Estimating disease survey intensity and wildlife population size from the density of survey devices: Leg-hold traps and the brushtail possum

Wildlife disease surveillance requires accurate information on the proportion of managed populations sampled or their population density, parameters that are typically expensive to measure. However, these parameters can be estimated using spatially explicit modelling of capture probabilities, based on the distribution and deployment times of capture devices, given accurate information on the relationships between these variables. This approach is used in New Zealand's surveillance programme aimed at confirming areas free of bovine tuberculosis (bTB¹) in brushtail possums (Trichosurus vulpecula). However, there is uncertainty about the accuracy of the underpinning parameters characterizing possum trappability (g), given the distance between where a trap is placed and the possum home range centre. Sampling intensity (SI: the percentage of the population sampled during a population survey) and sigma (σ; 95% home range radius/2.45) were measured, using leg-hold traps deployed under a set protocol to standardize survey effort, at four sites containing previously radio- and GPS-collared individuals. Those data were used to derive an estimate of the nightly probability of capture of possums in a trap set at their home range centre (g₀). Those estimates were compared to the standard assumptions currently used as defaults in the day-to-day approach used by bTB managers. Home-range size (and therefore σ) varied widely between sites (range 3.6-49.4 ha), probably largely in response to differences in possum density. Field measured SI also varied widely between sites, and was closely positively correlated with home range size (R² = 0.967; P =  0.017); wide-ranging possums were more trappable than sedentary ones. We found that g₀ was inversely related to σ, but the magnitude of increases in g₀ with declining σ appeared to be insufficient to compensate for the fewer places at which each possum could be trapped when those home ranges were small. SI was, therefore, not constant across sites where a standard survey effort was applied. The assumed relationship between g₀ and σ in the current spatial model may, therefore, need reassessment. The management implication of these result is that the sampling effort required to attain a target sampling intensity is dependant on the target animal density, and for bTB management of possums in New Zealand, is under-estimated by the current default parameters in a model of freedom-from-disease for higher density possum populations.

Movements and habitat preferences of pests help to improve population control: the case of common brushtail possums in a New Zealand dryland ecosystem

BACKGROUND

Introduced brushtail possums are controlled in New Zealand to mitigate their spread of bovine tuberculosis in livestock. Given the low rainfall and extreme variation in seasonal temperatures in dryland areas of the South Island, the habitats of possums in these areas differ in many respects from those in the rest of New Zealand. We investigated the movements and habitat preferences of possums at two dryland sites to identify where they aggregate following population control by using GPS collars and cards chewed by possums. At one site, possum numbers were reduced from high levels by 65%, and at the other site, possums had already been reduced to low levels for some time beforehand but were further reduced to maintain them at low levels. This resulted in different possum densities.

RESULTS

Possum home ranges were about 3 times smaller at the higher-density site, but average ranges expanded by 27% following initial control. Home ranges were already large at the lower-density site but did not expand further after maintenance control. No preference for habitat types was apparent at the higher-density site, but at the lower-density site possums selected rock and shrubby habitats and avoided open grassy areas.

CONCLUSIONS

Home range sizes and habitat preferences were density dependent: the lower the density, the larger was the home range; and habitat preferences were highly variable between individuals, but less so for possums at low density. Preference for shrubs and rocks is likely to benefit population control if population control devices are focused on these habitat types. © 2016 Society of Chemical Industry.

Foliar Nutritional Quality Explains Patchy Browsing Damage Caused by an Invasive Mammal

Introduced herbivores frequently inflict significant, yet patchy damage on native ecosystems through selective browsing. However, there are few instances where the underlying cause of this patchy damage has been revealed. We aimed to determine if the nutritional quality of foliage could predict the browsing preferences of an invasive mammalian herbivore, the common brushtail possum (Trichosurus vulpecula), in a temperate forest in New Zealand. We quantified the spatial and temporal variation in four key aspects of the foliar chemistry (total nitrogen, available nitrogen, in vitro dry matter digestibility and tannin effect) of 275 trees representing five native tree species. Simultaneously, we assessed the severity of browsing damage caused by possums on those trees in order to relate selective browsing to foliar nutritional quality. We found significant spatial and temporal variation in nutritional quality among individuals of each tree species examined, as well as among tree species. There was a positive relationship between the available nitrogen concentration of foliage (a measure of in vitro digestible protein) and the severity of damage caused by browsing by possums. This study highlights the importance of nutritional quality, specifically, the foliar available nitrogen concentration of individual trees, in predicting the impact of an invasive mammal. Revealing the underlying cause of patchy browsing by an invasive mammal provides new insights for conservation of native forests and targeted control of invasive herbivores in forest ecosystems.

Incorporating Field Studies into Species Distribution and Climate Change Modelling: A Case Study of the Koomal Trichosurus vulpecula hypoleucus (Phalangeridae)

Species distribution models (SDMs) are an effective way of predicting the potential distribution of species and their response to environmental change. Most SDMs apply presence data to a relatively generic set of predictive variables such as climate. However, this weakens the modelling process by overlooking the responses to more cryptic predictive variables. In this paper we demonstrate a means by which data gathered from an intensive animal trapping study can be used to enhance SDMs by combining field data with bioclimatic modelling techniques to determine the future potential distribution for the koomal (Trichosurus vulpecula hypoleucus). The koomal is a geographically isolated subspecies of the common brushtail possum, endemic to south-western Australia. Since European settlement this taxon has undergone a significant reduction in distribution due to its vulnerability to habitat fragmentation, introduced predators and tree/shrub dieback caused by a virulent group of plant pathogens of the genus Phytophthora. An intensive field study found: 1) the home range for the koomal rarely exceeded 1 km in in length at its widest point; 2) areas heavily infested with dieback were not occupied; 3) gap crossing between patches (>400 m) was common behaviour; 4) koomal presence was linked to the extent of suitable vegetation; and 5) where the needs of koomal were met, populations in fragments were demographically similar to those found in contiguous landscapes. We used this information to resolve a more accurate SDM for the koomal than that created from bioclimatic data alone. Specifically, we refined spatial coverages of remnant vegetation and dieback, to develop a set of variables that we combined with selected bioclimatic variables to construct models. We conclude that the utility value of an SDM can be enhanced and given greater resolution by identifying variables that reflect observed, species-specific responses to landscape parameters and incorporating these responses into the model.

