Chapter 6 - Populations and Communities Flashcards
What is a population?
A group of organisms of the same species living in a particular area.
How can population numbers vary?
Population numbers can remain relatively stable or can grow or decline over time.
What factors influence population growth?
Birth rate
Death rate
Immigration
Emigration
What is one of the most studied examples of population growth?
The growth of bacteria (or yeast) in nutrient medium in closed conditions, for example, in a beaker or conical flask.
One of the most studied examples of population growth is the growth of bacteria (or yeast) in nutrient medium in closed conditions, for example, in a beaker or conical flask. Draw a graph showing the characteristic pattern of population growth typically seen in these circumstances.
Textbook page 89
What units is population growth measured in?
Population growth (or decline) is measured as the change in the number of organisms
How does the population of bacteria in a nutrient medium in closed conditions increase?
Bacteria will divide (split) to produce two new daughter cells.
One of the most studied examples of population growth is the growth of bacteria (or yeast) in nutrient medium in closed conditions. How many distinct phases can the pattern of growth be divided into?
Four
One of the most studied examples of population growth is the growth of bacteria (or yeast) in nutrient medium in closed conditions. What are the four distinct phases that the pattern of growth can be divided into?
Lag phase
Exponential (or log growth) phase
Stationary (or stable) phase
Decline phase
What is the lag phase?
• In this phase there will be a very slow increase in number (numbers may even decrease for a time). This is a stage when nutrient assimilation takes place - this may involve the bacteria activating genes and producing the appropriate enzymes to metabolise a particular food substrate.
What is the exponential (log) phase?
- The bacteria divide exponentially.
- There is no restriction to growth (for example, abundant resources present and insignificant waste accumulation) and the bacteria can divide to produce new bacteria at the maximum rate.
- A bacterium can divide to produce two new bacteria as often as once every 20 minutes and consequently the increase in numbers can be exponential.
What is the stationary (stable) phase?
- In this stage food supplies may begin to become limiting so the number of new individuals produced falls.
- Waste products and toxins may have also accumulated to a level that restricts growth.
- During the stationary phase the ‘birth’ and death rates approach equilibrium.
What is the decline phase?
- The death rate exceeds the birth rate and the population declines, sometimes very rapidly in a population ‘crash’.
- In the bacterial population discussed as our example, this can be due to the accumulation of toxic waste and/or the nutrient supply running out.
What stages comprise the sigmoidal (S-shape) growth curve of a population?
The lag phase
The exponential phase
The stationary growth phase
When does the sigmoidal (S-shape) growth curve comprising the lag, exponential and stationary phases of growth apply to the population of a species?
In particular circumstances, such as when colonising a new area.
The populations of different species all share the same characteristic growth phases and principles with reference to population growth. However, different factors can contribute to particular parts of the growth curve for different species. Give some examples of these factors.
For example, the lag phase can be due to the time taken for egg or larval production or the gestation period or even the time to grow and reach sexual maturity.
Why do most natural populations remain in the stationary phase?
Most natural populations are most likely to remain in the stable phase rather than progress into the decline phase - a phase typically associated with ‘closed’ conditions.
What scale can be used when representing the change in numbers of microbes over time?
A logarithmic scale
Why must a logarithmic scale be used when representing the change in numbers of microbes over time in certain circumstances?
As the increase can be over several orders of magnitude.
How is the change in numbers of microbes over time responded on a graph?
- Typically the time (on the x-axis) is scaled as normal but number of microbes (on the y-axis) has a log scale.
- Special ‘semi-log’ graph paper can be used for this purpose.
Outline of semi-log graph on textbook page 90
How can a normal graph be distinguished from a semi-log graph?
Exponential growth is seen as a straight line only when using a log scale on the y-axis
Define the term ‘biotic potential’
The maximum rate of growth of the population as seen in the exponential phase reflects the population’s biotic potential - the reproductive potential (rate) of a population under optimum environmental conditions with unlimited resources.
Define the term ‘environmental resistance’
- The environmental restrictions on population growth, for example, as evident in the stationary phase, create what is described as an environmental resistance.
- Environmental resistance is the restriction by the environment on the population reaching its maximum growth rate and its biotic potential.
Why does environmental resistance occur?
- Environmental resistance can be due to many factors including nutrient shortage or accumulation of waste but also climate, competition from other organisms, predation and disease.
- The factors that influence populations can be grouped into two main categories.
- Abiotic factors are factors in the chemical or physical environment and are loosely referred to as non-living; examples include water, nutrient, light and oxygen availability.
- Biotic factors are the effects of other organisms whether the same or other species, for example, food supply, competition, predation.