonal changes that delay or even inhibit reproduction, or suppress the immune system thereby increasing death rate and reducing birth rate.

Density-independent factors are unrelated to population size and have the same effect on individuals regardless of the population size. Density-independent factors include weather, climate and natural disasters such as freezes, seasonal changes, hurricanes, fires, floods, etc. The time and severity of the density-independent factor determines the proportion of the population affected. In some populations, density-independent factors control population size before density-dependent factors become important considerations.

Many populations remain stable in size, close to the carrying capacity, controlled mainly by density-dependent factors. They often display short term fluctuations as a result of density-independent factors such as an extremely dry summer or cold winter. Density-dependent and density-independent factors sometimes work in combination to control the population size. For example, a severe winter may reduce numbers in the population (density-independent) but may benefit the surviving members by reducing the competition for food in the following spring (density-dependent).

Population Cycles.....

Many populations (birds, lemmings, and mammals) show regular fluctuations in density. These are know as population cycles. For example, the lemming has a 4 year cycle, cicadas have a 13 or 17 year cycle. These cycles may be due to hormonal changes in response to increased density, to change in the food quality, a lag in response to predator population density, or an adaptation to reduce predation.

Humans....

The human population has been growing exponentially for centuries but will not be able to continue indefinitely because the same factors that affect other populations also affect the human population. The main reasons for the exponential growth in the human population has been the development of agriculture which began 10,000 years ago. This increased the birth rate and reduced the death rate.

 More recently the death rate has been reduced, especially infant mortality, as a result of advances in nutrition, medical care, and improved sanitation coupled with a relatively high birth rate. Population ecologists are unable to agree on the carrying capacity for the human population. Limited food resources is the normal limiting factor for most populations, but improved agricultural practice and technology have increased food resources for the human population. Humans have the capacity to consciously control reproductive capacity through contraception or family planning. One thing is agreed upon - the growth of the human population will eventually reach its carrying capacity. It is hoped that as the population reaches its carrying capacity it will do so smoothly and level off rather than overshoot the carrying capacity resulting in famine, disease, and conflict.

Introduction of a New Species......

into new areas is perhaps the greatest single factor to affect natural populations. More than 1500 insect species and more than 25 types of fish have been introduced into North America as well as more than 3000 species of new plants. The majority of accidental introductions may fail. However, once an introduced species becomes established, it's population growth is explosive. An example of a species introduced to the island of Newfoundland is the moose which have grown to a large population.

There are several other examples of species introduced in Newfoundland and Labrador. These include frogs and shrews as well as the Hemlock Looper and Spruce Budworm. Visit the following websites below to investigate the environmental, social, and economic impacts these non-native species have had on the provinces ecosystem.

• http://www.gov.nl.ca/snp/Animals/moose.htm
• http://www.gov.nl.ca/snp/Animals/snowshoe_hare.htm
• http://www.gov.nl.ca/env/Env/PollPrev/pesticides/protecting_the_forests.asp
• http://www.enature.com/fieldguide/showSpeciesGS.asp?curGroupID=5&curPageNum=7&recnum=MA0164

Activity

Text Book Reading:

Nelson Science 10 Concepts and Connections: Section 1.12; pages 38-41

Activity 1:

Complete the making a Histogram on page 39 of text which is supplemented in the Student Record of Learning on page 55-56

Activity 2:

Complete the Understanding Concepts on page 41 of text which is supplemented in the Student Record of Learning on page 57-58

Activity 3:

Complete question #21 a, b, and c page 62 of text

Activity 4:

Complete Black Line Master worksheets # 1.12a, 1.12b, 1.12c and 1.12d

Activity 5:

Examine the graph below and answer the questions that follow

1. hat does this graph represent?
2. What is happening to the moose and wolf populations from 1955 to 1995? Why do you think this is so?
3. Why is it important that there should be overall greater numbers of moose  than wolves?
4. 1. What happens to the wolf population as the moose  population increases and decreases?
   2. What do you think will happen to the numbers of moose if there were no wolves at all?
5. What is the name of the relationship that exists between moose and wolves?

Test Yourself

1. Which term best describes a population of flightless birds living on a remote island?
   1. closed
   2. open
   3. unstable
   4. evolving
2.  What mathematical expression best describes population growth?
   1. (deaths + emigration) - (births + immigration)
   2. (births + emigration) + (deaths + immigration)
   3. (births + immigration) - (deaths + emigration)
   4. (deaths + emigration) + (births + immigration
3. Which best defines carrying capacity?
   1. The single factor that limits the size of the population.
   2. The maximum number of individuals that the ecosystem can support over a relatively long period of time.
   3. The maximum number of reproductive episodes a population can undergo in one year.
   4. The range of abiotic factors within which an organism can survive.
4. Which factor is least likely to determine biotic potential?
   1. The maximum number of offspring per reproductive episode.
   2. The number of times an organism reproduces each year.
   3. The age at which reproduction begins.
   4. Favorable conditions of light and temperature
5. Which represents a density-dependent factor?
   1. forest fire
   2. disease
   3. flood
   4. volcano