PDA

View Full Version : help me(reading passage 3, test 4, cambridge ielts1



math_dn12
18-02-2008, 04:51 PM
You should spend about 20 minutes on Questions 28-39 which are based on Reading Passage
3 below.
Population viability analysis
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.
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

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) ...


ḿnh ko biết trả lời 3 câu hỏi nay. help me!!!

miumiu
26-02-2008, 10:04 PM
Đáp án là
36. Could be extinct
37. Distribution
38. Logging take place