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predation, coexistence, fish, zooplankton

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Examples from fishless aquatic habitats show that competition among zooplankton for resources instigates rapid exclusion of competitively inferior species in the absence of fish predation, and leads to resource monopolization by the superior competitor. This may be a single species or a few clones with large body size: a cladoceran such as Daphnia pulicaria, or a branchiopod such as Artemia franciscana, each building its population to a density far higher than those found in habitats with fish. The example of zooplankton from two different fish-free habitats demonstrates the overpowering force of fish predation by highlighting the consequences of its absence. Released from the mortality caused by predation, a population of a superior competitor remains at a density equal to the carrying capacity of its habitat, in a steady state with its food resources, consisting of small green flagellate algae, which are successful in compensating high loss rates due to grazing, by fast growth. In such a situation, the high filtering rate of Daphnia or Artemia reduces resources to levels that are sufficient for assimilation to cover the costs of respiration (threshold food concentration) in adults but not in juveniles. This implies long periods of persistence of adults refraining from producing live young, because production of instantly hatching eggs would be maladaptive. Severe competition for limiting resources imposes a strong selective pressure for postponing reproduction or for producing resting eggs until food levels have increased. Offspring can only survive when born in a short time window between such an increase in food levels and its subsequent decline resulting from population growth and intense grazing by juveniles. Such zooplanktons become not only a single-species community, but also form a single cohort with a long-lifespan population. The observations support the notion that diversity may be sustained only where predation keeps densities of coexisting species at levels much below the carrying capacity, as suggested by Hutchinson 50 years ago.