Platypus in a changing world: impacts of land use and climate

Abstract

Human activity affects natural environments in a range of ways, including through the impacts of climate change and land-use conversion. Of particular concern are the potential impacts these changes will have on top predators and, in turn, on food webs and ecosystems. This thesis examines the effects of drought and land use on the food-web role and body condition of a top aquatic predator, the platypus (Ornithorhynchus anatinus). The platypus is reliant on freshwater environments for prey and habitat and is likely an important top order consumer in eastern Australia. Stable isotope analysis was used to investigate the trophic relationships of platypuses when subjected to drought and land-use change. During drought, platypuses had lower body condition and fed at a lower trophic level. While basal resources did differ slightly for the drought and post-drought food webs, terrestrial vegetation was supporting the platypuses’ main prey species under both conditions. A shift was observed from lentic (standing water) invertebrate taxa to lotic (running water) invertebrates after the drought, which underscores the flexibility of the platypuses’ diet. The effects of urbanisation were complex. There was no effect detected of urbanisation extent (measured as catchment total imperviousness) on platypus condition. However a significant relationship was observed between platypus condition and distance from the central business district of Melbourne, interpreted as an impact of time since urbanisation occurred. There was some evidence of an effect of urbanisation on invertebrate community composition, but platypus trophic relationships appeared to largely be driven by prey availability rather than urbanisation per se. A comparison between platypus from catchments with urban, agricultural and native forest land cover revealed that platypus body condition, food chain length and isotopic niche width varied with land-use type. Platypuses in agricultural catchments were in better condition than those from forested and urban systems. To explore a potential mechanism for the effect of urbanisation on platypus, a platypus-substrate interaction experiment was carried out. Platypus showed a preference for pebble substrates and fed effectively in those settings. A feature of urban systems is high inputs of fine sediment and loss of pebble and cobble habitats. This may reduce the quality of habitat for platypuses in urban settings. The present study demonstrates that the resource and food-web flexibility of the platypuses' diet is the key to its ability to survive under drought and land-use stressors. However this also shows that platypuses may be vulnerable to the accruing effects of those impacts through time. The greatest challenges for platypus populations into the future are likely to be ongoing habitat modification, changes in drought frequency as a consequence of climate change, and competition for freshwater resources with consumptive users.