Nonpredatory Interspecific Interactions among Plants and Animals in Freshwater Communities - Freshwater Ecology: Concepts and Environmental Applications - page 425

100

90

Diatoms

80

Cyanobacteria

70

60

50

40

30

Bare Sand

20

Filamen to us Gr e e n s

10

0

10 30 50 70 90 110 130

Time Since Flood (D)

FIGURE 20.8 A successional sequence of algal groups in a desert stream following a flood
event (reproduced with permission from Fisher and Grimm, 1991).

How indirect interactions alter predictions made by considering only
direct interactions can be illustrated by the case of one predator with two
competing prey species. A predator may cause extinction of its prey if the
predator and prey are present only in one habitat. Likewise, a competitor
may cause extinction of another species if the second species is an inferior
competitor. If two prey species are present, the predator may switch to pre-
fer the more abundant prey species. This switching can occur because it
can be more energy efficient for a predator to forage preferentially for the
more abundant prey (see Chapter 19). Because the preferred prey species
is not necessarily the competitive dominant, the predator may promote the
coexistence of both species.
Indirect interactions can give rise to mutualistic relationships in lake
communities (Lane, 1985). Mathematical analyses of lake food webs
confirm this for a variety of pelagic ecosystems. Although the details of
such analyses are beyond the scope of this topic, this analysis does sug-
gest that trophic cascades in food webs are not the only important indi-
rect interactions.
A specific example of indirect and complex interactions is illustrated
by the common algal macrophyte Cladophora, its epiphytes, and grazers
of the epiphytes (Dodds, 1991; Dodds and Gudder, 1992). In this case, the
epiphytes generally compete for nutrients and light with the Cladophora .
However, the Cladophora is N limited, and some of the epiphytes fix N
and so may ultimately provide N to the Cladophora . Furthermore, epi-
phytes reduce drag on Cladophora while decreasing advective transport
through the Cladophora filaments. Thus, the epiphytes may harm or help
the Cladophora, depending on the environmental conditions. Invertebrate
insect larvae remove sediments and epiphytes from the surface of the
Cladophora and remineralize nutrients; this may facilitate Cladophora
growth. At high densities, when other food sources are unavailable to in-
vertebrates, they will eat the Cladophora . The Cladophora provides habi-
tat and protection from predation for the invertebrates. Thus, the three