Ferret and Stoat Research - Abstracts
Byrom, A. E. 2002: Dispersal and survival of juvenile feral ferrets Mustela furo in New Zealand. Journal of Applied Ecology 39: 67-78.
- Introduced feral ferrets (Mustela furo) are a significant pest of both conservation and economic importance in New Zealand. Ferrets prey on indigenous wildlife, and they also carry bovine tuberculosis, a disease which may threaten New Zealand's international beef, dairy and venison markets.
- Very little is known about the role of dispersal and survival of juvenile ferrets in the recovery of ferret populations after control operations, and how these parameters might affect ferret impacts on native wildlife and their role in the spread of Tb.
- Fifty-two juvenile ferrets were radio-collared at emergence from their natal dens on six study sites during two years in 1997/98 and 1998/99. On three sites, most introduced mammalian predators (including ferrets) had been removed by kill-trapping from October to January to protect critical bird nesting areas. Three sites were left as non-removal sites. Dispersal and survival of juvenile ferrets was therefore measured at low and normal ferret densities.
- Survival of juvenile ferrets showed a strong density-dependent response to predator removal. In year 1, survival of juvenile ferrets on removal sites from emergence to the end of the dispersal period (about 4 months) was 86% (C.I. 63-100%), whereas survival on non-removal sites was 19% (0-38%). In year 2, survival was 100% on removal sites, and 71% (45-97%) on non-removal sites. Dispersal of juvenile ferrets was not related to population density.
- A frequency distribution of dispersal distances of juvenile ferrets showed a typical negative-exponential pattern. The median dispersal distance of juvenile ferrets was 5.0 km (range 0.5-45.0 km). There was no apparent sex bias in survival, dispersal distances, or timing of dispersal in either season.
- Density-dependent survival of juvenile ferrets should be a key parameter of interest in determining frequency and seasonal timing of ferret control. Ferret control to conserve native wildlife and minimise Tb spread should be carried out in late autumn after dispersal, to provide a longer lag time before juveniles reinvade an area.
- This study fills a critical gap in the understanding of ferret dynamics in New Zealand. It has direct implications for management of this important introduced predator, and it augments a currently sparse literature on the dispersal and survival of mammalian carnivores.
Barlow, N. D.; Norbury, G. L. 2001: A simple model for ferret population dynamics and control in semi-arid New Zealand habitats. Wildlife Research 28: 87-94.
Introduced ferrets (Mustela furo) in New Zealand are subject to population control to reduce their threat to native fauna and the incidence of bovine Tb in livestock. To help in evaluating control options and to contribute to a multi-species model for Tb dynamics, a simple Ricker model was developed for ferret population dynamics in a semi-arid environment. The model was based on two data sets and suggested an intrinsic rate of increase for ferrets of 1.0 ? 1.3/year and a carrying capacity of 0.5 to 2.9/km2. There was evidence for direct density-dependence in both data sets and the effect appeared to act mainly on recruitment. Dependence of predator rate of increase on wild rabbit (Oryctolagus cuniculus) density was exhibited in one of the two data sets, together with a numerical response relating current density of predators asymptotically to current density of rabbits, their primary prey. Predators in this data set included both cats and ferrets, estimated from spotlight counts, but the other data set demonstrated a direct proportionality between predator (cat and ferret) spotlight counts and minimum ferrets known to be alive by trapping. The model suggested, firstly, that populations are hard to suppress by continuous culling, with at least a 50% removal per year necessary to effect a suppression of 50% in long-term average density. Secondly, if control is episodic rather than continuous, culling in autumn gives a greater degree of suppression over time (280%, accumulated over time) than culling in spring (180%). A differential equation version of the model provides a component for a general Anderson/May bovine Tb/wildlife (possum/deer/ferret) model.
Fitzgerald, G.; Fitzgerald, N.; Wilkinson, R. 2002: Social acceptability of stoats and stoat control methods: Focus group findings. Science for Conservation 207. Department of Conservation, Wellington, New Zealand. 45p.
In May and June 2001 seven facilitated focus-group discussions were conducted throughout New Zealand to examine the perceptions of the New Zealand public and interest groups of feral stoats and other mustelids, and attitiudes to their control. Three of the group discussions were with the general public and four with particular interest groups. Overall, stoats were perceived negatively, expecially for their predation on native fauna. Participants in the public groups were largely uninformed about stoats, in contrast to those in the interest groups. Trapping and poisoning, the current forms of control, were perceived as having problems, with trapping tending to be preferred over poisoning. Proposed new forms of of control under investigation, in particular the use of introduced diseases and genetically engineered organisms designed to kill stoats or reduce their fertility, received a largely negative response from the groups due to concerns about a lack of research and knowledge about potential risks and non-target effects. There was a clear preference for improving the current forms of control. Participants across all groups felt the public should be involved in any decision making about GE-based controls, but this would first require wider education about the stoat problem and any proposed biocontrol technologies.
