New Zealand's Larger Brushtail Possums

Australian visitors to New Zealand often observe that feral brushtail possums there appear larger, darker, and fluffier than their native Australian counterparts. Individuals caught in New Zealand have weighed over 6kg, significantly more than the 1.5 to 4kg range for most mainland Australian brushtails. Wildlife biologist Bruce Warburton confirmed the presence of large individuals, noting that while 4.5kg is considered large, those 5kg or more are rare and often found in areas with abundant food during invasion phases.

Individuals caught in New Zealand have weighed over 6kg, significantly more than the 1.5 to 4kg range for most mainland Australian brushtails.

Introduction and Impact in New Zealand

Brushtail possums were introduced to New Zealand from Australia in the 1800s for a fur trade. Unlike Australia, where their population has declined by 80 percent due to predators like feral foxes and cats, possums thrived in New Zealand, reaching peak populations of 50-70 million in the 1980s. New Zealand has few native mammals (excluding seals and bats), which significantly contributed to the possums' proliferation.

As an invasive species, brushtail possums have negatively impacted New Zealand's environment and economy. They are vectors for tuberculosis in farmed cattle, cause forest degradation by stripping vegetation, prey on native birds (eggs, chicks, and adults), and consume large native insects and snails.

Jessi Morgan of the Predator Free New Zealand Trust stated that New Zealand's abundant food sources and lack of significant predators allow possums to spend more time feeding and growing.

Factors Contributing to Increased Size

Several factors are believed to contribute to the larger size of New Zealand's brushtail possums:

• Bergmann’s Rule: This ecological principle suggests that warm-blooded animals in colder climates (closer to the poles) tend to be larger to conserve heat. New Zealand's colder, wetter forests, being closer to the South Pole than mainland Australia, align with this rule. Research from 1985 observed this trend, noting rapid evolution within 30-35 generations.
• Genetics: The Tasmanian subspecies, which is often larger than mainland Australian brushtails and known for dark, fluffy fur, was among the first introduced to New Zealand in 1837. Despite subsequent introductions of mainland animals, Tasmanian genetics remain dominant in many areas.
• Abundant Food and Lack of Predators: The New Zealand environment offers a rich food supply and minimal natural predators, facilitating increased growth and size.

Ongoing Evolution and Eradication Efforts

Brushtail possums are capable of rapid evolutionary change. PhD student Shelby Middleton indicated that physical characteristics, such as tail length or skull structure, can adapt over generations when isolated or exposed to new environmental pressures.

Currently, New Zealand's brushtail possum population has decreased to an estimated 28.5 to 48 million, but they still incur an economic cost of approximately NZ$110 million annually. The New Zealand government aims to eradicate invasive species, including possums, by 2050.

However, experts like Bruce Warburton express skepticism about achieving complete eradication on the main North and South Islands within his lifetime, though it might be feasible on smaller islands.