Colonial Introduction: Unintended Consequences on New Zealand's Ecosystems

When European colonists first settled in New Zealand, they discovered a vast land that seemed empty to them. Unable to appreciate the unique ecosystems in front of them, they saw it as an opportunity to stock the land with creatures from back home. So strong was this sentiment that many referred to it as ‘a Game reserve waiting to be stocked.’

Amongst these introductions was the Common Brush-tail Possum. First introduced in 1837, alongside rabbits and hares, it was seen as a prime way to establish a fur industry. Possums are nocturnal marsupials, weighing between 1.5 - 4.5 kg. Colouring can range from silver-grey to reddish-black. Remarkably long-lived, given their size, male possums can live up to nine years, while females may live up to twelve years of age.

In Australia, foxes, snakes, and dingoes all prey on possums. Furthermore, the Australian environment simply cannot support large possum populations. The extreme heat, lack of a ready food supply, and predators keep numbers low naturally. In response to these harsh conditions, possums have evolved to become omnivores. This allows them to survive on a vast range of foods, many of which have limited nutritional properties.

The New Zealand environment could not be of greater contrast to the Australian outback. Upon their arrival, possums found themselves in a predator-free lush temperate rainforest, with a year-round supply of high-nutrient foods. These optimum conditions allowed possum populations to rapidly explode, especially once they reached critical mass. By 1948, they’d occupied 54% of New Zealand. By 1980, they would cover 91% of the country. That’s an additional 225,000 square kilometres colonized in just thirty years!

Brushtail Possums have colonised a quarter of a million kilometres in thirty years.

The Silent Invaders: Understanding Possum Behaviour and Its Impact

Early Europeans seem to have been blissfully unaware of the spread of possums; much of this is likely due to their nocturnal and secretive nature. In low numbers, possums are hard to detect. Despite hundreds of liberations across the 19th and 20th centuries, possums were largely unknown to the New Zealand public until the 1970s. It’s even been suggested that legends of a native mammal called the Waitoreke, from the early 19th century, may have resulted from unknowing early interactions with possums.

The harsh Australian outback rarely provided a stable food supply year-round; in response, possums evolved to become seasonal browsers. Consequently, individual possums tend to inhabit multiple dens across large territories. Moreover, this same harsh environment provided little protection from daytime predators. Over millions of years, possums adapted to a nocturnal lifestyle to avoid predation.

Possums are rarely seen in the daytime. When they are, it tends to be indicative of either illness or disturbance of a nesting site. Possums sleep all day, rising near dusk. Though even then, possums remain near their home dens, cleaning themselves and watching for predators. When night eventually falls, they may travel several kilometres in search of food, returning home near dawn to sleep again.

Nocturnal behaviour played a key role in avoiding the detection of early Europeans. In low numbers, it may not be apparent that possums are present. A recent West Coast Study suggested that it can take possums ten years of browsing before their presence is readily observable. However, by thirty years of possum presence, substantial canopy damage became apparent. Interestingly, this colonisation was from one area with a high possum population to a second location where they were entirely absent. In reality, much of the spread of possums across New Zealand would have been from low population areas to areas of absence. This suggests that early possum colonisation would have been largely undetectable.

Moreover, early releases tended to occur in remote areas away from civilisation. For much of the first fifty years after introduction, the low density of possums across the South Island, coupled with their seasonal nature, would have given time for foliage to recover. Young male possums disperse widely in search of new home ranges, while females tend to experience high mortality. Until regions began to reach carrying capacity, their presence would have been largely undetected.

Possums can go largely unnoticed due to their nocturnal nature 

Possum Proliferation: The Remarkable Reproductive Capacity

The Australian outback has a “boom-bust” cycle of existence. Life is a short, brutal experience, with long seasons of want and short periods of plenty. Species that adapted too rapidly to reproduce in times of plenty survived, while those that did not quickly became extinct.