Cost-Effective Large-Scale Occupancy-Abundance Monitoring of Invasive Brushtail Possums (Trichosurus Vulpecula) on New Zealand's Public Conservation Land

There is interest in large-scale and unbiased monitoring of biodiversity status and trend, but there are few published examples of such monitoring being implemented. The New Zealand Department of Conservation is implementing a monitoring program that involves sampling selected biota at the vertices of an 8-km grid superimposed over the 8.6 million hectares of public conservation land that it manages. The introduced brushtail possum (Trichosurus Vulpecula) is a major threat to some biota and is one taxon that they wish to monitor and report on. A pilot study revealed that the traditional method of monitoring possums using leg-hold traps set for two nights, termed the Trap Catch Index, was a constraint on the cost and logistical feasibility of the monitoring program. A phased implementation of the monitoring program was therefore conducted to collect data for evaluating the trade-off between possum occupancy–abundance estimates and the costs of sampling for one night rather than two nights. Reducing trapping effort from two nights to one night along four trap-lines reduced the estimated costs of monitoring by 5.8% due to savings in labour, food and allowances; it had a negligible effect on estimated national possum occupancy but resulted in slightly higher and less precise estimates of relative possum abundance. Monitoring possums for one night rather than two nights would provide an annual saving of NZ$72,400, with 271 fewer field days required for sampling. Possums occupied 60% (95% credible interval; 53–68) of sampling locations on New Zealand’s public conservation land, with a mean relative abundance (Trap Catch Index) of 2.7% (2.0–3.5). Possum occupancy and abundance were higher in forest than in non-forest habitats. Our case study illustrates the need to evaluate relationships between sampling design, cost, and occupancy–abundance estimates when designing and implementing large-scale occupancy–abundance monitoring programs.

Genetic and ontogenetic variation in an endangered tree structures dependent arthropod and fungal communities

Plant genetic and ontogenetic variation can significantly impact dependent fungal and arthropod communities. However, little is known of the relative importance of these extended genetic and ontogenetic effects within a species. Using a common garden trial, we compared the dependent arthropod and fungal community on 222 progeny from two highly differentiated populations of the endangered heteroblastic tree species, Eucalyptus morrisbyi. We assessed arthropod and fungal communities on both juvenile and adult foliage. The community variation was related to previous levels of marsupial browsing, as well as the variation in the physicochemical properties of leaves using near-infrared spectroscopy. We found highly significant differences in community composition, abundance and diversity parameters between eucalypt source populations in the common garden, and these were comparable to differences between the distinctive juvenile and adult foliage. The physicochemical properties assessed accounted for a significant percentage of the community variation but did not explain fully the community differences between populations and foliage types. Similarly, while differences in population susceptibility to a major marsupial herbivore may result in diffuse genetic effects on the dependent community, this still did not account for the large genetic-based differences in dependent communities between populations. Our results emphasize the importance of maintaining the populations of this rare species as separate management units, as not only are the populations highly genetically structured, this variation may alter the trajectory of biotic colonization of conservation plantings.

Determination of villous rigidity in the distal ileum of the possum (Trichosurus vulpecula)

We investigated the passive mechanical properties of villi in ex vivo preparations of sections of the wall of the distal ileum from the brushtail possum (Trichosurus vulpecula) by using a flow cell to impose physiological and supra-physiological levels of shear stress on the tips of villi. We directly determined the stress applied from the magnitude of the local velocities in the stress inducing flow and additionally mapped the patterns of flow around isolated villi by tracking the trajectories of introduced 3 µm microbeads with bright field micro particle image velocimetry (mPIV). Ileal villi were relatively rigid along their entire length (mean 550 µm), and exhibited no noticeable bending even at flow rates that exceeded calculated normal physiological shear stress (>0.5 mPa). However, movement of villus tips indicated that the whole rigid structure of a villus could pivot about the base, likely from laxity at the point of union of the villous shaft with the underlying mucosa. Flow moved upward toward the tip on the upper portions of isolated villi on the surface facing the flow and downward toward the base on the downstream surface. The fluid in sites at distances greater than 150 µm below the villous tips was virtually stagnant indicating that significant convective mixing in the lower intervillous spaces was unlikely. Together the findings indicate that mixing and absorption is likely to be confined to the tips of villi under conditions where the villi and intestinal wall are immobile and is unlikely to be greatly augmented by passive bending of the shafts of villi.

Quantifying the direct transfer costs of common brushtail possum dispersal using least-cost modelling: a combined cost-surface and accumulated-cost dispersal kernel approach

Dispersal costs need to be quantified from empirical data and incorporated into dispersal models to improve our understanding of the dispersal process. We are interested in quantifying how landscape features affect the immediately incurred direct costs associated with the transfer of an organism from one location to another. We propose that least-cost modelling is one method that can be used to quantify direct transfer costs. By representing the landscape as a cost-surface, which describes the costs associated with traversing different landscape features, least-cost modelling is often applied to measure connectivity between locations in accumulated-cost units that are a combination of both the distance travelled and the costs traversed. However, we take an additional step by defining an accumulated-cost dispersal kernel, which describes the probability of dispersal in accumulated-cost units. This novel combination of cost-surface and accumulated-cost dispersal kernel enables the transfer stage of dispersal to incorporate the effects of landscape features by modifying the direction of dispersal based on the cost-surface and the distance of dispersal based on the accumulated-cost dispersal kernel. We apply this approach to the common brushtail possum (Trichosurus vulpecula) within the North Island of New Zealand, demonstrating how commonly collected empirical dispersal data can be used to calibrate a cost-surface and associated accumulated-cost dispersal kernel. Our results indicate that considerable improvements could be made to the modelling of the transfer stage of possum dispersal by using a cost-surface and associated accumulated-cost dispersal kernel instead of a more traditional straight-line distance based dispersal kernel. We envisage a variety of ways in which the information from this novel combination of a cost-surface and accumulated-cost dispersal kernel could be gainfully incorporated into existing dispersal models. This would enable more realistic modelling of the direct transfer costs associated with the dispersal process, without requiring existing dispersal models to be abandoned.

Combining aspirin with cholecalciferol (vitamin D3)--a potential new tool for controlling possum populations

The introduced Australian brushtail possum is a major vertebrate pest in New Zealand, with impacts on conservation and agriculture being managed largely through poisoning operations. Cholecalciferol (vitamin D3) is registered for use in controlling possums and despite its many advantages it is expensive and relatively inhumane. Combination of a high proportion of aspirin with a low proportion of cholecalciferol was effective in killing high proportions of groups of acclimatised, caged possums: this is attributed to both an unexpectedly high toxicity of the type of cholecalciferol used, and a proposed synergistic mechanism between the two compounds. Death was caused by localised damage to heart ventricles by aspirin, and inhibition of tissue repair by both aspirin and cholecalciferol. The observed toxicosis had lower impact on the welfare of possums than either compound administered alone, particularly aspirin alone. Residue analyses of bait remains in the GI tract suggested a low risk of secondary poisoning by either compound. The combination of cholecalciferol and aspirin has the potential to meet key requirements of cost-effectiveness and humaneness in controlling possum populations, but the effect of the combination in non-target species has yet to be tested.

An exploration of the microrheological environment around the distal ileal villi and proximal colonic mucosa of the possum (Trichosurus vulpecula)

Multiple particle-tracking techniques were used to quantify the thermally driven motion of ensembles of naked polystyrene (0.5 µm diameter) microbeads in order to determine the microrheological characteristics around the gut mucosa. The microbeads were introduced into living ex vivo preparations of the wall of the terminal ileum and proximal colon of the brushtail possum (Trichosurus vulpecula). The fluid environment surrounding both the ileal villi and colonic mucosa was heterogeneous; probably comprising discrete viscoelastic regions suspended in a continuous Newtonian fluid of viscosity close to water. Neither the viscosity of the continuous phase, the elastic modulus (G') nor the sizes of viscoelastic regions varied significantly between areas within 20 µm and areas more than 20 µm from the villous mucosa nor from the tip to the sides of the villous mucosa. The viscosity of the continuous phase at distances further than 20 µm from the colonic mucosa was greater than that at the same distance from the ileal villous mucosa. Furthermore, the estimated sizes of viscoelastic regions were significantly greater in the colon than in the ileum. These findings validate the sensitivity of the method and call into question previous hypotheses that a contiguous layer of mucus envelops all intestinal mucosa and restricts diffusive mass transfer. Our findings suggest that, in the terminal ileum and colon at least, mixing and mass transfer are governed by more complex dynamics than were previously assumed, perhaps with gel filtration by viscoelastic regions that are suspended in a Newtonian fluid.

Predicting summer site occupancy for an invasive species, the common brushtail possum (Trichosurus vulpecula), in an urban environment

Invasive species are often favoured in fragmented, highly-modified, human-dominated landscapes such as urban areas. Because successful invasive urban adapters can occupy habitat that is quite different from that in their original range, effective management programmes for invasive species in urban areas require an understanding of distribution, habitat and resource requirements at a local scale that is tailored to the fine-scale heterogeneity typical of urban landscapes. The common brushtail possum (Trichosurus vulpecula) is one of New Zealand's most destructive invasive pest species. As brushtail possums traditionally occupy forest habitat, control in New Zealand has focussed on rural and forest habitats, and forest fragments in cities. However, as successful urban adapters, possums may be occupying a wider range of habitats. Here we use site occupancy methods to determine the distribution of brushtail possums across five distinguishable urban habitat types during summer, which is when possums have the greatest impacts on breeding birds. We collected data on possum presence/absence and habitat characteristics, including possible sources of supplementary food (fruit trees, vegetable gardens, compost heaps), and the availability of forest fragments from 150 survey locations. Predictive distribution models constructed using the programme PRESENCE revealed that while occupancy rates were highest in forest fragments, possums were still present across a large proportion of residential habitat with occupancy decreasing as housing density increased and green cover decreased. The presence of supplementary food sources was important in predicting possum occupancy, which may reflect the high nutritional value of these food types. Additionally, occupancy decreased as the proportion of forest fragment decreased, indicating the importance of forest fragments in determining possum distribution. Control operations to protect native birds from possum predation in cities should include well-vegetated residential areas; these modified habitats not only support possums but provide a source for reinvasion of fragments.

A new multi-scale measure for analysing animal movement data

We present a new measure for analysing animal movement data, which we term a 'Multi-Scale Straightness Index' (MSSI). The measure is a generalisation of the 'Straightness Index', the ratio of the beeline distance between the start and end of a track to the total distance travelled. In our new measure, the Straightness Index is computed repeatedly for track segments at all possible temporal scales. The MSSI offers advantages over the standard Straightness Index, and other simple measures of track tortuosity (such as Sinuosity and Fractal Dimension), because it provides multiple characterisations of straightness, rather than just a single summary measure. Thus, comparisons can be made among different segments of trajectories and changes in behaviour can be inferred, both over time and at different temporal granularities. The measure also has an important advantage over several recent and increasingly popular methods for detecting behavioural changes in time-series locational data (e.g., state-space models and positional entropy methods), in that it is extremely simple to compute. Here, we demonstrate use of the MSSI on both synthetic and real animal-movement trajectories. We show how behavioural changes can be inferred within individual tracks and how behaviour varies across spatio-temporal scales. Our aim is to present a useful tool for researchers requiring a computationally simple but effective means of analysing the movement patterns of animals.

The hypothalamic-pituitary-ovarian axis and manipulations of the oestrous cycle in the brushtail possum

The main purpose of this review is to provide a comprehensive update on what is known about the regulatory mechanisms of the hypothalamic-pituitary-ovarian axis in the brushtail possum, and to report on the outcomes of attempts made to manipulate by hormonal means, these processes in the possum. Over the last 15 years, several unique features of possum reproductive physiology have been discovered. These include an extended follicular phase despite elevated concentrations of FSH during the luteal phase, and early expression of LH receptors on granulosa cells of small antral follicles, suggesting a different mechanism for the selection of a dominant follicle. The use of routine synchronisation protocols that are effective in eutherians has failed to be effective in possums, and so the ability to reliably synchronise oestrus in this species remains a challenge.

Adult survival and microsatellite diversity in possums: effects of major histocompatibility complex-linked microsatellite diversity but not multilocus inbreeding estimators

Adult survival is perhaps the fitness parameter most important to population growth in long-lived species. Intrinsic and extrinsic covariates of survival are therefore likely to be important drivers of population dynamics. We used long-term mark-recapture data to identify genetic, individual and environmental covariates of local survival in a natural population of mountain brushtail possums (Trichosurus cunninghami). Rainfall and intra-individual diversity at microsatellite DNA markers were associated with increased local survival of adults and juveniles. We contrasted the performance of several microsatellite heterozygosity measures, including internal relatedness (IR), homozygosity by loci (HL) and the mean multilocus estimate of the squared difference in microsatellite allele sizes within an individual (mean d (2)). However, the strongest effect on survival was not associated with multilocus microsatellite diversity (which would indicate a genome-wide inbreeding effect), but a subset of two loci. This included a major histocompatibility complex (MHC)-linked marker and a putatively neutral microsatellite locus. For both loci, diversity measures incorporating allele size information had stronger associations with survival than measures based on heterozygosity, whether or not allele frequency information was included (such as IR). Increased survival was apparent among heterozygotes at the MHC-linked locus, but the benefits of heterozygosity to survival were reduced in heterozygotes with larger differences in allele size. The effect of heterozygosity on fitness-related traits was supported by data on endoparasites in a subset of the individuals studied in this population. There was no apparent density dependence in survival, nor an effect of sex, age or immigrant status. Our findings suggest that in the apparent absence of inbreeding, variation at specific loci can generate strong associations between fitness and diversity at linked markers.

