New Zealand’s Supersized Possums: A Tale of Invasion and Evolution
Holidaying Aussies often remark on a surprising difference when encountering brushtail possums in New Zealand: they’re bigger, darker, and fluffier than their counterparts back home. This isn’t just perception; New Zealand’s feral possum population has, in some cases, grown to impressive sizes, with individuals exceeding 6kg recorded in the 1980s – significantly larger than the typical 1.5 to 4kg found on the Australian mainland.
From Fur Trade to Feral Plague
The story of the brushtail possum in New Zealand is a classic tale of unintended consequences. Introduced from Australia in 1837 to establish a fur trade, the possums quickly thrived in the absence of natural predators and a plentiful food supply. By the 1980s, their numbers had swelled to an estimated 50 to 70 million, rivaling the sheep population.
Why Are New Zealand Possums So Large?
Whereas smaller possums still exist in New Zealand, the prevalence of larger individuals has sparked curiosity. Bruce Warburton, a retiring wildlife biologist and pest controller with 48 years of experience at New Zealand’s Bioeconomy Science Institute, confirms the presence of these sizable animals. “A 4.5kg animal is a big possum, and you really notice them,” he noted. He attributes the larger size to an overabundance of food in areas where the possum population was still expanding.
Bergmann’s Rule and Possum Size
One potential explanation for the size difference lies in Bergmann’s Rule, a principle stating that warm-blooded animals tend to be larger in colder climates to conserve heat. New Zealand’s cooler, wetter forests compared to much of Australia could contribute to this phenomenon. Scientific research published in 1985 supports this, finding that brushtails in the south of New Zealand were generally larger than those in the north. Similar trends are observed within Australia, with Tasmanian brushtails growing larger than those in northern regions.
Genetic Influence and Rapid Evolution
The initial introduction of brushtail possums to New Zealand primarily involved the Tasmanian subspecies. This genetic legacy, combined with the favorable environmental conditions, may be driving the observed size increase. Shelby Middleton, a PhD student at Edith Cowan University researching brushtail subspecies, suggests that possums can rapidly evolve. She points to the example of Barrow Island, where possums have almost lost their tails due to the lack of trees, and Tasmania, where the brushtail has evolved to look like a different species over 10,000 years of isolation. Changes in New Zealand, potentially affecting teeth and skulls as they adapt to different foods, are also possible.
The Cost of Invasion: Economic and Environmental Impact
Despite a decline to around 28.5 to 48 million today, feral possums continue to inflict significant damage. They cost the New Zealand economy approximately NZ$110 million annually, spreading tuberculosis to farmed cattle and devastating native ecosystems by stripping forests bare and preying on native birds and insects.
Eradication Efforts and Future Outlook
The New Zealand government has set a goal to eradicate invasive species, including possums, by 2050. While Warburton believes eradication may be feasible on smaller islands, he expresses skepticism about achieving it on the North and South Islands in his lifetime.
Other Aussie Invaders in New Zealand
The brushtail possum isn’t the only Australian animal causing concern in New Zealand. Magpies and wallabies are also common in some regions, and incursions of the Queensland fruit fly and bogong moth are regularly reported.
Frequently Asked Questions
- Why are New Zealand possums so destructive?
- Possums spread diseases like tuberculosis to livestock, damage native forests by consuming vegetation, and prey on native birds, eggs, and insects.
- Is it possible to eradicate possums from New Zealand?
- Eradication is considered possible on smaller islands, but achieving it on the North and South Islands is a significant challenge.
- What is Bergmann’s Rule?
- Bergmann’s Rule states that warm-blooded animals tend to be larger in colder climates to conserve heat.
Did you know? Brushtail possum fur is still used in knitwear in New Zealand, often blended with farmed merino wool.
Pro Tip: If you encounter a possum while travelling in New Zealand, observe it from a distance and avoid feeding it. Report any sightings to local authorities.
Desire to learn more about New Zealand’s unique wildlife challenges? Explore our other articles on invasive species and conservation efforts.
