Environmental Engineering Reference
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Figure 23.8 The definition of 'white noise' and 'red noise' in
census data for biological populations.
Source: After Pimm and Redfearn (1988).
Figure 23.9 The 'red noise' stability of the skylark: (a)
population densities 1962-88, with 1966 set at 100; (b) the
same data plotted as standard deviation of logarithms of
density.
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 populations have no
'equilibrium level' but can build up to levels which make them susceptible to random
crashes and possible extinction. In the case of British birds, widespread habitat changes
occurred during the second half of the twentieth century, perhaps at a faster rate than ever
before, and they continue to affect the number and distribution of birds.
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