Raiders of the last ark: the impacts of feral cats on small mammals in Tasmanian forest ecosystems.

Feral individuals of the cat Felis catus are recognized internationally as a threat to biodiversity. Open, non-insular systems support a large proportion of the world's biodiversity, but the population-level impacts of feral cats in these systems are rarely elucidated. This limits prioritization and assessment of the effectiveness of management interventions. We quantified the predatory impact of feral cats on small mammals in open, non-insular forest systems in Tasmania, Australia in the context of other factors hypothesized to affect small mammal densities and survival, namely the density of a native carnivore, co-occurring small mammals, and rainfall. Change in feral cat density was the most important determinant of small mammal density and survival. We calculated that, on average, a 50% reduction in feral cat density could result in 25% and 10% increases in the density of the swamp rat Rattus lutreolus and long-tailed mouse Pseudomys higginsi, respectively. Low-level culling of feral cats that we conducted on two of our four study sites to experimentally alter feral cat densities revealed that swamp rat survival was highest when feral cat densities were stable. We conclude that feral cats exert downward pressure on populations of indigenous small mammals in temperate forest systems. However, alleviating this downward pressure on prey by culling a large proportion of the feral cat population is difficult as current methods for reducing feral cat populations in cool temperate forest systems are ineffective, and potentially even counterproductive. We suggest using an adaptive approach that regularly and robustly monitors how feral cats and small mammals respond to management interventions that are intended to conserve vulnerable prey species.

Nutrients cause grassland biomass to outpace herbivory.

Human activities are transforming grassland biomass via changing climate, elemental nutrients, and herbivory. Theory predicts that food-limited herbivores will consume any additional biomass stimulated by nutrient inputs ('consumer-controlled'). Alternatively, nutrient supply is predicted to increase biomass where herbivores alter community composition or are limited by factors other than food ('resource-controlled'). Using an experiment replicated in 58 grasslands spanning six continents, we show that nutrient addition and vertebrate herbivore exclusion each caused sustained increases in aboveground live biomass over a decade, but consumer control was weak. However, at sites with high vertebrate grazing intensity or domestic livestock, herbivores consumed the additional fertilization-induced biomass, supporting the consumer-controlled prediction. Herbivores most effectively reduced the additional live biomass at sites with low precipitation or high ambient soil nitrogen. Overall, these experimental results suggest that grassland biomass will outstrip wild herbivore control as human activities increase elemental nutrient supply, with widespread consequences for grazing and fire risk.

Diversity and Community Composition of Vertebrates in Desert River Habitats.

Animal species are seldom distributed evenly at either local or larger spatial scales, and instead tend to aggregate in sites that meet their resource requirements and maximise fitness. This tendency is likely to be especially marked in arid regions where species could be expected to concentrate at resource-rich oases. In this study, we first test the hypothesis that productive riparian sites in arid Australia support higher vertebrate diversity than other desert habitats, and then elucidate the habitats selected by different species. We addressed the first aim by examining the diversity and composition of vertebrate assemblages inhabiting the Field River and adjacent sand dunes in the Simpson Desert, western Queensland, over a period of two and a half years. The second aim was addressed by examining species composition in riparian and sand dune habitats in dry and wet years. Vertebrate species richness was estimated to be highest (54 species) in the riverine habitats and lowest on the surrounding dune habitats (45 species). The riverine habitats had different species pools compared to the dune habitats. Several species, including the agamid Gowidon longirostris and tree frog Litoria rubella, inhabited the riverine habitats exclusively, while others such as the skinks Ctenotus ariadnae and C. dux were captured only in the dune habitats. The results suggest that, on a local scale, diversity is higher along riparian corridors and that riparian woodland is important for tree-dependent species. Further, the distribution of some species, such as Mus musculus, may be governed by environmental variables (e.g. soil moisture) associated with riparian corridors that are not available in the surrounding desert environment. We conclude that inland river systems may be often of high conservation value, and that management should be initiated where possible to alleviate threats to their continued functioning.

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 Australia's 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.

Size breeds success: multiple paternity, multivariate selection and male semelparity in a small marsupial, Antechinus stuartii.

Mating in the marsupial genus Antechinus is a synchronous annual event that is characterized by monoestry in females and abrupt postmating mortality in males. Male semelparity (multiple copulations during a single breeding season per lifetime) is often assumed to occur as a consequence of the intense mating effort expended by males in the rut, but the forces selecting for this remain elusive. Here, we investigate selection in male brown antechinus, Antechinus stuartii, and test two hypotheses for the evolution of semelparity: intermale competition and sperm competition. If intermale competition drives semelparity, we predicted that males would be under strong selection for large body size. If sperm competition is important, we predicted that selection would be strongest on scrotal size, a surrogate for testes volume. Using microsatellite markers, we found that 92% of females in free-living conditions mated with multiple males, producing litters of eight that had up to four fathers. These observations confirm the potential for sperm competition. Using selection analysis, we then found paternity success in 119 males to be related most strongly to body mass and scrotal size, thus providing support for both hypotheses. Large males presumably experience increased paternity success by gaining more matings or prolonged copulations via mate guarding, while large testes may allow increased sperm investment per copulation. Increased levels of free corticosteroid hormones in males facilitate the extreme mating effort during the short period of rut, but lead to immune suppression and consequently to the phenomenon of postmating mortality.

