Populations & Communities

• What is a population?

  A population is a group of individuals of the same species which occupy a particular habitat.
• What can happen to population numbers?

  Population numbers can remain relatively stable, or can change over time.
• In what conditions will species of the same population reproduce?

  As they are of the same species, they will reproduce as long as there are available resources e.g. nutrients, but also compete (intra-specific) if resources are in limited supply.
• What is an example of a population growing in closed conditions?

  The growth of bacteria or yeast in a nutrient medium in a beaker.
• What does the growing of yeast or bacteria in closed conditions produce in a graph? (2)

  - This produces the characteristic pattern of population growth, which is measured as the change in the numbers of organisms.

  
  

  - Remember that the time frame involved will vary according the species.
• What phases is population growth split into?

  Population Growth can be divided into four main phases as shown in the graph

  
• What is the lag phase?

  This is the time taken for nutrients to be used (assimilated) as well as the production of new offspring (e.g. egg production; egg/larvae development; gestation period in mammals).
• What is the exponential/log phase?

  Each new member of a population has the reproductive capacity to generate more individuals (e.g. 1 bacterium divides to form 2, these 2 divide to form 4 and so on).
• What is growth like during the exponential phase?

  In this phase, growth is rapid as there is little competition since there are sufficient resources.
• How do the numbers increase in the exponential phase?

  The numbers increase by the intrinsic rate of natural increase (r), as the population is exhibiting its biotic potential - the reproductive capacity of a population under optimum conditions.
• What is the stationary phase?

  The increased numbers in the population result in environmental resistance.
• What is the stationary phase caused by? (3)

  - An accumulation of waste

  - Lack of resources (e.g. nutrients)

  - Increased competition (in this case, intraspecific competition)
• What is the result of environmental resistance?

  The result of environmental resistance is a reduction in birth rate and an increase in death rate - the outcome being the population number remaining constant.
• What is the decline phase? (3)


  - The reduced availability of resources as well as an increase in toxic waste materials results in birth rate falling to zero.

  
  

  - There is also an increased death rate.

  
  

  - This results in a steep decline in population numbers (they "crash")
• What has the population said to have reached?

  The population is said to have reached its carrying capacity (K) - the maximum number of organisms the environment can support.
• What does the sigmoidal curve apply to?

  This sigmoidal curve applies to the populations of many species in particular circumstances such as when colonising a new area, or the time to grow and reach sexual maturity.
• Which phase do most populations stay in?

  Most populations however, will stay in the stationary phase and not reach the decline phase.
• What shape of growth curve is possible in theory + what factors would have to be in place?

  In the absence of any limiting factors a J- shaped population growth curve is theoretically possible - this represents the biotic potential of the population.
• What could a J-shaped curve represent? (2)

  - Yeast growth in an open (continuous) culture where waste and dead cells are constantly removed + fresh medium is constantly added (Graph A).

  
  

  - A population of mammals where they are protected from their natural predators to encourage population growth (Graph B).

  
• What does graph A illustrate + what happens if a larger flask or extra resources are provided? (2)

  - Graph A illustrates the carrying capacity of a population of yeast.

  
  

  - If we provided extra resources, or a larger volume of medium was provided (a larger flask) this would dilute the waste/toxins produced as well as providing more resources and so there would be a higher carrying capacity.

  
• How can you find out what causes a plague of mice or locusts?

  We have to find out what causes the changes in the numbers of births or deaths.
• Give an example of causes of fluctuations in population using birds. (3)

  - The huge increases in the number of birds such as great tits following a bumper crop of beech seeds, the staple winter food of this bird.

  
  

  - The extra food availability following a bumper crop means that fewer juvenile birds will starve to death in the winter, therefore more birds will survive until the following spring to breed and the population will increase in size.

  
  

  - Therefore in boom years the number of births exceeds the number of deaths.
• What does the past example of population fluctuation highlight?

  This highlights how the type of resource can influence the population.
• If resources are renewable, what is likely to happen to the population?

  Leaf fall in deciduous woodland providing a constant food supply for detritivores or earthworms, then the population is more likely to remain in the stationary phase for longer (a stable population).
• What may a J-shaped curve also be characteristic of?

  A J shaped curve is also characteristic of many protoctistan populations (e.g. algae).

  
• Explain the J-shaped growth curve for a protoctistan population during spring.

  There will be a very rapid growth in spring as there is abundant nutrient availability in the water, both temperature and light levels are increasing and there are relatively few herbivores (zooplankton) in the water at his time of year.
• Explain the J-shaped growth curve for a protoctistan population during midsummer.

  In midsummer, the population may crash (with no stationary phase) and fall rapidly as the nutrient supply becomes exhausted, but the herbivores also increase in number and accumulate waste.
• What was a key determinant of growth pattern in the prior example.

  Resource availability is a key determinant of the growth pattern.
• What is the influence of temperature on populations?

  Temperature is not a resource, but will determine the metabolic rate in organisms and so the rate at which they develop.
• How does temperature affect laboratory populations? (2)

  - In laboratory populations it can be demonstrated that the rate of increase (in the exponential phase) will rise at higher temperatures, but that a higher temperature will not influence the size of the maximum population (in the stationary phase).

  
  

  - This is determined by the resources such as available nutrients.
• How does temperature affect populations in warm springs?

  A warm spring will produce rapid increases in insect populations which will then benefit the growth of populations of insectivorous birds.
• What are the 4 factors affecting population growth + ∴ population size?

  - Birth rates (B)

  - Immigration (I)

  - Death rates (D)

  - Emigration (E)
• How is change in population size is estimated?

  (births + immigration) - (death + emigration)
• What equation will a population in equilibrium have?

  Births + Immigration = Deaths + Emigration
• Give three examples for which the equation can be applied to?

  - In bacteria cultured in a laboratory, migration is not an issue, so the balance is dependent upon the births and deaths.

  
  

  - The rapid increase in the populations of migratory bird species in spring and summer shows an obvious seasonal effect on population size and is a combination of both births & migration.

  
  

  - For other species, the seasonal effect is determined by the balance of births and deaths.

  - Many species of animals give birth in spring or early summer so there are large populations at these times of year, when temperatures are suitable, and resources are plentiful.
• How can seasonal effects be represented?

  Seasonal effects can be represented by survivorship curves, which show the percentage of individuals surviving over a year, as the season's progress.
• What can other survivorship curves show?

  The number of individuals of a particular species surviving over a period of years.
• What does the graph show? (2)

  - A typical survivorship curve for tawny owls over their first 5 years of life.

  
  

  - % survival rates represent the number of chicks from one particular year that survive over successive years as they age.

  
• What other factors may contribute to fluctuations in population size?

  Population sizes can also change from year to year, and could be due to the normal predator prey relationship (see later), changes in food supply, or abiotic factors (e.g. severe temperature changes.)
• What does r & K-selection relate to

  The selection of traits which promote success in particular environments and originates from work on island biogeography by MacArthur and Wilson.