Discrimination Between Exploitative and Interference Competition Between Cladocera and Keratella Cochlearis

Rotifers and cladocerans are important components of most freshwater plankton communities. Large cladocerans can suppress rotifers through both mechanical interference competition (IC) and exploitative competition (EC) for shared food resources. We assessed the relative extent by which small— and large—bodied cladocerans suppressed the rotifer Keratella cochlearis by these mechanisms, using circulating algal suspensions in dual—chamber laboratory cultures and natural animal densities. The small—bodied cladoceran Ceriodaphnia dubia suppressed Keratella exclusively through EC, while a slightly larger species (Daphnia ambigua) did it primarily by this mechanism. Adults (Daphnia galeata mendotae and Daphnia pulex) and subadults (Daphnia pulex) of two much larger bodied cladocerans suppressed Keratella primarily through IC. The ratio of initial Keratella to adult Daphnia pulex density had little effect on the relative importance of the mechanisms–IC always greatly exceeded EC. Suppression by EC occurred in all treatments only after food was depleted below the concentration necessary to support maximal rotifer population growth, while suppression by IC occurred at both high and low food concentrations. These two mechanisms by which cladocerans interact with rotifers should affect the composition of natural rotifer communities in markedly different ways; IC should favor invulnerable rotifer species, many of which are large bodied, while EC should favor rotifer species with low food requirements, which tend to be small bodied.