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© 2019 Anna Yeoman for the Central Otago Ecological Trust

Home  >  Otago drylands  >  Origins

To discover why we have unique drylands here in inland Otago, we can wind the clock back and follow two tectonic movements, those creepingly slow but absolutely unstoppable motions of the Earth’s crust.

Melicytus alpinus. Anna Yeoman

The first is the movement of crust that split Zealandia off from the super-continent of Gondwana, and carried us to our lonely spot in the Southern Ocean. The second is the grinding together of the Pacific and Indo-Australian plates that has created our high mountain backbone, the Southern Alps. 

 

The first of these movements, our split from Gondwana, has meant that for the last 80 million years our flora and fauna have evolved in relative isolation, giving rise to the high number of unique species on our shores. The second crustal movement, the uplift of the Southern Alps, has created the rainshadow effect that gives inland Otago its almost continental climate.

Drylands of the Alexandra basin. Anna Yeoman

Evolving alone

 

About 80 million years ago the small continent of Zealandia began to split away from the super-continent of Gondwana. The process took some time, but as the Tasman Sea opened up, all land bridges were eventually lost. We were alone. On this continental fragment the plants and animals continued to evolve, largely cut off from the rest of the world.  ​

 

Sometime after we cut ties with Gondwana came the global mass extinction event that is most famous for wiping out the dinosaurs. With 75% of the Earth’s plant and animal species wiped out in one event, the remaining plants and animals quickly diversified to fill the ecosystem niches. Across most of the globe one animal group particularly took off, and that was the fur-covered milk producers, the group we know as mammals. 

 

Yet for reasons that still baffle scientists, this was not the case on Zealandia. We have two living species of bat, and some intriguing fossil evidence for an ancient species described as a waddling mouse. Yet mammals did not come to dominate our ecosystem here as they did elsewhere in the world. 

 

In the absence of mammals

Instead, Zealandia became a land of birds, reptiles, amphibians and invertebrates. On a scale unmatched anywhere in the world, birds became our mega-fauna. With a wingspan of nearly three metres and claws the size of a tiger’s, the Haast eagle became the top predator. Others became scavengers, grazers, foragers and insect eaters, ranging in size from the quarter-of-a-ton Giant moa to thimble-sized wrens. Without mammalian predators or competitors, species could diversify into new and often surprising forms: the kea became the world’s only mountain parrot, the kakapo its only flightless one, and a recent discovery shows we also had the world’s largest parrot, a whopping one-metre tall Heracles inexpectatus.

 

Our insect fauna also diversified in surprising ways, producing 1000 species of snails. Beetles and grasshoppers grew large, and in many cases, flightless. A relative of the cricket grew to become the heaviest insect in the world, the whopping 70-gram Giant weta, twice as heavy as a common house sparrow. Amphibians and reptiles abounded, frogs and tuatara, geckos and skinks, turtles and even a crocodile. 

With a wingspan of nearly three metres and claws the size of a tiger’s, the Haast eagle became the top predator.

Alone but not inapproachable

 

With our long history of isolation in the Southern Ocean, it is surprising to realise that a large proportion of our animal and plant species have in fact arrived in New Zealand since our split from Gondwana. While tuatara and geckos came with us from Gondwana, we think skinks arrived about 30 million years ago. Mānuka, rātā, flax and pōhutukawa are late arrivals, along with saddleback, kōkako and huia. Blown by storms, carried on rafts of plants and soil, possibly set on their way by rare flood or tsunami events, they have somehow made it across the ocean to Zealandia. 

 

Once here, these plants and animals have adapted to life on our shores, evolving separately from their relatives in other lands and forming species now found only in New Zealand. So while our isolation is not as complete as once thought, it is significant enough to have given rise to our extremely high levels of endemism, of species that occur nowhere else in the world. 

Kanuka at Flat Top Hill. Anna Yeoman

Uplift

 

So with our largely unique community of living things on board, the second key tectonic movement of this story got under way. As the tectonic plates continued their shuffling and jostling, borne on the currents of warm magma below, Zealandia found itself situated on a pivot point of action. We were in the hot seat, as it were, of a new plate boundary that had formed between the Australian and Pacific Plates.

 

As the plates continued to converge, the beginnings of the Alpine Fault were created, and about 5 million years ago the Southern Alps began to rise. Slowly at first, the huge forces pushed up the land. The plate boundary we found ourselves on was complex, to the north of us the Pacific Plate was being pushed under the Australian Plate, and to the south it was the Australian Plate under the Pacific. In the middle, down the spine of the emerging South Island, the plates were sliding past as well as pushing into each other.

