Clare J. Veltman

UNEXPECTED CONSEQUENCES OF VERTEBRATE PEST CONTROL: PREDICTIONS FROM A FOUR-SPECIES COMMUNITY MODEL

Although indirect effects are important structuring forces in ecological communities, they are seldom considered in the design of pest control operations. However, such effects may cause unpredicted and deleterious changes in other populations that could reduce or even negate the benefit to endangered species for which control is undertaken. Furthermore, the complexity and nonlinearities inherent in interacting ecological communities may cause thresholds in the strength of pest control, on either side of which indirect effects could vary greatly in their magnitude and desirability. We constructed a four-species simulation model for a common pest community in New Zealand beech (Nothofagus spp.) forests: house mice, ship rats, stoats, and brushtail possums. When the model was perturbed to simulate common control techniques, marked increases in the abundance of nontarget pest species were observed at the next forest mast. Higher mouse numbers were observed following both toxin (1080) application and rat kill-trapping, and higher rat numbers were observed following stoat kill-trapping, due to a release from predation in all cases. In comparison, a marked decrease in stoat abundance at the next forest mast was observed following simultaneous control of rats and mice, due to the effects of decreased prey abundance on the stoat population. For rat control, the size of the indirect effect on mouse numbers increased monotonically with control strength. Because the curvature of the relationship is slight, the relationship between the direct benefits of control and the indirect costs incurred would remain relatively unchanged regardless of the strength of control employed. For simultaneous mouse and rat control, however, high levels of control (as initially simulated) were predicted to cause decreased peak stoat abundance at the next mast event, whereas intermediate and low levels of control were predicted to cause increased stoat abundance. Hence, this study demonstrates two points of concern for pest managers. First, indirect effects of control operations do have the potential to reduce the planned-for benefit. Second, thresholds in the strength of control employed can potentially occur, across which indirect effects switch from being of conservation benefit to being of conservation concern.

Collaborative Learning as Part of Adaptive Management of Forests Affected by Deer

Adaptive management requires the merger of management with science to provide robust knowledge about the effect of management actions. It can also be applied as a model of collaborative learning to support effective resource management. Using the example of adaptive management of native forests affected by introduced deer in New Zealand, we set out to identify some of the tensions that become apparent when adaptive management is applied in this way. We describe the process of adaptive management as it was applied in this case study. Drawing from project documentation and participant reflections on the learning process, we highlight three key lessons: (1) the need to create ‘space’ – i.e. a permissive environment that allows for an evolving process rather than a formalised and legalistic one; (2) that adaptive management cannot be expected to progress in a standardised way but instead, role clarity will emerge over time and this will contribute to an emerging vision of contribution that participants see for their project; and (3) the collaborative learning component of adaptive management poses a new challenge for science as rather than providing solutions to management issues, scientists contribute technical expertise and methods as part of the management

Potential of delta-decanolactone and (Z)-7-dodecen-1-yl acetate to attract feral goats (Capra hircus)

A lure is needed for cost-effective control of feral goat populations dispersed in forests. We evaluated the odour delta-decanolactone and the pheromone (Z)-7-dodecen-1-yl acetate in a series of trials using penned Angora goats on two New Zealand farms. Filter paper discs at each of two feeders stocked with lucerne chaff dispensed 2 mL of odour or pheromone in solution. The goats could also forage for grass. We counted goats at each feeder at 30-min intervals, and analysed how often the number at one exceeded the other. Delta-decanolactone attracted female goats significantly more often than the control solution (5% ethanol) in 1 of 3 trials and was not attractive when it was combined with oxytocin and gamma-amino-n-butyric acid in 1 trial. (Z)-7-dodecen-1-yl acetate was significantly more attractive than the control in 1 of 2 trials on females and 2 of 3 trials on males, but addition of oxytocin and gamma-amino-n-butyric acid did not enhance the effect. A mixture of the odour and the pheromone attracted females in 1 of 3 trials. Inconsistent results between similar trials indicate the substances tested were unlikely to be universally attractive under field conditions.

Search strategies for conservation detection dogs

Monitoring rare or cryptic species can be challenging, especially with limited time and resources. Dogs are often used for this purpose, but methods are highly variable. There is a need to optimise search methods for dog teams so that time and resources are used as efficiently as possible. Some degree of standardisation is also desirable so that search results are comparable between different times and places. The discipline of search theory has developed effective methods to maximise the probability of detecting a search object and/or maximise the efficiency of a search. However, these advances have not been explicitly applied to the use of dogs to search for plants and animals in the wild. Here, we provide a brief introduction to search theory, then discuss how ideas from search theory might be used to standardise and optimise the use of conservation detection dogs. We describe approaches that have been used, discuss their strengths and weaknesses, and suggest priorities for further research. Standardised methods based on search theory could increase the effectiveness of conservation detection dogs, and make search results more comparable across different locations and times.

