Te Taha Tawhiti

The noble false widow spider (Steatoda nobilis) has found urban habitats profitable and has become one of the most successful invasive spiders around the globe. One that is continuing to expand its range thanks to the transport and habitat given it by humans, and one that carries with it a menagerie of microbes.

Invasiveness combines an ability to get to a place and ability to survive and breed there. Animals that associate closely with humans are referred to as synanthropic and are especially good at travelling with people and so continue to share their urban environment, wherever in the world that is. (Right: Male Steatoda nobilis from Porirua, Wellington. (c) S.A.Trewick).

Humans unwittingly enable certain species to invade, but, it appears that human environments are not all equal. Species such as the noble false widow cannot thrive everywhere; they have physiological limits and climatic conditions are especially important for ectothermic creatures including spiders. Planet-scale climate influences where the most suitable habitat is.

Remarkably, a study published in 2019 using ecological niche modelling predicted that many parts of New Zealand were likely to be suitable for Steatoda nobilis. They have been proved right; the noble false widow is living in cities across New Zealand, nestled in urban gardens and around houses.

New Zealand now has five species of introduced false widow spider, all associated with modified habitats across the country. Each must have arrived by some form of accidental transport, but then colonised successfully.

False widow spiders are so-called because they resemble Latrodectus widow spiders such as the black widow L. lilianae (Left. (cc) Jorozko). The similarity does not end there. Although generally considered less dangerous than widow spiders, the venom of Steatoda nobilis contains toxins similar to those produced by Latrodectus lilianae.

The spread of Steatoda nobilis is paralleled by increasing concern about its human health and ecological impact as an invasive species. Not only can the species inflict a painful bite with Latrodectus-like symptoms but it has emerged that associated bacterial infection can lead to necrosis (tissue death). Of 22 bacterial species detected on S. nobilis, 12 are known to be pathogenic in humans. Sometimes the infections resulting from bites do not respond to standard antibiotic treatments, and the level of antibiotic resistance displayed by some Steatoda-associated bacteria is significant for medical science.

It is also significant for ecosystems. Biodiversity loss and accelerated environmental change are hallmarks of the Anthropocene, and rapid global climate shifts are expected to increase the rate at which invasive species replace native species.

Brushtail possums (Trichosurus vulpecula) are one of the most troublesome pests in New Zealand and despite the governments ‘PredatorFree2050’ aspirations we are a long way from eradicating this invasive species. The very effective use of synthetic fluoroacetate (1080) poison has short-term impact on total numbers.

Invasive pests commonly result from accidental introductions or opportunistic colonisation involving few individuals, but Acclimatisation societies worked assiduously to import and release brushtail possums throughout the country.

‘We shall be doing a great service to the country in stocking these large areas … with this valuable and harmless animal’  reported the Auckland Acclimatisation Society in 1917. New Zealand forest were considered to be under utilised and a widely held belief was that a valuable fur trade would result from possums. The legacy of this naivety is a species that damages New Zealand agriculture, tourism, biodiversity and ecosystem resilience through transmission of bovine TB, spread of bacterial pollutants to waterways and destruction of native animal and plant populations.

A view of the current success of possums in New Zealand can be found here: PossumNZ

The efforts of the individuals and societies primarily in the latter 19 Century equipped the invasive New Zealand possum population with huge genetic diversity. Multiple origins of the possums in Australia are evident from the variation in fur colours, that reflect different subspecies. Release in New Zealand created a genomic melting pot that enabled mixing of of these variants which freely interbreed. Evidence for this mixing comes from population genetics, and analysis of morphological evidence that show a continuum in variation.

Fur colour is probably of little consequence to possum survival in New Zealand but it provides a visual clue of genetic diversity. Possibly the most concerning genetically controlled attribute of brushtail possums for New Zealand is in there potential to tolerate high levels of 1080 poison.

So far, the fact that possums came from southeast Australia and Tasmania has benefited control measures as possums in those areas have low tolerance of natural 1080 because it is rare in the plants they eat. The genomic potential for resistance is obvious from possums in Western Australia that eat plants that naturally have high 1080 levels. WA possums have high tolerance of the poison. See POSSUMS STOMACH 1080

Remarkably, since the 1980s no new data have been obtained that directly tell us how tolerant of 1080 possums in New Zealand are. In fact the research done in the development of 1080 baits in New Zealand has never been published and only summary data are available (captured in table above).

Watch this space…

Alpine insects are losing ground.

Climate change is here. Increasing global temperatures has already resulted in alpine species disappearing from their lowest elevations. 

In Aotearoa New Zealand this is most apparent among the endemic alpine-adapted grasshoppers, that live above the tree-line on mountains and have evolved to survive repeated freezing and thawing.

Data spanning 52 years show the lowest elevation recorded for New Zealand’s largest grasshopper (Sigaus villosus) in Canterbury has moved 290 metres up the  mountain. This means this flightless black-eyed alpine specialist is now missing from habitat it was using in the 1960s because rising temperatures have already reduced the habitat suitable for this endemic species.

Although some alpine species can move up mountains to track cooler conditions higher up, that is only possible if the mountain is tall enough. In some places they have reached the top already, resulting in a squeeze on space as lower elevations get too hot. This change to the distribution of alpine insects is documented world-wide as shown by a newly published review in the science journal Evolution & Ecology: Meza-Joya et al. 2025

The review of alpine insect species from around the world shows that more than half have lost some of their lowest elevation habitat (over 10 or more years). Upslope expansion was observed in 56% of studied alpine species – but upslope expansion has not been recorded for New Zealand’s largest alpine grasshopper which can live at 2130 m above sea level. Fewer than 30 peaks in the Southern Alps are over 3000 m asl so there is not a limitless supply of potential habitat.