What happened when possums arrived in a land of comparative abundance? Their nature did not change, but their ability to rapidly adapt played a major role in their ability to outcompete other species and dominate the New Zealand landscape. Selective pressures over the past two hundred years have allowed possums to display some remarkable trends. On average, New Zealand possums are a third heavier than their Australian counterparts. Moreover, research suggests that silver-grey possums tend to be found in drier eastern regions, while reddish-brown possums tend to be found in the wetter areas of New Zealand.

In Australia, possums typically only reach breeding condition once a year. Whereas in New Zealand, a female possum may breed in early winter, again in September, and in some instances, even share her den with a female offspring from the previous winter. Females that lose young may replace them with remarkable speed, coming back into heat within a few weeks or months. In optimum conditions, a female North Island possum may produce six young a year, while their South counterparts can reproduce four times a year.

Such is the pace by which possums can reproduce that a population that has experienced near extermination from control operations can bounce back in six years. This does include repopulation from neighbouring regions. But even so, it’s a remarkable statistic.

Brush-tail Possums can get pregnant six times in a year.

Dietary Dominance: From Australian Survivors to New Zealand Invaders

The Australian Outback is a hard place to survive. To maximize their chances of survival in this harsh environment, possums have adapted to a wide-ranging omnivore diet, often with limited nutritional sustenance. Possums have been known to consume trees, plants, fruits, and grasses. In addition, they have also been known to feed on insects, lizards, and even birds.

Upon introduction in New Zealand, they discovered themselves in a vast untouched landscape, which provided them with an almost unlimited diet of highly nutritious foods. In fact, it has been the hardy nature of possums that has allowed them to adapt to such a broad variety of foods, helping them become such a dominant force across our country.

Today it’s estimated that forty million possums inhabit New Zealand. Hardly an ecosystem exists in New Zealand that has not become infested by them. Not even farmland is safe. It’s believed that possums cause in excess of thirty-five million dollars of direct agricultural damage a year and tens of millions more in damage indirectly to cattle and deer herds, through the spread of tuberculosis. Furthermore, possums have become a major threat to native birds, both through directly preying on nests and chicks, the collapse of the forest canopy through feeding, and direct competition for the same resources.

Possums exhibit high levels of learned behaviour. It’s been demonstrated that parents teach their offspring about both new food opportunities and how to avoid threats. Adult females have been observed educating offspring how to circumvent traps and poison. What’s more, once a possum begins to feed on a new food source, other possums will quickly follow suit.

This fascinating capacity to sustain themselves on a diverse diet contributes to the widespread distribution of possums across New Zealand. It also helps us understand the high carrying capacity of possums across our country. Carrying capacity is the maximum number of possums a place can naturally sustain. Studies vary widely on what the true carrying capacity of New Zealand is likely to be. Once it was believed to have been between seventy and ninety million possums; nowadays, this figure is believed to be based upon faulty data. A more probable figure is somewhere between forty-five and sixty million possums.

Thankfully, control methods keep numbers much lower than this, with the national possum population sitting anywhere between thirty and forty million animals. Though this is only achieved with hundreds of millions of dollars spent eradicating introduced pests annually. A substantial proportion of this funding is spent on possum control. Without this funding, possums would pose a greater threat to our biodiversity.

Possums have colonised even the most remote areas of New Zealand

Here to Stay: A Persistent Challenge for New Zealand

We cannot understand possums, nor the threat they pose to our unique wildlife, without examining the causes of their effectiveness as colonisers. The nocturnal nature of possums kept them hidden from the public for over a hundred years. Even when they did reach carrying capacity, their ability to adapt to new foods and their remarkable reproductive abilities have permitted them to remain firmly entrenched across New Zealand despite control efforts.

It seems that without emerging technologies, possums will persist for a long time in New Zealand.

Enjoy this style of writing?

Support us by purchasing from our online store which allows us to devote more time to stories like this.