Chris R. Dickman

Alien predators are more dangerous than native predators to prey populations

Alien predators are widely considered to be more harmful to prey populations than native predators. To evaluate this expectation, we conducted a meta-analysis of the responses of vertebrate prey in 45 replicated and 35 unreplicated field experiments in which the population densities of mammalian and avian predators had been manipulated. Our results showed that predator origin (native versus alien) had a highly significant effect on prey responses, with alien predators having an impact double that of native predators. Also the interaction between location (mainland versus island) and predator origin was significant, revealing the strongest effects with alien predators in mainland areas. Although both these results were mainly influenced by the huge impact of alien predators on the Australian mainland compared with their impact elsewhere, the results demonstrate that introduced predators can impose more intense suppression on remnant populations of native species and hold them further from their predator-free densities than do native predators preying upon coexisting prey.

Extreme climatic events shape arid and semiarid ecosystems

6 Climatic changes associated with the El Nino Southern Oscillation (ENSO) can have a dramatic impact on ter- restrial ecosystems worldwide, but especially on arid and semiarid systems, where productivity is strongly lim- ited by precipitation. Nearly two decades of research, including both short-term experiments and long-term studies conducted on three continents, reveal that the initial, extraordinary increases in primary productivity percolate up through entire food webs, attenuating the relative importance of top-down control by predators, providing key resources that are stored to fuel future production, and altering disturbance regimes for months or years after ENSO conditions have passed. Moreover, the ecological changes associated with ENSO events have important implications for agroecosystems, ecosystem restoration, wildlife conservation, and the spread of disease. Here we present the main ideas and results of a recent symposium on the effects of ENSO in dry ecosystems, which was convened as part of the First Alexander von Humboldt International Conference on the El Nino Phenomenon and its Global Impact (Guayaquil, Ecuador, 16-20 May 2005).

The use of habitat mosaics by terrestrial vertebrate fauna: implications for conservation and management

Many species of vertebrates require multiple habitats to obtain different resources at different stages of their life-cycles. Use of habitat mosaics takes place on a variety of spatial and temporal scales, from a daily requirement for adjacent habitats to seasonal use of geographically separated environments. Mosaics of habitats are also required in some species to allow ontogenetic habitat shifts, while in others each sex may have specific requirements that are met by different habitats. The extent and nature of animal movements are key (but generally poorly known) factors affecting the vulnerability of species to landscape change. The requirement by many species for multiple habitats suggests that their conservation will be most effective in a mosaic environment and that protection of certain high profile habitats alone, such as rainforest, will be insufficient to achieve conservation goals. Management regimes that result in homogenization of habitats should be avoided. Priority should be given to research that identifies the extent to which species can locate habitat mosaics, at different spatial scales and arrangements, in modified environments.

Impact of exotic generalist predators on the native fauna of Australia

This paper reviews the impacts of three species of introduced mammalian predators on native fauna in Australia. The feral cat Felis catus, introduced over 200 years ago, is linked with early continental extinctions of up to seven species of mammals, regional and insular extinctions of many more species of mammals and birds, and the failure of management programs attempting to reintroduce threatened native species to parts of their former ranges. Evidence for cat-impact is largely historical and circumstantial, but supported by observations that afflicted native species are, or were, small (<200 g) occupants of open habitat and hence likely to be especially vulnerable to cat predation. The red fox Vulpes vulpes was released successfully in 1871. Its subsequent spread into all except parts of arid and tropical Australia coincided with local and regional declines of medium-sized (450 – 5,000 g) mammals, birds and chelid tortoises. The fox has also created recent failures of many management attempts to recover threatened native species. Unequivocal demonstration of fox-impact has been obtained in removal experiments, especially on rock-wallabies Petrogale lateralis. The dingo Canis lupus dingo, introduced 3,500–4,000 years ago, probably caused the extinction of the thylacine Thylacinus cynocephalus and Tasmanian devil Sarcophilus harrisii on mainland Australia. It effectively suppresses extant populations of large mammals, such as kangaroos, and emus, over large areas. Impacts of all three predators are wrought primarily by direct predation. Negative impacts appear to be increased in spatially fragmented forests where native species are restricted to remnant vegetation, and in arid landscapes when native species become restricted temporarily to scattered oases during drought. Alternative prey, especially rabbits Oryctolagus cuniculus, enhance negative impacts on native species by supporting large populations of the predators. It is concluded that feral cats and especially foxes have major negative impacts on certain small and medium-sized native vertebrates in Australia, whereas dingoes have major negative impacts on large species. Dingoes could have positive effects on smaller native species if they significantly suppress populations of foxes and cats. Further quantification of both the direct and indirect impacts of the three predators on native fauna is needed, and should be obtained from experimental field studies.

