POPULATION
ECOLOGY (2)
SOCIAL ORGANISATION,
COMMUNITY STRUCTURE &
ECOLOGICAL SUCCESSION
SOCIAL ORGANISATION/SOCIAL BEHAVIOUR
Social behaviour is described as the interaction of two or more animals,
usually of the same species.
The Formation of Herds or Flocks as a Predator Avoidance Strategy
Fish – schools of fish tend to confuse predators especially through the
various formations they create as they move. In this way individuals within
the school may be less vulnerable to predators than an individual fish that is
not part of the school.
Zebras - the stripes of zebras as they move in herds confuse the predators
visually. The stripes tend to visually break up the form of the animal so that
the individuals cannot be distinguished. A herd of zebras tends to confuse the
predators whereas a zebra that is alone is easy prey to predators.
The Formation of Packs as a Successful Hunting Strategy
Predators kill other species for their food and often use clever strategies to
capture their prey:
working co-operatively in a group to increase the chances of successful
hunting
choosing vulnerable animals (old, young, sick, injured)
migrating to areas where there is a lot of prey
using camouflage to avoid being detected while stalking the prey
Animals with Dominant Breeding Pairs
Some animals claim a particular space and then defend this territory from
other
individuals, usually of the same species
The animal may use aggression, singing or marking by scent to mark and
defend its
territory = territoriality
A male animal, may claim a particular spot. By its aggressive behaviour,
loud noises or
showy behaviour it may attract a female member of the species. Together,
Division of Labour Among members of a Colony
A honeybee colony
A single adult queen (fertile female)
About 80 000 worker bees (all females)
A few drones (males)
Function of the queen bee is to lay eggs. Some of the eggs are fertilised
using sperm
stored from previous mating with drones. These fertilised eggs develop
into diploid
worker bees (females). Some of the eggs laid are unfertilised. They
develop into haploid
drones (males).
Function of worker bees: they nourish the larvae, they produce wax and
act as builders,
act as food gatherers
Function of the drones is to produce sperm to fertilise the eggs produced
by the queen.
Therefore, the bees in a colony display division of labour – the different
types of
individuals within the colony are each adapted to perform different
functions.
COMMUNITY STRUCTURE
COMMUNITY – consists of a group of populations living together in the same
habitat.
POPULATION – is a group of organisms of the same species living together
in the same
habitat and capable of random interbreeding
CONSUMERS/HETEROTROPHS – they rely on other organisms for their
source of food since
they are not capable of manufacturing
their own organic
molecules from inorganic substances
PREDATION
Interpreting a Predator-Prey graph
• The number of prey increases first:
An environment with food, space and a lack of predators will allow the
prey population to increase rapidly / exponentially
• The number of predators only rises after the prey numbers have
increase / high. There is a delayed reaction to the rise in prey
Predators are still in the initial lag phase. There are few sexually mature
individuals reproducing, and gestation is taking long
If new to an environment, predators might also need to acclimatise first
There is competition for prey
• Prey number starts to decrease
Due to higher environmental resistance (lack of food, space…, increased
predation)
• Predator numbers start to decrease once prey numbers are low or busy
decreasing rapidly
As prey decreases, food becomes a limiting factor for the predator.
Increased competition between predators for food causes higher mortality
rates amongst predators
Predators may also emigrate out of the area where more food can be
found
• Stationary phases are extremely short and fluctuations of predator and
prey numbers tend to be high
Environmental resistance increases on prey populations as predators
have access to large supply of prey.
Prey numbers decrease as predator numbers increase.
When predator numbers grow too large and there is not enough prey
anymore, their numbers decrease rapidly.
* Cyclic ossicilation
COMPETITION
Intraspecific Competition = competition between organisms of the same
species depending
on the same resources like food, space, shelter,
water and
access to mates.
For example: all of the blow-fly larvae competing for the same supply of meat
within a bottle
show a decreased rate of growth and an increase in mortality
of larvae occurs.
Sometimes one or more individuals “wins” the resources available and all
other individuals die or leave. This usually happens when competition is for
breeding space, not for food.
For example: a young and stronger pair of woodland owls may challenge an
older pair. If the younger pair wins, the older pair is driven out and the
younger pair takes over the breeding space within the woodland.
Interspecific Competition – competition between organisms of different
species which have
similar ecological requirements. If the two
populations are
dependent on the available resources and
emigration is
prevented, the decline and elimination of one
species occur.
