4.5 FACTORS CONTROLLING POPULATION SIZE
1. Ecosystems are dynamic and subject to change.
2. Candidates should be able to define the term ‘population’.
Number of Organisms of the same species in one given area.
3. Population numbers will fluctuate and this is dependent on various factors.
Biotic/Abiotic Factors (10-12)
For a specific population, there is a SET POINT, A certain equilibrium population that the environment
can support for. Population Oscillations are regulated by NEGATIVE FEEDBACK.
4. Population numbers are dependent on the birth and death rates, and immigration into the
population and emigration away.
Once a population is established, the growth of that particular population is determined by the birth and
death rates. However, individuals can leave and enter a population (Except Microbes under Aseptic
Conditions). Immigration is the movement of individuals INTO a population, whilst emigration is the
movement OUT of a population. Migration is periodic, Emigration is one way process. Overcrowding and
other factors may act as a stimulus for Emigration.
5. Populations will increase in size whilst death rate is lower than birth rate.
Log Phase
6. Some factors will slow down population growth rate and some will cause a population crash.
Sigmoid curve – Shows effect of limiting factors
J Curve – No Limiting Factors , Catastrophic Decline – BOOM or BUST.
7. Candidates should be able to draw a generalised graph of population growth (sigmoid growth
curve) and label the stages of growth as shown by a liquid culture of yeast or bacteria.
Lag Phase – Little or no cell growth
- Time for adjusting/adapting to medium
-creating the essential enzymes needed
-Protein Synthesis
Log Phase – Exponential Cell Growth
-Plentiful supply of Nutrients
-No Limiting Factors
Stationary Phase – Cell growth = Cell Death
-Equilibrium between birth and death established
-one or more limiting factor(s) in place
Death phase – Decrease in Numbers
-Dying > Birth.
-Increasing limiting conditions
-adverse conditions.
8. Candidates should be able to plot graphs of population growth rate when provided with
appropriate data.
9. Candidates should be able to interpret graphs of changes in population growth rate.
10. Weather, predation, parasitism (disease), food supply, living space and competition may affect
population growth.
Environmental Resistance – Factors which affect population growth
11. The effect of density dependent factors varies with the size of the population whereas the effect
of density independent will be the same regardless of the population size .
Density Dependent – Varying Effects dependent
-Competition B
on the size of the population
Density Independent – Same Effects irrespective
-Disease
B
of population size.
-Parasites
B
-Weather
A
-Food Supply B
-Temperature A
-Floods/Drought
A
-Habitat Disruption. A
Biotic Factors – LIVING
Abiotic Factors – NON LIVING
12. Candidates should be able to define the term ‘carrying capacity’; explain how this is dependent on
the availability of resources (which therefore act as density-dependent factors); how populations
might then fluctuate about this set point.
Carrying Capacity – Limit to the number of individuals an area can support.
-It has reached its maximum size due to limiting factors
13. Candidates should be able to describe the relative advantages and disadvantages of chemical and
biological control in terms of efficiency, environmental damage, ease of use and effects on human life.
ADVANTAGES
DISADVANTAGES
-Highly Specific
-Slow to build up in number
-Long Term (once Equilibrium Est.) -Relatively few successful examples
BIOLOGICAL -Inexpensive in the Long Term
-Expense – High Level of Skill Needed
CONTROL
(Expensive to start up)
-Detailed Knowledge of the life cycle needed
-No Environmental Contamination -Agents aren’t known for all pests
-Can be used in glass houses
-Frequent input needed
-Little use to individual producers.
-Effective
-Non Specific = Potentially eradicate beneficial
-Quick
insects
CHEMICAL
-Relatively Cheap
-Potential resurgence of the pest
CONTROL
-Small Scale
-May become resistant to pesticide
-Low Skill Level Needed
-May Kill Fish/Birds/Mammals
-Long Term Exposure harm Humans.
14. The advantages of chemical control include rapid eradication of a pest over a specified localised
area and relative cost effectiveness. The disadvantages include the development of resistance and
toxicity risks to non target organisms.