Heyward, R. P.; Norbury, G. L. 1999: Secondary poisoning of ferrets and cats after 1080 rabbit poisoning. Wildlife Research 26: 75-80.
Rabbits (Oryctolagus cuniculus) threaten pastoral and conservation values in New Zealand. High-density rabbit populations are therefore controlled, typically by poisoning with sodium monofluoroacetate (1080). Wild ferrets (Mustela furo) and feral cats (Felis catus) often rely on rabbits as a main food source, and so contribute to rabbit population control. However, ferrets and cats are also conservation pests in their own right, and ferrets are potential reservoirs of bovine tuberculosis. Secondary poisoning of predators during rabbit poisoning may therefore have implications for these issues.
The incidence of secondary poisoning was determined by using radio telemetry to assess the survival of 68 ferrets and 21 cats on two treatment and one control site in the dry tussock grasslands of New Zealand. The treatment sites were aerially poisoned with 1080-coated carrot baits (0.02% wt/wt) to control rabbits. The control site was not poisoned. Ferrets and cats were monitored at 2-weekly intervals for at least 1 month before, and 2 months after the poison operations. Muscle samples from ferrets and cats that died within 50 days of poisoning on the treatment sites were assayed for 1080. Seven to eleven percent (n=28) of ferrets on one site and 8-15% (n=26) of ferrets at the other site apparently died of secondary 1080 poisoning. Natural mortality rates of ferrets ranged from 34-81 % per annum. While we have evidence that secondary poisoning of cats does occur, we monitored insufficient numbers of cats to reliably estimate mortality rates.
Declines in predator
numbers are commonly
observed after rabbit
poisoning. This study
indicates that secondary
poisoning contributes to
these declines.
Norbury, G. 2001: Conserving dryland lizards by reducing predator-mediated apparent competition and direct competition with introduced rabbits. Journal of Applied Ecology 38: 1350-1361.
1. Native skinks Oligosoma spp. in New Zealand's dry grasslands have declined in range and abundance since the arrival of humans. I hypothesized that introduced rabbits Oryctolagus cuniculus exacerbate this decline by supporting introduced mammalian predators for which they are primary prey; by sudden declines in abundance that cause predators to switch to skinks; by grazing vegetation, thereby reducing skink refuges from predators; and by reducing skink food and shelter from climatic extremes. The first three effects cause enhanced skink predation and represent indirect or apparent competition. The fourth effect represents direct competition.
2. Interactions between two introduced predators (ferrets Mustela furo and cats Felis catus), rabbits and two secondary prey (McCann's skink O. maccanni and common skink O. nigriplantare polychroma) were studied. By measuring skink consumption, and predator, rabbit and skink numbers, offtake of skinks by predators under varying rabbit and skink densities was calculated.
3. Predation by ferrets and cats was inversely density-dependent because predation accelerated at low skink densities. Therefore, predation could potentially exterminate skink populations if densities fell below some critical range. As skink densities rose, predation became an increasingly less important source of mortality.
4. Predation of skinks increased markedly after sudden declines in rabbit abundance, because predators remained abundant but switched to feeding on skinks. Although a temporary effect, repeated cycles of intense rabbit control and population recovery may have chronic detrimental effects on skink population viability.
5. Optimal rabbit management for maintaining viable skink populations is likely to require avoidance of large swings in rabbit abundance by maintaining populations at low, stable, levels. Fewer rabbits mean fewer predators and greater refuge (less apparent competition), and improved food and shelter (less direct competition). If large swings in rabbit abundance cannot be avoided, the effects of prey-switching could be reduced by controlling predators when rabbit numbers decline.
6. Better understanding and management of primary-secondary prey systems, where the primary prey consumes the habitat of secondary prey, will require recognition of the extra complexities that underlie these systems.
Norbury, G.; McGlinchy, A. 1996: The impact of rabbit control on predator sightings in the semi-arid high country of the South Island, New Zealand. Wildlife Research 23: 93-97.
Extensive rabbit poisoning operations were conducted under the Rabbit and Land Management Programme between 1990 and 1993 in the semi-arid high country of New Zealand. Repeated surveys of 371 kilometres of spotlight transects that initially supported more than 40 rabbits per spotlight km showed a 95% decline in sighted rabbits. This was followed within a year by a 71% decline in sighted cats and ferrets.
