Pp. 1–12 in Glen, A. S. and Dickman, C. R. (Eds) (2014). Carnivores of Australia: Past, Present and Future. (CSIRO Publishing: Collingwood). Chapter 1 The importance of predators Alistair S. Glen 1 and Chris R. Dickman 2 1 Landcare Research, PO Box 69040, Lincoln 7640, New Zealand 2 School of Biological Sciences A08, University of Sydney, NSW 2006, Australia Abstract The importance of predators for ecosystem function is being recognised increasingly around the world. Predators can have a strong influence not only on their prey but also on one another, with cascading effects on many species and ecosystem processes. Removal of top-order predators can alter the behaviour and abundance of other species, including herbivores and smaller predators. In turn, this can have profound impacts on vegetation and small vertebrate communities. Here, we briefly outline the importance of predators in shaping the past and present biota of Australia. In prehistoric times, Australia was inhabited by a diverse range of avian, mammalian and reptilian carnivores. Although most of the largest carnivores died out in the Pleistocene or earlier, Australia still supports a vast array of flesh-eaters ranging in size from tiny insectivorous mammals to hulking seven-metre crocodiles. These animals vary enormously in their diets, habitat preferences and hunting behaviours. Accordingly, they have very different ecological impacts. Predators have been instrumental in the declines and extinctions of many species, but may also be the saviours of many others. For example, dingoes may help to maintain biodiversity by suppressing large herbivores and smaller predators. Just as predator communities have changed over evolutionary time, they will continue to change in future. The impacts of humans and introduced species are still playing out on the Australian landscape, and many carnivores are threatened with extinction. The shape of things to come will depend largely on the wildlife management practices of today. To maintain or restore functioning ecosystems, wildlife managers must consider the ecological importance of predators. The importance of predators The importance of predators for ecosystem function is receiving ever-increasing recognition. For decades, a ‘doomed surplus’ paradigm predominated in ecology; this stated that predators had little influence on prey populations and took only individuals that were surplus to the population while prey abundance was determined by density-dependent processes such as intraspecific competition and density-independent processes such as weather (Errington 1946). Although this may be true in some systems (e.g. Banks 1999), there is no longer any doubt that predators often do limit or even regulate prey populations (Krebs et al. 1995; Pech et al. 1995; Terborgh et al. 1999; Ale and Whelan 2008; Salo et al. 2010). (Limiting factors determine a population’s equilibrium density; regulation is a density-dependent process that forces the population towards equilibrium (Sinclair and Pech 1996)). It is increasingly clear, in addition, that predators can strongly influence not only their prey but also one another (Palomares and Caro 1999; Glen and Dickman 2005; Donadio and Buskirk 2006; Ritchie and Johnson 2009). Through processes of competition and intraguild predation, predators can have a profound influence on each other’s abundance, distribution and resource use. Because the impacts of different predator species on prey populations are not necessarily equivalent, this in turn can have a strong effect on prey species, and ultimately even on the cover, composition and diversity of vegetation. Such interactions, which extend through multiple links in a food web, are called trophic cascades (Paine 1980; Pace et al. 1999; Schmitz et al. 2000).