Spatial distribution of defense chemicals and markers and the maintenance of chemical variation

Exploring the spatial distribution of variation in plant secondary metabolites is critical for understanding the evolutionary ecology of biochemical diversity in wild organisms. In the present study, concentrations of foliar sideroxylonal, an important and highly heritable defense chemical of Eucalyptus melliodora, displayed strong, fine-scale spatial autocorrelation. The spatial patterns observed could promote associational effects on herbivore foraging decisions, which may influence the selection pressures exerted on sideroxylonal content. Multiple chemical traits have roles in certain eucalypt-herbivore interactions, and the spatial characteristics of the herbivore foraging environment are therefore determined by these different factors. We used a model of E. melliodora intake by common brushtail possums (Trichosurus vulpecula), based on the combined effects of two chemical traits, to explore this idea and found that the spatial patterns were different to those of sideroxylonal alone. Spatial genetic autocorrelation, examined using microsatellites, was strong and occurred at a fine scale, implying that restricted gene flow might allow genetic patches to respond to selection relatively independently. Local two-dimensional genetic autocorrelation, explored using a new heuristic method, was highly congruent with the pattern of local phenotypic variation observed for sideroxylonal, suggesting that the genetic variance underlying the sideroxylonal variation is similarly structured. Our results suggest that the spatial distribution of genetic and phenotypic variation could influence both the selective pressure imposed by herbivores on eucalypt defenses and the potential of populations to respond to natural selection. Spatial context should be considered in future studies of plant-herbivore interactions.

Physiology and pharmacology of the brushtail possum gastrointestinal tract: relationship to the human gastrointestinal tract

Oral formulations are typically based on studies from eutherian animal models. This review introduces information relating to oral formulations for a marsupial species, the Australian brushtail possum (Trichosurus vulpecula) that has arisen from research into new methods for controlling this species - a major vertebrate pest in New Zealand. Morphologically, the gastrointestinal tract of the brushtail possum is similar to that of hindgut fermenting eutherian species, but there are some striking differences in function. Limited data suggests that the pharmacokinetics and bioavailability of administered drugs are similar to that in eutherian species, but there is some evidence that possums may have specific mechanisms for handling the intake of plant toxins and xenobiotics. The development of oral formulations for a free-ranging pest species presents several challenges above those encountered in the development of therapeutic formulations for humans and domestic animals. Use of a marsupial animal model may lead to new strategies for oral formulations in humans.

Cycle of the seminiferous epithelium in a marsupial species, the brushtail possum (Trichosurus vulpecula), and estimation of its duration

We investigated the cycle of the seminiferous epithelium in a marsupial, namely the brushtail possum (Trichosurus vulpecula), using semithin sections of seminiferous tubules embedded in Spurr's resin. Using 14 steps of spermatid development as markers, we were able to class tubular cross-sections into 10 well-defined stages of the seminiferous epithelial cycle. The duration of one cycle was 13.5 days, as determined by injections of [(3)H]-thymidine and autoradiographic examination of the most advanced sperm cells at 2 h and 17 days after injection. The durations of stages I-X were 21.4, 66.4, 54.1, 47.0, 29.8, 28.5, 25.3, 25.0, 12.0 and 15.9 h, respectively, estimated by the relative percentage of occurrence of each stage. It was estimated that the life spans of the main germ cells were as follows: type B spermatogonia, 5.4 days; primary spermatocytes, 16.7 days; secondary spermatocytes, 0.7 days; and spermatids, 21.4 days. The results suggest that the kinetics of spermatogenesis in marsupials show a similar pattern to that in eutherians.

Resource distribution influences mating system in the bobuck (Trichosurus cunninghami: Marsupialia)

Mammalian mating systems are thought to be shaped by the spatial distribution and abundance of key resources, which in turn influence the spacing behaviour of individuals. In particular, female home range size is predicted to reflect the availability of key resources. We documented the availability and distribution of food and shelter resources for two neighbouring populations of bobucks, or mountain brushtail possums, Trichosurus cunninghami, that were characterised by different mating systems: our "forest population" was socially monogamous, whereas the "roadside population" was polygynous. Both silver wattle, Acacia dealbata, the main food resource for bobucks, and den-trees, which provided shelter, occurred at significantly higher density at the roadside site. The pattern of distribution of these two resources also differed between the sites. Both food and den-trees were scattered evenly throughout the roadside habitat. In contrast, den-trees were located predominantly at one end of the forest site, while silver wattle trees were located at the other. There was no significant difference in the amount of silver wattle, or in the number of den-trees, located within the home ranges of individual females at the two sites. However, forest females had home ranges, on average, almost three times the size of those of roadside females. At the roadside site, the size of female home ranges varied inversely with the density of silver wattle, indicating that these females ranged over as large an area as necessary to gain access to sufficient silver wattle trees. There was no such relationship among forest females. These populations provide a clear example of resource distribution determining female home range size. This influenced the number of female home ranges a male's home range overlapped with, which in turn determined the mating system. Such clear links between resource availability and mating system have not previously been established in a marsupial.

Exogenous purines induce differential responses in the proximal and distal regions of the possum sphincter of Oddi

1. The aim of this study was to compare the effect of exogenous ATP and adenosine on spontaneous motility of the proximal and distal regions of the possum sphincter of Oddi (SO). 2. ATP or adenosine (1 microm-1 mm) was applied to distal-SO or proximal-SO muscle rings in organ baths in the absence or presence of tetrodotoxin (TTX) or P1/P2 antagonists. 3. Both ATP and adenosine altered spontaneous activity, predominantly in proximal-SO rings. 4. Exogenous ATP induced a bi-phasic response consisting of a brief TTX-sensitive excitatory component, and a longer-lasting TTX-insensitive inhibitory component. 5. The excitatory ATP response likely involves P2X receptors, whereas the late inhibitory response likely involves P2Y receptors. 6. Exogenous adenosine decreased spontaneous SO activity, via a TTX-insensitive mechanism. 7. Exogenous purines modulate SO motility, acting primarily in the proximal region of the SO, via neural and non-neural mechanisms and multiple purine receptor subtypes.

