Environmental Engineering Reference
In-Depth Information
Box 8.1 Types of Ecosystem Stability
There are three ways that ecosystem stability may be measured:
Coefficient of Variation
—Species abundances (population numbers or total biomass)
may fluctuate over time due to disturbances. The degree of fluctuation can be meas-
ured statistically by scaling the magnitude of fluctuation (measured as the standard
error about a mean abundance) relative to mean abundance. Smaller coefficients of
variation indicate smaller fluctuations and hence imply greater stability.
Resistance
—Species have some capacity to withstand a disturbance. Resistance
quantifies that capacity. A small change in species abundance due to some distur-
bance implies greater resistance than does a larger change.
Resilience
—Species abundances may be changed by disturbances. But, species also
have the capacity to recover their abundances after a disturbance. The rate at which
a species or an ecosystem recovers is a measure of resilience. Species or ecosystems
that recover quickly are more resilient than those that recover more slowly.
compensate for species losses lower down. More diverse systems tend to be
more stable than less diverse systems.
The Diversity-Stability Hypothesis was tested using a large-scale field ex-
periment at the Cedar Creek Long-Term Ecological Research site in Min-
nesota, United States (Tilman 1996). In this system, plant species are
consumers of soil nutrients in a food web context similar to that envisioned
by MacArthur.The experiment comprised different numbers of plant species
(from two to twenty)—and hence different numbers of consumer-resource
linkages with soil nutrients—planted out among 207 field plots. Tilman
measured the degree of fluctuation in total plant biomass in a plot from one
year to the next. In this case, stability was measured using the coefficient of
variation (CV) in plant biomass (see Box 8.1).The experiment revealed that
total plant biomass in plots with greater plant diversity tended to fluctuate
less (have lower CVs) than biomass in plots with lower diversity.
In many parts of the world, periodic drought is an important stressor on
ecosystems. Frank and McNaughton (1991) evaluated how such stress af-
fected the stability of a grassland ecosystem by tracking changes in plant
species composition before and after the 1988 drought in Yellowstone Na-
tional Park, United States.Within three different locations in the park they
