Environmental Engineering Reference
In-Depth Information
ness components (survival and reproduction) cause a directional reversal in
population density.Thus the populations are continually drawn back or at-
tracted to equilibrium. Hence, we apply the special term
attractor
to equi-
libria of deterministically oscillatory systems (Edelstein-Keshett 1988).
Populations also can be influenced by
stochastic processes
. For example, sup-
pose an unexpected storm passed through a location causing temperatures
to plummet. Suppose some number of individuals died because they were
poorly adapted to cope with such temperature change.The storm, an ex-
trinsic factor, effectively increased the average mortality rate in the popu-
lation.
Weather forecasters will tell you that specific weather events such as se-
vere storms (or comparatively more benign events such as rainfall, dry con-
ditions, etc.) are not highly predictable over the long term.Although we can
count on regular seasonal changes in weather, we cannot expect that a storm
event will hit the same location with the same ferocity and duration on the
same date each year.There is considerable variability from one year to the
next in rainfall, duration of winter, and so forth. Because the duration and
intensity of this extrinsic factor is highly variable in space or time, it is un-
predictable and so cannot be considered a deterministic factor.We call this
a stochastic factor (Edelstein-Keshett 1988).
Stochastic factors also have impor-
tant implications for population dy-
namics beyond simply altering
mortality rates within populations.
Suppose that individuals of a herbi-
vore species rely on grassland forage
to sustain themselves and reproduce.
The nutritional quality of the grass-
land forage can be quite high in dry
(good) years when the forage is cured
by the sun's heat (except in the ex-
treme case of drought when there is
no forage and mass starvation ensues).
In wetter (poor) years, the nutritional value is lower because the forage qual-
ity becomes diluted by water in the plant tissues. Because nutritional qual-
ity of the resource determines herbivore birth and death rates, it plays an
important role in shaping the carrying capacity of the landscape. In other
words, an extrinsic factor such as weather can limit the quality or abundance
An extrinsic factor such as weather
can limit the quality or abundance
of food resources and thus deter-
mine the carrying capacity in any
single year. Furthermore, the
stochastic nature of weather
causes the carrying capacity of
the environment to fluctuate or
oscillate up and down from one
year to the next.