 

In recent millennia the uplift has accelerated, and the Southern Alps are one of the most rapidly rising mountain ranges in the world. It is estimated that in the last 2-3 million years, they have risen by 20 000 metres, with similarly rapid rates of erosion keeping them at their current height. 

We were in the hot seat, as it were, of a new plate boundary that had formed between the Australian and Pacific Plates.

Rainshadow lands

 

It is the presence of the Southern Alps on our western boundary that gives inland Otago its dry climate. Not only are they improbably high for such a narrow country, they have been uplifted at cross angles to the prevailing winds. New Zealand is situated at latitudes known as the “Roaring Forties”, a region of the globe dominated by strong westerly winds. 

 

When moisture-laden weather systems from the Tasman Sea or the Southern Ocean reach our western coast, they are tossed into the barrier of the Southern Alps. They are forced to rise, and as they gain height the air within them cools, causing the moisture to condense and fall as rain, and at higher altitudes, as snow. Fiordland and the West Coast receive this moisture, it fills their tumbling rivers and sustains the rich rainforests. 

 

By the time the air masses have made it over the crest of the Alps there is little moisture left for the east. As it descends it also warms, further locking up any moisture that does remain. This phenomenon is known as a ‘rainshadow effect’, and creates an area of arid climate on the leeward side of mountain ranges. While rainfall on the western faces of the Southern Alps can be more than 10 000 millimetres a year, only 100 kilometres to the east in Central Otago it can be as low as 400 millimetres a year. 

Land of extremes

 

The same factors that cause the rainshadow effect also produce strong, drying winds known as Fohn winds. As the westerly airflow crosses the Southern Alps and descends to the eastern lowlands, it rises in temperature and accelerates creating the South Island’s infamous ‘nor’wester. Tearing washing from the line and agitating school children, these often gale force winds also have an incredibly drying effect on the landscape. They occur most commonly in springtime, and the dryland vegetation has had to adapt to their forceful, drying nature. 

 

Like ripples from the Southern Alps, a series of broad topped mountain ranges lie across Central Otago, all following the same north-east trajectory. Between these roughly 2000 metre schist-topped ranges lie gentle basins and wide river valleys drained by the Clutha and Taieri Rivers. Along the northeastern corner of the region there is another series of ranges of a similar height which trend northwest. On the range tops temperatures stay reasonably low all year round, and the highest peaks hold snow cover for a reasonable proportion of the year. 

 

In the basins below, conditions are very different. It is these lowland areas that experience both the hottest summers and the coldest winters. High summer temperatures are a product of the low altitude, long sunshine hours and the region’s distance from the moderating effects of the coast. During the cold winters, again a result of the region’s distance from the ocean, the low temperatures are exacerbated by temperature inversions. A temperature inversion is when a layer of cold air gets trapped below warmer air, and often occurs in valleys or basins during times of settled weather. Such conditions are common in the lowland basins of inland Otago, and cause the frosts, and hoar frosts, the area is known for. 

 

All these extremes of climate put massive pressure on plants and animals, and the species that have evolved to survive the highly variable and challenging conditions of Otago’s drylands show resilience, adaptability and often creative solutions to these climatic pressures. 

Tearing washing from the line and agitating school children, these often gale force winds also have an incredibly drying effect on the landscape.

Department of Geology, University of Otago. Rise of modern mountains on Otago’s northeast margin. Retrieved from: https://www.otago.ac.nz/geology/research/gold/geology-and-gold/modern-mountains-rise.html

 

GNS Science Te pū ao. The Geology of New Zealand. Retrieved from: https://www.gns.cri.nz/Home/Our-Science/Land-and-Marine-Geoscience/Regional-Geology/The-Geology-of-New-Zealand

 

Matt McGlone (2007). Evolution of plants and animals - Split from Gondwana. Te Ara - the Encyclopedia of New Zealand. Retrieved from: http://www.TeAra.govt.nz/en/evolution-of-plants-and-animals/page-2

 

Richard Holdaway (2007). Extinctions - Pre-human extinction events. Te Ara - the Encyclopedia of New Zealand. Retrieved from: http://www.TeAra.govt.nz/en/extinctions/page-2

 

Science Learning Hub – Pokapū Akoranga Pūtaiao (2009). New Zealand’s unique ecology. Retrieved from: https://www.sciencelearn.org.nz/resources/1440-new-zealand-s-unique-ecology

 

Szabo, M.J. (2013). Haast's eagle. In Miskelly, C.M. (ed.) New Zealand Birds Online. Retrieved from: http://www.nzbirdsonline.org.nz/species/haasts-eagle

 

Worthy, T. et al. (2019). Evidence for a giant parrot from the Early Miocene of New Zealand. Biology Letters, 15(8). Retrieved from: https://royalsocietypublishing.org/doi/10.1098/rsbl.2019.0467

Bibliography