Environmental Engineering Reference
In-Depth Information
Fig. 5.4 Portion of original
energy that remains as path
length increases (From
Braner [
11
], reprinted with
permission). There is still
noticeable energy in the
system after 10 steps
100
10
1
Cone Springs
0.1
Silver
Springs
0.01
Barataria
Bay
0.001
1
23
45678
Path Length
9 101112131415
cycled by exiting and reentering the same compartmental node. In other words,
about 3,000 kcal/m
2
/y of the total system throughflow is comprised of energy due
to cyclic pathways which retain the energy in the system. This additional boost is
important to the overall function of the Cone Spring ecosystem. A noticeable
contribution of cycled flow is a common phenomenon in all ecosystems. Another
way to demonstrate this importance of cycling, and the fundamental shift it has on
how an ecosystem should be viewed, was given by Braner [
11
]. While
investigating the same five-compartment Cone Spring model above, he showed
that cyclic pathways identified by flow analysis reveal that the original boundary
flow persists in the system much longer than obviously apparent. For contrast, in
a five-compartment food chain model - a type often used, incorrectly, to represent
an ecosystem- the longest path could only be four steps in length fromX1
!
X
2
!
X3
X5. Real ecosystems have more complex structures with cycles. After
those four steps, the original flow from compartment 1 would exit the system at
compartment 5. According to Braner, more than 10% of the flow remains in the
Cone Spring ecosystem after four steps. In fact, approximately 1% of the original
flow remains after 9 steps and 0.001% is left after 15 steps (
Fig. 5.4
). A similar
result is shown for two other ecosystems in the same figure. Therefore, the cycles,
evident from flow analysis, play a very important role in the system having enough
resource to function and provide ecosystem services.
!
X4
!
Ecosystem Goal Functions
Flow analysis has another useful feature related to understanding ecosystem dynam-
ics. Odum [
12
] proposed 24 different attributes which describe the ecosystem
development, for aspects such as community energetics, nutrient dynamics, and
overall homeostasis. The attributes dealing with energetics, which change during the
