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17.3.3 Biotic Factors
- An organism may have different relationships or
interactions with its surrounding organisms that
affect its distribution or abundance
- Relationships could be close, casual, beneficial or
harmful
Predation - the relation in which the predator kills
& feeds on the prey
Predator-prey relationship determines the relative
abundance of predator and prey:
increased predators, decreased preys
decreased predators, increased preys
Competition
- a relationship in which two organisms
compete with each other for needs such as
food, water , space, light or mate
- the more similar the needs, the greater is the
competition;
- same species has greater competition than
different species
Interspecific competition: competition between
different species
* The competitive exclusion principle - leads to
extinction of one or the other (p 14)
Intraspecific competition: competition between
members of the same species
In the food web:
 If small insects are removed, number of
caterpillars will increase because of less
competition but number of birds will decrease
because less food is available
Commensalism
- an association in which the commensal benefits
while the host is unaffected
- crab: attachment and locomotion for the barnacle
 Barnacles (commensal) are benefited from the
food remains and locomotion
 Crabs (host) are not harmed or benefited.
Mutualism
- an association in which both are
benefited by living together
- sea anemone: food and mobility
hermit crab: protection by sting
cells/camouflage
Root nodules:
- bacteria obtain sugars from algae
- host plant gains fixed nitrogen to make
proteins
Lichen:- algae makes food by photosynthesis
fungus extracts water & minerals from
substratum
Parasitism
- parasite live inside or outside the host for food
and shelter
- example: tapeworm (parasite) in man (host)
Antibiosis
Organisms produce chemicals which repel other
organisms.
These may be directed against members of their
own species.
Many mammals, for example, use chemicals to
mark their territories, with the intention of
deterring other members of the species from
entering.
Ants produce a hormone (pheromone) to
warn off other members of the species.
Fungi, e.g. Penicillium produces antibiotic
(penicillin) to prevent bacterial growth in
their vicinity
Hoverfly mimicking a wasp
Others
1. Dispersal of organisms, particularly plants
2. Pollination of flowering plants by insects
3. Mimicry - by certain flies to resemble wasp
in order to warn off the harmless flies to
avoid being stung
4.Human influence,e.g. hunters, farmers,
developers, polluters, etc.
17.3.4 Species diversity index: is the
number and range of different species
found in an ecosystem.
It is helpful when considering the
interaction of the edaphic, climatic and
biotic factors which influence an
ecosystem.
In general, a stable ecosystem has a wide
range of different species each with a
similar population size. A less stable
ecosystem (due to pollution or extreme
climatic conditions) has just a few
species with very large populations.
17.4.1 Measurement of Environmental Parameters
Abiotic factors are important in determining both
the distribution of the organisms and their
physical and physiological adaptations.
Temperature
- diurnal and seasonal temperature variations are
significant in affecting different species of plants
and animals
- equipment: mercury thermometer,
maximum-minimum thermometer, miniaturized
thermistor
pH meter in use
pH
-measure pH of a solution by universal
indicator, pH paper, pH meter, etc.
Light
-measure its duration and intensity; duration by
predication from Royal Observatory;
intensity by photographic light meter
Humidity
Relative humidity: the water content of a
given volume of air relative to the same
volume of fully saturated air
- equipment: whirling hygrometer
Wind and Water Speed
- wind speed:
- anemometer or wind
gauges
- water speed:
- time the movement of a
floating object over a
measured distance
Salinity
- using a conductivity meter: greater salinity
has greater conductivity
Oxygen Level
- using an oxygen meter or chemical method
(Winkler method)
17.4.2 Sampling Methods
It is virtually impossible to identify and
count every organism in a habitat.
For this reason only small sections of the
habitat are usually studied in detail to
represent the whole area.
Sampling techniques are used:
Quadrats
- a sturdily built
wooden frame,
can be folded for
easy
transport
and storage
Using a quadrat along
a belt transect
Quadrats
- When placed on the ground, the species
present within the frame are identified and
their abundance recorded
- Sampling could be random or systematic
Line Transect
- useful where a transition of flora and/or
fauna occurs
- a string or tape is stretched out along the
ground in a straight line;
record the organisms touching or covering
the line all along its length or at regular
intervals
- Profile transect: when there is appreciable
height change along the transect and thus
affecting the distribution of its species
Using a quadrat
along a belt transect,
e.g. ladder transect
(every 5m)
Belt Transect
It is a strip, usually a metre wide, marked by
putting a second line parallel to the other. The
species between the lines are carefully recorded,
working a metre at a time.
Alternatively, a frame quadrat in conjunction with a
single line transect could be used.
Point Frames
- for grassland field study of dense vegetation
17.4.3 Collecting Methods
Collecting all organisms within a habitat is
normally impractical and therefore small areas
are selected.
Remember to return all material to its original
position after searching & collecting sufficient
specimens.
Some collecting apparatus for general use are listed
below:
1. specimen tube
2. screwed-topped jars
3. polythene bags
4. forceps
5. paint brush
6. bulb pipette
7. pooter
8. widger
11. enamel dish
14. Tullgren funnel
17. pitfall traps
9. sieve
12. beating tray
15. Baermann funnel
18. netting
10. hand lens
13. light traps
16. mammal traps
17.5 Estimating Population Size
The exact methods used for
estimation depend not only the
nature of the habitat but also on
the organisms involved,
e.g. animals - population ;
plants - percentage cover
17.5.1 Using Quadrats
- By sampling an area using quadrats and
counting the number of individuals within
each quadrat, it is possible to estimate the
total number of individuals within the area
- confined to plants and sessile, or very
slow-moving animals;
- fast-moving animals are disturbed and run
away
17.5.2 Capture-recapture Techniques
- useful for mobile animals which can be marked
- capture, marked, released, randomly recaptured
and marked individuals recorded
no. of marked individuals recaptured
total no. of individuals in 1st sample
-------------------------------------------- = -----------------------------------------total no. of individuals in 2nd sample
estimated size of population
(the Lincoln Index)
17.5.2 Capture-recapture Techniques
Factors affecting the accuracy of the estimation:
deaths, migration,
individuals become more liable to predation, etc.
Examples:
-
-
-
arthropods marked on their backs with
non-toxic paint,
fish have tags attached to opercula,
mammals have tags clipped to their ears,
birds have their legs ringed
17.5.3 Abundance Scales
The population size may be fairly accurately
determined by making some form of frequency
assessment.
These are subjective and involve an experimenter
making some estimate of the number of
individuals in a given area, or the % cover of a
particular species.
This is especially useful where individuals are very
numerous, e.g. barnacles on a rocky shore, or
where it is difficult to distinguish individuals, e.g.
grass plants in a meadow.
The assessments are usually made on
an abundance scale of 5 categories:
 Abundance,
 Common,
 Frequent,
 Occasional,
 Rare.
Barnacles exposed at
low water
17.6 Populations and Communities
 A population is a group of individuals of
the same species occupying a particular
area at the same time.
 A community consists of a number of
populations. It comprises all the plants and
animals which occupy a particular area.
17.6.1 Population growth
 Provided the birth rate exceeds the death rate,
 a population will grow in size. If only a few
individuals are present initially:


