Preliminary Results On The Effects Of Environmental Parameters On Food-Web
Complexity In The Florida Everglades
Alissa J. Williams, Joel C. Trexler
Florida International University, Miami, FL 33199
Examination of food-web dynamics is vital to the understanding of basic
ecosystem processes. Food-web dynamics are a result of ecological processes and
interactions within the ecosystem. Changes within an ecosystem can produce
variation in the mechanisms of feeding
relationships. Food webs effectively
determine system energy flow and material
transfer, hierarchy of trophic relationships,
and species interactions. Therefore, detailed
knowledge of the factors affecting food-web
structure and complexity are extremely
important in the characterization of any
ecological system.
The Florida Everglades are a protected
wetland ecosystem characterized by unique
patterns of climate, hydrology, nutrient
regime, and trophic structure. Unlike many
other freshwater ecosystems, studies in the
Everglades have depicted a system
supporting unusually high standing crops of
algae in periphyton mats, yet very low
standing crops of consumers. To explore
the origins of this distinctive trophic
structure, we studied the relationship
between food-web structure and the abiotic
environment.
Fig. 1. Map of the 20 study sites
To estimate spatial variation in the length of
Everglades marsh food webs, samples of
representative groups from basal,
intermediate, and top trophic levels were
Fig. 2. 1m2 throwtrap used to sample
collected in Fall 2002 from 20 sites in
fishes and macroinvertebrates
three regions in the Everglades: Taylor Slough
(TS), Shark River Slough (SRS), and the Water Conservation Areas (WCA) (Fig.
1). Food webs in any ecosystem are inherently complex, and it is virtually
impossible to account for every species or functional group without relying upon
significant assumptions. Therefore, to create an index of food-web complexity,
we chose to sample target species at key points in the food web. These species
were chosen based upon previously published Everglades diet studies. The basal
portion of the Everglades food web is largely made up of a periphyton complex,
comprised of both edible and inedible algal species, bacteria, and detritus. Due to
this complex mat structure, it is difficult to separate the algal species and bacteria,
possibly the base of the food web, from the structural components. Therefore,
primary consumers were used as the basal level of the food web for this study.
Specimens of small fishes and macroinvertebrates were captured using throwtrapping and large predatory fishes by electrofishing methods (Fig. 2).
Trophic relationships
among organisms for
each marsh food web
were estimated using
stable isotope and gut
content analysis.
Bulk Periphyton
Traditionally, food
Possible base of food
webs were projected
web
using only gut content
analysis, but more
Vascular plants
recently, stable isotopes
(C, N, S) have shown
-25
13C
to be a useful method
From Loftus 2000
for food-web analysis.
Fig. 3. Diagram of Everglades food-web by stable
This technique, used in
isotope methods
conjunction with gut
Loftus, W.F. 2000. Accumulation and fate of mercury in an
content analysis,
Everglades aquatic food web. Ph.D. Dissertation. Florida
provides a quantitative,
International University. Miami, FL 33199
standardized method
applicable across ecosystems. Stable isotope delta () values indicate the ratio of
the heavy/light isotope assimilated by living material. The light isotopic forms of
nitrogen increase in frequency relative to the heavy one with increasing trophic
level, revealing the trophic position of a species relative to the baseline organism
(~3-4‰). Carbon isotopes remain relatively constant with increasing trophic
position (~1‰), helping to reveal the base of the food web leading to that
organism (e.g., algal or bacterial/detrital loops). Trophic structure can then be
reconstructed based upon these predictable changes in elemental magnitude (Fig.
3). Food webs were analyzed and reconstructed at each site using an estimator of
trophic position. Periphyton productivity was measured at each site using
light/dark bottle incubation experiments. Hydrological data were obtained for
each study site to characterize hydroperiod. Food webs were then compared
among sites to examine possible relationships between food-web structure,
hydroperiod, and local nutrient status.
 A: Nitrogen
5
Preliminary results (first
4
sampling season) show
significant variation in
3
Shark River
trophic relationships
2
Slough
among sites. Much
Taylor
1
Slough
variation is observed in
0
13C values, indicating a
-1.000
-0.500
0.000
0.500
1.000
1.500
2.000
2.500
-1
possible shift in the
Water Conservation
relative role of detrital
-2
A reas
vs. algal components at
-3
 A: Carbon
the base of the food web.
MD
TS
SR S7
SR S8
SRS37
W CA1
WCA5
WC A4
WC A3
This shift may also result
from variation in the
Fig. 4. Preliminary corrected stable isotope data for
relative availability of
amphipods for nine sites
heavy and light isotopes
at each site (a non-biological explanation). To illustrate spatial variation in
isotope signatures, we adjusted the average amphipod value from each site to the
grand mean for that species (Fig. 4). These data revealed wide variation within
and among regions (TS, SRS, & WCA) in both 13C and 15N values amphipods.
This variation may be due to changes in environmental parameters, or to a
difference in diet at each site. Stable isotope data for mosquitofish at each site
was corrected for environmental variation by subtracting the amphipod deviation
values. 15N values for mosquitofish revealed little variation among sites (~23‰), with the exception of site SRS 37 which exhibited significantly lower 15N
values than other eight sites. 13C values for mosquitofish revealed a much wider
range, indicating possible differences in the relative roles of detrital vs. algal
components as the food-web base.
This research was funded in part by NSF Florida Coastal Everglades Long Term
Ecological Research Program, USGS CA 1445-CA09-95-0112, Subagreement
No.1 , and a US EPA STAR Fellowship
Alissa, Williams, Florida International University, University Park, Miami, FL
33199, Phone: 305-348-4032, awill013@fiu.edu