Complex interactions among mammalian carnivores in Australia, and their implications for wildlife management

Mammalian carnivore populations are often intensively managed, either because the carnivore in question is endangered, or because it is viewed as a pest and is subjected to control measures, or both. Most management programmes treat carnivore species in isolation. However, there is a large and emerging body of evidence to demonstrate that populations of different carnivores interact with each other in a variety of complex ways. Thus, the removal or introduction of predators to or from a system can often affect other species in ways that are difficult to predict. Wildlife managers must consider such interactions when planning predator control programmes. Integrated predator control will require a greater understanding of the complex relationships between species.

Top predators as biodiversity regulators: the dingo Canis lupus dingo as a case study.

Top‐order predators often have positive effects on biological diversity owing to their key functional roles in regulating trophic cascades and other ecological processes. Their loss has been identified as a major factor contributing to the decline of biodiversity in both aquatic and terrestrial systems. Consequently, restoring and maintaining the ecological function of top predators is a critical global imperative. Here we review studies of the ecological effects of the dingo Canis lupus dingo, Australias largest land predator, using this as a case study to explore the influence of a top predator on biodiversity at a continental scale. The dingo was introduced to Australia by people at least 3500 years ago and has an ambiguous status owing to its brief history on the continent, its adverse impacts on livestock production and its role as an ecosystem architect. A large body of research now indicates that dingoes regulate ecological cascades, particularly in arid Australia, and that the removal of dingoes results in an increase in the abundances and impacts of herbivores and invasive mesopredators, most notably the red fox Vulpes vulpes. The loss of dingoes has been linked to widespread losses of small and medium‐sized native mammals, the depletion of plant biomass due to the effects of irrupting herbivore populations and increased predation rates by red foxes. We outline a suite of conceptual models to describe the effects of dingoes on vertebrate populations across different Australian environments. Finally, we discuss key issues that require consideration or warrant research before the ecological effects of dingoes can be incorporated formally into biodiversity conservation programs.

Keystone effects of an alien top-predator stem extinctions of native mammals

Alien predators can have catastrophic effects on ecosystems and are thought to be much more harmful to biodiversity than their native counterparts. However, trophic cascade theory and the mesopredator release hypothesis predict that the removal of top predators will result in the reorganization of trophic webs and loss of biodiversity. Using field data collected throughout arid Australia, we provide evidence that removal of an alien top-predator, the dingo, has cascading effects through lower trophic levels. Dingo removal was linked to increased activity of herbivores and an invasive mesopredator, the red fox (Vulpes vulpes), and to the loss of grass cover and native species of small mammals. Using species distribution data, we predict that reintroducing or maintaining dingo populations would produce a net benefit for the conservation of threatened native mammals across greater than 2.42 × 106 km2 of Australia. Our study provides evidence that an alien top predator can assume a keystone role and be beneficial for biodiversity conservation, and also that mammalian carnivores more generally can generate strong trophic cascades in terrestrial ecosystems.

Mammals of particular conservation concern in the Western Division of New South Wales

