Environmental Engineering Reference
In-Depth Information
result of being free from fish predation. Further, there is a decrease in invertebrate
population to the left, which is a result of an increase in fish populations after the
fish have been freed from piscivore top-down predation.
18.4 Discussion and Outlook
Trophic cascading effects are an important feature of the trophic structure of
aquatic ecosystems. After describing the working mechanisms, we applied the
theory with a spatially-explicit model that we used for long-term simulations of
10 years for the dynamics of the basic structure of the Everglades food web.
A specific feature of the Everglades is the annual fluctuation in water level,
which is also fundamental to many other wetlands; e.g. the Pantanal (Heckman
1998), Do˜ana (Serrano and Serrano 1996). We have used our simplified model
representation to study such a system using a minimal food web model on a
100
100 cell landscape, which has an elevation gradient and is exposed to
fluctuating seasonal water levels. The model structure and parametrisation were
suggested from empirical knowledge derived from the Everglades system. Some
important features stand out in the computer simulations.
l
The fish, in particular, but also crayfish, show distinctive pulses at the edge of the
drying front as water levels decline. Some small pulse-like behaviour can be
observed following the flooding front during rising water level, but the pulses are
much smaller. The difference is due to two main effects. First, nearly all fish that
are not stranded move to lower elevation in response to the drying front. On the
other hand, when cells are re-flooded, a smaller fraction of the fish invades the
newly flooded cells. A perhaps more important reason for the weaker pulses with
rising water is that fish moving up the flooding front deplete the supply of fish in
the donor cells, which are resupplied slowly by diffusion from the flooded cells
at lower elevations. When water levels are falling, the retreating fish pile up with
fish already present in the still-flooded cells, which in turn pile up in the cells
below as water continues to drop.
l
Trophic cascades can be discerned in the simulations, but they are relatively
moderate compared to such cascades that we have discussed before. One
reason for this may be that our food web is not a linear trophic chain, but
contains omnivory as well; that is, piscivorous fish consume all of the fish
species, as well as crayfish. Although omnivory is known to work against the
strength of trophic cascades (Strong 1999; Nystroem et al. 1996), lake systems
contain omnivory as well, so this does not completely explain our results.
Another reason, however, is that diffusion is an important mechanism in our
model. Smaller fish can move in and out of the zone in which there is a high
concentration of piscivores, which may tend to smooth out the top-down effects,
which is supported by other studies (Howeth and Leibold 2008; Holyoak et al.
2005; Leibold et al. 2004).