High definition mapping of circular and longitudinal motility in the terminal ileum of the brushtail possum Trichosurus vulpecula with watery and viscous perfusates

Longitudinal and radial movements during spontaneous contractions of isolated segments of terminal ileum of the brushtail possum, a species of arboreal folivore, were studied using high definition spatiotemporal maps. Segments obtained from specimens were continuously perfused with solutions of various apparent viscosities at 3 cm and 5 cm hydrostatic pressure. A series of sustained tetrodotoxin-sensitive peristaltic events occurred during perfusion. The leading edge of each peristaltic event progressed by a succession of rhythmic surges of circular contraction with concerted concurrent phasic longitudinal contractions. Three types of peristaltic event were observed, with differing durations of occlusion and patterns of cyclic, in phase, circular and longitudinal contractions. Each peristaltic event was preceded by a change of shade on the D map that indicated circumferential dilatation. Differences in the slopes of these phasic shade changes from those occurring during peristalsis indicate that this distension is passive and likely results from aboral displacement of fluid. Tetradotoxin insensitive longitudinal contraction waves of frequency 9.2 min(-1) occurred during and in the absence of peristalsis, originating at a variety of sites, and propagating either in an orad or aborad direction but predominantly in the latter. Perfusion with 1% guar gum, at 5 cm hydrostatic pressure caused the lumen to become distended and the generation of peristaltic events to cease pending reduction of the hydrostatic head to 3 cm but longitudinal contractile activity was preserved. Neither the frequencies nor the rates of progression of circular and longitudinal contractile events, nor the temporal coordination between these events, varied with the apparent viscosity of the perfusate or altered in a manner that could facilitate mixing.

Uterine and vaginal insemination optimised in brushtail possums (Trichosurus vulpecula) superovulated with pregnant mare serum gonadotrophin and porcine luteinising hormone

Artificial insemination of brushtail possums (Trichosurus vulpecula) is being developed as an assisted breeding model for endangered marsupials, as well as a bioassay for testing fertility control vaccines to manage overabundant populations. Procedures were optimised in animals superovulated with pregnant mare serum gonadotrophin (PMSG) and porcine luteinising hormone (pLH). Of three intervals examined, yields were maximal following uterine insemination at 27–29.5 h after pLH treatment (four eggs, two to three embryos per female). Compared with no insemination, uterine-inseminated animals ovulated 30–36 h rather than 28–34 h after pLH treatment. For the vaginal route, yields were maximal following insemination at 10–13 h after pLH treatment (six to seven eggs, four embryos per female) than at five other intervals, and when using acclimatised females during the autumn breeding season. This protocol was suitable for testing fertility control vaccines in April–June and was influenced by the housing location of animals, the presence of an active corpus luteum and PMSG batch, but not other factors (year of trial, Freund’s adjuvant treatment, changes in bodyweight, dose of PMSG kg–1). Embryos developed to the eight- to 16-cell or unilaminar blastocyst stage after uterine or vaginal insemination, respectively. With the timing of artificial insemination optimised, new methods to synchronise or induce oestrus and ovulation are required to achieve year-round testing of fertility control vaccines or birth of offspring.

Conflicting demands on detoxification pathways influence how common brushtail possums choose their diets

Most herbivores eat more and survive better when they have access to a variety of foods. One explanation involves the detoxification of plant secondary metabolites (PSMs). By feeding from a variety of plants that contain different classes of PSMs, animals can use multiple detoxification pathways and presumably consume more food. Although popular, this theory is difficult to test because it requires knowledge of the detoxification pathways of each PSM in the diet. We established that common brushtail possums (Trichosurus vulpecula) use various combinations of oxidation, hydrolysis, and conjugation with glucuronic acid (GA) or glycine to detoxify six PSMs. Compared to their ingestion of a single PSM, possums ate more when offered a choice between two diets containing PSMs that require apparently independent detoxification pathways (benzoate and 1,8-cineole, benzoate and p-cymene, benzoate and orcinol, benzoate and salicin, or orcinol and 1,8-cineole). However, possums still did not eat as much of these diets as they did of a basal diet free of PSMs. This suggests that detoxification pathways are never independent, but are separated instead by degrees. In contrast, possums offered a choice of two PSMs that require competing detoxification pathways (1,8-cineole and p-cymene, 1,8-cineole and salicin, or orcinol and salicin) ate no more than when offered diets containing one of the compounds. There was an exception: even though both rutin and orcinol are detoxified via conjugation with GA, the feeding behavior of possums did not suggest competition for detoxification pathways. This implies that the supply of GA is not limiting. This study provides the first convincing evidence that herbivorous mammals can eat more by selecting mixed diets with a diversity of PSMs that make full use of their detoxification potential. It also emphasizes that other behavioral and physiological factors, such as transient food aversions, influence feeding behavior.

How do soil nutrients affect within-plant patterns of herbivory in seedlings of Eucalyptus nitens?

This study assessed how the palatability of leaves of different age classes (young, intermediate and older) of Eucalyptus nitens seedlings varied with plant nutrient status, based on captive feeding trials with two mammalian herbivores, red-bellied pademelons (Thylogale billardierii), and common brushtail possums (Trichosurus vulpecula). Seedlings were grown under three nutrient treatments (low, medium and high), and we determined how palatability was related to chemical and physical characteristics of the leaves. Pademelons ate more older leaves than young and intermediate leaves for all treatments. This pattern was best explained by sideroxylonals (formylated phloroglucinol compounds known to deter herbivory by other marsupials), and/or essential oil compounds that were present in lower concentrations in older leaves. In the low-nutrient treatment, possums also ate more of the older leaves. However, in the medium- and high-nutrient treatments, possums ate more intermediate leaves than older leaves and showed a behavioural preference for young leaves (consuming younger leaves first) over intermediate and older leaves, in spite of high levels of sideroxylonals and essential oils. The young leaves did, however, have the highest nitrogen concentration of all the leaf age classes. Thus, either sideroxylonals and essential oils provided little or no deterrent to possums, or the deterrent was outweighed by other factors such as high nitrogen. This study indicates that mammalian herbivores show different levels of relative use and damage to leaf age classes at varying levels of plant nutrient status and, therefore, their impact on plant fitness may vary with environment.

