What this bizarre case of mating says about NZ's biodiversity crisis

The native New Zealand grasshopper P. otagense is seen here mating with its larger counter-part, P. marginale. Photo / Supplied

The mating of two distinctly different native species of grasshopper might simply seem a bizarre, almost vulgar quirk of New Zealand nature.

But scientists say the phenomenon they've been observing points to a much bigger and more important crisis facing our country.

It's a tragic symbol of what's become of our natural landscapes and the rich biodiversity they supported for millions of years.

With the ongoing transformation of our countryside into agricultural land has come the displacement of entire ecosystems.

But what does the cross-breeding of two lowland native grasshoppers have to do with that?

A tale of two grasshoppers

It's a complex story.

Several years ago, Massey University researchers began looking at the distribution of genetic variation between the two flightless grasshopper species belonging to the Phaulacridium genus.

Phaulacridium marginale is found across a wide area spanning the North Island and most of the South Island, typically in lowland grasslands.

Its much smaller cousin, P. otagense, has long been confined to small pockets in the Mackenzie Basin and Central Otago, mostly on dry slopes and riverbeds.

"These are interesting to us because the genus is shared with Australia, where there are several endemic species, so we were interested in how New Zealand and Australian taxa were related," Massey evolutionary biologist Professor Steve Trewick explained.

Their work revealed how the species in both countries held a shared lineage, implying that the ancestors of those in New Zealand would have crossed the Tasman Sea, no earlier than seven million years ago.

A green P. marginale grasshopper. Photo / Supplied

They also found how the more restricted P. otagense had a higher level of genetic diversity than the wider ranging P. marginale.

Trewick said this pattern suggested that when P. marginale had pushed out its range from a small population to a larger area, it had done so while carrying fewer genetic variants.

"Meanwhile, P. otagoense probably persisted in its specialised, semi-arid environment which is and was probably the largest natural non-forest habitat in lowland New Zealand, excluding wetlands."

In a new study, he and his colleagues have delved deeper, drawing on spatial location information and climate modelling to explore how the habitat used by each of the species has been distributed and changed over time.

That work turned up even more genetic diversity in the P. otagense - but something else yet more compelling.

Trewick said there was clear evidence of introgression, or the sharing of genetics, between the two species as a result of hybridising.

This mixing had happened as P. marginale had reached the enclaves of P. otagoense, making its habitat an even smaller stronghold in the process.

A pair of P. marginale grasshoppers. Photo / Supplied

It's something that could likely never have happened in the lush, natural, forest-dominated world that was once New Zealand's landscape.

Modelling showed that, far back in our natural history, P. marginale was able to survive across much of the country's lowland area, but not in dense forests.

"Forest would effectively exclude grasshoppers; even if the climate range was suitable, the vegetation and access to solar radiation for warming would not be," Trewick said.

"So loss of forest is the likely explanation for range change."

The first shock to our prehistoric wilderness came with the arrival of Polynesian settlers, who used fire to open the landscape.

The native flora responded, and in dry areas in the South Island, there was an eastward expansion of tussock grassland that eventually enabled grasshoppers, open-country skinks, beetles, cicadas and other small creatures to move as well.

But far bigger changes happened when European pastoralists began using fire, fertiliser and seed of non-native grasses to make the grasslands more suitable for agriculture.

Trewick said this appeared to have been a heyday for P. marginale, which were happy to munch on exotic grasses and herbs like plantain and dandelion.

A rare red form of endemic grasshopper a rare red form of P. otagoense. Photo / Supplied

The new corridors paved the way for more range expansion, which meant more ecological competition, and more reproductive mixing.

It epitomised the devastating consequences that human development had meant for complex and fragile ecosystems across the country.

"Large scale landscape changes associated with agriculture have an overlooked and forgotten impact; directly through replacement of entire ecosystems, and indirectly through alteration of connectivity among locations and surviving habitat patches," Trewick said.

A view of the plains around Waimate. In Canterbury, areas of the plains home to weta and skinks were being further transformed by agriculture. Photo / File

"There is an active field of research on habitat fragmentation, but the flipside of fragmentation is connection and there is global concern about biological homogenisation."

In New Zealand, the loss of forests and wetlands was known, but the impact of this was not apparent to most of us simply because our personal experience was of wide open grasslands.

"These once held huge native biodiversity that has been replaced by a handful of exotic species," he said.

"Consequently reserve land mostly represents areas of least appeal to agriculture rather than representing the full range of native habitats and biodiversity that once existed."

The future only looked bleaker for species like P. otagense.

Our vanishing nature

New Zealand is estimated to have more than 80,000 native animal, plant and fungal species, but only about 30,000 of those have been described, named and classified.

Our country is internationally renowned for its remarkable rates of endemism, or native life.

But since the arrival of humans, and the wave of pests we brought with us, the rate of decline of species and habitats has accelerated enormously.

In just a few centuries, New Zealand has lost 14 species of moa, the world's largest ever eagle, the laughing owl, the New Zealand thrush, several species of wrens, three frog species, one bat species, at least 12 invertebrates such as snails and insects, the world's largest ever gecko and the New Zealand trout.

Source / Steve Trewick

The most recent extinction was the South Island kokako, last seen in the 1980s.

With more than 1000 threatened animal, plant and fungi species in New Zealand today - that we know of - protection and conservation is a complex task.

It's the main reason for a new effort to rid our country of pest predators by 2050.

A warming climate would bring a new range of threats and problems, but Trewick said the main driver continues to be what it always had been: landscape change.

Today, New Zealand consists of 26.8 million ha, of which agriculture covers just over half, exotic forestry makes up 8.1 per cent and native forest covers 29.6 per cent, with the rest of the land including environments such as mountains, towns and cities.

While recent estimates show sheep and beef farming land has shrunk by 4 million ha since 1990, dairy and forestry land has increased by 1.3 million ha.

"The last 20 years have seen massive intensification with pastures ploughed and reseeded, fertilised and irrigated to create the most productive systems, primarily for cows," Trewick said.

"These modified and disturbed pastures have very low suitability for the remaining open country native fauna - grasshopper egg masses, for example, are destroyed by ploughing and trampling.

Source / Department of Conservation

"In central South Island, the process has moved on a pace and seen 'improvement' of large areas that were formerly habitat for P. otagoense and other endemic grasshoppers and insects of this region, even though they were already occupied by many introduced weeds."

And once such habitats were replaced with another, scientists like Trewick could only surmise what once lived there.

"For example in Canterbury, until recently part-modified areas of the plains supported remnant populations of native animals including local ground weta and skink species.

"These had population foci in piles of boulders and stones set aside during earlier pastoralisation.

"Many of these are now being removed to allow expansion of modified pastures and use of large-scale mechanised irrigation systems."

Ultimately, human-driven environmental changes were as rapid as they were far reaching, he said.

"In addition to directly causing extinction and so loss of biodiversity by removal of habitat, these changes can result in novel species interactions that also contribute to diversity loss.

"This is important for New Zealand where our emphasis on agriculture has meant local extinction and other effects have gone unobserved.

"This example provides a glimpse at the extent of biological change in New Zealand since European settlement."