Part A
To make political
decisions about the extent and type of forestry in a region it is important to
understand the consequences of those decisions. One tool for assessing the
impact of forestry on the ecosystem is population viability analysis (PVA).
This is a tool for predicting the probability that a species will become
extinct in a particular region over a specific period. It has been successfully
used in the United States to provide input into resource exploitation decisions
and assist wildlife managers and there is now enormous potential for using
population viability to assist wildlife management in Australia’s forests.
A species becomes extinct when the last
individual dies. This observation is a useful starting point for any discussion
of extinction as it highlights the role of luck and chance in the extinction
process. To make a prediction about extinction we need to understand the
processes that can contribute to it and these fall into four broad categories
which are discussed below.
Part B
A Early
attempts to predict population viability were based on demographic uncertainty
Whether an individual survives from one year to the next will largely be a
matter of chance. Some pairs may produce several young in a single year while
others may produce none in that same year. Small populations will fluctuate
enormously because of the random nature of birth and death and these chance
fluctuations can cause species extinctions even if, on average, the population
size should increase. Taking only this uncertainty of ability to reproduce into
account, extinction is unlikely if the number of individuals in a population is
above about 50 and the population is growing.
B
Small populations cannot avoid a certain amount of inbreeding. This is
particularly true if there is a very small number of one sex. For example, if
there are only 20 individuals of a species and only one is a male, all future
individuals in the species must be descended from that one male. For most
animal species such individuals are less likely to survive and reproduce.
Inbreeding increases the chance of extinction.
C
Variation within a species is the raw material upon which natural selection
acts. Without genetic variability a species lacks the capacity to evolve and
cannot adapt to changes in its environment or to new predators and new
diseases. The loss of genetic diversity associated with reductions in
population size will contribute to the likelihood of extinction.
D
Recent research has shown that other factors need to be considered. Australia’s
environment fluctuates enormously from year to year. These fluctuations add yet
another degree of uncertainty to the survival of many species. Catastrophes
such as fire, flood, drought or epidemic may reduce population sizes to a small
fraction of their average level. When allowance is made for these two
additional elements of uncertainty the population size necessary to be
confident of persistence for a few hundred years may increase to several
thousand.
Part C
Beside these processes we need to bear in
mind the distribution of a population. A species that occurs in five isolated
places each containing 20 individuals will not have the same probability of
extinction as a species with a single population of 100 individuals in a single
locality.
Where logging occurs
(that is, the cutting down of forests for timber) forest dependent creatures in
that area will be forced to leave. Ground-dwelling herbivores may return within
a decade. However, arboreal marsupials (that is animals which live in trees)
may not recover to pre-logging densities for over a century. As more forests
are logged, animal population sizes will be reduced further. Regardless of the
theory or model that we choose, a reduction in population size decreases the
genetic diversity of a population and increases the probability of extinction
because of any or all of the processes listed above. It is therefore a
scientific fact that increasing the area that is loaded in any region will
increase the probability that forest-dependent animals will become extinct.
Questions 28 – 31
Do the following statements agree with the views of
the writer in Part A of Reading Passage 3? In boxes 28-31 on your answer sheet
write
YES if
the statement agrees with the writer
NO if
the statement contradicts the writer
NOT GIVEN if
it is impossible to say what the writer thinks about this
Example Answer
A link exist between the consequences of
decisions YES
and the decision-making process itself.
28 Scientists
are interested in the effect of forestry on native animals.
29 PVA
has been used in Australia for many years.
30 A
species is said to be extinct when only one individual exists.
31 Extinction
is a naturally occurring phenomenon.
Questions 32 – 35
These questions are based on Part B of Reading Passage
3. In paragraphs A to D the author describes four processes which may
contribute to the extinction of a species. Match the list of processes (i-vi)
to the paragraphs. Write the appropriate number (i-vi) in boxes
32-35 on your answer sheet.
NB There are more processes
than paragraphs so you will not use all of them.
|
32
Paragraph
A 33
Paragraph
B |
Processes i Loss of ability to adapt |
Questions 36 – 38
Based on your reading of Part C, complete the
sentences below with words taken from the passage.
Use NO MORE THAN THREE WORDS for each
answer. Write your answers in boxes 36-38 on your answer sheet.
While the population of a species may be on the
increase, there is always a chance that small isolated groups ... (36) ...
Survival of a species depends on a balance between the
size of a population and its ... (37) ...
The likelihood that animals which live in forests will
become extinct is increased when ... (38) ...
Question 39
Choose the appropriate letter A – D and write
it in box 39 on your answer sheet.
39 An alternative heading
for the passage could be:
A The protection of native
flora and fauna
B Influential factors in
assessing survival probability
C An economic rationale for
the logging of forests
D Preventive measures for
the extinction of a species