Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
Variety of Life and Relation of Organisms with their Environment ...... Page 36
Ecology
Syllabus :
Ecosystem - the meaning of the terms : biosphere, biome, ecosystem, community and
population. the concept of habitat and niche of an organism; an outline of population growth and
the factors affecting it; an outline of biotic and abiotic factors in ONE local ecosystem and their
effects on the distribution and abundance of organisms in that ecosystem; the use of an
appropriate sampling method, such as the quadrat, line transect and belt transect, to study the
distribution and abundance of organisms.
Energy flow and nutrient cycling – the transfer of energy between different trophic levels and its
relative efficiency; the importance of producers, consumers (including detritivores) and
decomposers in the cycling of nutrients.; the concepts of food chain, food web and trophic level;
the pyramid of number, pyramid of biomass, and pyramid of energy; the nitrogen and carbon
cycles.
Interdependence of organisms – the interactions between organisms: predation, competition,
commensalism, mutualism and parasitism.
Succession – a simple account of ecological succession, including primary and secondary
succession, and climax community.
Introduction
Ecology is the study of the relationships of living organisms with each other and
their non-living or physical surroundings.
The relationship of ecology to other branches of biology is summarised below.
Genetics
Molecular
B io lo gy
Cytology
Histology
Anatomy
genes
cells
Biochemistry
and
physiology
tissues
organs
organisms
populations
communities
Ecology ecosystems
Biosphere
[Note] The whole planet, the Earth, operates as an ecosystem. The oceans, forests, grasslands etc. are
smaller ecosystems which are linked, by energy flow and exchange of materials, to form the
overall planetary ecosystem.
Ecosystem
It comprises both the physical environment and all the organisms in a given area. The
organisms interact with each other and with the physical environment, forming a
relatively stable system.
In other words, a natural unit comprising biotic components (including all the
organisms) and abiotic components (including all the physical factors), which interact
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
Variety of Life and Relation of Organisms with their Environment ...... Page 37
with each other through the flow of energy and cycling of materials, in a defined area
is an ecosystem.
Terms to be known :
Biome : a large and distinct terrestrial region characterized by a dominant form of
vegetation determined by typical climatic conditions
Biosphere : the largest ecosystem , it includes all ecosystem on the Earth; it is a global
ecological system in which the organisms interact with each other and
with the physical environments.
Habitat : a physical environment in which an organism lives.
Ecological niche : in an ecosystem, different species of organisms fulfill different
ecological functions; the functional role that a species plays within
a community is called its ecological niche
: the ecological niche of an organisms is determined by three factors :
1. the place that it lives , i.e. its habitat
2. its behaviour or its entire way of life;
3. its interactions with the community and the habitat, e.g. its
feeding habit (what type of food it feeds on).
Population : a group of organisms of one species occupying a defined area and usually
isolated to some degree from other similar groups.
Community : any group of organisms belonging to a number of different species that
coexist in the same habitat or area and interact through trophic and
spatial relationships.
Exercise :
(90 I 7a)
Distinguish between niche and habitat.
[2 marks]
(93 I 6b)
Distinguish between biome and habitat
[2 marks]
Population growth:
The following are some methods for estimating the population density of organisms :
A. Direct counting :- usually used in opened area for large animals or plants which populations are
small
B. Marking-recapture method :- firstly, some individuals ‘a’ are capture from a population ‘N’, marked by spots of
colour or rings and released
- after a certain time, a second capture is made, if this time ‘b’ individuals are
captured, of which ‘c’ are marked, then
a/N = c/b or N = ab/c
where N is the original population of the organism
- this method is commonly used in animals like bats, birds and fish, in making the
calculation, following assumptions should be made :
a. organisms mix randomly within the population
b. sufficient tome must elapse between capture and recapture
c. organisms disperse evenly within the geographical area of the population
d. it is only applicable to populations restricted geographically
e. changes in population size i.e. migration, birth and death are negligible.
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
Variety of Life and Relation of Organisms with their Environment ...... Page 38
C. Sampling :- most widely used method
- the density of a population in a small area is counted and is used to represent the
value of the whole area inhabited by the species
- the main difficulty lies in selecting a sample that is as representative as possible
of the whole population generally many samples from different typical locations
are counted to give the average
Factors affecting population growth :
A. Environmental resistance :- is the collective name given to those factors which actively decrease the fertility
and survival of the individuals, in turn limit the growth of a population
a. Physical factors: such as temperature, light-intensity, shelter, nutrients, gases,
water and accumulation of toxic waste, etc.
[Note] A habitat under a given set of environmental conditions, can only support the population to
reach a saturation point known as the carrying capacity. The carrying capacity of a population
in a habitat will change when there are changes in environmental conditions.
b. Biotic factors :
i/ predation = they prey on a particular species play a critical part in keeping
down the population of that species
= if the predation relationship is worked out under an artificial
condition, the predator could find all the prey and ate them all and
then it dies out due to starvation i.e. both species died
= however, when the prey and predator interaction in nature, the
following curves result
UB p132 fig 10.3
Fig. 26 Relationship between prey and predator populations.
= In natural ecosystem, the prey would survive because they could
escape, thus the preys could reproduce and grow elsewhere.
Therefore both the predators and preys could exist together, but
followed some periodic oscillations.
• the number of predators is usually less than the number of prey
• the shape of the two curves is similar, but there is a time lag
between the two i.e. the curve for the predators lags behind that
of the prey
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
Variety of Life and Relation of Organisms with their Environment ...... Page 39
Number of individuals
Exercise : (99 I 4)
In an experiment to study the interaction of organisms, a student introduced some
zooplankton X into a culture of zooplankton Y kept in a jar. The graph below shows
the population changes of X and Y in the jar based on the data obtained from this
experiment. It is known that these two zooplanktons are not competitors, and they
have similar masses and similar turnover rates.
(a) Identify the relationship between X and Y.
[1 mark]
Zooplankton Y
Zooplankton X
Time
(b) (i) Sketch a graph to show what you would expect to find in the population
pattern of X and Y if they were allowed interact in the natural
environment. (Do not use graph paper.)
[2 marks]
(ii) Account for the difference in population pattern when X and Y were
allowed to interact in the natural environment compared to when they were
kept in the jar.
[3 marks]
ii/ competition = individuals in a population are continually competing with each
other, there are two types :
( 1 ) intra-specific competition
: competition between individuals of the same species
( 2 ) interspecific competition
: competition between individuals of different species
: the intensity of competition depends on the degree to which
the two species share on that common resources
B. Biotic potential (reproductive potential) :Refers to the inherent power of a population to increase in numbers when the age
ratio is stable and all environmental conditions are optimal. It is affected by :
1. time required for the individual to reach sexual maturity
2. the number of living individuals in a population
3. the average survival time (life span) of individuals
4. age of the individual
Population growth curve :
When the birth rate exceeds the death rate, a population will grow in size.
Sigmoid (S-shaped) curve :- the curve can be seen to be quite S-shaped, it can be divided into 4 main phases
1. lag phase == if only a few individuals are present initially, the rate of growth
will be very slow
2. log phase (exponential phase) == as the numbers increase, more individuals
become available for reproduction and the
population grows at an ever increasing rate,
provided no factor limits growth
Buddhist Chi Hong Chi Lam Memorial College A.L. Bio. Notes (by Denise Wong)
Variety of Life and Relation of Organisms with their Environment ...... Page 40
[Note] growth cannot continue indefinitely because there is a limit to the number of individuals that
any area can support, this is the carrying capacity of that area.
3. stationary phase == beyond this point certain factors limit further population
growth, the size of the population may then stabilize at a
particular level
4. death phase == the high population level may cause the carrying capacity of the
environment to decline, so the population falls
UB p129 fig 10.1
Fig. 27 Growth of a population (sigmoid curve)
Age Pyramids :
Age pyramids refers to the distribution of individuals in a population into age classes.
Generally, the youngest are at the base and the oldest at the top, males in the left and
females on the right.
Ecologically the individuals in a population can be divided into three groups :
pre-reproductive (immature), reproductive (mature) and post-reproductive (old).
Post-reproductive
Reproductive
Pre-reproductive
male
female