Environmental Engineering Reference
In-Depth Information
Plate 23.1
A young snowy owl on its nest on Devon Island,
Canadian Arctic. Snowy owls live almost entirely on
lemmings and occasional hares. Populations are unstable, as
they fluctuate greatly in response to lemming cycles.
Photo: Ken Atkinson.
whereas land ecosystems show 'white' noise, i.e. truly random effects and impacts.
However, recent work on the BTO census data for British birds by the ecologists Pimm
and Redfearn suggests that the variability of land 'noise' is coloured too. Figure 23.9
shows the example of the skylark. Figure 23.9a shows the densities on farmland for the
years 1962-86, with the scale set at 100 for 1966. Figure 23.9b shows the data for the
same population plotted as 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
