INTENSE PREDATION CANNOT ALWAYS BE DETECTED EXPERIMENTALLY - A CASE-STUDY OF SHOREBIRD PREDATION ON NEREID POLYCHAETES IN SOUTH-AFRICA

The effect of predation by curlew sandpipers Calidris ferruginea L. an d grey plovers Pluvialis squatarola (L.) on populations of nereid worm s Ceratonereis keiskama (Day) and C. erythraeensis (Fauvel) was studie d at the Berg River estuary, South Africa, by comparing observations o f shorebird-foraging intensity with the results of a population study of two species of nereid worms within and outside bird exclosures. The study was carried out during the four-month period prior to northward migration of shorebirds. Population structure of the two nereid speci es differed considerably. Ceratonereis keiskama reproduced earlier tha n C. erythraeensis and only young individuals were present during the study. By contrast, old C. erythraeensis were available to the birds a t the start of the experiment and young animals entered the population during the experiment. Despite selective predation on certain size cl asses of nereids by the birds, no significant changes in the populatio n structure of either nereid were detected by the cage experiment. Num bers and biomass of both Ceratonereis spp. in paired controls and cage s tracked each other and did not diverge as predicted. A consistent di fference in the depth stratification of the two nereids may, however, have been due to predation pressure. Curlew sandpipers were calculated to remove 3112 nereids per m2 during the three months, equivalent to 4.4 g (dry weight) per m2. This represents 58% of the initial numbers and 77% of the initial biomass of nereids. Although predation on nerei ds by waders was exceptionally high at the Berg River estuary, any dep letion in numbers or biomass of nereids caused by these predators was masked by the reproduction of the nereids. The fact that the predators ' high energy requirements prior to northward migration coincide with the period of peak production of invertebrate prey makes the Berg Rive r estuary an exceptionally favourable wintering area.