By Jamie Steer 22/01/2019

Introduced species can cause significant changes in their receiving environments. For any new species, a wide range of effects can be contemplated – both positive and negative. In the absence of evidence, caution is surely needed.

Introduced by accident in the mid-1960s, ‘plague skinks’ (Lampropholis delicata) have been known for all but about the last eight years as ‘rainbow skinks’ in New Zealand. I’ll get to that recent name change in a minute. For now I’m going to stick with calling them rainbow skinks.

Rainbow skinks are tiny, egg-laying lizards. They live for about 2-4 years in the wild, enjoying busy, active little lives – munching bugs, sun basking and just generally keeping busy scuttling about the place. Anyone who has seen them around Auckland on a hot day will know that they are also super quick.

First introduced to Auckland, rainbow skinks have since spread north to Northland and south to the Waikato and Bay of Plenty. Some other populations have also established in Whanganui, Palmerston North and Foxton Beach. And you may have recently heard that they’ve been introduced to Havelock and the outskirts of Blenheim in the South Island too.

Much like the arrival of rainbow skinks in New Zealand in the first place, their spread within the country has largely been due to people moving them around – whether by accident or otherwise.

What’s the problem?

Rainbow skinks were formerly protected under the Wildlife Act 1953. But this protection was removed by amendments to the Act in 2010. At this time they were also listed as an ‘unwanted organism’ under the Biosecurity Act 1993.

The official reason provided for the removal of legislative protection for the species was that it ‘poses a competition risk to native skinks’. And this is the most common argument for considering the species to be harmful in New Zealand, though it is by no means the only one.

Other reasons that have been suggested are:

  • Predation (of native invertebrates)
  • Disease and parasite vectors (to native lizards and humans)
  • Supporting higher populations of predators (because the predators consume them).

What’s the evidence?

A recent study on biosecurity strategies for managing rainbow skinks noted that the documented impacts of invasive herpetofauna (reptiles and amphibians) have been until recently ‘largely speculative’. Referring to another study, the author noted that of the 322 species of introduced invasive herpetofauna, ‘only 14 have had impacts either demonstrated or inferred, and only three [brown tree snakes, cane toads, and American bullfrogs] have been studied in moderate detail’.

Despite this lack of evidence, the author maintained that the effects of invasive herpetofauna are ‘highly underestimated’. This same confidence, in the absence of evidence, seems to characterise attitudes towards the impacts of rainbow skinks in particular.

In New Zealand, only one now 15-year-old study has been conducted to attempt to determine their effects. That study, on the effects of competition between rainbow skinks and native copper skinks, found no evidence for impacts – only that the two species hunted the same types and sizes of prey, and foraged in broadly the same way.

Rainbow (plague) skink, Raglan, New Zealand.

As another scientist noted, ‘there is not yet any research showing a measurable impact of the plague [rainbow] skink on native New Zealand lizards’ – meaning that any deleterious effects of competition between rainbow and native skinks amounts to conjecture. But surely, as ‘not yet’ implies, it is a mere formality…

What about the theory that rainbow skinks prey on native invertebrates? Well, the finding here is similar: ‘we have no information on the impacts of dense populations of plague [rainbow] skinks on native invertebrate communities’. The notion that rainbow skinks are disease or parasite vectors, or that they support higher predator densities, seem to be similarly free of empirical support.

But what about elsewhere? Maybe their effects have been shown in another part of their introduced range? Answer: not really. Rainbow skinks have been introduced to two other areas: Hawaii and Lord Howe Island. In Hawaii, early suspicions that the skinks displaced native skinks have proven merely circumstantial. On Lord Howe Island, suspicions that rainbow skinks may be impacting the island’s invertebrates are only anecdotal.

Summarising the effects of rainbow skinks in New Zealand, a recent review notes that there is simply ‘no conclusive data for the negative impact of plague [rainbow] skink in New Zealand or elsewhere in its invasive range’.

So, um, what are we doing?

This, you might think, raises an interesting question – and you can perhaps see where I’m going with this: Why is the rainbow skink listed as an ‘unwanted organism’ at all?

Yes, it’s possible that it may have effects on native species, but there is no evidence to support any of the suggested possibilities, either in New Zealand or elsewhere.[1] And there seems, it must be said, to be little motivation to gather any either. There have been a number of studies on rainbow skinks over the last two decades, but next to none of that work appears to have been directed towards gathering an evidential basis to support the claim that rainbow skinks are harmful.

Why would that be? Well, one possibility is that research to prove damage is clearly no longer necessary here. Already established is that rainbow skinks may cause harm. After all, it appears to have been the sole basis for considering them to be a problem and for removing their protections under the Wildlife Act. When may is the standard for decisions on invasiveness, evidence becomes almost incidental. Why would you even bother?

Surely not, you say. We must at least have a strong hunch here? I mean perhaps competition, the primary reason for suspicion, is recognised as an important factor in the decline of New Zealand lizard species? Well, no, even this is hard to substantiate. The best evidence available suggests that native lizard declines are due primarily to habitat loss and predation.

Enter the plague skink

As if an apparent ambivalence to evidence wasn’t funky enough, someone recently decided to change the lizard’s name, thus all but reifying its pariah status. Somewhere between mid-2010 and about 2017 a decision was seemingly made – by party or parties unknown – to change ‘rainbow skink’ to ‘plague skink’. This change has been reflected in all recent Government communications on the lizard.

According to one scientist, this is because the new name reflects ‘both its invasiveness’ – though there is no evidence of harm – ‘and the high densities that it can reach’ – though this is not in itself a reason for considering a species to be harmful. But the old name, we are told, is part of the problem too.

