Predation and Food Webs - Freshwater Ecology: Concepts and Environmental Applications - page 392

Some macrophyte species are unpalatable to herbivorous animals be-
cause they are chemically defended against consumption. Macrophytes can
either synthesize or accumulate toxins (Hutchinson, 1975; Porter, 1977;
Kerfoot et al., 1998) and may be too tough for some herbivores to process
(Brönmark, 1985). Some macrophytes, such as the filamentous green alga
Cladophora, can be a poor food source to herbivores because they have
low nitrogen and phosphorus content (Dodds and Gudder, 1992).

DETRITIVORY AND OMNIVORY

Input of terrestrial organic material (allochthonous) into rivers, lakes
(Wetzel, 2001), streams, and wetlands can be significant. Groundwaters
can also receive coarse organic particulate materials from terrestrial vege-
tation (Eichem et al., 1993). This material is consumed by many organisms
and forms the base of the food web in cases with high input rates or where
primary production is limited. Analysis of benthic and pelagic food webs
of a subtropical lake suggests that omnivory
and detritus feeding are a general feature of
aquatic food webs (Havens et al., 1996a).
Most orders of aquatic insects (Cummins
and Klug, 1979) and other groups of inver-
tebrates contain omnivorous organisms that
consume detritus. For example, in the de-
tailed food web documented for a riffle in an
Ontario creek, the majority of the primary
consumers are omnivorous (Fig. 19.2). Any
benthic organism that does not exclusively
specialize on macrophytes, periphyton, or
predation on animals probably consumes de-
tritus as a significant portion of its diet. A
few fishes are detritivores as well, notably
the carp species used in Asian polyculture
discussed in Chapter 21.
The microbial community generally
must first condition allochthonous materials
before they can be profitably consumed (Al-
lan, 1995). Thus, detritivores often eat the
microbes that eat the detritus. In streams,
fungi are a very important component of the
microbial community that conditions leaves
and wood entering from the surrounding ri-
parian vegetation. Detritus-ingesting fish,
such as larvae of common carp (Cyprinus
carpio) and talapia (Oreochromis niloticus),
ingest significant numbers of bacteria in the
detritus they consume (Matena et al., 1995).
Many aquatic animals eat more than one
type of food during their life span. Some or-
ganisms, such as crayfish, can be predators,

One approach to keep stocked fish from re-
producing is to use triploid grass carp pro-
duced with thermal or temperature shocks of
newly fertilized eggs. The triploids are unable
to reproduce and are the only grass carp al-
lowed in some states (Allen and Wattendorf,
1987). Scientific equipment and training are
needed to determine if grass carp are actually
triploid.
A problem with use of grass carp to control
macrophytes, in a management sense, is inap-
propriate overstocking. Some people perceive
macrophytes as a nuisance in lakes and ponds.
If many grass carp are added, all vegetation is
removed. This clears the way for large algal
blooms to occur in the absence of suppression
by macrophytes (except see Lodge et al., 1987,
for an alternative outcome). A rational ap-
proach is to accept a moderate amount of
macrophytes as a healthy component of nat-
ural ponds and wetlands and control them only
if they become so thick as to completely pre-
clude desired uses. In this case, judicious use
of grass carp may be warranted. Sufficiently
low densities should be used so that not all
macrophytes are removed. Nonfertile triploids
should be stocked so that fish will not multiply
and completely remove the macrophytes and,
worse, spread to other habitats and become a
nuisance.