Lead exposure of mainland Australia's top avian predator

Lead (Pb) toxicity, through ingestion of lead ammunition in carcasses, is a threat to scavenging birds worldwide, but has received little attention in Australia. We analyzed lead exposure in the wedge-tailed eagle (Aquila audax), the largest raptor species found in mainland Australia and a facultative scavenger. Eagle carcasses were collected opportunistically throughout south-eastern mainland Australia between 1996 and 2022. Lead concentrations were measured in bone samples from 62 animals via portable X-ray fluorescence (XRF). Lead was detected (concentration >1 ppm) in 84% (n = 52) of the bone samples. The mean lead concentration of birds in which lead was detected was 9.10 ppm (±SE 1.66). Bone lead concentrations were elevated (10-20 ppm) in 12.9% of samples, and severe (>20 ppm) in 4.8% of samples. These proportions are moderately higher than equivalent data for the same species from the island of Tasmania, and are comparable to data from threatened eagle species from other continents. Lead exposure at these levels is likely to have negative impacts on wedge-tailed eagles at the level of the individual and perhaps at a population level. Our results suggest that studies of lead exposure in other Australian avian scavenger species are warranted.

Irrupting prey populations in the absence of a mammalian apex predator drive shifts in prey selection by eagles

Removal of apex predators can have far-reaching effects on the organization and structure of ecosystems. This occurs because apex predators can exert strong suppressive effects on their prey and competitors and perturbation of these interactions can shift the balance of interactions between dyads of species at lower trophic levels and trigger trophic cascades. Dingoes (Canis dingo) are Australia’s largest mammalian carnivore. Because they are a pest to livestock producers, dingo populations are suppressed in many regions. Suppression of dingo populations has been linked to a suite of ecosystem changes due to ensuing population irruptions of their prey and competitors. Here, we investigate the impact that the suppression of dingoes has on the diet of wedge-tailed eagles (Aquila audax) in Australia’s Strzelecki Desert. Wedge-tailed eagles are generalist predators that readily shift their diet in relation to prey availability. We assessed the abundance of species frequently preyed on by eagles and quantified prey remains at eagle nests located on either side of a dingo-proof fence where dingoes were common and rare, respectively. Wedge-tailed eagles consumed more species where dingoes were rare compared to where dingoes were common. Kangaroos (Macropodidae) and western bearded dragons (Pogona vitticeps) were more abundant and were consumed more frequently by eagles where dingoes were rare. Introduced European rabbits (Oryctolagus cuniculus) were the prey item most frequently identified at eagle nests. However, rabbits were more abundant and their remains were found at a higher proportion of nests where dingoes were common. Our results provide evidence that shifts in the composition of vertebrate assemblages associated with the presence/absence of dingoes, particularly the irruption of kangaroos, influence the diet of wedge-tailed eagles. More generally, by showing that the presence/absence of dingoes can influence the diet of wedge-tailed eagles, our study highlights how pervasive apex predators’ effects on ecosystems can be.

Making inference from wildlife collision data: inferring predator absence from prey strikes

Wildlife collision data are ubiquitous, though challenging for making ecological inference due to typically irreducible uncertainty relating to the sampling process. We illustrate a new approach that is useful for generating inference from predator data arising from wildlife collisions. By simply conditioning on a second prey species sampled via the same collision process, and by using a biologically realistic numerical response functions, we can produce a coherent numerical response relationship between predator and prey. This relationship can then be used to make inference on the population size of the predator species, including the probability of extinction. The statistical conditioning enables us to account for unmeasured variation in factors influencing the runway strike incidence for individual airports and to enable valid comparisons. A practical application of the approach for testing hypotheses about the distribution and abundance of a predator species is illustrated using the hypothesized red fox incursion into Tasmania, Australia. We estimate that conditional on the numerical response between fox and lagomorph runway strikes on mainland Australia, the predictive probability of observing no runway strikes of foxes in Tasmania after observing 15 lagomorph strikes is 0.001. We conclude there is enough evidence to safely reject the null hypothesis that there is a widespread red fox population in Tasmania at a population density consistent with prey availability. The method is novel and has potential wider application.

Is wedge-tailed eagle, Aquila audax, survival and breeding success closely linked to the abundance of European rabbits, Oryctolagus cuniculus?