That name, you see, is:

‘both confusing and misleading. It is confusing because the name ‘rainbow skink’ is also used for a distantly related African lizard, and misleading because the Australian species is not particularly colourful. It is predominantly coppery-brown, but does reflect an iridescent rainbow sheen when in bright light.’

But does this logic stand to reason? Are we likely to confuse a rainbow skink seen, say, in a garden in West Auckland with a rainbow skink you might see in a garden in Zimbabwe? Common names often overlap between similar but unrelated species. The New Zealand robin, for example, is not a ‘true’ robin. That is one of the reasons why we have scientific names – for people who need to know the actual taxonomy.

Rainbow (plague) skink, Raglan, New Zealand.

And is the emphasis on ‘rainbow’ so misleading? Yes, its iridescent sheen is only visible in bright light, but it is striking and distinctive. Just take a look at the image on the Department of Conservation’s web page. For me, it is somewhat ironic for a New Zealander to be arguing that a local species is ‘not particularly colourful’ anyway. This is the same charge that is often levelled (and contested) at our native species.

And was ‘plague skink’ the only alternative anyway? No, in fact ‘delicate skink’ is the name commonly used both in its native range in Australia and in its introduced range on Lord Howe Island. Surely that would have been a better choice.

But wait though:

‘there is more to the new name than clarifying ambiguity – there is psychology there also. Would you be more concerned about a colony of plague skinks or a colony of delicate skinks being detected on a conservation island?’ 

It’s here then that the real reason for the name change becomes clear. And it is really about rhetoric. ‘Rainbow skink’ implies something that could be valuable, where ‘plague skink’ implies only a negative value. It prevents anyone from even attempting to consider any positive attributes.

Perversely, it also makes it all but impossible to investigate those possibilities. What, for instance, would you think of the chances of a grant application for research seeking to determine the positives of something named after a collection of the worst diseases in human history? Not likely is it.

As for the welfare of the skinks, the rhetoric of a disease works in much the same way that military metaphors do elsewhere in conservation. Among other reasons, they suppress our ability to feel compassion for these animals. In fact, such considerations would seem mildly perverse. Hands up who wants to consider the welfare of the plague?

Perhaps the decision to consider rainbow skinks unwanted is merely precautionary though? We don’t know so we’ll assume guilty until proven innocent. But if that is the case, shouldn’t we also do similarly for all the other reasonably abundant introduced species in New Zealand too? I’m sure we could conceive of many ways that they might contradict native species as well. And where do we stop with precaution? What are the limits?

A related question might be: precautionary from whose perspective? Once an animal is labelled ‘invasive’ or ‘pest’ or ‘unwanted’ it faces the near certainty of suffering on account of it. Thus rainbow skinks on Great Barrier island were recently subjected to experimental death-by-chicken. In contrast, the species that it apparently (in this case) impacts only may actually be affected. And what is an appropriate standard of evidence to determine harm? Is no evidence appropriate?

Final thoughts

Whatever your perspective on them, rainbow skinks are likely a permanent addition to New Zealand’s lizard fauna. Like ourselves, they’re probably here for good. And while it is sensible to prevent their further human-assisted spread, perhaps we need to be thinking longer term about how we are going to choose to interpret these newer arrivals in their now-established ranges.

Yes, their effects may well be a mixed bag here – some positive, some negative. We don’t know. Rather than pulling out the pitchforks, I’d suggest we should instead be seeking clarity on what their actual effects on the environment are. Perhaps we can add some potential positives to the potential negatives list too. With introduced species, it’s not just a choice between benign or negative. If it were, we wouldn’t allow any further introductions (e.g., biocontrol agents).

And the naming situation? I think it’s unbalanced. ‘Plague skink’ clearly seeks to both predetermine and ossify the way we interpret these lizards (i.e., negatively). That’s harmful to the lizards, to public perceptions of the lizards, and to scientists ability to study them dispassionately. On the other hand, perhaps ‘rainbow’ or ‘delicate’ speaks too forcefully to positive attributes. If that’s the case, maybe a neutral common name might be more appropriate. That might at least allow for a more open and impartial assessment of their values.


[1] In fairness, ‘unwanted organism’ also has a very specific technical meaning under the Biosecurity Act. That is: ‘any organism that a chief technical officer believes is capable or potentially capable of causing unwanted harm to any natural and physical resources or human health’ (emphasis mine). As noted above, this could be interpreted to include almost any species in New Zealand – introduced or native. In that respect, the decision is really more around what constitutes ‘capable’.

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Featured image: Rainbow skink © Flickr

0 Responses to “The plague skink: Science or scaremongering?”

  • In the absence of knowledge on whether an introduced species is likely to have a negative impact on native biodiversity or ecosystems, the precautionary approach is the best response . In other words: treat as likely to be harmful unless there is evidence it is not. In particular for a species that is relatively new . This is international best practcie – as e.g. in the IUCN guidelines. While this means that you may act against some species that would be relatively harmful, this is far more preferable than not acting on one until the impact is obvious – by which time it usually is too late to eradicate. Invasion Biology 101.

    • Maybe Invasion Biology 101 needs a refresh. While the precautionary approach remains sensible, just how precautionary are we going to be? If all wild introduced species may cause harm should they all be listed as unwanted organisms? Or just ‘relatively new’ ones? How relatively new?

      Invasion biologists are quick to note that only a small proportion of introduced species prove to be invasive. But in practice we seem to be treating them all as invasive unless someone can prove otherwise. The distinction between introduced and invasive is thus blurred. And how much proof would we need to show that a species is not, on balance, harmful? Sometimes we search for proof of harm, rarely for proof of benefit.

  • It would be good to know how plaque skinks impact biodiversity. I work a trapping line in Kohimarama (Aukland) where we have a lot of plague skinks, I have been wondering if the introduced invertebrates are more resilient to the skinks than the native invertebrates.