Several mechanisms have been proposed to explain the observed difference in strength of trophic cascades between lentic and lotic food webs, and these are based largely on attributes of the predator and prey species and the systems that they occupy. It has been argued that such predator effects are greater 19 and act over shorter timescales 16 in aquatic than terrestrial systems, and are greater in lentic benthos than in streams 19. For example, a predator that directly suppresses populations of herbivorous prey species could be expected to indirectly favour plant biomass and diversity 17, 18. Such 'trophic cascades' have been defined as “indirect species interactions that originate with predators and spread downward through food webs” 16. The effects of predators often extend beyond their immediate prey to lower trophic levels. ![]() Indeed, Morin 14 argued that vertebrate predation was the cornerstone of aquatic community theory, and an overview of aquatic and terrestrial ecosystems found that the strongest interactions occurred in association with invertebrate herbivores and endothermic vertebrate predators 15. ![]() Some invertebrates 11, as well as many species of fish and other vertebrates, have strong predatory effects 12, 13. These effects of predators were first noted in limnological studies in the 1960s 6, 7, and subsequent research has confirmed predation to be a major structuring force within many freshwater communities 8, 9, 10. 4, or if predator-presence influences the behaviour or other traits of prey individuals (non-consumptive effects,e.g. Such influences can arise if predators selectively depredate prey species from the array of prey taxa that is available (consumptive effects,e.g. Predation can strongly influence the size and dynamics of prey populations and the structure of prey communities 1, 2, 3. Platypuses can clearly have both strong and subtle effects on aquatic food webs that may vary widely between ecosystems and locations, but further research is needed to replicate our experiments and understand the contextual drivers of this variation. Platypuses did not cause redistribution of sediment via their foraging activities. Contrary to expectation, predation effects were stronger in the stream than the lake no effects were found on algae in either ecosystem due to weak effects of platypuses on herbivorous invertebrates. Platypuses had strongly suppressive effects on invertebrate prey populations, especially detritivores and omnivores, but weaker or inconsistent effects on invertebrate taxon richness and composition. Hypotheses were tested using novel manipulative experiments involving platypus-exclusion cages. ![]() ![]() We investigated the effects of platypus (Monotremata: Ornithorhynchus anatinus) on its benthic invertebrate prey, and tested predictions that this voracious forager would more strongly affect invertebrates-and indirectly, epilithic algae-in a mesotrophic lake than in a dynamic stream ecosystem. Although such effects are well known in aquatic ecosystems, few studies have explored the influence of predatory aquatic mammals, or whether the same predator has similar effects in contrasting systems. Predators can have strong impacts on prey populations, with cascading effects on lower trophic levels.
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