Environmental Engineering Reference
In-Depth Information
waterbirds and caimans, and they exhibit algal blooms. Eventually, all but
the deepest pools dry and then fill in the following wet season, when the
cycle begins again.
Successional patterns of macrophytes in riparian wetlands are found
elsewhere. In the Rhone River in France, successional sequence depends on
the degree of connectivity to the main channel. The diversity is highest in
frequently flooded habitats because more propagules are available to es-
tablish plants (Bornette
et al.,
1998b). In a more spatially restricted study,
four species of macrophytes were found to coexist in a side channel:
Spar-
ganium emersum
was found in the least disturbed areas of the channel,
Hippuris vulgaris
and
Groenlandia densa
were found in moderately dis-
turbed areas, and
Luronium natans
was found in the most disturbed areas
(Greulich and Bornette, 1999). This pattern illustrates the relationship be-
tween competitive ability and successional processes. Without disturbance
and succession,
S. emersum
would probably be the only species present be-
cause it is the competitive dominant.
Understanding successional patterns may be crucial in wetland restora-
tion schemes (Middleton, 1999). Two general lines of thought are taken:
The first is that succession depends on the initial inhabitants, and the sec-
ond is that a natural successional series will restore a wetland if the abi-
otic features are reproduced. Thus, some people assume that the manager
must plant the species desired, and others assume that species will natu-
rally populate a restored wetland (Middleton, 1999). The reality probably
lies in-between these two points on a continuum. If a severely disturbed
wetland is to recover, reintroduction of lost or rare species may be neces-
sary. Alternatively, in a wetland of any size, it is difficult to control exactly
which species become established over the years and where.
Rivers and streams may also undergo succession on seasonal timescales.
In small streams in deciduous forests (Fig. 20.7), the light regime and leaf
input vary over the seasons. High light and low litter inputs favor peri-
phyton and species that consume it (scrapers). High litter inputs favor
species that depend on leaves for nutrition (shredders). Seasonal flooding
may also alter the diversity and biomass of species in such a stream. In-
vertebrates in temperate streams can be categorized into three groups:
slow-seasonal, fast-seasonal, and nonseasonal life cycles (Anderson and
Wallace, 1984). Many slow-seasonal insects reproduce in the fall and grow
through the winter (e.g., winter stoneflies and some Trichoptera). These
species may specialize on the large amount of leaf litter entering the stream
in fall. Fast-seasonal insects have a prolonged diapause and a short period
of rapid growth followed by reproduction. These species may reproduce in
spring, summer, or fall. Nonseasonal species may have very long life spans
or very short and overlapping generations. Data collected from a small
Michigan stream suggest that total invertebrate biomass remains approxi-
mately constant throughout the year despite the dominance of different
species in specific seasons (Cummins and Klug, 1979). For instance, insect
larvae that are shredders attain maximum biomass in spring after a winter
of feeding on detritus.
Seasonal flooding is also an important feature of desert streams. A pre-
dictable sequence of colonization events by algae occurs in Sycamore
Creek, Arizona (Fig. 20.8). Quick colonizers include diatoms. Filamentous
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