The diet of the platypus Ornithorhynchus anatinus was studied by examination of material collected from the cheek pouches of animals captured while foraging in streams in Kangaroo Valley, NSW, Australia. Platypuses consumed benthic invertebrates from 55 families in 16 orders, with virtually no prey being derived from the terrestrial environment. We also sampled invertebrates in pool, riffle and stream edge habitats to identify where prey were obtained. Invertebrates in the diet were most similar to those collected along stream edges and in pools compared with the faster-flowing riffles, suggesting that platypuses focused their foraging activities largely in these deeper water habitats. Although there was no seasonality in the assemblage structure of macroinvertebrates, the diet of platypuses varied between seasons, notably between winter and summer, suggesting that some dietary selectivity is seasonal. Dietary differences between the sexes were not detected. Overall, our results suggest that some dietary selection occurs in the platypus with respect to both foraging habitat and season. Seasonal selectivity may reflect different metabolic demands on platypuses at different times of the year. In contrast, habitat selectivity may reflect difficulty of prey access and risk of prey escape in fast-flowing riffles, higher energy costs and risk of predation associated with exploiting this habitat, and prey avoidance responses that are more rapid in the shallow riffles than in the deeper water pools and stream edges. These alternatives await evaluation by future research.

The serological findings demonstrate that these animals are constantly exposed to infection with Leptospira in their environment, but it is not known if platypuses suffer from clinical leptospirosis or if they mount an immune response, but are unaffected by the bacteria. The prevalence of Leptospira infection among the platypus population could not be precisely estimated because of the unknown number of individuals inhabiting the Wollondilly River inside the survey property. Domestic livestock, mostly cattle, may be the major source of Leptospira infection. The effects of this disease on population dynamics and on reproduction in wild platypuses are not well understood. The role of other wildlife in the transmission and maintenance of Leptospira in the environment is unknown.

in driving trophic cascades 22 : mesotrophic lakes and low biodiversity ecosystems are most likely to demonstrate cascading effects between trophic levels 23. Cascading effects are also produced if interaction strengths are large, and target species monopolise the available resources 21. A recent review 24 has stressed the importance of cross-taxonomic and cross-system approaches to provide broader views of the role of predators in ecosystems, hence, studies on the same top predator species in contrasting ecosystems should be of considerable value. We take this approach here, and examine specifically the influence of a mammalian top predator-the platypus (Monotremata: Ornithorhynchus anatinus)-on the structure of invertebrate communities in lotic and lentic systems in eastern Australia. Platypuses are medium-sized (body mass: males 0.8-3 kg, females 0.6-1.7 kg 26 , semi-aquatic, largely nocturnal predators that feed almost exclusively on benthic invertebrates 27-30. Platypuses may forage continuously for over half the diel cycle 31 with up to 75 foraging dives per hour 32 , consuming 13-19% of their body weight each day 33,34. During late lactation daily food intake can reach 90-100% of body weight 35. Although distributed widely in freshwater systems in eastern Australia, platypus numbers are declining, and the species' status is not secure 25. Given their high metabolic requirements and, in some locations, high abundance, platypuses are likely to have strong impacts on benthic invertebrate communities through direct consumption and by eliciting predator avoidance behaviours. As platypuses may select for different invertebrates 29 , they could potentially alter invertebrate community composition such as by reducing the abundance of herbivores which may, in turn, trigger trophic cascades that extend to autotrophs 36,37. To investigate these possibilities, we explored whether the presence of platypuses influenced both invertebrates and epilithic algae in a westerly flowing stream in New South Wales (lotic) and a sub-alpine lake in Tasmania (lentic). In addition, platypus foraging behaviour, which involves vigorous overturning of substrata by the forepaws and sifting by the bill, disturbs aquatic substrata and sediments 38. As this activity may move and redistribute sediments, it could further affect the microhabitats used by benthic invertebrates 39 , this possibility was also examined. Using the platypus as a model predator in these contrasting lentic and lotic ecosystems, our specific hypotheses were that platypuses would: