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Populations of rodents were studied for periods of 7−9 years at each of three sites in arid Australia. All species fluctuated dramatically in abundance, being absent or in low numbers during droughts but erupting after significant rainfall. Strong correlations were obtained between capture rates and cumulative monthly rainfall residuals, with time lags, that had been modified by an exponential decay function to model the post-rain depletion of resources. The introduced Mus domesticus erupted within only two months of exceptional rainfall at one site, whereas the native rodents Notomys alexis, Pseudomys hermannsburgensis and P. desertor generally responded 3Œ10 months after rain. The faster response of M. domesticus reflects its high reproductive potential compared with the native rodents, and perhaps greater immigration. Differences in magnitudes and delays in response to rainfall among the native species were due probably to differences in initial population sizes and times available for response and, for P. desertor, to between-site differences in the quality or quantity of food resources. Population declines in all species during dry periods probably followed resource shortages. Despite the usually strong linkage between rainfall and population dynamics, rain failed to trigger population responses in rodents for prolonged periods at two of the sites, and was not clearly associated with an eruption of P. hermannsburgensis at the third. The lack of a population response at one site was attributed to predation, but events at the other two remain unexplained. We conclude that long-term studies should play an important role in describing temporal changes in rodent populations in arid Australia and, with field experiments, in evaluating how rainfall and other factors combine to effect the changes.

In parts of the world such as the Pacific Islands, Australia, and New Zealand, introduced vertebrate predators have caused the demise of indigenous mammal and bird species. A number of releases for reestablishment of these mammal species in mainland Australia have failed because predators extirpated the new populations. The nature of the decline of both extant populations and reintroduced colonies provides information on the dynamics of predation.Predator-prey theory suggests that the effects of predation are usually inversely dependent on density (depensatory) when the prey are not the primary food supply of exotic predators. Thus, such predators can cause extinction of endemic prey species. Three types of evidence can be deduced from the predator-prey interactions that allow predictions for conservation: (1) whether per capita rates of change for prey increase or decrease with declining prey densities, (2) whether predation is depensatory or density-dependent, and (3) the overall magnitude of predation. If this magnitude is too high for coexistence, then the degree of predator removal required can be predicted. If the magnitude of predation is sufficiently low, then the threshold density of prey that management must achieve to allow predator and prey to coexist can also be predicted. We analyzed published reports of both declining populations and reintroduced colonies of endangered marsupial populations in Australia. The observed predation curves conformed to the predictions of predator-prey theory. Some, such as the black-footed rock-wallaby ( Petrogale lateralis ), were classic alternate prey and were vulnerable below a threshold population size. Others, such as the brush-tailed bettong ( Bettongia penicillata ), have a refuge at low numbers and thus offer the best chance for reintroduction. Our predictions suggest a protocol for an experimental management program for the conservation of sensitive prey species: (1) determination of net rates of change of prey with declining population, (2) improvement of survivorship through habitat manipulation, (3) improvement of survivorship through predator removal, (4) determination of the threshold density above which reintroductions can succeed, and (5) manipulations to change interactions from Type II to Type III. The task in the future is to determine how to change the vulnerability of the prey so that they can have a refuge at low numbers. Predicción de los Efectos de la Depredación en la Conservación de una Presa en Peligro de ExtinciónResumen: En algunas partes del mundo como son las Islas del Pacífico, Australia y Nueva Zelandia, los veretebrados depredadores introducidos han ocasionado la desaparición de especies indígenas de mamíferos y aves. Un gran número de liberaciones para el restablecimiento de estas especies de mamíferos en tierras continentales de Australia han fracasado debido a que los depredadores han extirpado las poblaciones nuevas. La naturaleza de la declinación tanto de las poblaciones existentes como de las colonias reintroducidas provee inform...

This study investigated the population dynamics of three species of dasyurid marsupials in sand ridge habitat of the Simpson Desert, western Queensland, over a 10-year period between March 1990 and December 1999. The lesser hairy-footed dunnart (Sminthopsis youngsoni), was captured most consistently over the period of study, followed by the wongai ningaui (Ningaui ridei), and the mulgara (Dasycercus cristicauda). Rates of recapture were low (4.5–22.2%), probably because individuals of each species are very mobile. All species bred in late winter or early spring when animals were aged at least 8–10 months, and independent juveniles first appeared usually in summer. S. youngsoni reared a second litter in late spring or early summer in 3 of the 10 years studied, when the availability of food was likely to have been high; neither N. ridei nor D. cristicauda were known to attempt a second litter within a season. To explore factors that might influence population dynamics, we compared capture rates of each species with measures of rainfall, temperature, vegetation cover, abundance of predators [feral cats (Felis catus), red foxes (Vulpes vulpes), and goannas (Varanus spp.)], dragons, other dasyurids and indices of food abundance. The abundance of S. youngsoni appeared to depend primarily on the cover of spinifex 7–9 months earlier, that of D. cristicauda was related most strongly to rainfall 7–9 months earlier, while that of N. ridei was related to minimum temperature lagged by 1–3 months. While the dynamics of other arid-zone mammals are driven demonstrably by interactions between rainfall, resource availability and predation, our findings suggest that dasyurids have limited flexibility in their life histories and are influenced more subtly and by factors such as facilitation that are just beginning to become apparent.

Populations of feral cats, foxes and dingoes were assessed at four sites in the northern Simpson Desert from May 1995 to October 1996 using track counts. Counts were based on presence/absence of footprints on sandplots placed randomly throughout each of the four sites, with all habitats being sampled in accordance with their availability. Sandplots were repositioned between sampling periods so that data were temporally independent. This unbiased method was used to assess the reliability of more conventional indices of carnivore abundance based on spotlighting and counts of footprints on roads, which were run concurrently with the random sandplot counts. The sandplot data were also used to assess habitat use of the three study species. Counts on regularly spaced sampling plots along roads were correlated with random sandplot counts for foxes, but not for cats. Conversely, an index of activity based on counting individual tracks along roads was correlated for cats, but not for foxes. Spotlighting counts appeared to have little relationship with random sandplot counts for either species, with random sandplot counts and spotlight counts of foxes being correlated for only one of the four sites and no correlations being apparent for cats. The sandplot data showed that cats used dune crests preferentially over other habitats, while foxes preferred both dune crests and roads. Dune crests are probably used as natural runways by both species. Dingoes used roads preferentially over other habitats. It was concluded that differences in the use of roads and other runways by the carnivores can potentially lead to biases in indices collected along roadways. In relation to the random sandplot counts, foxes were overestimated by road counts and spotlighting relative to cats, the latter bias being related possibly to temporal differences in the activity patterns of the two species.