Commensal and mutualistic interactions among terrestrial vertebrates.

Although overlooked in many field studies, commensal and mutualistic interactions occur frequently between species of terrestrial vertebrates. Potential advantages for individuals in mixed-species associations are very diverse, and include reduction in parasite load, reduced risk of predation, and increased access to food and other resources. Recent theoretical and experimental studies reveal the prevalence of such interactions among terrestrial vertebrates, and also confirm their importance within communities thought previously to be dominated by interspecific competition.

Mechanisms of competition among insectivorous mammals.

This study investigates the mechanisms of competition between congeneric pairs of insectivorous mammals in two communities in Australia and England. Direct field observations showed that physical interactions between species do not occur, whereas conspecific encounters are frequent. In field enclosures the smaller, subordinate species in each community (Antechinus stuartii: Marsupialia: Sorex minutus: Eutheria) remained alert in the presence of the dominant species (A. swainsonii, S. araneus), and moved quickly away when the latter approached. The rate of prey capture by subordinate individuals also increased immediately after removal of the dominants. Hourly removals of some individuals of the dominant species in each community over 24 h produced hourly increases in the numbers of subordinate individuals that were captured. The rapidity of these responses suggests strongly that the dominant insectivores in each community interfered with the resource use of the subordinate species. Biomass of invertebrates increased inconsistently or slowly within 3 months of removal of the dominant insectivores; hence the rapid responses by subordinate individuals in experiments were not due to simple exploitation or tracking of resource levels. The subordinate insectivores probably detected and avoided contact with dominants instantaneously using auditory or olfactory cues; reciprocal avoidance of congeneric odours was demonstrated using odour-scented traps. Insectivorous mammals may usually compete by interference (or encounter competition, sensu Schoener 1983). For dominant species within communities the cost of interference is minimal and the benefit of gaining exclusive access to resource-rich microhabitats is high. Conversely for subordinate species the benefit of temporarily exploiting the same rich microhabitats may exceed the small costs of vigilance and movement to nearby refugia.

The role of photoperiod in the timing of reproduction in the Dasyurid Marsupial Antechinus stuartii.

Antechinus stuartii has one highly synchronized mating period which occurs at the same time each year. An analysis of the time of reproduction in 162 populations of A. stuartii shows that the onset of the mating period is correlated with the rate of change of photoperiod, rather than with critical photoperiodic length. The rate of change of photoperiod is different for two designated forms of this species and can be used as a predictor of these animals' reproductive timing. A rate of change model further explains the rigid and highly sunchronized nature of the mating period and, by providing a mechanism for reproductive isolation, offers an explanation for the evolution of the two forms of this species.

An experimental manipulation of the intensity of interspecific competition: effects on a small marsupial.

An investigation was made of the effects of reducing and enhancing the intensity of competition on a small marsupial, Antechinus stuartii, from a larger and competitively dominant congener, A. swainsonii. Populations of these species were monitored in two study areas (one control, one experimental) in forest near Canberra, Australia, between February and July in 1980 and 1981. In the experimental study area in 1980 I reduced the numbers of A. swainsonii relative to A. stuartii (thus reducing the intensity of interspecific competition), but in 1981 I augmented the numbers of A. swainsonii (thus increasing the intensity of competition). No manipulations were made in the control study area, and the numbers of both species remained similar there in both years. When the intensity of interspecific competition was reduced, the A. stuartii population increased in size. Increases occurred also in individual movements, home range areas, diurnal activity and in the proportion of large terrestrial prey (larvae, Amphipoda) in the diet. An increase in use of structurally complex forest floor habitats also coincided with decreased arboreal activity. In contrast, when the intensity of competition was enhanced, most of these population and resource shifts were reversed. These findings suggest that reduction in the intensity of interspecific competition allows A. stuartii access to terrestrial sources of food favoured by A. swainsonii, whereas enhancement leads to exclusion of A. stuartii from the forest floor. Competition occurs by interference. This may result in fixed per capita competitive effects of A. swainsonii on A. stuartii, and account for the observed changes in a very broad range of population and resource parameters.

A test of a competition model with reference to three species of small mammals in south-eastern Australia.

This study tests the hypothesis that competition occurs in a community of three species of small mammals (Antechinus stuartii and A. swainsonii, Marsupialia; Rattus fuscipes, Rodentia) in south-eastern Australia. The hypothesis was tested by using a simple model of competition that is based on two premises: I. competition results in a negative numerical or a negative spatial association between species, and II. competition is more intense where the densities of the competing species are relatively high. The model combines both premises and predicts that measures of association between competing species will be more negative where the species exist at high population densities than derived from the model and applied to data gathered in two study areas with contrasting mean animal densities. There was no evidence of numerical association among the species (three tests), but evidence of negative spatial association (three tests our of four). These latter findings were consistent with the predictions of the model. All species preferred the same broad kinds of habitat (macrohabitat), but they segregated into smaller, finer grained patches (microhabitat) when their numbers were relatively high. Such segregation was more evident between the two species of marsupials than between either species of marsupial and the eutherian rat. These findings suggested that competition occurs by interference, and reinforce the idea that an understanding of the behaviour of individuals is important to understanding competition between species.