Species diversity in the Florida Everglades, USA - Environmental ...

274 M. T. Brown et al. Ecosystem biodiversity in the Florida Everglades
Figure 8. Expected-to-Observed Emergy Throughput (EOET) vs. Transformity for the graminoid marsh.
cally, the nearly order of magnitude difference between
wet and dry season transformities – with dry seasons exhibiting
signifi cantly lower transformity values – indicates
dramatic differences in resource consumption rates
and process effi ciencies that may be artifacts of the data
used. While there is considerable uncertainty in the specifi
c values computed for each component, we note that
the transformity values obtained are in strong agreement
with similar calculations made using other techniques
(Odum, 1996).
While network analyses (Ulanowicz, 1986; Dame
and Patten, 1981) and emergy refi nements proposed
herein offer little insight into the particular cybernetic
controls provided by each species in the system, transformity
has been proposed as an informative measure of
the expected importance of each component (Odum,
1996). Ulanowicz (1980), in developing the ecosystem
concept of ascendency, recognized the need to scale fl ow
diversity at the system scale by total system throughput
to adequately compare between systems. We propose that
system-scale fl ow diversity further requires adjustment
for energy quality (transformity) before meaningful
cross-trophic comparisons of importance and betweensystem
comparisons of development or stress can be
achieved. As such, we propose an emergy-based ascendency
framework as an avenue of further research (see
Christensen, 1994, for a preliminary effort).
Deviance from the Shannon index maximum, as applied
herein, may be a useful indicator of system condition.
This may be true at the ecosystem scale and at the
scale of particular ecosystem component. Our effort to
identify specifi c organisms that exhibit deviation from
expected importance (EOET, measured as a fraction of
expected productivity adjusted for quality) showed increases
in EOET with transformity. This observation
suggests that the ecosystem is systematically depauperate
in the upper trophic levels, a conclusion consistent
with the documented losses in wading bird populations,
alligators and top carnivores. Notably, those species with
high normative value in society (i.e., those for which
wildlife management plans and on which considerable
public scrutiny are focused) are generally those with high
EOET values, indicating their rarity relative to expected
emergy fl ow contributions. For purposes of evaluating
ecosystem condition, we would expect that during recovery
the trend of EOET with transformity would tend towards
no association (r 2 ~ 0). However, were the ecosystem
to continue to degrade, we would expect an
increasingly signifi cant positive association between
EOET and transformity as lower quality ecosystem components
are selected for, and ecosystem hierarchy and
organization diminish.
While we would not postulate that all components of
an ecosystem would be present in equal importance (and
therefore there might be signifi cant variability in EOET
even in healthy ecosystems), we do expect that intact
ecosystems would demonstrate approximately equal
emergy at all trophic levels, and consequently no systematic
association between EOET and transformity. If
this were not the case, net production at one trophic level
would be failing to generate production at the next
trophic level.