Wildlife detector dogs and camera traps: a comparison of techniques for detecting feral cats

ABSTRACT A major challenge in controlling overabundant wildlife is monitoring their populations, particularly as they decline to very low density. Camera traps and wildlife detector dogs are increasingly being used for this purpose. We compared the cost-effectiveness of these two approaches for detecting feral cats (Felis catus) on two pastoral properties in Hawkes Bay, North Island, New Zealand. One property was subject to intensive pest removal, while the other had no recent history of pest control. Camera traps and wildlife detector dogs detected cats at similar rates at both sites. The operating costs of each method were also comparable. We identify a number of advantages and disadvantages of each technique, and suggest priorities for further research.

I smell a rat! Estimating effective sweep width for searches using wildlife-detector dogs

Abstract Context. Dogs are often used to find rare or cryptic species, but search methods are not standardised, making it difficult to interpret and compare results. Standardised approaches are needed to optimise search effectiveness and/or efficiency. Designing an optimal search strategy requires knowledge of the effective sweep width, which is related to the probability of detection (POD) at various distances between the searcher and the search object. Aims. Our primary aim was to estimate effective sweep width for wildlife-detector dogs searching for rodents. We also tested whether dogs differed in their reaction on encountering a laboratory rat (Rattus norvegicus) or a wild-caught Norway rat (wild-type R. norvegicus). Methods. We conducted field trials using trained rodent-detector dogs to locate dead laboratory rats. We used the numbers of detections and non-detections at distances of 0–100 m to estimate detection probability and effective sweep width. Key results. Dog teams located 100% of rats (regardless of strain) placed directly in their search path. POD declined rapidly with an increasing distance, yielding an observed detection rate of 33% at 10 m, and close to zero at ≥20 m. The data were best described by an exponential decay function. Effective sweep width was estimated to be 16.8 m (95% confidence interval 12.3–21.4 m), corresponding to a strip extending 8.4 m on either side of a walked track. Handlers could not consistently judge whether a dog had encountered a laboratory rat or a wild rat. Conclusions. Our results suggest that when dogs are >10 m from a source of rat odour, POD declines sharply. We estimate that the effective distance explored when searching for a stationary rodent is 8.4 m either side of the search path. Implications. This information will allow users to optimise the search pattern that dog teams should follow for a given search scenario.

A management experiment reveals the difficulty of altering seedling growth and palatable plant biomass by culling invasive deer

Abstract Context. There is concern that deer are shifting forests towards undesirable trajectories, and culling of deer is often advocated to mitigate these impacts. However, culling deer is expensive and sometimes controversial. To reliably ascertain whether such action is beneficial, management-scale experiments are needed. We conducted a management experiment to evaluate the benefits of culling deer in four New Zealand forests. Aims. Our experiment tested the predictions that culling deer should increase (1) canopy tree seedling height relative growth rate (SHRGR), and (2) the foliar biomass of understorey species palatable to deer (FBP). Methods. Each forest was divided into two 3600-ha areas, with deer culling randomly assigned to one area. Deer abundances were indexed using faecal pellet counts, and forest variables were measured at the start and end of the 8-year experiment. Deer were already at low abundance in one forest and were not culled there. We used structural equation modelling (SEM) with Bayesian variable selection to update our a priori graphical forest–deer model with data from all four forests. Key results. Deer abundances were significantly reduced in one forest but increased or did not change in the other two forests in which deer culling occurred. Culling deer did not increase seedling height relative growth rate (SHRGR) or the foliar biomass of understorey species palatable to deer (FBP) in the three areas subject to deer culling compared with the three areas not subject to deer culling. SEM revealed no significant relationships between local-scale deer abundance and either SHRGR or FBP. Rather, tree basal area and the foliar biomass of unpalatable understorey species were important predictors of FBP and SHRGR, respectively, in some forests. Conclusions. Our study revealed that culling deer, as currently practiced by New Zealand land managers, did not generate the desired responses in New Zealand forests, possibly due to deer not being culled to sufficiently low densities and/or because forest dynamics and abiotic drivers determined plant growth more than deer. Implications. Managers should consider actions other than ineffective deer culling (e.g. creating canopy gaps) to alter the dynamics of New Zealand forests. Alternatively, managers will need to substantially increase culling effort above what is currently practised for this activity to substantially reduce deer populations and thus potentially alter forest dynamics.