Suppression of rotifer populations by Daphnia: A review of the evidence, the mechanisms, and the effects on zooplankton community structure1

Abstract

There is strong evidence that rotifers cannot become abundant members of freshwater zooplankton communities in the presence of large (≳ 1.2 mm) Daphnia. Indirect evidence comes from field observations and experiments showing that rotifers are replaced by Daphnia in seasonal successions, that rotifers are much less common in communities with than without Daphnia, and that the experimental addition of zooplanktivorous fish causes the removal of Daphnia and a simultaneous increase in rotifer abundance, while the experimental removal of fish has just the opposite effect. Direct evidence comes from field experiments showing that the removal of Daphnia from communities causes a marked increase in the fertility and abundance of rotifers, and from laboratory experiments showing that Daphnia can kill and rapidly exclude rotifers in mixed‐species cultures. Rotifers are suppressed by large Daphnia both through exploitative competition for shared, limiting food resources and through mechanical interference. This interference can cause high mortality rates on susceptible rotifer species, even at low (1–5 ind. liter‒1) Daphnia densities and should affect the species structure of rotifers able to coexist with Daphnia in natural communities. Rotifers seem to be much less inhibited by small 1.2 mm) cladocerans and often co‐occur with them at high densities in natural communities in the absence of large Daphnia. There are several reasons why small cladocerans are less likely than large Daphnia to competitively suppress rotifers: small cladocerans generally do not mechanically interfere with rotifers, are less likely to deplete food resources, are themselves more likely to be suppressed by exploitative competition with rotifers, and usually are more susceptible to invertebrate predation. High rotifer abundance in the absence of Daphnia competition may have an important secondary effect in natural communities—the development of populations of invertebrate predators dependent on rotifer prey.