Environmental Engineering Reference
In-Depth Information
Table 23.5 Perturbations affecting ecosystem stability
Natural
Human-made
Drought
Deforestation
Freezing
Overgrazing
Fire
Agrochemicals
Insect pests
Acid precipitation
Disease
Oil pollution
where
S
= stability,
Pi
= proportions of energy passing through the
i
th species. Two
different stability situations are shown in Figure 23.7. The killer whale receives energy
equally from five separate sources. This is a system with maximum choice, low
information content and maximum uncertainty. In contrast the wolf subsists mainly on
caribou, whose migration paths it follows, with lesser amounts of energy from a range of
small mammals. This is a system of little choice, high information content and little
uncertainty. The differences are reflected in the stability values of 0·70 and 0·47.
MacArthur hypothesized that any failure of one energy pathway would be less severe the
greater the number of pathways and the more even the distribution of energy between the
pathways.
A second index of stability is the degree to which a biological population fluctuates,
i.e. the variability of population density over time. This can be measured by the standard
statistical measures of variance (σ
2
), standard deviation (σ) or coefficient of variation (cv)
where:
where cv = coefficient of variation, σ = standard deviation and = mean density. The
variability of biological populations is important because it depends not only on internal
properties of the ecosystems (intrinsic factors)
Figure 23.7
Two food webs with contrasting stabilities. The
killer whale receives food from a range of equally energetic
sources and is potentially stable, whereas the wolf is
overdependent on one source and is potentially unstable.