SPATIAL SCALE OF THE PATCHINESS OF PLANT POISONS: A CRITICAL INFLUENCE ON FORAGING EFFICIENCY

Generalist mammalian browsers and folivores feed on a range of chemically different plant species, which may assist them in diluting toxins and diversifying nutrient consumption. The frequency and order in which their diets are mixed are important determinants of intake. As a result, the degree of plant heterogeneity in an environment, and the spatial scale at which this occurs, should directly influence herbivore foraging decisions. We tested whether altering the Spatial scale of plants, and thus plant secondary metabolites (PSMs), affected foraging efficiency of a generalist folivore, the common brushtail possum (Trichosurus vulpecula). First, we demonstrated that possums were able to consume more from a mixed diet of two chemically different species, Eucalyptus globulus and E. tenuiramis, than when either of these species was offered alone. We then tested whether altering the spatial scale between E. globulus and E. tenuiramis, as small- or large-scale plant heterogeneity "patches," affected possum foraging behavior and, ultimately, their foraging efficiency. Possums increased their foraging efficiency when the spatial scale of plant heterogeneity was small rather than large. We argue that the ability to regularly switch diets, when plant spatial distribution is at a small scale, reduces the negative effects of PSM ingestion. We predict that the heterogeneity of plant patches, in relation to PSM distribution, and the scale at which this occurs across a landscape, are critical factors that influence foraging efficiency and, ultimately, fitness of mammalian herbivores. This research provides a fundamental link between plant chemistry, foraging, and habitat heterogeneity.

Plasma concentrations of prolactin in brushtail possums (Trichosurus vulpecula) in different physiological states

Prolactin (Prl) has been implicated in reproduction in many mammalian species and is illustrated by the distinctive patterns of secretion during the breeding season, the oestrous cycle and lactation. The recent development of a homologous RIA for measuring the circulating Prl concentrations in brushtail possums has facilitated the reliable measurement of Prl in plasma during different physiological states in this species for the first time. Determination of Prl concentrations during lactation involved the collection of weekly blood samples from eight female possums from the time of parturition through either one or two consecutive lactational cycles. Prl was at baseline levels during early lactation (weeks 0-14 post-partum), and then increased markedly to maximum concentrations at weeks 19-21 before returning to nadir levels at a time coincident with the weaning of pouch young (weeks 23-27). The profile of Prl secretion over the oestrous cycle and in particular at the time of the preovulatory LH surge was obtained from 14 possums during the reproductive cycle, in which preovulatory follicle development and ovulation were monitored by laparoscopy. There was no distinct daily pattern of Prl secretion during the oestrous cycle; however, in 3/4 possums in which a typical preovulatory LH surge was measured, a biphasic preovulatory Prl surge was also observed. The preovulatory Prl surge commenced 2-6 h prior to, and had returned to baseline close to the onset of, the preovulatory LH surge, and a second surge of Prl occurred concomitantly with the delayed preovulatory FSH surge. Seasonality of Prl levels was established from weekly blood samples collected from six barren female possums, and concentrations of Prl were lower during the breeding season compared to the non-breeding season. Additionally, a circadian pattern of Prl secretion was evident in both female and male possums, with Prl levels higher in the morning compared to the afternoon. In conclusion, interpretation of endogenous secretory patterns suggests that Prl may be important during late lactation and at impending ovulation, but the involvement of the circannual rhythm of Prl in the regulation of seasonality in the brushtail possum remains to be determined.

Pancreatobiliary afferent recordings in the anaesthetised Australian possum

The sensory innervation to the pancreatobiliary system is poorly characterized. Afferent signals from the gastrointestinal tract and biliary tree are transmitted to the central nervous system via the vagus and spinal nerves. We aimed to record afferent discharge in order to characterize the vagal and splanchnic afferent signals from the possum upper gastrointestinal tract, biliary tree and pancreas. In 21 anaesthetised possums nerve fibres were teased from the vagus or splanchnic nerve for multi-unit recording. Mechanical stimuli consisted of balloon distension of the gallbladder and duodenum (2-7 ml) and fluid distension (0-20 mm Hg) of the bile or pancreatic ducts. Approximately 60% of fibres from all nerves displayed spontaneous discharge. Spinal afferent responses to mechanical stimuli were infrequent (n=13). Increased discharge occurred in response to duodenal (12/99 fibres) or gallbladder (7/96 fibres) distension, but not to bile duct (0/73 fibres) or pancreatic duct (0/51 fibres) distension. Vagal afferent responses to distension of the duodenum or stomach (5-30 ml) were more common (n=8). Increased discharge was recorded in response to duodenal (49/134 fibres), or gastric (22/70 fibres) distension. Responses to gallbladder distension were less frequent (6/99 fibres) and as with the spinal afferent no response to bile duct (0/66) or pancreatic duct (0/70) distension were recorded. We conclude that mechanosensitive afferents in the pancreatobiliary system are relatively rare, particularly within the ducts, and/or that they are adapted to monitor stimuli other than luminal distension.

Gastrointestinal transit in the common brushtail possum measured by gamma scintigraphy

This paper reports an example of the application of pharmaceutical technology to wildlife management, specifically the design of an oral delivery system for the common brushtail possum in New Zealand. Designing an oral delivery system requires a knowledge of the time taken for particulates to reach target sites within the gastrointestinal tract (GIT). The transit time for fluid and indigestible particles of two different size ranges was determined in the common brushtail possum (Trichosurus vulpecula). Technetium-labelled (99mTc) anion exchange resin particles (75-125 or 500-700 microm diameter) or solution (99mTc-labelled diethylenetriamine pentaacetic acid, 99mTc-DTPA) was administered orally. At predetermined times after dosing (3, 6, 12, 24 or 32 h), the distribution of radioactivity throughout excised gastrointestinal tracts was determined by gamma scintigraphy. The transit profile was similar for the three formulations investigated. Unlike other closely related hindgut fermenting marsupials, there was no evidence to support the presence of a colonic separating mechanism in the common brushtail possum. Gastrointestinal transit was independent of body mass, gender and time of day that the dose is given. To target the hindgut for oral delivery of protein and peptide biocontrol agents, the formulation would need to protect the bioactive for approximately 12 h prior to release.

