EFFECTS OF PREDATOR INTERACTIONS, PREY PALATABILITY AND HABITAT STRUCTURE ON SURVIVAL OF NATTERJACK TOAD BUFO-CALAMITA LARVAE IN REPLICATEDSEMINATURAL PONDS

The significance of predation and aquatic habitat structures to the su rvivorship of natterjack toad Bufo calamita larvae was investigated by manipulating predator numbers and pond characteristics in a series of replicated semi-natural pools over three consecutive years. Two speci es of fish (common tarp Cyprinus carpio and perch Perca fluviatilis) i ncreased the survival of small tadpoles severalfold by selectively con suming predatory invertebrates, bur a third species of fish (rudd Scar dinius erythrophthalmus) devoured tadpoles and invertebrates indiscrim inately. Survival of larger tadpoles later in larval development was l ess affected by the reduction of invertebrate predation pressure from carp and perch, probably because abiotic factors (pond desiccation and anoxia) were stronger agents of tadpole mortality. In ponds of low pH (ca 4.5) there was greatly increased spawn mortality and reduced tadp ole growth rates but no significant change in tadpole predation compar ed with circumneutral controls. Neutralisation of acid ponds to pH 7 b y addition of Ca(OH)(2) restored spawn viability and tadpole growth ra tes to control levels without affecting predation level. Addition of o rganic nutrients stimulated tadpole growth rates significantly in olig otrophic ponds but not sufficiently to improve survival of small larva e in the Face of predation. Extensive growths of macrophytes increased predator numbers up to more than twofold but effects on tadpole morta lity rates differed between experiments. Replacement of natural substr ates by concrete basins substantially increased tadpole survival throu ghout development, probably because both predation by invertebrates an d abiotic mortality factors were ameliorated. Predation was a strong f orce early in natterjack tadpole development irrespective of chemical and biological conditions within ponds, but became much less important compared with abiotic factors as an agent of mortality at later times .