Environmental Engineering Reference
In-Depth Information
levels. Due to the distinct seasonality with dry and wet phases, the seasonal changes
in the distribution of flooded and non-flooded areas are extremely pronounced. As
the elevation gradient of the Everglades landscape is only minimal, small differ-
ences in mean water levels can alter the fraction of flooded habitat drastically.
Surviving organisms must be able to cope with these altering hydrological condi-
tions (Trexler et al. 2002), which also determine the available foraging area for
wading birds and thus, have influence on their breeding cycles. In this context, a
substantial decline of the traditional bird communities in the Everglades has
occurred over the past several decades (Ogden 1994). There is empirical evidence
that in wetlands large planktivorous and piscivorous fish that are sensitive to
seasonal changes in water depths, periodically move in and out of local areas in
which the water depth changes (Trexler et al. 2005; DeAngelis et al. 2007). This
affects the temporal pattern on which trophic cascades influence the Everglades
food web (Dorn et al. 2006; Chick et al. 2008). Within the cycle of annual re-
flooding of extensive wetland areas, trophic cascades caused by invading fish can
lead to significant changes in the whole aquatic food web structure of the Ever-
glades (DeAngelis et al. 2010). Depending on individual traits of the fish species,
they can disperse and exploit different habitat types; e.g. opportunistic fish species
can disperse into and exploit re-flooded areas first, while gleaner species, which are
good at exploiting resources at low levels, are more successful in dominating
permanently flooded wetland areas. The specific combination of heterogeneity in
elevation and fluctuations of water level can lead to a community of multiple
coexisting species feeding on the same resource (DeAngelis et al. 1998; Jopp
et al. 2010).
We now investigate the dynamics of such a food web structure with an annual
standard water level fluctuation of 0.6 m amplitude. To do this we introduce a
spatially-explicit model framework (for detailed model descriptions, see: DeAngelis
et al. 2010; Jopp et al. 2010), that includes the main physical factors, seasonal water
level fluctuations and a linearly increasing topographic elevation. The model con-
sists of a simple aquatic food web, as well as rules of movement for certain species
populations in the simulated spatial environment. The food web consists of six
groups: a functional group of invertebrates, three separate small fish species,
which differ in their traits (F1
¼
an intermediate fish, F2
¼
a good disperser,
F3
a good exploiter), crayfish, and a piscivorous fish species, which is the top
predator. There is also a periphyton functional group, which is a mixture of algae and
microbes that serves as a food source for fish and crayfish. Non-living pools exist for
detritus and nutrients. For each of these components, there is a set of specific
differential equations (see Chap. 6: Ordinary Differential Equations), which
describes the interactions between the components. The functional responses of
the small fish and crayfish are not Holling type 2, but rather Beddington-DeAngelis
(Beddington 1975; DeAngelis et al. 1975). This assumes that the fish are somewhat
territorial and thus self-limiting, and helps to stabilize the dynamics of the model.
The functional response of the top predator follows a Holling type 2 function. The
whole model is spatially explicit, with the food web dynamics occurring on a grid of
100
¼
100 cells. Each spatial cell is assumed to be 200
200 m, which results in an