Ontogeny and pathway of formation of 5alpha-androstane-3alpha,17beta-diol in the testes of the immature brushtail possum Trichosurus vulpecula

The testicular androgen 5alpha;-androstane-3alpha,17beta-diol (androstanediol) mediates virilisation in pouch young of a marsupial, the tammar wallaby, and is the principal androgen formed in immature rodent testes. To chart the pattern of androstanediol formation in another marsupial species, the testes or fragments of testes from brushtail possums (Trichosurus vulpecula) that spanned the age range from early pouch young to mature adults were incubated with (3)H-progesterone and the products were identified by high-performance liquid chromatography. The only 19-carbon steroids identified in pouch young and adult testes were the Delta(4)-3-keto-steroids testosterone and androstenedione. However, androstanediol and another 5alpha-reduced androgen (androsterone) were synthesised by testes from Day 87-200 males and these appeared to be formed from the 5alpha-reduction and 3-keto reduction of testosterone and androstenedione. In the prostate and glans penis of the immature male, (3)H-androstanediol was converted to dihydrotestosterone. We conclude that the timing of androstanediol formation in the possum testis resembles the process in rodents rather than in the tammar wallaby and that any androstanediol in the circulation probably acts in target tissues via conversion to dihydrotestosterone.

Viscoelastic behaviour aids extrusion from and reabsorption of the liquid phase into the digesta plug: creep rheometry of hindgut digesta in the common brushtail possum Trichosurus vulpecula

Temporal variation in the rheometric properties of the proximal and distal colonic digesta of an arboreal marsupial folivore, the common brushtail possum, was examined to assess flow behaviour during peristalsis, segmentation and other aspects of intestinal motility. The time-dependent rheometric characteristics on application of a constant shear stress within the physiological range showed an initial elastic and subsequent viscoelastic phase, which fitted Burger's model of creep compliance. Similarly, the time-dependent rheometric characteristics on recovery from shear stress fitted with a generalised two-component Maxwell model of elastic and viscoelastic components for creep recovery. Differences in the relative magnitudes of the viscoelastic components during recovery from those during shear indicated that the physical properties of the digesta plug changed with sustained shear stress, a phenomenon, which is likely to result from extrusion of the liquid phase from the solid elements of the digesta plug. There was significant viscoelastic recovery during the initial 4 s following cessation of stress, which would allow for prompt concomitant reabsorption of the liquid phase into the digesta plug. This supports a hypothesis of alternate extrusion and reabsorption of the liquid phase of the digesta plug. This would promote both nutrient absorption across the intestinal wall (from liquid extrusion) and enzyme permeation and digestion (from liquid absorption into the plug). However, the presence of a slower component of viscoelastic recovery indicates that liquid phase reabsorption into the digesta plug is incomplete if the interval before a subsequent contraction is less than 150 s, in which case unreabsorbed liquid may be driven either orally or aborally. This would at least partly account for differences in retention times of liquid and solid phase digesta markers reported for the gastrointestinal tracts of numerous vertebrate species.

Diet switching in a generalist mammalian folivore: fundamental to maximising intake

Generalist mammalian herbivores exploit a diverse diet. A generalised feeding strategy utilises a mixed diet to obtain a range of nutrients and to reduce the detoxication load of similar groups of plant secondary metabolites (PSMs). There is limited research investigating how mammalian herbivores achieve this dietary mixing in their daily foraging activities. We investigated the patterns of, and behaviours associated with, dietary mixing in a generalist mammalian folivore, the common brushtail possum (Trichosurus vulpecula). Possums were offered foliage of two eucalypt species (Eucalyptus globulus and E. regnans) as either (a) Full choice: both species offered for 8 h; (b) Restricted choice: both species offered for 2x2 h blocks; (c) G-R no choice: E. globulus offered for the first 4 h, E. regnans offered for next 4 h; and (d) R-G no choice: E. regnans offered for first 4 h, E. globulus offered for next 4 h. We hypothesised that possums would maximise intake on the Full choice diet, where time availability was greatest in combination with a choice of foliage. We also hypothesised that diet switching, defined as the frequency of changing between food types while feeding, would play a fundamental role in maximising intake. Possums achieved maximum intake on the Full choice diet and minimum intake on the No choice diets. Although intake was similar between the Restricted choice and No choice diets, possums achieved this intake in half the amount of time when able to switch regularly between foliage on the Restricted choice diet. We conclude that a generalist herbivore's ability to effectively switch diets when foraging is fundamental to maximising intake. Hence, the degree of plant heterogeneity in an environment, and the spatial scale at which it occurs, may affect an herbivore's foraging decisions and, ultimately, influence its foraging efficiency.

Histology of the pouch epithelium and the mammary glands during chemically induced oestrus in the brushtail possum (Trichosurus vulpecula)

Changes in the epithelium of the maternal pouch and the mammary gland of brushtail possums (Trichosurus vulpecula) were examined after animals were treated to induce ovulation with follicle-stimulating hormone (FSH), luteinizing hormone (LH), pregnant mares' serum gonadotrophin (PMSG) and oestradiol. The mammary glands were similar in appearance to those described in eutherian mammals and in previous studies on other marsupials. Exposure of possums to these compounds, particularly PMSG, appeared to result in changes in the mammary glands that could be associated with milk/secretion production. In contrast, the pouch epithelium had a similar histological appearance to that of epithelium from other parts of the body regardless of whether the animal was exposed to stimulants. These preliminary observations are discussed in the context of the purported role of the pouch epithelium and the mammary gland in production of secretions at oestrus and provision of immunological protection to the neonatal marsupial.

Periodic fluid extrusion and models of digesta mixing in the intestine of a herbivore, the common brushtail possum (Trichosurus vulpecula)

The digesta in four gut compartments (proximal and distal halves of small intestine, caecum, and proximal colon) of a wild hindgut fermenting herbivore, the common brushtail possum (Trichosurus vulpecula), were investigated by rheometry and permeametry. Digesta from all compartments were highly viscous and exhibited shear-thinning. Apparent viscosity was positively related to dry matter content, and increased from proximal small intestine to colon. Dynamic rheological measurements showed that in small intestinal digesta the elastic modulus was greater than the viscous modulus and their ratios were characteristic of weak gels, indicating that digesta could sustain compression. The apparent viscosity of distal small intestinal digesta was markedly lower when measured by capillary viscometry than by rotatory viscometry, indicating that plug flow was likely to be facilitated by lubrication from a peripheral layer of less viscous fluid; i.e., there was an augmented plug flow. Permeametry showed that fluid was extruded from all digesta on compression at physiological pressures, that there was significant permeability of proximal and distal small intestinal digesta, but that digesta became progressively compacted during permeation, with a concomitant reduction in permeability as dry matter content increased. It is proposed that conditions within the small intestine differ from those of an ideal plug flow reactor as radial mixing and turbulence cannot occur. Instead, we suggest that segmentation and peristalsis aid radial mixing of the fluid phase by compressing the solid phase, with extrusion of fluid through the digesta plug. This extrusion may be followed by resorption of fluid back into the plug when the elasticity of the solid phase of digesta is Hookean, thus aiding the mixing of secreted enzymes with insoluble substrates within the plug.

