Iii Cambridge Practice Tests for ielts

READING PASSAGE 3
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