Context Some ecologists argue that nesting success and abundance of wedge-tailed eagles (Aquila audax) are strongly linked to the abundance of introduced wild rabbits (Oryctolagus cuniculus). Consequently, concerns were expressed about eagle population viability when the biological control agent rabbit haemorrhagic disease virus (RHDV) heavily reduced rabbit numbers. However, observations following the spread of rabbit haemorrhagic disease (RHD) in Australia and Spain (where Aquila adalberti is an equivalent of A. audax) question this assertion. Eagle numbers did not fall even though rabbits declined regionally by up to 90% in both countries. Aims To reconsider the assumption of a strong link between rabbit abundance and wedge-tailed eagle breeding and population maintenance. Dispelling misconceptions, if any, about the eagles’ dependence on rabbits would benefit the future management of both eagles and rabbits. Methods We reviewed the literature associated with claims that eagles were heavily dependent on rabbits and asked whether these views could be substantiated given the lack of changes in eagle abundance following the spread of RHD. Data on eagle egg-clutch size and nesting success were also reviewed. Conclusions There is little evidence that eagles depend heavily on rabbits as prey. Instead, as rabbits decline, more kangaroos, reptiles and birds are eaten, partly because more native prey becomes available. Eagles have a high proportion of rabbits in their diets mainly where degradation of natural ecosystems, including that caused by rabbits, results in native prey being rare or unavailable. There has been minimal variation in average clutch size following major perturbations in rabbit population size. Implications Rather than perpetuating the idea that high populations of rabbits are needed for wedge-tailed eagle conservation, resources would be better re-directed into understanding continental-scale eagle population dynamics. This would provide a more rational framework to assist decisions on future biological control agents for rabbits.

Population recovery of the yellow-footed rock-wallaby following fox control in New South Wales and South Australia

Context Introduced herbivores and carnivores have significantly altered ecosystems across Australia and have been implicated in the decline and extinction of many species, particularly in the arid and semiarid zones. The experimental confirmation of agents of decline is a fundamental step in threatened species management, allowing for an efficient allocation of resources and effective species recovery. Aims Following unsuccessful attempts to increase the abundance of yellow-footed rock-wallaby populations through concerted goat control across the southern extent of their range, the primary aim of our study was to determine whether fox predation was limiting the recovery of wallaby populations. Methods Intensive fox-control programs (1080 baiting) were initiated around wallaby subpopulations in New South Wales and South Australia. Wallaby numbers were monitored for a three-year period before and after the initiation of fox control, and ANCOVA used to examine for differences between the subpopulation’s rate of increase (r). Observational data were used to determine the demographic effects of fox predation on wallaby colonies in New South Wales. Key results Wallaby subpopulations that were treated with fox control increased significantly, while experimental control subpopulations remained at consistently low levels. Juvenile and subadult wallabies were apparent within the treated New South Wales subpopulation, but almost absent from the control subpopulation, suggesting that fox predation focussed primarily on these age classes. Conclusions Fox predation is a major limiting factor for southern wallaby populations and fox control should be the primary management action to achieve species recovery. Implications Although a significant limiting factor, the regulatory effect of fox predation was not assessed and, as such, it cannot be confirmed as being the causal factor responsible for the historic declines in wallaby abundance. The determination of whether fox predation acts in a limiting or regulatory fashion requires further experimentation, but its resolution will have significant implications for the strategic allocation of conservation resources.

Clever strategists: Australian Magpies vary mobbing strategies, not intensity, relative to different species of predator

Anti-predator behaviour of magpies was investigated, using five species of model predators, at times of raising offspring. We predicted differences in mobbing strategies for each predator presented and also that raising juveniles would affect intensity of the mobbing event. Fourteen permanent resident family groups were tested using 5 different types of predator (avian and reptilian) known to be of varying degrees of risk to magpies and common in their habitat. In all, 210 trials were conducted (across three different stages of juvenile development). We found that the stage of juvenile development did not alter mobbing behaviour significantly, but predator type did. Aerial strategies (such as swooping) were elicited by taxidermic models of raptors, whereas a taxidermic model of a monitor lizard was approached on the ground and a model snake was rarely approached. Swooping patterns also changed according to which of the three raptors was presented. Our results show that, in contrast to findings in other species, magpies vary mobbing strategy depending on the predator rather than varying mobbing intensity.

Interactions between chuditch (Dasyurus geoffroii) and introduced predators: a review

The chuditch is a carnivorous marsupial that has suffered catastrophic decline since European settlement in Australia. The impacts of introduced foxes and feral cats are thought to have contributed to this decline. Although chuditch populations appear to respond favourably to fox control, ecological theory predicts that other predators, including feral cats, are also likely to increase in abundance when foxes are removed. The flow-on effects of these predicted increases are not known, but are potentially significant for chuditch and for other native fauna. Here, we review the evidence for limiting effects of foxes and feral cats on chuditch populations. We also discuss the likely effects on chuditch populations (direct and indirect) of current fox control practices. Using recent advances in carnivore community ecology, we make testable predictions regarding likely interactions within the predator guild. Experiments should adopt a whole-of-community approach, seeking to clarify the effects of fox control on the entire suite of native and introduced predators, as well as on the abundance and diversity of prey.