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Populations

Unit 4 - Populations and the environment
1 Populations
Populations and ecosystems
Ecology - study of inter-relationships between organisms and their environment (abiotic +
biotic)
Abiotic - non-living components, e
...
temperature, rainfall, soil pH
Biotic - living components, e
...
competition, predation
Biosphere - life-supporting layer of land, air and water that surrounds the Earth
Ecosystem - all interacting biotic and abiotic features in a specific area
2 major processes:
• flow of energy through the system
• cycling of elements within the system
• Within an ecosystem are a number of species
...

• population - a group of interbreeding
organisms of the same species living in a
habitat
• The boundary between two populations may
be unclear
• community - all populations of different
organisms living and interacting in a particular
place at the same time
• habitat - the place where a community of
organisms live
• within each habitat are microhabitats, e
...
the mud at the bottom of the stream
• ecological niche - describes how an organism fits into the environment, i
...
where it
lives and what it does there
...

• No 2 species occupy exactly the same niche, some may show different tolerance levels
to environmental factors

1

Populations
Investigating populations
• As long as the samples taken are representative of the habitat as a whole, any
conclusions drawn will be valid
• Quadrats - 3 factors considered
1
...
number of sample quadrats to record within the study area - the larger the
number, the more reliable
...
position of each quadrat within the study area - to produce statistically
significant results, random sampling must be used
...
Lay out two long tape measures at right angles, along two sides of the study
area
2
...
Place a quadrat at the intersection of each pair of coordinates and record
the species within it
Systematic sampling across transects
It is sometimes more important to measure abundance and distribution in a systematic
manner especially where some form of transition in the communities of plants and animals
takes place
...
g
...
Organisms over
which the line passes are recorded
• belt transect - strip (1m wide) marked by putting a second line parallel to the first
...
g
...
This method is useful when the number
of organisms is hard to count (e
...
grass)
...
It is useful where a species is hard to count
...
The larger the sample, the more representative
2

Populations
Mark-release-recapture techniques
Animals are difficult to sample using the previous methods as they move away when
approached and others are often hidden making them difficult to find and identify
...
Some time later, a given number of individuals is collected randomly and
the number of marked individuals caught is recorded
...


Number marked in 1st sample (m1)
Size of whole population (N)

=

Number marked in 2nd sample (m2)
Total caught in 2nd sample (n2)

OR
Size of whole population (N)

=

Number marked in 1st sample X Total caught in 2nd sample
Number marked in 2nd sample (m2)

Assumptions:
• proportion of marked to unmarked in the second sample is the same as the proportion
of marked to unmarked in the population as a whole
• marked individuals released from the first sample distribute themselves evenly amongst
the remainder of the population and have sufficient time to do so
• the population has a definite boundary so that there is no immigration/emigration
• few deaths/births
• the method of marking is not toxic nor does it make the individual more liable to
predation
• the mark is not lost during the investigation
Analysing data
1
...
Comparisons are made more precisely using statistical analysis of the data - mean
and standard deviation is calculated
3
...
The factor of chance can be checked statistically and the results show whether these
differences are due to some particular factor or are just a matter of chance
5
...
Statistical tests can
be used to calculate the strength and direction if any correlation between two
variables (e
...
Spearman rank)
3

Populations
Ethics and fieldwork
To minimise the impact of an ecological investigation on the environment:
• Organisms should be studied in situ
...
Even if they are dead
...
LAG PHASE - the initially small number of individuals reproduce to slowly build up
their numbers
2
...
The population size doubles during each interval of time
...

3
...
The decline may be due to the food supply, limiting factors, or increased
predation
...


4

Populations
Population size
No population continues to grow indefinitely because certain factors limit growth, e
...
the
availability of food, light, water, oxygen and shelter, and the accumulation of toxic waste,
disease and predators
...
In plants and cold-blooded animals, temperature fall below
optimum, enzymes work more slowly so metabolic rate is reduced
...
At higher temps, enzymes work less efficiently as they
are denatured so again population grows more slowly
...
The
further the temp of the external temperature from their optimum, the more energy
organisms expend in trying to maintain their body temp; leaving less energy for
individual growth so they mature more slowly and reproductive rate slows
...

• Light - ultimate energy source for ecosystems
...
Populations that feed on
plants therefore also increases
...
A population of organisms in which the pH is
the closest to their optimum will be larger
...
Humidity affects
transpiration rates and evaporation of water from the body of animals
...

• Biotic factors
• Competition
A
...
The availability of these
resources determines the size of the population
...
g
...
Interspecific competition - individuals of DIFFERENT species compete for
resources such as food, light, water etc
...
If conditions remain the same, this will lead to the complete
removal of one species - the COMPETITIVE EXCLUSION PRINCIPLE
...

To show how a factor influences the size of a population, it is necessary to link it to the
birth rate and death rate of individuals in a population
E
...
the effects of intraspecific competition - the red squirrel and the grey squirrel have
been competing for food and territory
...

• Prey have become more adept at avoiding predators by: camouflage,
protective features (e
...
spines) and concealment behaviour
• Evidence collected on predator and prey populations in a lab does not
necessarily reflect what happens in the wild
• Predator-prey relationship
- Predators eat prey, reducing prey population
- With fewer prey, more competition between predators for remaining prey
- Predator population is reduced as some individuals are unable to obtain
prey for survival
- With fewer predators, fewer prey are eaten
- Prey population therefore increases
- With more prey, predator population increases
• In natural ecosystems, organisms eat a range of foods so fluctuations in
population size are less severe

• Disease and climatic factors also play a part
in the cyclic fluctuations
...
The population evolves to be
better adapted to the prevailing conditions
...
development of agriculture
2
...

Factors affecting the growth and size of human populations
• Birth rate and death rate
• Immigration and emigration

population growth = ( births + immigration ) — ( deaths + emigration )

Percentage population growth
rate (in a given period)

=

population change during the period
population at the start of the period

x 100

Factors affecting birth rates

Factors affecting death rates

Economic conditions - countries with lower income
tend to have higher birth rates

Age profile - the greater the proportion of elderly, the
higher the death rate

Cultural and religious backgrounds - some cultures
encourage large families while others oppose
contraception

Life expectancy at birth - residents in economically
developed countries tend to live longer

Social pressures and conditions

Food supply + diet

Contraception + Abortion

Safe drinking water and effective sanitation

Political factors - education and taxation policies

Medical care + education
Natural disasters
War

birth/death rate per 1000

=

number of births/deaths per year
total population in the same year

x 1000

7

Populations
Population structure
• In developed nations, there has been an increase in life expectancy
...
This change is called demographic
transition
...

• However it is uncertain that demographic transition will take place in all countries

8

Populations
• Age-population pyramids show the percentage of males and females in each age
group
• Stable population - birth rates and death rates balance so there is no increase or
decrease in population size
...
Typical of less developed countries
...
This occurs in certain
economically more developed countries such as Japan

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