Environmental Engineering Reference
In-Depth Information
but also on the nature and frequency of the perturbations (extrinsic factors). Populations
vary more in climatically unpredictable ecosystems (arctic, subarctic, arid, semiarid) than
in predictable ones (tropical rain forests), suggesting that extrinsic factors may govern
variability more than intrinsic ones (Plate 23.1).
EXAMPLES OF STABILITY
Information concerning the stability of biological populations (mammals, birds, insects)
is notoriously difficult to obtain. It requires long-term studies to monitor long-term
effects; long runs of population data are needed because short-term studies can be poor
indicators of long-term trends. Charles Elton collected data from the Hudson's Bay
Company in Canada to record population trends for the chief fur-bearing animals (see
Chapter 24). He argued that, as hunting and trapping effort is not likely to change much
from year to year, company records of furs and skins bought would be good indices of
population numbers.
Overall population trends have long been of interest to ecologists, who wonder
whether they reflect long-term environmental changes or human impact. Steele has
argued that there are two basic trends - 'red noise' and 'white noise'. 'Red noise' is the
stability situation where the variability of populations increases with time; 'white noise'
is the stability situation where variability does not increase with time. In the former case
the amplitudes of variability increase with time; in the latter case, the amplitudes are
constant. Figure 23.8 plots the standard deviations of the logarithms (SDL) against the
period over which the calculation was made. SDL increases with time for the 'red' but
not for the 'white' population. Steele put forward the hypothesis that the oceans exhibit
'red' noise,
