Peter Sweetapple

Top down or bottom up? Comparing the impacts of introduced arboreal possums and ‘terrestrial’ ruminants on native forests in New Zealand

We review and contrast the impacts on New Zealands native forests of the two main types of introduced mammalian herbivore; ‘arboreal’ browsers, represented solely by the brushtail possum Trichosurus vulpecula, and ‘terrestrial’ ruminants, represented by deer (Cervidae) and goats Capra hircus. Because of their large size and sophisticated digestive anatomy, the ruminants are able to consume virtually all of the foliage of preferred native plants that is available to them and thereby exert a strong ‘top down’ regulatory effect on forest composition, particularly where they can supplement their diet with fallen leaves from the forest canopy. Possums appear to be less efficient as folivores than the ruminants as a consequence of their smaller size and simpler digestive anatomy. This may sometimes severely limit their top-down influence on plant abundance. However, possums appear able to make much greater use of foliage where they are able to combine an abundant but nutritionally inadequate leaf resource with high-quality foods such as fruit. For both possums and deer, the size and nature of supplemental food sources (i.e. other than growing foliage) appears crucial in determining both herbivore carrying capacity and the severity of their impacts on forest composition. Particularly important is whether or not the availability of the key supplemental foods is directly and negatively affected by herbivore abundance. We explore the implications of this hypothesis for herbivore management.

Unexpected consequences of control: competitive vs. predator release in a four-species assemblage of invasive mammals

Invasive species are frequently the target of eradication or control programmes to mitigate their impacts. However, manipulating single species in isolation can lead to unexpected consequences for other species, with outcomes such as mesopredator release demonstrated both theoretically and empirically in vertebrate assemblages with at least two trophic levels. Less is known about the consequences of species removal in more complex assemblages where a greater number of interacting invaders increases the potential for selective species removal to result in unexpected changes in community structure. Using a replicated Before-After Control-Impact field experiment with a four-species assemblage of invasive mammals we show that species interactions in the community are dominated by competition rather than predation. There was no measurable response of two mesopredators (rats and mice) following control of the top predator (stoats), but there was competitive release of rats following removal of a herbivore (possums), and competitive release of mice following removal of rats.

Effect of prefeeding, sowing rate and sowing pattern on efficacy of aerial 1080 poisoning of small-mammal pests in New Zealand

Context Aerial poisoning using sodium fluoroacetate (1080) is an important but controversial technique used for large-scale control of brushtail possums (Trichosurus vulpecula) and other pests in New Zealand. The technique reliably produces near total kills of possums and rats, provided that many tens of baits (and therefore many tens of individually lethal doses) are sown for each target animal present. Aim The aim of this study was to further refine aerial 1080 poisoning by determining the effect of prefeeding, sowing rate, and sowing pattern on effectiveness. Methods Eighteen experimental treatments comprising all possible combinations of three sowing rates (1, 2, and 5 kg/ha of bait), three frequencies of non-toxic prefeed (0, 1, and 2) and two sowing patterns (parallel and cross-hatched) were applied to each of two forested areas. Treatment effectiveness was assessed from changes in the rate of interference recorded on baited cards for three species: possum, ship rat (Rattus rattus) and mouse (Mus musculus). Key results Outcomes were highly variable, ranging from increases in pest activity to near total reductions. Possum reductions were highest where one or two prefeeds were used, and at the higher sowing rates (2 or 5 kg/ha), but with some interactions between these factors. For rats, two prefeeds resulted in the highest reductions but sowing rate had no effect. For mice, post-poisoning indices were often high, indicating low effectiveness. Conclusions Some treatments were highly effective so poor kills were unlikely to have resulted from pests not encountering bait, or the bait being unpalatable. Rather they appeared to reflect sub-lethal poisoning either as a result of low acceptance (as a result of a lack of familiarity and/or satiation) or bait fragmentation. We infer that for possum and rats prefeeding helps reduce this risk of sub-lethal poisoning not only by increasing familiarity, but also (in conjunction with high sowing rates) by increasing the bait encounter rate, particularly for possums. Implications There is scope to further reduce the amount of toxic bait sown and the cost of poisoning, without compromising efficacy, by fine-tuning the balance between prefeeding and sowing rate based on which species are being targeted and, for possums, reducing bait fragmentation.

Seedling ratios: a simple method for assessing ungulate impacts on forest understories

Abstract Introduced ungulates threaten the indigenous biota of remote oceanic islands such as Hawaii and New Zealand. The effectiveness of sustained animal-control programs to protect these forests is reduced by the lack of affordable and robust tools to monitor their outcomes. We developed a simple method (the seedling ratio index) to monitor forest understory condition. This index compared species richness of tall seedlings (30–200 cm) with that of short seedlings (<30 cm) for groups of species in different ruminant-feeding-preference classes (high, moderate, low). We assessed the methods utility by measuring seedling ratio indices at sites covering a wide range of forest types, ruminant species, and ruminant densities in Hawaii and New Zealand. The relationship between seedling ratio indices and pig (Sus scrofa) abundance in Hawaii also was investigated. Seedling ratio indices for high-preference plants were negatively correlated with ruminant abundance (r=−0.93). The regression equation fitted to these data successfully predicted changes in seedling ratio indices for high-preference species following managed reductions in ruminant densities. For low-preference species, seedling ratio indices remained near or above zero over the range of ungulate abundance examined. The method appears to provide a robust indicator of probable impact of ungulate browsing on forest understories, although other measures are required to assess the level of understory disturbance resulting from rooting by pigs.

Hunters and hunting patterns in part of the Kaimanawa Recreational Hunting Area

Abstract A postal survey of 156 hunters who used the three southeastern-most hunting blocks in the Kaimanawa Recreational Hunting Area (RHA) in 1986 and 1987 achieved a 74% response rate. Comparison of age distributions from this and earlier hunter surveys confirmed an apparent decline in recruitment of young hunters entering the sport during the 1980s. Most hunters had considerable experience (>10 years) and were motivated not only by hunting success but also by the aesthetic value of the outdoor experience. The prime motivation for hunting in the Kaimanawa RHA was the presence of sika deer, and more than two-thirds of the hunters came from outside the Tongariro/Taupo Conservancy. Hunting pressure over current sika deer range in the central North Island may decline if this species continues to disperse naturally or through illegal liberations to other areas. Most (80%) hunters felt that deer densities were acceptable and were satisfied with the present unrestricted hunting system. Fewer than half (41%) t...

Possum demographics and distribution after reduction to near–zero density

Abstract Demographic responses of a population of New Zealand’s foremost vertebrate pest, the introduced brushtail possum (Trichosurus vulpecula), after being reduced to very low levels, were investigated over 2.5 years in a central North Island mixed conifer-broadleaved forest. Densities 4–9 months after population reduction were estimated to be c. 0.08 possums/ha, possibly indicating that most individuals were isolated from one another immediately after population reduction. We found no evidence of inverse–density–dependent factors reducing the rate of recovery of the possum population; 87% of 23 adult females captured during the study were carrying pouch young or had recently bred. The estimated exponential annual rate of population increase during the study was 0.59. Changes in habitat used, decreased trappability with increasing time since first detection, and data from radio–collared animals all indicate that some possums were particularly mobile during the study. We hypothesise that, despite the usually sedentary nature of adult possums, adults made large-scale range shifts following population reduction, presumably to enable aggregation of the survivors into breeding groups.

Dietary changes in response to population reduction in the possum Trichosurus vulpecula in New Zealand

Abstract Context. Efforts to protect or restore degraded plant communities by population control of invasive herbivores frequently fail to achieve their goals. Aims. We seek to quantify changes in diet of an introduced herbivore, the brushtail possum (Trichosurus vulpecula), following population control, and determine how these may contribute to variable responses in plant condition. Methods. Stomach contents of possums from five areas of indigenous forest in northern New Zealand were analysed to measure diet before and after the application of possum control. Key results. The contribution of fruit, and foliage of some early successional forest species, to total possum diet increased up to 27-fold following possum population control. This was accompanied by declines in consumption of the main pre-control possum foods (foliage from common canopy trees). Dietary changes were a combination of an immediate response to control (1 year) and a strengthening of these initial changes with increasing time since control. Conclusions. Possums in the study areas changed diet following population control, from a diet dominated by foliage of common canopy tree species to one dominated by fruits, and foliage of uncommon early successional plants. Pest control instantaneously increased the per capita availability of all foods, and probably permitted absolute increases in some foods through plant recovery, enabling possums to substitute scarce, high-preference foods for abundant but less preferred canopy foliage. Implications. Following control of a pest herbivore, dietary changes reduce benefits for the most vulnerable preferred plant foods, but enhance benefits for less favoured plants. Intense pest control can permit some recovery of highly preferred foods, despite increased per capita consumption of these foods by survivors of control.

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 Zealands 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.