Population Ecology (pp 280 – 288)
Learning outcomes
1. Define the biological term “population”.
2. Describe the attributes that characterize
population.
3. Describe the factors that determine
population structure.
4. Interpret survivorship and population
growth curves.
• Population - A population is a group of
organisms which belong to the same
species and live in the same location, e.g.
all of the sperm whales living off the coast
of Kaikoura.
• Species - Males and females of a species
are capable of breeding with each other
and the offspring are fertile.
Attributes or Features of Population
• Population size or Total abundance number of members in a population.
• Population range – the area in which
members of a population are found.
Boundaries are determined by limiting
factors (these are usually environmental
factors that a species needs to survive).
Attributes or Features of Population
• Population structure – determined by
density, spacing/distribution and age
structure.
Density
• This is the number of individuals of a
population per unit area/volume
e.g. tigers per square kilometre, mussels
per square metre, paramecium per cubic
ml of water, etc.
Distribution / Spacing
• Random distribution – The presence of
one individual does not directly affect the
location of another individual. Uncommon
in animals, but often seen in plants.
• Clumped distribution – Individuals tend
to stay in groups.
• Regular or Uniform distribution –
Occurs in species that keep and defend
territories.
Age structure (pp 283-284)
• The proportion or percentage of individuals in
each age group in the population.
• Population is usually divided into three groups:
* pre-reproductive
* reproductive
* post-reproductive
Survivorship (pg 286)
Some populations have many young but few
survive e.g. fish, turtles
Some have few young but most survive to old
age e.g. elephants, humans
In some populations the rate of loss is almost
constant at all age groups e.g. rodents
Survivorship Curves
These are graphs that tell us how long
individuals tend to survive in a population.
• Type 1 – Mortality is low in early stages.
• Type 2 – Mortality is relatively constant at
all life stages.
• Type 3 – Mortality is high in early life
stages.
Survivorship Curves
•Type 1
•Type 2
•Type 3
Population dynamics / change (pg 285)
•
•
•
•
Natality (birth rate)
Mortality (death rate)
Immigration (entering a population)
Emigration (leaving a population)
Population growth =
Births – Deaths + Immigration - Emigration
Population dynamics may be
apparent from age structure
• Pyramid shaped – increasing population
• Bell shaped – stable population
• Urn shaped (small at the bottom) –
contracting population
Carrying capacity
• All populations are capable of increasing
in numbers as long as there are sufficient
resources and other factors do not limit the
population.
• The size of a population present at any
given time in an area is a balance between
its reproductive potential and the
environmental resistance. This balance
is called the carrying capacity.
• Reproductive potential – The maximum ability
of organisms to reproduce themselves under
optimum conditions.
• Environmental resistance – amount of
resources available in the environment, range of
abiotic factors such as temperature, humidity,
etc.
• Carrying capacity – the maximum size of a
population, determined by the interaction of the
above mentioned two factors.
Population growth curves (pg 287)
• The way a population grows is shown on a
graph where the population size is plotted
against time.
Population size is limited by: (pg 282)
density-dependent factors
•
•
•
•
•
Disease
Predators
Parasites
Food supply
Intraspecific (within
species) Competition
– The greater the population,
the greater effect these
factors have.
– Eg. Black plague in the
Middle Ages – more deaths
in cities
density-independent factors
Catastrophic events
– Volcanic eruption, storms,
floods, drought
Physical Factors
– Rainfall, temperature,
salinity, acidity
– Chemical pesticides
– Major habitat disruption
(clear felling for logging,
mining, oil spill etc.)
• Most are abiotic factors
Factors affecting population distribution
1. Tolerance – ability to survive variation in
environmental conditions.
Optimum range: best environmental
conditions for population
Zone of physiological stress:
uncomfortable but survivable.
Lower/upper limit of tolerance: you
die!
Acclimation
• If conditions change slowly it is possible
for the tolerance of an organism to
change.
Factors affecting population distribution
2. Liebig’s Law of the Minimum
‘the functioning of an organism is limited
or controlled by whatever essential
environmental factor is present in the
least favourable amount’
•
Scurvy, bush sickness
Factors affecting population distribution
3. Geographical barriers
Populations may not be present in a
location just because they can’t get there
due to a physical barrier such as ocean
or mountain.