15.The advantages of biological control include its specificity, with no resistance and no
environmental damage or residues. A disadvantage is the initial cost of the research required to
evaluate the agent’s biology, in order to introduce adequate numbers into the pest population at the
optimum time. Growers must be convinced that prevention of financial loss does not require
complete eradication, but that the maintenance of control may necessitate repeat application.
16. Candidates should understand what is meant by organic
breakdown and describe its importance to the ecosystem in the
recycling of mineral nutrients.
Chemical elements are available only in LIMITED amounts, and so
life depends on the recycling of essential chemical elements.
17. Microorganisms play an important role in the process of
decay, releasing compounds of essential chemical elements from
the bodies of dead organisms.
18. Candidates should be able to draw a labelled diagram of the
carbon cycle linking the processes of photosynthesis, respiration,
decomposition, fossilisation and combustion (link to 5.8).
Photosynthesis
- Plants take up CO2 and use it to make sugars
Respiration
-Breakdown of Sugars to release energy and produce CO2 as a by product
Decomposition
-Bacteria and Fungi break down dead and decaying matter
Fossilisation
-Plants and Animals die but don’t decay. Action of heat and pressure over millions of years create
fossils/fossil fuels
Combustion
-Car Engines/Factories etc. Burn the Fossil Fuels and release the CO2 back into the environment
19. Nitrogen is found in all amino acids from which proteins are made. Nitrogen is available to plants
only in the form of ammonium and nitrate ions; these ions being taken up by the roots.
Nitrogen is needed to make Amino Acids, Nucleic acids and Proteins.
20. Candidates should link the uptake of nitrates with protein synthesis and the synthesis of nucleic
acids.
21. The nitrogen cycle is the flow of organic and inorganic nitrogen within an ecosystem where there
is an interchange between nitrogenous compounds and atmospheric nitrogen.
22. The main processes of the nitrogen cycle are putrefaction, nitrification, nitrogen fixation and
denitrification.
Putrefaction
-Bacteria and Fungi decompose dead plants and
animals, faeces and urine into ammonium ions ( NH4+).
Nitrification
-Ammonium is converted to Nitrites, then Nitrates
by Nitrifying Bacteria. These Bacteria require AEROBIC
conditions.
Nitrogen Fixing
-Atmospheric Nitrogen is converted straight to
nitrogen compounds by nitrogen fixing bacteria. There
are Free Living and Symbiotic Nitrogen Fixing Bacteria
Denitrification
- Nitrogen is lost from the Ecosystem by Denitrification. This is a problem in waterlogged soils.
This requires ANAEROBIC conditions.
23. Candidates should explain the activities of nitrifying, denitrifying and nitrogen-fixing bacteria and
should know the generic names Nitrosomonas, Nitrobacter, Azotobacter and Rhizobium.
Nitrobacter – Nitrification – Nitrites to Nitrates
Nitrosomonas – Nitrification – Ammonium to Nitrites
Azotobacter – Nitrogen Fixing – Nitrogen to Ammonium *Free Living
Rhizobium – Nitrogen Fixing – Nitrogen to Ammonium *Symbiotic in the root nodules of Legumous
plants.
Pseudomonas – Denitrifying – Nitrates to Nitrogen
24. Ploughing and drainage are important in aeration of the soil, producing the aerobic conditions
required by nitrifying bacteria.
Denitrification requires ANAEROBIC conditions. This means that in waterlogged soils, Denitrification is
favoured more than nitification, and so Nitrates/Nitrites/Ammonium is converted back to atmospheric
nitrogen.
Humans can also improve the circulation of Nitrogen by
-Fixing atmospheric nitrogen artificially using chemical processes that convert it to fertilizer
-large amounts of animal waste used as manure
-Sewage disposals boost Nitrogen Supplies
-Farming Practices such as
-Planting fields with Clover (legumous plant) to encourage nitrogen fixing
-Draining land and reducing anaerobic conditions
-Ploughing fields in order to improve aeration of the soil.