Jim Hone

Feral pigs in Namadgi National Park, Australia: dynamics, impacts and management

Feral pigs (Sus scrofa) have spread through Namadgi National Park (NNP) in south-eastern Australia since the early 1960s at a mean rate of 4 km/year. Pigs were abundant (approximately 1–2 pigs km−2) during the mid-1980s. Research from 1985 to 2000 inclusive has demonstrated a positive curved relationship across years between the frequency of occurrence of pig rooting and pig abundance (R2=0.48; P<0.001) and the extent of ground rooting; more pigs, more rooting. The ground rooting decreased plant species richness. There was a negative curved relationship between plant species richness and the extent of pig rooting at two sites (R2=0.81; P<0.0001 and R2=0.67; P<0.0001), with plant richness declining to zero with intensive pig rooting. Since the mid 1980s intensive pig control work has resulted in a significant (R2=0.39; P<0.001) decline in pig abundance with an annual instantaneous rate of change (r) of −0.15 between 1985 and 2000 inclusive. The results and their implications for biodiversity conservation and feral pig management are discussed.

Spatial and temporal aspects of vertebrate pest damage with emphasis on feral pigs

1. Aspects of the spatial and temporal patterns of damage by vertebrate pests, especially feral pigs, are examined. 2. The spatial extent and pattern of ground rooting, as an index of damage, by feral pigs can be predicted by simple modelling. This allows determination of locations of maximum and minimum disturbance, and possible sites for concentration of pig damage control. 3. The response of damage to control depends on a relationship between the levels of damage and pest abundance or the rate of change in damage and pest abundance. 4. Analysis shows that statistically significant relationships occur for the latter but not the former for feral pigs. The relationship is non-linear, and predicts an equilibrium abundance of pigs for maintenance of rooting frequency and a variable response in rooting to a unit change in pig abundance associated with control. 5. The implications for vertebrate pest control are discussed.

Rate of Increase and Fertility Control

1. The relationships between the rate of increase of populations of vertebrate pests and net reproductive rate, and fecundity and survivorship, are examined. 2. The relationships are non-linear so the response of a population to pest control, e.g. by fertility control, will be non-linear. The response will increase as fertility declines, if the generation interval or survivorship is constant. 3. If survivorship increases as fecundity declines, the effects of fertility control on rate of increase will be reduced. 4. Practical and strategic implications are that fertility control of pests may be most effective in a pest population previously reduced in abundance

Visibility bias in aerial survey: mark–recapture, line-transect or both?

Mark-recapture and line-transect sampling procedures both provide estimators for visibility bias in aerial surveys, and have coexisted in the literature for decades. Mark-recapture estimators of abundance tend to be negatively biased in this context as a result of unmodelled heterogeneity. Line-transect sampling can also be negatively biased if detection probability on the line is less than 1.0. Numerous papers have described hybrid approaches using mark-recapture and line transect methods but there have been some subtle but important differences that may not be apparent to the practitioner. We have used wild horse survey data collected in south-eastern Australia and some imaginary data to highlight these subtle differences. We demonstrate the advantage of using the hybrid approach, which uses the strengths of both mark-recapture and line-transect procedures by fitting a detection function (with p(0) = 1) to the line-transect data to estimate the shape of the detection function, and uses a separate detection function for the mark-recapture data to estimate the intercept (p(0)).

Evaluation of predator numerical responses

We evaluated hypotheses of the dynamics of predators (lynx) relative to prey (snowshoe hares) and predator abundance in the Yukon, Canada. The hypotheses were that predator (lynx) dynamics are influenced by prey density, or by both prey and predator densities. Annual lynx population growth rate (r), estimated from lynx counts, was positively related to previous hare density and negatively related to previous lynx density, as described by the best-fitting additive model (R2 = 0.85). Annual lynx growth rate (r) estimated from lynx tracks was positively related to the ratio of hares per lynx in the best-fitting model (R2 = 0.55). There was most support for the prey- and predator-dependent hypothesis of predator dynamics. Projected lynx tracks showed similar trends to observed abundance but lagged one year, emphasising the need for evaluation of projected predator trends.

Under what conditions do climate-driven sex ratios enhance versus diminish population persistence?