Wild cats and ferrets harbour bovine tuberculosis and impact upon protected native fauna. The means by which these apparent declines in predator numbers occur is important in assessing the role that rabbit control might play in managing for Tb and nature conservation.
Norbury, G. L.; Norbury, D. C.; Heyward, R. P. 1998: Behavioral responses of two predator species to sudden declines in primary prey. The Journal of Wildlife Management 62(1): 45-58.
Feral ferrets (Mustela furo) and cats (Felis catus) are pests in New Zealand because they carry bovine tuberculosis and prey on native fauna. We examined their behavioral responses to large-scale poisoning of their primary prey, feral European rabbits (Oryctolagus cuniculus). We monitored the movements of 70 radiocollared ferrets and 28 radiocollared cats for 2 years during pre- and posttreatment periods for 1 nontreatment (Earnscleugh) and 2 treatment (Bendigo, Grays Hills) populations in tussock grassland habitat in New Zealand's South Island. There were no changes (P > 0.05) in the mean home range size of ferrets on the Bendigo site where poisoning reduced rabbit numbers by 77% (70 ha prepoisoning and 67 ha postpoisoning), or on the Earnscleugh site where rabbit numbers remained high (71 ha prepoisoning and 61 ha postpoisoning). However, when rabbit numbers were reduced by 99%, ferrets on Grays Hills increased home range size from 85 to 230 ha (P < 0.05), and their mean distance from range center from 475 to 993 m (P < 0.001). While most ferrets at Grays Hills remained resident (i.e., occupied discrete home ranges and remained close to range centers), the proportion of Amobile@ ferrets (i.e., those moving > 1.8 km from range centers but later returning, or those that dispersed for longer periods or permanently) in the population increased from 1 of 25 to 6 of 18 (P = 0.015). For cats, treatment effects were less clear because sample sizes were low and the data were highly variable. However, 3 adult cats at Grays Hills dispersed up to 9.2 km from range centers after rabbit numbers were reduced by 99%. No obvious behavioral changes of predators were observed where rabbit declines were less pronounced or indeed where rabbit abundance remained high. This observation suggests that gross changes in predator movements occur only after very large disturbances to their primary prey, but there was insufficient replication of treatments to separate treatment effects from site effects. Nevertheless, our qualitative results indicate that very large reductions in rabbit populations in this habitat are likely to cause a small number of predators to disperse into adjacent areas. In the absence of a clear understanding of the role of ferrets and cats in the epidemiology of bovine Tb, and of their impacts on native fauna, the implications of this induced dispersal are unknown. Intuitively, however, any consequences are likely to be adverse.
Norbury, G.L.; Norbury, D.C.; Heyward, R.P. 1998: Space use and denning behaviour of wild ferrets and cats. New Zealand Journal of Ecology 22(2): 149-159
We monitored the behaviour of 62 radio-collared ferrets and 25 radio-collared cats in dry, tussock grassland habitat in New Zealand's South Island. The total home range of adult male ferrets (102 + 58 ha, mean + 1 S.D.) was marginally greater than that of females (76 + 48 ha), and averaged 90 + 55 ha. Male ferret core ranges (27 + 15 ha) were larger than those of females (16 + 8 ha). Adult cat home ranges were similar between sexes, and were larger and more variable than those of ferrets (225 + 209 ha). Core range size of cats was similar between sexes and averaged 54 + 24 ha. The upper 95% confidence limits of the mean home range lengths of adult ferrets and cats were 2.7 km and 5.1 km, respectively, indicating the width of buffer zones where predator control should be extended to protect the boundaries of areas targeted for predator control in dry, tussock grasslands.
Although core areas were mostly discrete, home ranges were distributed randomly, and animals that shared space neither avoided nor attracted each other. Little evidence of territoriality may be related to high densities of primary prey. Too few cats were monitored to determine territoriality.
Ferrets used at least 9.4 + 3.2 dens, and cats used 11.5 + 3.0 dens during the study. Although 71% of dens were used only once, some were used up to nine times. Day-time resting by ferrets was mostly solitary. If transmission of bovine Tb occurs between adult ferrets, simultaneous sharing of dens during the day is unlikely to be a significant mode of transmission in this habitat. We were unable to determine the extent of den sharing by cats. Cats occupied den sites with more shrubs and rocks compared with ferrets. Predator control stations in dry tussock grassland habitat may therefore be more effective at killing cats than ferrets if placed in shrubby, rocky areas.
