Recovering forest reveals its history like chapters in a book if you take the time to read the pages of the story.  Kereru, the world's largest pigeon, seek out puriri trees to feed on their berries. Our eastern trapline sidles across the slopes of a steep stream valley as it heads up to the watershed ridge line. Once cleared farmland, these slopes are now reforested with mature canopy kanuka forest. Along the floor of the valley, remnants of the original forest are still present with these trees marked out by their size and height.
Station 15 on the trapline is at the confluence of two headwater streams and is a great place for a break from setting traps before the steep climb out of the valley. On the cool, moist stream bank stands a very large puriri tree that not only provides a massive structural canopy it tells the story of this forest. This puriri predates the clearance of the valley and it would have been a large tree when the axes arrived. It clearly became a mature tree inside a forest as its branches are erect and it does not have the spreading canopy of puriri that grow in the open. It has grown up into a light well that was created when an even older tree fell and tore a gaping hole in the original forest canopy. This tree is the first chapter of the forest’s story and it proves that mature broadleaf forest has persisted continuously on this site for at least centuries.
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In any discussion of predator control, cats are the elephant in the room, but ignoring them may be the worst option. In October 2019 we posted comment about hedgehogs being “The Forgotten Predator”. Now it is time to consider if feral cats are “The Ignored Predator”. That cats are missing from New Zealand’s Predator Free 2050 aspiration is an anomaly that needs to be addressed. Is it a lack of ecological data, a lack of know-how or a lack of political will that is behind this exception? A lack of ecological data cannot be the reason behind this omission because in 2016 a paper authored by Australian and New Zealand based research ecologists was published in the Proceedings of the National Academy of Sciences that clearly showed the impact that feral cats have on the loss of global biodiversity. Of contemporary vertebrate extinctions, invasive predators were implicated in almost 60%, and feral cats contributed to 60 (42%) out of those 142 extinctions. Therefore, feral cats were directly connected to a quarter of the recent extinctions of vertebrate species. These numbers are also probably underestimates as the authors found a further 23 critically endangered species that were listed as “possibly extinct”.  Little Barrier Island, a successful cat eradication and in 1980 the largest island from where cats had been removed. Image from Google Maps. Coronavirus is back as Covid-19 continues to emulate a fast running invasive mammal biosecurity incursion. On August 17 2021 the New Zealand Government announced that an infection of the Delta variant of the SARS‑CoV‑2 virus had been positively identified in the community. As a result, the whole country immediately moved from the least restrictive Level 1 Covid-19 alert level to the most restrictive Level 4 lockdown.  This event occurred almost 18 months after the virus had been first detected in New Zealand in late February 2020. New Zealand’s swift and decisive action to suppress coronavirus through the autumn of 2020 resulted in its successful eradication from the country. Over the ensuing months positive cases of infection were continuously detected at the border and these were mostly successfully controlled by the Managed Isolation and Quarantine system designed to reinforce the country’s closed international border.
Episodic mouse plagues in Australia may be a portent of the successful eradication of rats and stoats in New Zealand. AustraliaNews from the world's smallest continent regularly remind us that Australia truly is a land of extremes. The physical forces of nature regularly demonstrate their destructive power but nature's biological forces can also flex their muscles, none more so than when mice reach plaque proportions.
The causes and effects of human induced environmental changes are complex and interrelated with consequences that may not be obvious but are none the less important.
While Covid-19 is a human disease sweeping the world, it exhibits similarities with invasive predators and shows the difficulties of eradication and reinvasion prevention. The rapid establishment, development and acceleration of the global coronavirus pandemic has very quickly introduced people across the world to biological patterns that are all too familiar to predation scientists. And as with predation, for some individuals their interaction with this invasive organism has sadly been fatal, the outcome of most predatory interactions.
New Zealand’s response to Covid-19 has been world-leading and currently the novel coronavirus is considered to be eliminated from Aotearoa as there is no transmission of the virus within the community. Understanding how this was achieved illustrates many parallels between fighting to eradicate a virus and fighting to eradicate predators. So does the eradication of Covid-19 tell us anything about the eradication of invasive predators? Irruptive population growth is easy to plot with pen and paper but does it exist in the real world? The answer is a very definite "Yes" and some predators are experts at it. In August 2017 we posted about the exponential growth potential of populations that ecologists call an irruption. These generate a characteristic 'J’ shaped growth curve, or “hockey stick” graph of population size over time. While these curves are easy to plot with a calculator and a sheet of graph paper, do they actually exist in nature?
