Life Sciences
MATERIALS
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Population fluctuation and regulation
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Carrying capacity – maximum number of individuals of a
species that an ecosystem can continually supply resources to.
Certain factors prevent a population from unlimited growth and
exceeding its maximum growth rate – limiting factors and
together are called environmental resistance.
Seasonal fluctuations – Swallows fly from the cold European
winter to Southern Africa where the summer temperature is
more favourable.
Annual fluctuation – Migration of millions of wildebeest in East
Africa. This phenomen is determined by the availability of
grazing which is also dependant on rainfall.
J Gerber and J Goliath
Growth patterns/forms – Geometric or
J – shaped curve
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Population size increases
unrestricted and rapidly with
time.
A population can reach its full
reproductive potential because
of enough food, water and no
predators.
Environmental resistance
increases when numbers of
population increase.
Number of population can
decrease rapidly due to limiting
factors.
J Gerber and J Goliath
Graph of repeated growth pattern
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J Gerber and J Goliath
Logistic growth form (S - shaped)
Lag phase:
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Population acclimates, individuals
become sexually mature and seek
partners for mating.
Accelerating growth phase
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Population grows rapidly and
reaches its maximum growth rate.
Deceleration growth phase
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Growth rate decreases due to an
increase in environmental
resistance.
Equilibrium phase
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Population numbers reach the
carrying capacity.
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J Gerber and J Goliath
Geometric and logistic growth phase
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J Gerber and J Goliath
Methods to determine population size direct technique (census)
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Count of all individuals in a population and
includes human population census.
Aerial photography may be used in the
counting of larger animals in nature reserves.
J Gerber and J Goliath
Indirect technique – simple sampling
(quadrated method)
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A square frame (quadrat) is used
Place it on the ground and count the individuals
inside the quadrat
It gives the number of individuals (density) per unit
area
Repeat in different places at random and calculate
the average density
Method is used for plants and slow moving animals
J Gerber and J Goliath
Mark-recapture technique (Peterson)
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Demarcate a specific area.
Capture a number of individuals, count and mark them (first
sample).
Set the marked individuals free in the area.
Allow the marked individuals enough time to mix with unmarked
individuals in the area.
Capture a second number of individuals and count them
(second sample).
Count the number of previously marked individuals in the
second sample.
J Gerber and J Goliath
Mark-recapture technique
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Use this information to calculate the total size of the population
by means of the following formula:
P=mxs
t
P = estimated population size.
m = total number of animals captured and marked in first
sample.
s = total number of animals captured in second sample.
t = number of marked individuals in second sample.
J Gerber and J Goliath
Mark-recapture technique - validity
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Marking methods should not injured the organism.
Marks should be clear for the duration of the investigation.
Marking methods should not affect the movement of the
animals.
Marked organisms should be allowed enough time to mix.
Population must be closed.
Period between two samplings should be sufficiently brief – no
births or deaths.
J Gerber and J Goliath
Human population age and gender
distribution in:
1. an increasing population
A population pyramid with a small number of old people
indicates a population with a high birth rate, a high death
rate and a short life expectancy. This pattern is typical of
less economically developed countries (LDC) like South
Africa, South America and Asia (excluding Japan)
2. a stable population
There is approximately the same number of young people
and old people. About the same number of children is born
each year compared to the number of people who die
each year. Economical developed countries like Ireland
have this kind of pyramid
3. a decreasing population
There are more old people than young people. Each year
more people die than are born. Developed countries like
Germany have this kind of pyramid. Some southern African
countries, like Botswana (experiencing the effects of
HIV/AIDS) are also starting to show this kind of age-gender
pyramid.
Use these graphs to discuss the changing trends in
the SA population. Indicate the working group (20
– 60 years), mention how their economical
contributions will support the non-economical
groups (children and old people). Take the % of
jobless people into consideration and the fact that
only 5.6% of the population pays tax
Interactions within a Community
The five main types are:
 Predation
 Competition
 Parasitism
 Mutualism
 Commensalism
PREDATION
In predation, one
individual, the predator,
captures, kills, and
consumes another
individual, the prey.
Predators, Prey, and Natural Selection
 Natural Selection favors
adaptations that improve a
predator‘s efficiency at finding,
capturing, and consuming
prey.
 These adaptations include a
shark’s jaws, a scorpion’s
claws and stinger, and a
spider’s web and fangs
Prey-predation interaction
A
Graph interpretation
 The cycles of increase and decrease reflects a
predator –prey intercation
 e.g lions feeding on impalas, lynx feeding on
rabbits
 Prey population reaches higher density than
predator population
 Portion A = geometric phase- rapid increas of
prey, predator pop too low to hinder increase
 More prey means more food for predators and
the population increases
 More predators = increase in mortality of prey
 Less prey could cause predators to emigrate / die
, hence prey population can increase again
Competition
 Competition occurs when organisms in the same
community seek the same limiting resource.