Space is not the only problem. Higher up a mountain the lower the oxygen level – a factor that might prevent some species expanding. And, the shape of mountains means that habitat patches get smaller, fragmenting and reducing populations size.

New research shows that New Zealand Orthoptera don’t conform to the metabolic cold adaptation hypothesis.

Metabolic rate varies with body size but when size is considered in concert with other factors many cold-adapted organisms show faster metabolic rate than their warm-adapted cousins at the same test temperature. But this is not what we see in Aotearoa NZ.

In NZ most of our native wētā and cricket diversity is nocturnal and many species are tolerant of cool temperatures or even thrive in cold places. However, their metabolic rates do not support the theory that to cope with the challenges of the cold, alpine insects run their engines faster.

The metabolic cold adaptation hypothesis predicts that ectotherms from colder climates (high latitudes or elevations) have steeper thermal performance curves and elevated metabolic rates at the same test temperature compared to those from warmer environments . We found that New Zealand insects from higher elevations and latitudes had lower standard metabolic rates than expected.

As our planet rapidly warms we need to consider the thermal sensitivity of our wildlife. This is the capacity of an individual animal to change its metabolic rate as a response to increases in temperature. Two localized, declining wētā species (Deinacrida rugosa and Motuweta riparia) have high thermal sensitivity of metabolic rate. Climate change will elevate the average temperature wēta experience resulting in increased metabolic rates in these thermally sensitive species, thus requiring greater energy expenditure. This will be an added challenge to species already facing threats from habitat loss and novel predators.

There were no rodents in Aotearoa/New Zealand before humans arrived. The first people came with Rattus exulans (kiore, pacific rat), then later, different people brought different rodents (Rattus rattus; Rattus norvegicus; Mus musculus/domestics). Now there are rats in every corner of the country. 

About two years ago I got some new rat-traps and set them up around my home – near the house and in the forest fragment where I live.  I own 2.3 hectares of land that was once used for sheep farming, but the steep river terrace and gully would never have been very productive. The trees and punga in the gully were allowed to grow and they are expanding and replacing the gorse. I caught 29 rats in the first month.  I use snap traps mounted on tree trunks and baited with peanut-butter. After two years I have caught 300 rats. All but one of these 300 were Rattus rattus – the ship rat, the black rat, the roof rat, the long tailed rodent. The single Rattus norvegicus was killed in a stoat trap on the ground.

Reasons I kill rats, number one:

A dead rat can’t eat a native insect. In New Zealand forests Rattus rattus spends about 70% of its time off the ground and eats insects and plants. The insects it can find crawling about in the trees at night are tree wētā and ground wētā and cave wētā and stick insects and cockroaches and beetles and moths and caterpillars and crane flies and cicada and lots more.  Rattus rattus also eat seeds and fruit and flowers and eggs and little birds in nests.  But mostly it eats our native forest ectotherms – unless it’s dead. If the insects survive tonight, they might be food for the grey warbler or the piwakawaka (fantail) or the pōpokotea (whitehead) or tui tomorrow – but these vertebrates are native to New Zealand and hunt during the day. At night the nocturnal insects might be eaten by ruru (owl) or a spider or a gecko. I’m not anti-predation – but I’d rather it was an endemic species who benefited.

Reasons I kill rats, number two:

A live rat is home to the rat flea (Xenopsylla cheopis) and the flea is home to a bacterium called Rickettsia typhi. This bacterium causes the disease murine typhus when infecting humans. Symptoms are fever, nausea, headaches, and muscle pain. Although murine typhus can be fatal if not treated with antibiotics most recover fully.  Killing a rat results in the death of the fleas and the bacteria.  I don’t know how many fleas on my rats are infected with Rickettsia typhi but disease reduction could save health care costs. 

Reasons I kill rats, number three:

Every rat exhales carbon dioxide. 300 rats (about 30kg in total) are no longer contributing to greenhouse gas emissions. The invertebrates that the rats would have eaten get to walk away – they also release carbon dioxide but they are ectotherms, so their metabolic rate is much lower. The fruit and seeds the rats would have eaten might germinate and produce seedlings. By reducing the number of rats in my gully I’m reducing my carbon footprint.

Over the two years I have learnt how to rapidly distinguishing Rattus rattus from Rattus norvegicus – Even if a rat is half grown the tail-to-body ratio, fur colour on belly, shape of face and size of ears will provide the information to separate the two Rattus species.

My two local Rattus species

Although I have killed 300 rats there are still more out there. I know I am not eradicating them from my property as there will always be more moving in. Most Rattus rattus don’t move much further than 100m from their home, if there is plenty of food. But on all sides of my place there are rat populations producing more offspring than can be supported by local food – so from all around me hungry rats will be arriving. The entire rat population was removed from a small Palmerston North forest fragment in 1977 but it took only two months for rats to recolonised the forest (Innes & Skipworth 1983). The graph of number of rats killed per month shows that my local population is responding to the seasons – there is an increase in numbers in early autumn, fewer in winter but never a complete absence. Only if I built a fence could I make a forest fragment free of carnivorous mammals.  So, killing rats will not increase the local tui, kereru and the tomtit population but I have three good reasons to bait my traps again tomorrow.

Let the insects live, prevent disease and reduce carbon dioxide:

saving the planet one rat at a time.