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Community structure
The following organisms form part of a community
A community is a
group of
populations
(producers,
consumers and
decomposers)
living together in
a habitat
Predatorprey
relationships
Competition
1. Intraspecific competition: the competition between
organisms of the same species depending on the same resources
like food, space, shelter, water and access to mates.
2. Interspecific competition: the competition between
organisms of different species depending on the same resources
e.g. light, space, water, shelter, food
Competition: Intraspecific competition
Intraspecific competition: males
compete for territory.
Interspecific
competition:
In the African
savanna, hyenas
and vultures
compete with
one another for
the flesh of dead
animals such as
the dead
elephant
pictured here.
Interspecific competition
Lion and hyenas fighting for the same food
resources
Jackal and a hyena fighting for the
same food source
Specialisation for specific modes of life
When two similar species co-exist, they will start to specialize in their habitats and feeding
habits and undergo character displacement to reduce the severity of interspecific
competition.
This is demonstrated by Galapagos finches described by Charles Darwin. The ancestral bird
from the mainland of Ecuador had an intermediate-sized beak to consume insects and seeds
of all sizes. Notice, however, how the break size has changed, to reflect specialization in
feeding habits. (In fact, there is one finch, the woodpecker finch, which uses a twig as a tool
to dig out insects from holes in trees.)
Specialisation of the Galapagos
finches
1. Competitive exclusion: the competition in
which one of the two competing species is much
more successful that the other such that the
successful species survives and the other species
disappears.
2. Resource partitioning: The kind of
competition situation in which competing
species coexist in the same habitat since they
use the resources slightly differently
Paramecium aurelia and P. caudatum:
In a classic study in the 1930’s, Gauss
cultured P. aurelia and P. caudatum bot
h alone and together in culture
tubes. When grown separately, the
populations grew to a fairly predictable
density. However, when grown
together, P. caudatum always lost and
eventually went extinct.
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
Symbiosis: The close association of two organisms such
that one or both benefit form the association
Parasitism: a relationship in which one of the species
benefits and the other is harmed by the relationship
Mutualism: The symbiotic relationship in which both of the
species benefit form the association
Commensalism: Two species living together where one
species benefits and the other neither benefits nor suffers
disadvantage
Parasitism: malaria mosquito
sucking human blood
Life cycle of
the malaria
mosquito
Parasitism: Ticks feeding on animals or humans
A tick bite can cause tick bite fever in humans
Parasitism: swimming habits may influence
parasitic infections
Life cycle of
the bilharzia
parasite
World distribution of
bilharzia
Sanitation
may
influence
parasitic
infections
Parasitism: cholera
One species is harmed in a parasitic relationship –some
scientists regard it as predataion since the host may be killed
The
spreading of
cholera
from
Zimbabwe
Platyhelminthes: taenia
solium/tapeworm
Tapeworm with hooks with which it attach to
the wall of the small intestine
With its long, narrow shape the tapeworm is well adapted to
attach to the small intestine of the human
Life cycle of a tapeworm
Mutualism: oxpeckers on a giraffe. The oxpecker
gets food form the giraffe and the giraffe benefits
by getting rid of its parasites
Mutualism: Lichens is an organism made up of fungi
and algae. The fungus provides the external structure
of the lichen and provides the algae wit a protected
place to live. Algae photosynthesise and provide the
fungus with food.
Mutualism: Bees collect pollen and
nectar from flowers, flowers are
pollinated in the process
Some bees pollen collect in a
packet around their legs
Mutualism: Ants get nectar from the Acacia tree
as well as shelter in specialised swollen thorns.
Ants defend the tree against herbivorous and
wood boring insects.
Commensalism: The egret catch insects which are
disturbed by the activity of a large animal. The
herbivore are neither helped nor harmed by the egret
Commensalism: Remora fish swim next to sharks
or attach to them. It gets protection and scraps
of leftover food from the shark. The shark is
neither helped nor harmed by the remora fish
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.”
Extinct: a species of plant or animals that does
not exist any more as all the individuals have
died.
Biodiversity: the variety of different kinds of
living organisms (species0 that exist on earth.
The aesthetic value
placed on South Africa’s
biodiversity.
Discuss the values placed
on “large and fury’ animals
compared to “small and
slimy” animals
1. Which of these animals
will attract most tourists?
‘Small and
slimy” can
be very
interesting.
COMMUNITY CHANGE OVER TIME:
ECOLOGICAL SUCCESSION
1. Primary succession: the sequence of organisms that
occupy a new habitat.
2. Pioneer plant: a plant that can colonise bare soil and that
is part of the community that forms the first stage in the
process of succession
3. Secondary succession: the sequence of organisms that
occupy a disturbed habitat or when an established
community has been disturbed in a catastrophic manner.
4. Climax community: the final stage in the process of
succession that refers to a mature community of plants
that will remain stable with few, if any, changes over time
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
Random sampling: a sample that is not
deliberately selected but is selected by chance.
This means that any area being sampled or any
individual in a population being sampled has an
equal probability of being selected