Data from one of our own rodent control programmes demonstrates just how quickly an irruption can occur for fast breeding predators such as rodents. The control site was an area of regenerating coastal kanuka forest in Northland where the abundance of rodents was monitored monthly using 75 double snap trap stations spaced along a 1.5km trapline With New Zealand's predator free aspirations focused on rats, stoats and possums, hedgehogs may be a forgotten predator causing tomorrow's endangered wildlife. The European Hedgehog was introduced into New Zealand from Great Britain starting in 1869 when the Canterbury Acclimatisation Society released a pair at Lyttelton. Introductions continued until the 1890's to remind the colonists of their British homeland and as a biological control agent to eat slugs and snails in gardens.
While hedgehogs no doubt do eat slugs and snails in gardens they also eat a lot else besides, including the eggs and young of ground nesting birds, large invertebrates such as weta and native lizards. Their impact is particularly severe in open habitats, such as riverbeds, where birds may nest in colonies and offer easy pickings for these nocturnal hunters. The annual population cycle of forest rodents is one half of a predator prey cycle between rats and stoats that catches native wildlife in the crossfire. The most common rodent in New Zealand forests in the black rat, also called the ship rat, house rat or roof rat. Despite its name, it is often grey in colour and is identified by its tail being as long, or longer than its body. In contrast, the brown, or Norway rat has a tail that is shorter than its body.
In forests, ship rat populations cycle in numbers throughout the year. They breed rapidly during the warm days of summer before numbers decline through the cold weather of winter due to natural mortality and predation by stoats. A call to make New Zealand Predator Free by 2050 is an ambitious aspiration that is a complex problem with many hidden traps for the unwary. Forewarned is forearmed. To help conserve our unique biodiversity, the New Zealand government has taken an explicit stand against predators with the Predator Free 2050 initiative. This was announced in 2016 and is structured as a government and community partnership to advance predator control to the landscape level and eradicate predators from the country within the 34 years to 2050.
This lofty ambition has caught many people's imagination and has galvanised action across the nation in towns and cities, in the countryside and in the wilderness. However, despite the clamour for results it is worth pausing a moment to consider and better understand what this catch cry actually means. The ‘why' is pretty obvious - predators kill our native wildlife - but the ‘what’ and ‘how’ are less clear. Whereas 'r' is the driver of population growth, so 'K', the carrying capacity of the environment, is the handbrake to limit total population size. Last August we posted about exponential population growth where a constant rate of population growth, ‘r’, drives bigger and bigger increases in population size to give the classic ‘J’ shaped curve of population size.
While this scenario is usually only theoretical, or reserved for controlled lab experiments, it can be observed in natural populations that grow quickly from a narrow base, such as when a small number of founders colonise a new habitat. Predator-proof fences are critical for eco-sanctuaries but without monitoring behind the fence a predator incursion may go unnoticed with catastrophic consequences. We recently read a distressing article in a back issue of Wilderness magazine about a stoat intrusion at the predator fenced Orokonui Ecosanctuary in Otago. The stoat was detected after the sanctuary's population of South Island saddleback mysteriously disappeared.
Unfortunately a mated female stoat is able to delay the onset of her pregnancy until conditions to raise her young are optimal so it only requires one intrusion to result in the establishment of a small population. You can download a copy of the article here. For predator populations, 'r' is the driver of population growth and must be reduced if compensatory mortality is to be become additive and control is to be successful. Pest and predator control focuses on reducing the population size of a target pest or predator species. As a result it falls within the realm of population dynamics and to place this into context it is useful to consider some more of this interesting subject.
The size of any population is governed by the relative contributions of only four key factors. These are birth rate, also known as natality, death rate or mortality, immigration and emigration. Natality and immigration add individuals to the population whereas mortality and emigration remove them. Therefore to reduce a population, control must elevate mortality above natality and if possible also reduce or stop immigration. |
What's in a Name?The familiar saying "A bird in the hand is worth two in the bush" warns about the risks that come with trying to achieve more by challenging the status quo. Categories
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