Lag phase: very slow growth with a small
number of individuals
Exponential phase: grows at an ever increasing
rate, provided no factors limits growth since
more individuals are available for reproduction


Carrying capacity is the limit to the number of
individuals an area can support
Stationary phase: the size of the population
stabilizes at a particular level since certain factors
now limit further population growth

Death phase: the population decreases because
the high population causes the carrying capacity
of the environment to decline
Environmental resistance are the factors
which limit the growth of a particular
population,
 e.g. predation,
disease,
availability of light, food,
water,
oxygen and shelter,
the accumulation of toxic wastes and
even the size of the population itself.

17.6.2 Density-dependent Growth
 A population is a density-dependent when its
size (or density) affects its growth rate because
of density-dependent factors such as food
availability and toxic waste accumulation.
17.6.3 Density-independent Growth
 In this type of growth a population increases
until some factor causes a sudden reduction in
its size.
 Its effect is the same regardless of the size of
the population, e.g. temperature, fires, floods,
storms, etc.
17.6.4 Regulation of Population Size
 Fecundity is the reproductive capacity of
individual females of a species.
 Birth rate or natality is used to measure fecundity.
 Death rate or mortality is the number of
individuals of a species which die per unit time.
 Immigration occurs when individuals join a
population from neighbouring ones.
 Emigration occurs when individuals depart from
a population.
 A cycle occurs when the size of a population
fluctuates on a regular basis
17.6.5 Control of Human Populations
 Human beings are the top organism in many food
chains, they have little fear of predators.
17.6.5 Control of Human Populations (famine in Somalia)
 With advances in food production, technology, etc., the
human population has grown virtually unchecked in
recent times.
 The Malthusian principle (1798) states that with the
world's food supply to increase arithmetically, the human
population would increase geometrically, famine would
be inevitable.
17.6.6 Competition


Intraspecific competition:
Competition between members of the same species
Interspecific competition:
Competition between different species
17.6.7 Predation
It is important in producing cyclic change in the size
of a population.
Bobcat catching
Snowshoe hare
Hydra capturing
naphnia
Prey
and
predator
relationship:
predators eat preys
 preys decrease
 less food for predators
 less predators
 fewer preys eaten
 preys increase again
 predators increase again
 preys decrease again   
17.6.8 Competitive Exclusion Principle
17.6.9
Biological Control

It is a means of
managing
populations of
organisms which
compete for human
food or damage the
health of humans or
livestock.


The aim is to bring
the population of a
pest down to a
tolerable level by use
of its natural enemies.
The aim is not to
eradicate the pest but
at a level where the
pest has no major
detrimental effect.
Moths feeding
on ragwort
Examples:
1 Mosquito (vector of malaria) is controlled by
hydra which eats mosquito larvae
2 Larvae of many butterflies and moths (eats many
economically important plants) are controlled by a
bacterium which parasitizes the larvae
17.6.10 Communities and Succession
The community is a constantly changing dynamic
unit, which passes through a number of stages
from its origin to its climax.
Succession is the transition from one stage to the
next.
bare rock  lichen
 mosses
 ferns
 grasses
 shrubs
 trees
Pioneer community: the organism which
brings about the colonization of a new area,
e.g. lichen
Climax community: a balanced equilibrium
of species with few, if any, new varieties
replacing those established.
An oak wood is a
climax community
A climax community consists of animals as well as
plants.
Animals have undergone a similar series of
successional stages, largely dictated by the plant
types available.
Within the climax community there is normally a
dominant plant and animal species, or sometimes
two or three co-dominant species.
Primary succession: the succession where
bare rock / soil is first colonized.
Secondary succession: the subsequent
recolonization of a baren area previously
supporting life, e.g. after a forest fire
Deflected succession: normal succession
which has been artificially changed, e.g.
grasslands in UK for grazing by removing
the climax community of oak woodland.
Plagioclimax: the resultant sub-climax of
an artificially changed succession, e.g.
grassland for grazing
 A sere: is a series of successional stages
 A hydrosere is a series of successions in an
aquatic environment
 A halosere is a series of succession in a
saltmarsh environment
