In New Zealand, ferns are abundant in our landscape. They are a major part of our ecosystems. Scientists are interested in when and how our ferns arrived here – were the ancestors of our modern day ferns present on the New Zealand landmass that split from Gondwana 60–80 million years ago or did they arrive more recently?
Different theories have been put forward to explain the origins of our fern species, and as new evidence is presented or uncovered, these theories are developed, challenged or changed. Work being done by Dr Patrick Brownsey and Dr Leon Perrie is contributing to our understanding of the origins of our ferns.
Vicariance and the Moa’s Ark theory
The Moa’s Ark theory, popularised by David Bellamy in 1990, is based on the idea of vicariance. This theory proposes that New Zealand's animal and plant life rafted here when the piece of land that eventually became New Zealand split away from Gondwana 80 million years ago. Since that time, our plants and animals evolved largely untouched. This theory is often used to explain why New Zealand has flightless birds and other unusual fauna.
According to this theory, the living representatives of our ferns would be directly descended from the Gondwanan ferns. The ferns that New Zealand shares with the Australian mainland would also have evolved separately following the Gondwana split.
The theory of long-distance dispersal
An alternate theory to explain the origins of our ferns is long-distance dispersal – the movement of a plant or animal across a long distance of unsuitable habitat. Fern spores appear to be very well suited to long-distance dispersal as they are small and very lightweight. They scatter easily with the wind, and it has been shown that, high up in the sky, fern spores can travel all the way around the world within days.
According to this theory, fern spores from Australian fern species (for example) would be blown across the Tasman Sea, reach New Zealand and become established.
Proposing a hypothesis
In 2001, Patrick Brownsey gathered and examined existing New Zealand fern data, including fossil evidence, distribution patterns and cytological data. He presented the hypothesis that most ferns arrived in New Zealand relatively recently by long-distance dispersal.
Amongst other evidence in support of his hypothesis, he noted that Australia and New Zealand have about 90 species of ferns in common. This doesn’t fit with the Moa’s Ark theory as it’s highly improbable that the New Zealand fern species could have remained identical with those in Australia over 80 million years. It’s more likely that they are identical because they have dispersed much more recently across the Tasman Sea.
Testing the hypothesis
By examining DNA from different fern species, Leon was able to test Patrick’s hypothesis. This molecular approach uses differences between DNA sequences of living representatives to estimate a family tree (phylogeny) that can then be used to understand how plants have evolved over time. Based on the level of difference between DNA sequences and given a calibration point, usually from the fossil record, it’s possible to estimate when the local representative diverged from their overseas relative.
Using this approach, Leon was able to calculate an age when pairs of various New Zealand and overseas ferns were last part of the same population. What this revealed was that the majority of our ferns have been isolated in New Zealand for millions of years rather than tens of millions. Putting this in perspective, this data suggests that most of the ancestors of our present day fern species arrived here within the last 30 million years (some even within the last few million years) – not 80 million years ago when we split from Gondwana.
These results support Patrick’s hypothesis that the majority of our fern species have arrived relatively recently by long-distance dispersal. It is one more piece of evidence that contributes to establishing when and how New Zealand ferns arrived.
Nature of science
Advances in scientific research, such as molecular analysis, allow scientists to test, revise and occasionally discard theories. This ongoing process leads to a better understanding of how our world works.