By Nic Rawlence 22/08/2021

When most people think of the consequences of humans arriving on an isolated island paradise, it wouldn’t be making an insect flightless.

Most people would think about the rapid extinction of biodiversity and environmental modification that inevitably follows human arrival. In Aotearoa New Zealand this includes the sad loss of the giant megafaunal moa, pouakai Haast’s eagle, and the huia to name a few, as well as the widespread burning of forest. Others will mention the introduction of novel mammalian predators like the kiore Pacific rat, kurī Polynesian dog, and the myriad of sharp-toothed beasties Europeans brought with them. If people even think about the insects, it will be to wonder how many were munched into extinction by rats as they rapidly spread throughout Aotearoa in waves.

Once were treelines: There are few places left in Aotearoa where you can see a pre-human treeline. Image by Danilo Hegg.

It’s the middle of winter and Jon Waters and I are halfway up a Central Otago mountainside amongst the tussocks covered in fresh snow. Our fingers are frozen solid turning over rocks on the hunt for stonefly (Zelandoperla fenestrata) nymphs in the bubbling brook that tumbles down the mountainside, while our minds are thinking of hot chocolates at the Kissing Gate Café nearby and whether we’ll make it back to Dunedin before the next snowstorm. You’d be forgiven for thinking that this barren tussock and rocky tor landscape, made famous as the land of the horse lords in the Lord of the Rings movies, was natural. Majestic and beautiful as it is, this landscape is far from natural. Rather it’s the product of the widespread burning of forests 600-750 years ago.

Before the arrival of humans in Aotearoa, this area was covered in forest. Importantly, the tree line, where the forest gives way to natural alpine tussock and herb fields, was much higher than today. Bubbling creeks, like the one I froze my fingers in, ran down the many gullies that carved up the hill country. Native galaxiid fish called these little isolated streams home, feeding on stonefly nymphs, while the adults lived out their short adult life flying around at dusk protected by the forest canopy overhead.

As anyone that’s been to Central Otago during summer knows, it can get very hot and windy making al fresco dining and wine tasting all but impossible. Under their protective canopy of the forest, the bubbling brooks the stoneflies called home would have been sheltered. In contrast, those hardy individuals that lived in the windswept upper reaches of the stream above the climate-driven treeline lost their wings. All the better to stay well-grounded than be blown to the four corners of the compass on a regular basis.

I’ll huff and puff, and blow your house down: Living in the alpine zone can be hard work, especially when there’s no shelter around. Image by Nick Foster.

Detective work by palaeoecologists looking at sediment cores from swamps and lakes stretching back into the mists of Aotearoa’s prehistory showed that shortly after human arrival in the late 13th Century, the steady accumulation of pollen rain was joined by something else – particles of charcoal. Lots and lots of charcoal. This tell-tale signal was a clear sign of the human-induced burning that destroyed half the lowland and montane forest in New Zealand within a few centuries of Polynesian arrival that was only added to by Europeans. Any forest that survived in Central Otago became restricted to isolated refuges, away from pesky people. The new tree line could be significantly lower by several hundred metres depending on where these remnants were. In the geological blink of an eye, many of the stoneflies that were minding their own business in their sheltered little homes were thrust into a brave new world, one above the human-induced tree line where the wind regularly tried to huff and puff and blow them away.

Now new research, led by Brodie Foster of the University of Otago, has shown that the stoneflies that found themselves in this mad world evolved flightlessness and lost their wings in a very short time. While 600 years may seem a long time to you and me, when the generation time of a stonefly is a long two to three years, it’s a very short time indeed on the evolutionary toilet paper of time. Amazingly, the switch from flighted to flightless stoneflies matched local human-driven treelines (from 600-1000 metres above sea level), not the climatically controlled pre-human one. The sheer speed of this change is reminiscent of peppered moths in Europe during the Industrial Revolution that became darker in colour to blend in on soot-covered trees, compared to lighter-colored ones that no doubt failed at playing hide and seek with hungry predators.

Evolutionary tinkering: Selective pressure since forest clearance has resulted in wingless stoneflies (bottom) evolving from flying ancestors (top). Image by Brodie Foster.

So how did this remarkable change in stoneflies happen? It’s likely that the change in treeline from burning significantly altered the selective evolutionary pressure on the genes controlling flight and wing formation in the populations that experienced this pronounced shift. Stoneflies that became flightless would have preferentially survived into adulthood to reproduce and pass their flightless genes onto the next generation. Over multiple generations, the evolutionary trajectory of the stonefly populations above the treeline was changed forever.

New research like Brodie’s and colleagues is increasingly providing ever more fascinating details about the evolutionary consequences of human colonization of Aotearoa from the extinction of previously unknown species, to biological turnover events and now human-induced flightlessness in insects. I for one will be excited to see what scientists find next and can’t wait until our next family road trip to excitedly tell my natural history-obsessed kids about this cool new science. Humans causing insects to become flightless. Who would have thought?