This resource may be prey, water, light,
nutrients, nest sites, etc.
 Competition among members of the same
species is intraspecific.
 Competition among individuals of different
species is interspecific.
Intraspecific Competition
Competition between
organisms of the
same species
Interspecific Competition
Crocuta
crocuta
Competition between
organisms of the
different species
Panthera leo
Outcomes of Competition
 Niches of similar species may overlap. Two
species cannot compete for the same
limiting resource for long.
 eventually one species outcompetes the
other
 One species survive, other emigrates OR
dies out
 Even a minute reproductive advantage
leads to the replacement of one species by
the other.
 This is called the COMPETITIVE
EXCLUSION PRINCIPAL.
Evidence for
Competitive
Exclusion
the Russian ecologist, G.F.
Gausse demonstrated that
Paramecium aurellia
outcompetes and displaces
Paramecium caudatum in
mixed laboratory cultures,
apparently confirming the
principle.
Resource Partitioning
When two or more similar
species coexist, such as
these varieties of
warbler, each species
only uses part of the
available resources. This
is called resource
partitioning.
(species sharing
resources)
PARASITISM
 Parasitism is a species interaction that
resembles predation in that one individual is
harmed while the other benefits.
 However, in parasitism, the parasite feeds on
the host individual.
 This does not result in the immediate death of
the host. Rather, the parasite may feed on the
host for a long time instead of killing it.
Ectoparasites
 Ectoparasites are
external parasites.
 They live on their
hosts body, but do
not enter it.
 Examples include
ticks, fleas, lice,
lampreys, leeches
and mosquitoes
Ectoparasites
Endoparasites
 Endoparasites are
internal parasites,
and live inside the
host’s body
 Endoparasites
include bacteria and
other microorganisms, and
many worms
Resource partitioning amongst plants
Different species
of plants in the
same habitat will
compete for the
same resources
like light, water,
mineral salts, etc.
Different species
of plants grow to
different heights
or have roots that
are different
lengths so they
divide the
resources,
accessing them in
slightly different
ways
Resource partitioning: co-existing
shore birds
Human influence on community
structure
The elephant herd in the Kruger National Park
increased from 8, 000 to 12,500 in 2008 and to
19 000 in 2009. The elephant population is
increasing by 7% per year, and might reach 20 000
by 2012. This large herd cannot be sustained since
adult elephants consume 130 kg food a day and
they live for 55 – 65 years.
On 25 February 2008 the SA Government finally
concluded it would have to lift a 17 year-old
moratorium on the culling of the native elephant to
cope with its booming population. Minister van
Schalkwyk announced that killing of excess animals
would only be allowed once all other options
(translocation and contraception) had been ruled
out.
Minister van Schalkwyk said:”Our simple reality is
that elephant population density has risen so
mulch in some southern African countries that
there is concern about impacts on the landscape,
the viability of other species and the livelihoods
and safety of people living within elephant ranges.”
Mutualism
 Mutualism is a symbiotic relationship in which
both members of the association benefit.
 Often help organisms obtain food or avoid
predation.
 Bacteria in human intestinal tact.
 Need not be equally beneficial to both species.
 Cleaning Symbiosis
Pollination
 Pollination is the
most important of
the mutualistic
relationships.
 The plant
provides food for
the pollinators,
which in turn
carries the pollen
to another flower
COMMENSALISM
Is a relationship in which one species benefits
and the other is not affected
Succession
Primary succession begins in areas
consisting of bare, lifeless substrate such as
rocks or a car path. Organisms gradually
move into the area and begin to change its
nature,
Secondary succession occurs when a established
community has been disturbed in a catastrophic
manner, e.g. after a veld fire or a flood. In the
disturbance all the vegetation is destroyed, but all
or some of the soil remains. The same process
occurs as in primary succession, but as there is
soil for grasses and small plants to grow in, these
plants form the pioneer communities.
Pioneer plants
Lichens are pioneer plants as they are the first
organisms to colonise a bare area. Acidic
secretions from the lichens help to break down
the hard surface of the rocks and slowly bits of
soil accumulate, mosses may grow on these
small pockets of soil, enriching the quality and
quantity of the soil with the organic material
that they add to it. As time passes and the soil
becomes richer and deeper, other plants like
grasses and small herbaceous plants become
established in the larger pockets of soil , small
animals may also move into an area when
these plants become established.
An example of primary succession
Pioneer grass on a sand dune
Climax community
PHOTOTROPISM
EXPERIMENTS
DATA RESPONSE