Expanding wallaby populations
Six species of wallabies were introduced into New Zealand during the late 1800s, either for recreational hunting or as part of the then desire to acclimatise exotic species. Four of these species are found on Kawau Island, with dama wallaby also found around Rotorua, and Bennett’s wallaby found in South Canterbury / North Otago. Populations of both these species continue to spread on the mainland and have negative impacts on primary production and indigenous biodiversity.
Because of an increasing number of sightings outside these species’ containment areas (as designated by regional councils), there is growing concern over the increasing cost of their impacts and the increasing challenge of containing them. This concern has led to the Ministry of Primary Industries (MPI) contracting Dave and Cecilia Latham and Bruce Warburton to review the current distribution of these two species, and to predict their possible distributions in 5, 10, 20, and 50 years if allowed to spread at historical rates.
Maps of confirmed distributions along with additional data on recent sightings (live sightings and animals shot) were used to determine best- and worst-case distributions (Figures a and b). The historical distribution maps and recent GIS data on the distribution of these species at various stages since their establishment were used to generate rates of spread and to predict future distributions. It was assumed rates of spread would stay constant (i.e. containment would not become more effective nor rates of illegal liberations increase).
At present, Bennett’s wallaby occupy an estimated c. 5,322 km2 in the South Island centred on the Hunters Hills, South Canterbury. However, the large number of confirmed sightings and animals shot outside of this area suggest that they may occupy as much as 14,135 km2 (Figure a). Based on current estimated rates of spread, the distribution of Bennett’s wallaby in 50 years’ time is likely to be between 9,621 km2 and 20,631 km2, but possibly as large as 44,226 km2. The last value includes known illegal liberations and represents almost one-third of the South Island, or a 700% increase of the current known distribution.
At present, dama wallaby occupy an estimated c. 2,050 km2 in the North Island centred around the Rotorua lakes in Bay of Plenty. However, the large number of confirmed sightings and animals shot outside of this area suggest that they may occupy as much as 4,126 km2 (Figure b). Based on current estimated rates of spread, the distribution of dama wallaby in 50 years is likely to be between 3,265 km2 and 11,070 km2, but possibly as large as 40,579 km2. This last value represents more than one-third of the North Island, or a 1,700% increase of the current known distribution.
Based on habitat suitability models Bennett’s and dama wallabies could, if not managed, eventually occupy most of the South and North Islands respectively. The models predict that the only areas from which they may eventually be absent are those associated with high-production exotic grassland (e.g. dairy lands), urban areas, and high elevations.
The current total annual costs of the impacts of Bennett’s wallaby in the South Island were estimated to be c. $23,700,000 (which includes c. $22,200,000 in revenue lost to agriculture and c. $1,500,000 to ecosystem services and biodiversity values). If Bennett’s wallaby were allowed to spread without any active management, it was estimated that the total annual costs of their impacts in 10 years would increase to c. $67,000,000. If widespread remedial control was applied to reduce their densities within their predicted distribution in 10 years, the costs incurred would be c. $27,700,000. This suggests there is a large net benefit (c. $39,300,000) to managing them to control their unwanted impacts as opposed to not managing them.
The net benefit of containing Bennett’s wallaby would be even greater. That is, intensive control and surveillance within the present containment buffer would cost c. $6,200,000. This represents one-third of the expenditure that would be incurred if wallaby populations were allowed to expand for 10 years and then controlled ($18,000,000), or one-seventh the expenditure incurred if the populations were allowed to expand in the absence of management ($43,400,000).
Because wallabies are not a nationally important pest, very little research has been carried out on control strategies, tools, detection, impacts, and the economics of various management options. However MPI, through the Sustainable Farming Fund, has funded Manaaki Whenua – Landcare Research to determine the detection probabilities of using both aerial-based thermal imaging and dogs to better manage wallabies that have ‘escaped’ their containment areas. Determining such probabilities will enable farmers and managers to quantify the likelihood that they have eradicated wallabies when none are detected, and provide more effective methods for detecting them when at low numbers and close to eradication or when they have recently invaded an area.
This review was funded by the Ministry of Primary Industries.