For many species of reptile, crucial demographic parameters such as embryonic survival and individual sex (male or female) depend on ambient temperature during incubation. While much has been made of the role of climate on offspring sex ratios in species with temperature-dependent sex determination (TSD), the impact of variable sex ratio on populations is likely to depend on how limiting male numbers are to female fecundity in female-biased populations, and whether a climatic effect on embryonic survival overwhelms or interacts with sex ratio. To examine the sensitivity of populations to these interacting factors, we developed a generalized model to explore the effects of embryonic survival, hatchling sex ratio, and the interaction between these, on population size and persistence while varying the levels of male limitation. Populations with TSD reached a greater maximum number of females compared to populations with GSD, although this was often associated with a narrower range of persistence. When survival depended on temperature, TSD populations persisted over a greater range of temperatures than GSD populations. This benefit of TSD was greatly reduced by even modest male limitation, indicating very strong importance of this largely unmeasured biologic factor. Finally, when males were not limiting, a steep relationship between sex ratio and temperature favoured population persistence across a wider range of climates compared to the shallower relationships. The opposite was true when males were limiting – shallow relationships between sex ratio and temperature allowed greater persistence. The results highlight that, if we are to predict the response of populations with TSD to climate change, it is imperative to 1) accurately quantify the extent to which male abundance limits female fecundity, and 2) measure how sex ratios and peak survival coincide over climate.

Feral goat control in Egmont National Park, New Zealand, and the implications for eradication

Feral goats in the 34 169 ha Egmont National Park (North Island, New Zealand) have been subject to sustained ground-based hunting with dogs since 1925. We analysed trends in hunting success from 1961 to 1999. During 1961–86 the catch per unit effort (CPUE) declined from 7 kills hunter–1 day–1 to 50 years or 12 years, respectively. Annual immigration of goats would prevent eradication under all scenarios.

On bias, precision and accuracy in wildlife aerial surveys

Bias, precision and accuracy have been studied extensively in wildlife population estimation including aerial surveys. A review of the literature shows that the concepts of bias and precision are used broadly consistently. Aerial survey data from known populations of feral pig carcases and white-tailed deer show that few density estimates are unbiased and precise. Research is needed, however, to clarify how much bias and how much precision are enough for the various types of wildlife management activities. Accuracy is used in two closely related but different ways. One set of definitions of accuracy relates to deviations from the true value (bias) and the second set relates to squared deviations from the true value (bias and precision). The implications are that authors are encouraged to clearly state which definition of accuracy they use, or focus solely on bias and precision.

Is the relationship between predator and prey abundances related to climate for lynx and snowshoe hares

Context Predator dynamics may be related to prey abundance and influenced by environmental effects, such as climate. Predator–prey interactions may be represented by mechanistic models that comprise a deterministic skeleton with stochastic climatic forcing. Aims The aim of this study was to evaluate the effects of climate on predator–prey dynamics. The lynx and snowshoe hare predator–prey system in the Kluane region of the Yukon, Canada, is used as a case study. The specific hypothesis is that climate influences the relationship between lynx and hare abundance. Methods We evaluate 10 linear relationships between predator and prey abundance and effects of climate. We use data on lynx and snowshoe hare abundance over 21 years in the Yukon as the predator–prey system, and three alternative broad-scale climate indices: the winter North Atlantic Oscillation (NAO), the Pacific North American (PNA) index and the North Pacific index (NPI). Key results There was more support, as assessed by Akaike weights (ωi = 0.600), evidence ratio (=4.73) and R2 (=0.77) for a model of predator (lynx) and prior prey (hare) abundance with an effect of prior climate (winter NAO) when combined in a multiplicative, rather than in an additive, manner. The results infer that climate changes the amplitude of the lynx cycle with lower predator (lynx) abundance with positive values of winter NAO for a given hare density. Conclusions The study provides evidence that predator–prey dynamics are related to climate in an interactive manner. The ecological mechanism for the interactive effect is not clear, and alternative hypotheses are proposed for future evaluation. Implications The study implies that changes in climate may alter predator–prey relationships.

Mortality rates of feral ferrets (Mustela furo) in New Zealand

Life-table data from feral ferret populations in New Zealand were analysed to estimate their mortality rates, and to test for any additive effect of Mycobacterium bovis infection on observed mortality rates. The observed instantaneous mortality rate was best estimated by modelling mortality as a 2-phase step model with different rates for juveniles (μ1 = 1.45 year–1, 95% C.I. 1.2–1.7 year–1) and adults (μ2 = 0.55 year–1, 95% C.I. 0.4–0.9 year–1). This corresponds to a survival probability of 0.25 during the first year of life, rising to 0.55 year–1 thereafter, and a life expectancy of 0.95 years. At a population level, no additional mortality due to M. bovis infection was observed, suggesting either that the rate of disease-induced mortality was negligible, or that it was compensatory with natural mortality.