Constraint of feeding by chronic ingestion of 1,8-cineole in the brushtail possum (Trichosurus vulpecula)

Eucalyptus leaf-eating marsupials such as the brushtail possum (Trichosurus vulpecula) ingest large amounts of terpenes, especially 1,8-cineole (cineole)--the major component of many eucalyptus oils. Brushtail possums were acclimated to a non-Eucalyptus diet with increasing concentrations of cineole (0.5-4.0% wet weight) added over 18 d. We measured food and cineole consumption and urinary metabolites of cineole. Food intake decreased with cineole content, indicating that it was constrained by the maximum tolerable intake of cineole that was 3.8 +/- 0.2 g kg(-1) or 5.2 +/- 0.3 g kg(-0.75) (mean +/- SE, N = 6). The pattern of metabolites was similar at all cineole intakes (56% hydroxycineolic acids, 27% cineolic acids, 13% hydroxycineoles, and 4% dihydroxycineoles). In another experiment, possums maintained on artificial diet were abruptly presented with 4% cineole for 5 d. Food intake fell by 45 +/- 6% (mean +/- SE, N = 6) and mean cineole intake was 2.9 +/- 0.3 g kg(-1). There was evidence of induction of secondary oxidative pathways, as hydroxycineoles were the major metabolites (48% total) on the first day, but rapidly dropped to 15% on subsequent days as the acid metabolites increased. These findings indicate that ingestion of cineole is not constrained by selective saturation of individual enzymes involved in its multiple pathways of oxidation, but rather the total detoxification capacity appears to limit feeding on a cineole diet.

Inheritance Of Resistance To Mammalian Herbivores and Of Plant Defensive Chemistry In A Eucalyptus Species

Hybridization in plants provides an opportunity to investigate the patterns of inheritance of hybrid resistance to herbivores, and of the plant mechanisms conferring this resistance such as plant secondary metabolites. We investigated how inter-race differences in resistance of Eucalyptus globulus to a generalist mammalian herbivore, Trichosurus vulpecula, are inherited in their F1 hybrids. We assessed browsing damage of three-year-old trees in a common environment field trial on four hybrid types of known progeny. The progency were artificial intra-race crosses and reciprocal inter-race F1 hybrids of two geographically distinct populations (races) of E. globulus; north-eastern Tasmania and south-eastern Tasmania. Populations of trees from north-eastern Tasmania are relatively susceptible to browsing by T. vulpecula, while populations from south-eastern Tasmania are more resistant. We assessed the preferences of these trees in a series of paired feeding trials with captive animals to test the field trial results and also investigated the patterns of inheritance of plant secondary metabolites. Our results demonstrated that the phenotypic expression of resistance of the inter-race F1 hybrids supported the additive pattern of inheritance, as these hybrids were intermediate in resistance compared to the pure parental hybrids. The expression of plant secondary metabolites in the F1 hybrids varied among groups of individual compounds. The most common pattern supported was dominance towards one of the parental types. Together, condensed tannins and essential oils appeared to explain the observed patterns of resistance among the four hybrid types. While both chemical groups were inherited in a dominant manner in the inter-race F1 hybrids, the direction of dominance was opposite. Their combined concentration, however, was inherited in an additive manner, consistent with the phenotypic differences in browsing.

Inheritance Of Resistance to Mammalian Herbivores and of Plant Defensive Chemistry in an Eucalyptus Species

Hybridization in plants provides an opportunity to investigate the patterns of inheritance of hybrid resistance to herbivores, and of the plant mechanisms conferring this resistance such as plant secondary metabolites. We investigated how inter-race differences in resistance of Eucalyptus globulus to a generalist mammalian herbivore, Trichosurus vulpecula, are inherited in their Fl hybrids. We assessed browsing damage of 3-year-old trees in a common environment field trial on four hybrid types of known progeny. The progeny were artificial intra-race crosses and reciprocal inter-race F1 hybrids of two geographically distinct populations (races) of E. globulus north-eastern Tasmania and south-eastern Tasmania. Populations of trees from north-eastern Tasmania are relatively susceptible to browsing by T. vulpecula, while populations from south-eastern Tasmania are more resistant. We assessed the preferences of these trees in a series of paired feeding trials with captive animals to test the field trial results and also investigated the patterns of inheritance of plant secondary metabolites. Our results demonstrated that the phenotypic expression of resistance of the inter-race Fl hybrids supported the additive pattern of inheritance, as these hybrids were intermediate in resistance compared to the pure parental hybrids. The expression of plant secondary metabolites in the Fl hybrids varied among major groups of individual compounds. The most common pattern supported was dominance towards one of the parental types. Together, condensed tannins and essential oils appeared to explain the observed patterns of resistance among the four hybrid types. While both chemical groups were inherited in a dominant manner in the inter-race Fl hybrids, the direction of dominance was opposite. Their combined concentration, however, was inherited in an additive manner, consistent with the phenotypic differences in browsing.

Demography in relation to population density in two herbivorous marsupials: testing for source?sink dynamics versus independent regulation of population size

We compared demography of populations along gradients of population density in two medium-sized herbivorous marsupials, the common brushtail possum Trichosurus vulpecula and the rufous bettong Aepyprymnus rufescens, to test for net dispersal from high density populations (acting as sources) to low density populations (sinks). In both species, population density was positively related to soil fertility, and variation in soil fertility produced large differences in population density of contiguous populations. We predicted that if source-sink dynamics were operating over this density gradient, we should find higher immigration rates in low-density populations, and positive relationships of measures of individual fitness--body condition, reproductive output, juvenile growth rates and survivorship--to population density. This was predicted because under source-sink dynamics, immigration from high-density sites would hold population density above carrying capacity in low-density sites. The study included 13 populations of these two species, representing a more than 50-fold range of density for each species, but we found that individual fitness, immigration rates and population turnover were similar in all populations. We conclude that net dispersal from high to low density populations had little influence on population dynamics in these species; rather, all populations appeared to be independently regulated at carrying capacity, with a balanced exchange of dispersers among populations. These two species have suffered recent reductions in range, and they are ecologically similar to other species that have declined to extinction in inland Australia. It has been argued that part of the cause of the vulnerability of species like these is that they exhibit source-sink dynamics, and disturbance to source habitats can therefore cause large-scale population collapses. The results of our study argue against this interpretation.