Culled, efficiently

A new model that cuts down the costs of controlling invasive animals

By Sumana Narayanan
Published: Thursday 15 April 2010

imageInvasive species are one of the biggest threats to biodiversity. Means of human transport has only made it easier for some to hitch a ride and colonize new regions.

Several have deliberately been introduced by humans for food and as pets. Australia has a long history of such introductions. There are 304 invasive species in that country, says the Intern-ational Union for Conservation of Nature. Several can be traced to the First Fleet—11 ships that brought the first batch of British convicts to set up a penal colony in 1787.

Along with 300-odd convicts, rabbits entered Australia. Within a century, rabbits spread over most of the country—the fastest colonization worldwide of any invasive species. They were costing Aus-tralia over US $600m while ravaging fields and competing with livestock for food. In the 1990s, a disease-causing pathogen (calicivirus) was introduced in rabbits following which costs dropped to US $113m.

Other culling attempts compounded the problem. In 1935 cane toads were imported from Hawaii to control scarab beetles that are sugarcane pests. Now just the toads are costing the country half a million dollars annually. Towns conduct Toad Day Out events where people are invited to kill them. Native species of quolls, crocodiles and dingoes have decreased in numbers because they try to eat the poisonous amphibian.

Such haphazard control measures could be replaced by a more scientific and economical approach. A model, called star ( Spatio-Temporal Animal Reduction), was developed by researchers from the Charles Darwin University, the University of Adelaide and the South Australian Research and Development Institute. A simulation was run using data on invasive species—buffaloes, pigs and horses—at the Kakadu national park. The model gives wildlife managers three solutions: based on the budget available, the region most efficient to cull, and the costs for a specific population size.

“The model has not been field tested yet, but we have reasonable data on buffalo numbers and changes to know that we’re pretty close to reality,” said Corey Bradshaw, one of the authors and professor at the University of Adelaide.

Uncon-trolled, buffalo populations would double in less than a decade. The model predicts a moderate 20 per cent culling on a regular basis would cost US $4.58m, but with a moderately bigger budget of US $5.77m, a 40 per cent cull rate initially and 25 per cent thereafter would be achievable.

The study was published in Methods in Ecology and Evolution online on February 24. The paper has a couple of drawbacks. The results obviously depend on baseline data such as habitat, population densities and animal dispersal patterns. The authors admit that the model does not take into account the possibility of simultaneous culling when there are several invasive species within an area. For example, it makes sense to cull the different species at Kakadu simultaneously but the number of individuals from different species culled would then depend purely on chance.

Ankila Hiremath, an invasive plant species researcher, talked about the model’s application in India: “We lack good baseline data on invasive species, particularly plants. Also, models based on animals cannot be used for plants.” Hiremath is with Ashoka Trust for Research in Ecology and Environment in New Delhi.

Complications that cost

Invasives do not always remain pure invasives. They cross-breed with native species. Feral dogs, brought as companions and for livestock herding, have cross-bred with native dingoes reducing their number. Kangaroos are considered pests by some, sparking a heated debate. The 48 kangaroo species compete with cattle for pasture, damage crops and cause traffic accidents, costing the country US $76m every year.

A study in the Journal of Applied Ecology in 2009 estimated the cost of the disastrous culling experiment at Macquarie Island, an Australian territory, at US $21.9m. The island was overrun by feral cats which were culled but this led to a boom of rabbits.

A 2004 report by the Cooperative Research Centre for Pest Animal Control estimated the cost of invasives in Australia at US $657m.


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