Nonpredatory Interspecific Interactions among Plants and Animals in Freshwater Communities - Freshwater Ecology: Concepts and Environmental Applications - page 416

The idea that facilitation (any unidirectional positive effect of one
species on another) and perhaps mutualism may be important and over-
looked aspects of community interactions has received some attention
(Bertness and Callaway, 1994). Plants in stressful environments can facili-
tate each other (Callaway and Walker, 1997; Callaway, 1995). Few macro-
phyte and wetland plant communities have been studied with regard to fa-
cilitation, but it could be important in stressful freshwater habitats, as has
been demonstrated for estuarine marshes (Bertness and Hacker, 1994). For
instance, emergent freshwater marsh plants that are aerenchymous (trans-
port oxygen to their roots) can facilitate other emergent plants living
nearby by aerating the sediments (Callaway and King, 1996). As more re-
search is done on aquatic plant communities, examples of facilitation will
likely be documented.
Indirect facilitation may occur in streams. Crayfish exclude the green
alga Cladophora from pools. The exclusion of Cladophora facilitates the
growth of epilithic diatoms. The diatoms in turn support an increased bio-
mass of grazing insect larvae (Hart, 1992; Creed, 1994). Such complex in-
teractions may be a common feature of communities.
Similarly, invertebrates likely facilitate each other more often than has
been recognized. Facilitation was demonstrated in a group of three
oligochaete species (Fig. 20.3), but the mechanisms were not clear (Brinkhurst
et al., 1972). In this case, two of three species tested had greater weight gains
when grown with the others than when grown in a single-species culture. Fa-
cilitation may also be a common feature of stream invertebrates that process
litter. Shredders excrete fine particulate organic material that is ingested by

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L L+T L+P L+T+P T T+L T+P T+L+P P P+L P+T P+L+T

Limnodrilus Tubifex Peloscolex
FIGURE 20.3 Growth of three tubificid oligochaetes alone and in culture with the other
species. L, Limnodrilus hoffmeisteri; T, Tubifex tubifex; and P, Peloscolex multisetosus . Note
that in two of three experiments, growth was greater in the presence of the other two species
(data from Brinkhurst et al., 1972).