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standard deviations of the logarithms (SDL) of density
against the period over which the calculation is made. Pre-
1970 densities are ignored because before 1970 many bird
populations were recovering from the crash in the hard
winter of 1962-3. Had those data been included, the
increase in SDL would have been more marked. For both
nested years (2, 4, 8, 16) and non-nested years (2, 4, 8) SDL
increases with period. Pimm and Redfearn found the
same result for a range of birds, mammals and insects
from various countries. The implication is that land 'noise'
is also red, and populations show larger fluctuations as
time goes by. The implication is that biological popula-
tions have no 'equilibrium level' but can build up to levels
which make them susceptible to random crashes and
possible extinction.
In the case of the skylark, the BTO calculates that
Britain lost 1.5 M pairs from 1975 to 2000. There are many
possible reasons, but the use of pesticides seems not to
be the problem. A major habitat change has been the
expansion in arable areas of winter cereals, planted in
autumn. By May and June, the main breeding months of
the skylark, the winter wheat is very dense and intensively
managed. There is little food for the skylarks, which,
forced to nest in farm tracks, become susceptible to
predators. The Royal Society for the Protection of Birds
(RSPB) has advocated a Sustainable Arable Farming for
an Improved Environment (SAFIE) project, acceptable
in the new EU Single Farm Payment system, whereby
farmers leave unplanted patches of
0.3
White
0.2
Red
0.1
0.0
0
10
20
30
40
Length of census, years
Figure 22.8 The definition of 'white noise' and 'red noise'
derived from census data for biological populations.
Source: After Pimm and Redfearn (1988)
hypothesis that the oceans exhibit 'red' noise, whereas land
ecosystems show 'white' noise, i.e. truly random effects
and impacts. However, work on the British Trust for
Ornithology (BTO) census data for British birds by the
ecologists Pimm and Redfearn suggests that the variability
of land 'noise' is coloured too.
Figure 22.9 shows the example of the skylark. Figure
22.9a s hows the densities on farmland for the years
1962-86, with the scale set at 100 for 1966. Figure 22.9b
shows the data for the same population plotted as
low vegetation,
(a)
(b)
0.12
120
0.10
100
0.08
80
0.06
60
0.04
0.02
4 1962
1972
1982
248 6
248
non-nested
nested
Year
Length of census period
Figure 22.9
'Red noise' stability of skylarks
in Britain: (a) population
densities 1962-88, with 1966
set at 100; (b) same data
plotted as standard deviation
of logarithms of density.
{
16
8
Nested
4
2
Non-nested
24
8
 
 
 
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