Environmental Engineering FOURTH EDITION

6 ENVIRONMENTAL ENGINEERING

160

140 HARE

---- LYNX

3 120

m

J 100

80

3 60

E

3 40

20

.- L

1845 1855 1865 1875 1885 1895 1905 1915 1925 1935

Time in years

Figure 1-2. The hare and lynx homeostasis (source: D.A. MacLurich, “Fluctuations
in the Numbers of Varying Hare,” University of Toronto Studies, Biological Sciences
No. 43, Reproduced in S. Odum, Fundamentals of Ecology, 3rd ed., W.B. Saunders,
Philadelphia, 1971).

species in an ecosystem do not vary independently but rather fluctuate in an approx-
imate steady state in response to self-regulating or negative feedback (homeostasis).
Homeostatic equilibrium is dynamic, however, because the populations are also gov-
erned by positive feedback mechanisms that result from changes in the physical,
chemical, and biological environment (homeorhesis).
Homeostatic mechanisms can be illustrated by a simple interaction between two
populations, such as the hare and the lynx populations pictured in Fig. 1-2. When the
hare population is high the lynx have an abundant food supply and procreate. The lynx
population increases until the lynx outstrip the available hare population. Deprived of
adequate food, the lynx population then decreases, while the hare population increases
because there are fewer predators. This increase, in turn, provides more food for the
lynx population, and the cycle repeats. The numbers of each population are continu-
ally changing, making the system dynamic. When studied over a period of time, the
presence of this type of self-regulating feedback makes the system appear to be in a
steady state, which we call homeostasis.
In reality, populations rarely achieve steady state for any extended period of time.
Instead, populations respond to physical, chemical, and biological changes in the
environment along a positive feedback trajectory that will eventually settle into a new,
but again temporary, homeostasis. Some of these changes are natural (e.g., a volcanic
eruption that covers the lynx and hare habitat with ash or molten rock); many are
caused by humans (e.g., destruction or alteration of habitat, introduction of competing
species, trapping or hunting).
Ecosystem interactions obviously can also include more than two species; con-
sider, for example, the sea otter, the sea urchin, and kelp in a homeostatic interaction.
The kelp forests along the Pacific coast consist of 60-m (20043) streamers fastened to