Chapter 24 P1

Question 1

The human population is increasing at a significant rate.

Answer 1

• The global population has grown from one billion in 1800 to seven billion in 2012, and is predicted to reach 11 billion by the end of this century.
• Population growth like this cannot be sustained indefinitely as limiting factors, such as the availability of food, will prevent the population rising above a certain level.
• A limiting factor is an environmental resource or constraint that limits population growth.

Question 2

Population growth curve

Answer 2

If the growth of a new population over time is plotted on a graph, regardless of the organism, most natural populations will show the same characteristics. This is known as a population growth curve.
The graph can be divided into three main phases:

Question 3

Phase 1

Answer 3

a period of slow growth. The small numbers of individuals that are initially present reproduce increasing the total population.
As the birth rate is higher than the death rate, the population increases in size.

Question 4

Phase 2 -

Answer 4

a period of rapid growth. As the number of breeding individuals increases, the total population multiplies exponentially.
No constraints act to limit the population explosion.

Question 5

Phase 3

Answer 5

• a stable state.
• Further population growth is prevented by external constraints.
• During this time the population size fluctuates, but overall its size remains relatively stable.
• Birth rates and death rates are approximately equal.
• Slight increases and decreases can be accounted for by fluctuations in limiting factors, such the presence of predators.

Question 6

Limiting factors

Answer 6

• In theory, if all resources were in plentiful supply a population would continue to grow exponentially.
• However, this is rarely seen in nature.
• Instead, a short period of exponential growth occurs when conditions are ideal and the maximum growth rate is achieved.
• Limiting factors prevent further growth of a population and in some cases cause it to decline.
• Examples of limiting factors include competition between the organisms for resources, the build-up of the toxic by-products of metabolism, or disease.

Question 7

Limiting factors can be divided into abiotic and biotic factors:

Answer 7

• Abiotic factors - these non-living factors include temperature, light, pH, the availability of water or oxygen, and humidity.
• Biotic factors - these living factors include predators, disease, and competition.

Question 8

carrying capacity

Answer 8

The maximum population size that an environment can support

although individual years can show slight increases or decreases in population size. The population size remains stable as the number of births and deaths are approximately equal.

Question 9

Migration
Another important variable which affects population size is migration: includes

Answer 9

Immigration

Emigration

Question 10

Immigration

Answer 10

the movement of individual organisms into a particular area increases population size. For example, millions of Christmas Island red crabs (Gecarcoidea natalis) migrate each year from forest to coast to reproduce, dramatically increasing the coastal population of red crabs.

Question 11

Emigration

Answer 11

the movement of individual organisms away from a particular area decreases population size. For example, the Norway Lemming (Lemmus lemmus) emigrates away from areas of high population density or poor habitat.

Question 12

Density independent factors

Answer 12

• Density independent factors are factors that have an effect on the whole population regardless of its size.
• These can dramatically change population size.
• These factors include earthquakes, fires, volcanic eruptions, and storms.
• In some cases, these factors can remove whole populations of a species from a region.

Question 13

Human population growth

Answer 13

For many years the human population remained fairly stable, as is the case for other natural populations. The population was kept in check by limiting factors.

The development of agriculture, the industrial revolution, and advances in medicine have led to a population explosion. Like other species, the growth of the human population is a result of the imbalance between birth rate and death rate.

Question 14

Scientists use a measure to calculate how much a population has grown in a certain period of time. This is calculated using the formula:

Answer 14

Question 15

human population

Answer 15

• A number of factors affect the birth rate of the human population.
• In addition to ‘natural’ limiting factors, economic conditions, cultural and religious backgrounds, and social pressures can limit - or encourage - the birth rate.
• Different factors affect the death rate in a population.
• These include the age profile of the population (in general, the greater the proportion of elderly people, the higher the death rate), the quality of medical care, food availability and quality, and the effects of natural disaster or war.
• The future size of a population depends upon the number of women of child-bearing age.
• The age and gender demographic of a population can be represented using an age population pyramid,

Question 16

Types of competition

Answer 16

Competition is an example of a biotic limiting factor - it is a result of the interactions between living organisms. There are two main types of competition:
1 Interspecific competition - competition between different species.
2 Intraspecific competition - competition between members of the same species.

Question 17

Interspecific competition

Answer 17

• Interspecific competition occurs when two or more different species of organism compete for the same resource.
• This interaction results in a reduction of the resource available to both populations.
• For example, if both species compete for the same food source, there will usually be less available for organisms of each species.
• As a result of less food, organisms will have less energy for growth and reproduction, resulting in smaller populations than if only one of the species had been present.
• If two species of organism, however, are both competing for the same food source but one is better adapted, the less well adapted species is likely to be outcompeted.
• If conditions remain the same, the less well adapted species will decline in number until it can no longer exist in the habitat alongside the better adapted species.

Question 18

competitive exclusion principle

Answer 18

where two species are competing for limited resources, the one that uses the resources more effectively will ultimately eliminate the other.

Question 19

Red and grey squirrels in the UK

Answer 19

• An example of interspecific competition is the competition between red and grey squirrels for food and territory in the UK.
• In the 1870s the grey squirrel, a native of North America, was introduced into the wild in the UK.
• Its population quickly increased in numbers and resulted in the native red squirrel disappearing from many areas.
• This is primarily because the grey squirrel can eat a wider range of food than the red squirrel and as it is larger it can store more fat.
• This increases its chances of survival and therefore its ability to reproduce thus increasing its population.
• An increasing population of grey squirrels further reduces the food supply available to the red squirrels, reducing their ability to survive and reproduce.

Question 20

Intraspecific competition

Answer 20

• Intraspecific competition occurs when members of the same species compete for the same resource.
• The availability of the resource determines the population size - the greater the availability the larger the population that can be supported.
• This results in fluctuations in the number of organisms present in a particular population over time.
• The effects of intraspecific competition on a population are represented in Figure 3.

Question 21

effects of intraspecific competition on a population stage 1

Answer 21

• When a resource is plentiful in a habitat (such as food or space), all organisms have enough of the resource to survive and reproduce.
• This results in an increase in population size.

Question 22

effects of intraspecific competition on a population stage 2

Answer 22

• As a result of the increased population, there are many more individuals that share the food or space available. Resources are now limited; not enough is available for all organisms to survive. The population will decrease in size.

Question 23

effects of intraspecific competition stage 3

Answer 23

Less competition exists as the smaller population means less organisms are competing for the same resources. This means more organisms survive and reproduce, resulting in population growth.
This cycle of events will then repeat.