Summary 1.Interactions during mating are thought to be an important mechanism for transmission of tuberculosis (Tb) Mycobacterium bovis in the brushtail possum Trichosurus vulpecula . However, little information is available on the frequency of contacts between males and females in oestrus during the breeding season, and the relationship between mating contacts and population density. 2. We used radio-telemetry to record contacts between male and oestrous and nonoestrous female possums, and determined paternity of offspring using DNA analysis. This was repeated following the removal of c . 70% of the resident possums to determine the effect of reducing density on the contact rate. 3. We could not detect any significant differences in the contact rate between oestrous and non-oestrous females and males, either before or after the density reduction, even when paternity was positively identified from DNA analysis. This suggests that actual mating contacts could not be distinguished from other agonistic or affiliative contact behaviours. 4. Despite this, the relationships between male-female and male-male contact rates and population density were non-linear convex-up, implying that the contact rate during the breeding season did not decrease in proportion to reductions in density. This appeared to be driven by the enlargement of male ranges and a corresponding increase in male overlap of female ranges following the density reduction. 5. The form of the contact rate function will influence predictions of disease spread in epidemiological models for Tb in wildlife. This has major implications for the development of tactical approaches to disease management based on such models.
Summary1. Quantitative predictions of the effects of perturbations on communities of interacting species have often proved to be difficult. However, if precise predictions are not a requirement then qualitative models of community dynamics offer an alternative method for predicting species responses to perturbations. 2. We used two qualitative modelling approaches to predict the effects of predator control on the fledging rate of an endangered New Zealand bird, the North Island kokako. The first approach was based on loop analysis and provided predictions on the probable direction of change in species abundances to single species perturbations. The second approach, 'fuzzy interaction webs', used fuzzy logic in the framework of a fuzzy cognitive map to provide predictions on the probable magnitude of change in species abundances to perturbations. 3. Using both methods, we predicted the qualitative change in the equilibrium fledging rates of kokako under various regimes of single-and multispecies predator control (ship rats, brushtail possums and stoats). Single species control was insufficient to lift the fledging rate from 'low' to 'moderate'. However, simultaneous control of both ship rats and possums had the greatest influence on the fledging rates compared with any other combination as a result of the additional indirect effect of ship rat control on stoat abundance. 4. We propose qualitative modelling of community dynamics as a method suitable for predicting the effects of perturbations in complex ecological communities that can encapsulate diverse sources of knowledge about food web interactions. We believe that these methods are a useful set of heuristic tools that can be used to propose testable hypotheses about ecosystem functioning that can complement existing statistical and quantitative modelling approaches.
We apply a new algorithm for spatially simulating animal trapping that utilises a detection function and allows for competition between animals and traps. Estimates of the parameters of the detection function from field studies allowed us to simulate realistically the expected range of detection probabilities of brushtail possums caught in traps. Using this model we evaluated a common index of population density of brushtail possums based on the percentage of leg-hold traps catching possums. Using field estimates of the parameters of the detection function, we simulated the relationship between the trap-catch index and population density. The relationship was linear up to densities of 10 possums ha -1 . We also investigated the accuracy (bias and precision) of the trap-catch index for possums to estimate relative changes in population density (relative abundance) under conditions of varying detection probability, and compared these results with those obtained using a removal estimate of the population in the vicinity of trap lines. The ratio of trap-catch indices was a more precise estimator of relative abundance than the ratio of removal estimates but was positively biased (i.e. overestimated relative abundance). In contrast, the ratio of removal estimates was relatively unbiased but imprecise. Despite the positive bias, the trap-catch index had a higher power to determine the correct ranking between population densities than the removal estimate. Although varying detection probability can bias estimates of relative abundance using indices, we show that the potential for bias to lead to an incorrect result is small for indices of brushtail possum density based on trapping.
Summary1. We present methods for estimating disease transmission coefficients in wildlife, using Leptospira interrogans infection (a bacterial disease transmitted predominantly during social contacts) in brushtail possums Trichosurus vulpecula as a model system. 2. Using data from a field experiment conducted on a naturally infected possum population, we estimated disease transmission coefficients assuming either 'densitydependent' or 'frequency-dependent' transmission. 3. A model-selection approach determined that density-dependent transmission was the most appropriate form of the transmission of L. interrogans infection in brushtail possums. 4.We used the chosen model of transmission to examine experimentally the effect of tubally ligating female brushtail possums on the epidemiology of L. interrogans . The estimated transmission coefficient was 28% higher ( P = 0·16) in populations subject to tubal ligation, raising the possibility that fertility control of this type may increase disease transmission rates. 5. Altering mating behaviour through fertility control may have the potential to control diseases such as bovine tuberculosis in brushtail possums, although the potential of fertility control techniques to change disease transmission coefficients and disease epidemiology requires further investigation. This would require models that examine the combined effects of fertility control on population dynamics, social behaviour and disease transmission coefficients simultaneously.
Summary 1.Reducing the fertility of pest species using immunocontraceptive vaccines holds promise for the humane and effective control of vertebrate pest populations. However, despite much research into the development of contraceptive vaccines, there are few data illustrating the effectiveness of fertility control for managing wild populations of vertebrate pests. 2. In New Zealand, introduced brushtail possums Trichosurus vulpecula have been the focus for the development of contraceptive vaccines to provide a possible alternative to lethal control techniques. 3. The effect of fertility control on the population dynamics of brushtail possums was investigated experimentally using surgical sterilization to suppress breeding on six 12-ha study sites between January 1996 and October 2000. Sterilization treatments, consisting of 0%, 50% and 80% of resident females sterilized, were applied at a particular site, with adjustments made annually on recruits to maintain levels of sterility. Mark-recapture trapping was used to estimate age-specific demographic rates and their relative contribution to age-specific population growth rates ( λ ). 4. Sterility treatments resulted in reductions to per capita rates of local recruitment (surviving young per adult female) of 60% and 74% on average for 50% and 80% levels of sterility, respectively. However, immigration by yearling-aged possums compensated for reductions in local recruitment because of sterility treatments, leading to the stable ( λ = 1) population trajectories on sterility treatment sites. 5. There was some evidence that sterilized females had higher annual survival rates than fertile females, but otherwise there was no evidence of any other compensatory responses to imposed sterility. However, additional studies are needed to clarify possible densitydependent responses that could compensate for the effects of fertility control. 6. Synthesis and applications. Demographic compensation by pest populations can negate the effects of fertility control. Results from this study suggest that immigration can easily compensate for fertility control in pest species where adult survival is high and fecundity is low. This suggests that fertility control will need to be applied at large spatial scales to reduce this effect. Where adult survival makes a relatively high contribution to the population growth rate, the benefits of fertility control would be enhanced if integrated with conventional control to provide rapid initial population reduction.
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