Nonpredatory Interspecific Interactions among Plants and Animals in Freshwater Communities - Freshwater Ecology: Concepts and Environmental Applications - page 415

are a few of many that suggest that competition is an important, but con-
text-dependent feature of zooplankton communities.
Competition for food has led to clear specialization over evolutionary
time of benthic freshwater macroinvertebrates. Thus, invertebrates can be
classified into functional feeding groups based on mode of food acquisi-
tion, such as collectors, filterers, shredders, and scrapers. Within these
groups, competition can still occur. For example, net-spinning hydropsy-
chid caddis flies collect particles with silk nets of different mesh sizes, with
each mesh size characteristic of a species. Smaller mesh nets are more effi-
cient at collecting small particles, whereas large meshes capture large par-
ticles more effectively (Loudon and Alstad, 1990). Thus, species with fine
nets will likely compete more effectively for fine particles. Such competi-
tive specialization may determine which species dominate.
Competition can play a role in the establishment of anuran assem-
blages (Morin, 1983). In some ponds, predators remove the competitive
dominants. Predation thus allows less competitive species to coexist with
stronger competitors.
Competition among fish may be an important consideration for fish-
eries managers. For instance, Hodgson et al. (1991) demonstrated that in-
troduction of rainbow trout (Oncorhynchus mykiss) into lakes with 2- and
3-year-old largemouth bass (Micropterus salmoides) can lower the condi-
tion (weight to length ratio) of bass. The diet of the bass shifted from
Daphnia to odonate naiads, and bass condition was lower after the intro-
duction of trout into one lake, particularly when compared to a nearby
lake with no trout.
MUTUALISM AND FACILITATION
Mutualisms (both species have a positive effect on each other) are less
conspicuous in freshwater than in marine systems, possibly because the
continuous time for evolution of mutualisms has been less in freshwaters
than in marine systems (i.e., freshwater habitats have a shorter continuous
history than marine or terrestrial habitats). However, it seems that some of
the conditions for mutualism occur in freshwater. For example, fish that
clean other fish are common on marine reefs, and this mutualism involves
many species of fish from diverse taxonomic groups, but the same interac-
tion has not been identified in freshwaters despite comparable benefits to
freshwater fishes. Many of the mutualisms that occur in freshwaters in-
volve microorganisms and were discussed in Chapter 18.
Mutualisms based on behavior require coevolved systems and organ-
isms capable of complex behavioral patterns, such as fishes. Cichlids from
Lake Tanganyika demonstrate parental care, including guarding eggs and
fry from predators, and two species can brood in the same region and mu-
tually defend their broods (Keenleyside, 1991). Fish in the same lake have
evolved cooperation in which predators hunt in mixed groups and this co-
operation increases success (Nakai, 1993). Mixed-feeding schools may oc-
cur in other fish assemblages but have not been well studied. This is not a
completely unique adaptation; birds have demonstrated a similar coopera-
tive strategy of mixed-feeding flocks.