At the beginning:
shortly after acclimatisation, both species flourished since there was
sufficient food to
support the growth of both species
this is the geometric phase of logistic growth where there are little or no
limiting factors
for either species
however, the population of species A grows much faster than that of B
because it is able
to compete for the food better than species B
Thereafter:
both populations experienced a decreasing food supply because of the
higher density of
both populations
competition then became more intense and hence there was a change in
the growth
form – availability of food became a limiting factor
species A outcompeted B for the available food supply. The population size
of species A
thus increased slowly, while the population size of species B declined
during this time.
Ecological Niche, Competitive Exclusion Principle and Resource
Partitioning
The competitive exclusion principal states that when two species
When Paramecium aurelia and Paramecium caudatum are kept in separate
containers, their numbers increase rapidly. If the two species are put in the
same container P. caudatum outcompetes P. aurelia and the numbers of P.
aurelia decreases rapidly.
Resource Partitioning refers to the sharing of resources so that different
species can co-exist in the same area.
Examples:
Herbivores grazing in the same area.
Partitioning of light in a forest (Stratification)
Plants grow to a certain height due to their light requirements. Plants that
require less light, grow in the shade of taller plants
Symbiotic Relationships
Symbiosis is a close, long term biological relationship between individuals of
two or more species.
Parasitism = one species (parasite) benefits, while the other species (host)
is harmed.
Endoparasites – those that parasitise the host internally e.g. the tapeworm
Ectoparasites – those that parasitise the host externally e.g. leeches and
ticks
Dodder, an obligate parasite. It has no chlorophyll so it is completely
dependant on other plants (hosts) for food and water
Some scientists consider parasitism as a form of predation because both
involve pairs of species in which one species increases its population at the
expense of decreasing the other population of the other species.
A tapeworm has two hosts – pig & human. When it enters a human it lives
and feeds on
on the food on the small intestine of humans thus depriving such
individuals of their own
food supply. The infected person feels weak and tired because of shortage
of food.
The bilharzia parasite has two hosts – human & snail.
In both cases the spread of the disease in rural areas are greater because:
o poor sanitation
o poor playing facilities may result children in swimming in rivers infected
with the bilharzia
parasite
o keeping domestic animals such as pigs enable the life cycle to be
completed
Mutualism = is a symbiotic relationship between two organisms in which
both benefit from the relationship without harming each other.
Bacteria and roots of Leguminous Plants – Nitrogen-fixing bacteria
occur in roots of the
clover plant. The bacteria are able to convert nitrogen to nitrates which
the plant uses to
Commensalism = is a symbiotic relationship between two organisms in
which one benefits from the relationship without harming the other.
Shark and Remora fish – the remora fish attaches to the shark using its
sucking pads.
the remora fish feeds on the scraps of the
shark’s food.
Whales and Barnacles – barnacles are generally found attached to rocks
& depend on
food reaching it with the waves. Barnacles may
also be found
attached to the skin of whales. The barnacle
benefit by moving
around to fresh feeding grounds without
expending any energy
to do so.
ECOLOGICAL SUCCESSION
Primary succession
Refers to the changes in species composition over time in a habitat that was
not previously inhabited by organisms e.g. Sand dunes, new ponds or lakes,
bare rock surfaces such as volcanic lava or rocks scraped clean by glaciers.
No soil exists when primary succession begins.
Secondary succession
Is the change in species composition that takes place after some disturbance
removes the existing vegetation e.g. Abandoned agricultural fields or open
areas produced by forest fires.
The change that take place during any form of succession depend on a
variety of environmental factors – amount of moisture, temperature, wind
Succession usually takes place as follows:
The organisms that first inhabit an open or disturbed area are called
pioneers. The
pioneers species usually alter the environment in such a way that
conditions become more
favourable for a new species to move in.
The pioneers are usually replaced by annual plants.
And then herbaceous perennials.
Followed by shrubs and trees.
The last species to appear is referred to as climax species.
Together with the changing plant population are changes to the animal
population since
the introduction of new plant species creates habitat and food for new
animal species.
Each animal species that appears also becomes potential food for new
animal species.
At a certain time, no further changes take place in the composition of the
community
The succession is then said to have reached a climax and the community is
now regarded
as being stable.
The only thing that can change the community now is a disaster such as a
forest fire, or
humans chopping down trees.
