Predation and Food Webs - Freshwater Ecology: Concepts and Environmental Applications - page 391

HERBIVORY

Herbivory can be divided into consumption of macrophytes or micro-
scopic algae. Microscopic algae can be consumed as phytoplankton or as
periphyton. The adaptations of organisms for consuming small cells were
discussed in Chapter 18.
Although invertebrates are the primary consumers of phytoplankton,
planktivorous and omnivorous fishes may ingest suspended algae
(Matthews, 1998). Gizzard shad (Dorosoma cepedianum) can effectively
use the mucus on their gill rakers to trap cells as small as 20 m in diam-

eter. Several species of fish use similar strategies to filter and retain small
particles (Sanderson et al., 1991). Some small Talapia are also able to cap-
ture and ingest phytoplankton.
Numerous large organisms consume periphyton. The effects of grazers
on benthic algae have been extensively studied and can be divided into func-
tional and structural responses. The structural responses of periphyton to
grazers include (i) a general decrease in biomass (but not always);
(ii) changes in taxonomic composition (but
prediction of specific general taxonomic shifts
is difficult); (iii) changes in the form and struc-
ture (physiognomy) of communities, with a
general decrease in large erect forms; and
(iv) alteration of species richness and diver-
sity (perhaps with intermediate levels of graz-
ing leading to maximum diversity).
Functional responses of periphyton to
grazing include (i) a general decrease in pri-
mary production per unit area but not per
unit biomass, (ii) changes in nutrient con-
tent, (iii) increased rate of nutrient cycling,
(iv) increased rates of export of cells from
the assemblage, and (v) alteration of succes-
sional trajectories (Steinman, 1996).
Many fishes consume periphyton and
small macrophytes (Matthews et al., 1987;
Matthews, 1998). Herbivorous fishes that
consume periphyton are common in tropi-
cal waters. The minnow Campostoma
anomalum (the central stoneroller) is an im-
portant herbivorous fish in small streams in
the United States. This minnow can con-
sume significant amounts of periphyton and
will be discussed later with respect to its
role in food webs.
Many organisms consume macrophytes
in aquatic habitats, including crayfish, com-
mon carp, grass carp (Sidebar 19.1), lepi-
dopteran and trichopteran larvae, moose,
and snails. Many tropical fishes consume
macrophytes (Matthews, 1998).

Sidebar 19.1.
Using Grass Carp to Remove
Aquatic Vegetation

The grass carp (Ctenopharyngodon idella) is a
cyprinid that consumes aquatic vegetation as
an adult. This herbivorous fish has been intro-
duced into many areas to assist in removal of
unwanted macrophyte growth. It was first re-
leased in the United States in Arkansas in the
early 1960s and has since become widespread.
This species is a very effective herbivore.
For example, in Texas grass carp were stocked
at 74 fish per hectare in a reservoir with 40%
macrophyte cover. All macrophytes were con-
sumed within 1 year (Maceina et al., 1992).
There was a concurrent increase in cyanobac-
terial plankton and a decrease in water clarity.
There is concern regarding how much this
species can spread, and some states have
completely restricted its use for macrophyte
control. The temperature range for successful
reproduction is 19-30C (Stanley et al., 1978),
and because reproduction occurs mainly in
larger rivers, it was thought that the grass carp
was unlikely to spread when added to ponds.
However, ponds have breached and reproduc-
tive fish have escaped. As a result, grass carp
larvae have been found in the lower Missouri
River, and the species has likely become es-
tablished there (Brown and Coon, 1991).