Potential cane toad short to medium term control techniques - the biological feasibility and cost of exclusion as a mitigating control strategy

Abstract

Invasive cane toads Bufo marinus are spreading rapidly across northern Australia, raising serious public and scientific concerns for the long-term persistence of many potentially vulnerable wildlife populations. A number of mitigating solutions to the cane toad problem have been proposed, including direct killing of toads, biological control, or the establishment of secure areas from which cane toads are excluded. Here we address the exclusion strategy, seeking in particular to provide estimates of the cost of isolating long-term viable populations of 12 species of susceptible native fauna managed to remain free of cane toads, using advanced methods in population viability analysis modelling combined with our experience in wildlife management in the difficult environments of northern Australia. Our results reveal a relatively wide disparity across different taxa in the minimum habitat areas required for long-term persistence, ranging from as little as 16 km2 for mangrove monitors, to 220 km2 for northern quolls, to vast areas of up to 50 000 km2 for wide ranging species such as wedge-tailed eagles. An area the size of the Cobourg Peninsula (2 207 km2), which would be relatively cost-effective to isolate as a landscape-scale exclosure, appears to be large enough to support viable populations of most small mammal and reptile species such as quolls, goannas and predatory snakes (and presumably most amphibians and insects, which also have relatively small home range requirements or high average densities), though it would still fail to capture fully the areas for some of the largest free-ranging species, include most top avian predators such as wedge-tailed eagles. For Garig National Park, the cost of construction of a exclusion fence across the neck of the Cobourg Peninsula of 6 km length would be approximately $3.6–5.7 million, with annual maintenance costs in the range of $0.4-0.9 million. In reality the situation is more complex, because capital and recurring costs for the construction and maintenance of fencing are governed not only by the size of the area to be enclosed, but also by the choice of location (e.g. it is more efficient to fence off the neck of a natural partial exclosure such as the Cobourg Peninsula than it is to create a perimeter around an inland site) and the nature of the materials used in construction (e.g. sourcing local or recycled building materials is more cost-effective). Overall costs could also be reduced by created secure area exclusions that encompass the ranges of multiple species for which such conservation action is deemed warranted. Although the results presented herein represent preliminary estimates for the logistics of the “secure area” strategy of cane toad impact mitigation, they do nevertheless provide wildlife managers with some of the key information required to rationally and efficiently allocate time, money and habitat areas to maximise conservation benefits in the face of cane toad encroachments.