Abstract The Western Division of New South Wales is an administrative region of 325 000 km 2 on the eastern fringe of the Australian arid zone. Since European settlement in 1788, 71 species of native mammals have been recorded in the Division, seven more have been documented only as subfossils, and a further 15 species occur within 100 km of the Divisional boundary. At least 27 of the original species have become regionally extinct, and a further 11 have declined in distribution. As in other regions of Australia, species losses have been greatest for rodents and marsupials in a critical weight range of 35–5500 g, and least for bats. However, percentage losses among the terrestrial fauna are high relative to other regions, and probably reflect both the early settlement of New South Wales and the marginal distribution in the Division of 49% of the original fauna. Feral cats are implicated in the regional extinction of up to ten species of native mammals prior to 1857. Subsequent extinctions and range reductions are attributed to combinations of causes, including predation from cats Felis catus and red foxes Vulpes vulpes , competition and habitat degradation from rabbits Oryctolagus cuniculus , stock and other introduced herbivores, clearing of trees, changes in fire regimes and human persecution. We identified 28 species of particular conservation concern in the Division based on low abundance, distribution or survival prognosis. Nine species are of national significance, four of state, and 15 of regional, significance. The major current threats to these species are from grazing by stock, interference from feral mammals and clearing. Further land reservation is an important conservation measure, but must be complemented by more effective management of non-reserved land and by broad-scale management of feral species and other threatening proces es if the current species diversity is to be maintained. Long-term fauna surveys should be initiated throughout the Western Division to provide feedback on the effectiveness of management measures, and species reintroductions should be considered in situations where threats have been removed.

The Responses of Mammals to La Niña (El Niño Southern Oscillation)–Associated Rainfall, Predation, and Wildfire in Central Australia

Abstract In the Simpson Desert, central Australia, heavy rainfalls associated with the La Niña phase of the El Niño Southern Oscillation (ENSO) during 1999–2000 stimulated a pulse of primary productivity that prompted a rodent irruption during 2001, and subsequently an extensive wildfire in 2001–2002. More than 10,000 km2 of spinifex habitat were burned. In this study we examine a time series of climatic variables, and small mammal and predator dynamics in the desert in 1999–2002; and a before–after–control-impact study investigating the effects of the wildfire on small mammals. Rodents showed a delayed numerical increase in response to rainfall, whereas terrestrial predators showed a delayed numerical increase in response to rodent density. These delayed responses suggest the existence of bottom-up trophic pathways. However, a reduction in primary productivity and increase in predators appeared to suppress rodent numbers in 2001–2002, indicating that bottom-up effects can be temporarily reversed in this system. Wildfire had negative impacts on the abundance of the desert mouse (Pseudomys desertor) and the overall richness of small mammal species. Several other species of small mammals also appeared to show brief negative responses to wildfire. The impacts of wildfire on small mammals appear attributable to a loss of habitat for spinifex-dependent species and increased exposure to predation in burned habitats. Because extensive wildfires can be predicted from patterns of rainfall and fuel accumulation, we recommend that land managers be proactive in recognizing and reducing conditions of fire hazard. We recommend that small-scale prescribed burns should be carried out to reduce the extent of wildfires and also increase the chance of maintaining potential refuges from predators. In addition, control of introduced predators during and after irruptions of rodents will be crucial to prevent predator-driven crashes of their populations and those of secondary prey species. ENSO-related climatic forecasts appear to be useful cues that can be incorporated into fire and predator management strategies in arid Australia.

Resource pulses and mammalian dynamics: conceptual models for hummock grasslands and other Australian desert habitats.

Resources are produced in pulses in many terrestrial environments, and have important effects on the population dynamics and assemblage structure of animals that consume them. Resource‐pulsing is particularly dramatic in Australian desert environments owing to marked spatial and temporal variability in rainfall, and thus primary productivity. Here, we first review how Australias desert mammals respond to fluctuations in resource production, and evaluate the merits of three currently accepted models (the ecological refuge, predator refuge and fire‐mosaic models) as explanations of the observed dynamics. We then integrate elements of these models into a novel state‐and‐transition model and apply it to well‐studied small mammal assemblages that inhabit the vast hummock grassland, or spinifex, landscapes of the continental inland. The model has four states that are defined by differences in species composition and abundance, and eight transitions or processes that prompt shifts from one state to another. Using this model as a template, we construct three further models to explain mammalian dynamics in cracking soil habitats of the Lake Eyre Basin, gibber plains of the Channel Country, and the chenopod shrublands of arid southern Australia. As non‐equilibrium concepts that recognise the strongly intermittent nature of resource pulsing in arid Australia, state‐and‐transition models provide useful descriptors of both spatial and temporal patterns in mammal assemblages. The models should help managers to identify when and where to implement interventions to conserve native mammals, such as control burns, reduced grazing or predator management. The models also should improve understanding of the potential effects of future climate change on mammal assemblages in arid environments in general. We conclude by proposing several tests that could be used to refine the models and guide further research.