Moa shaped some of our rare plants
New Zealand biologists have powerfully returned serve in one of the longest-running debates about the origins of Aotearoan plants.
Moa birds may have played a large part in the evolution of our divaricate plants after all. Divaricates are shrubs and short trees that have small leaves and wiry, tangled branches that grow in zigzag patterns.
They account for more than 10% of the country’s native woody plants, a far higher proportion than anywhere else in the world. Since early colonial times, scientists have wondered why this archipelago contains so many.
One persistent theory was the growing habit was a defensive adaptation to browsing by moa. The birds didn’t get much nutrition from a mouthful of divaricate shrub and browsed other plants instead.
Another tenacious theory proposed that the habit was an adaptation to a dry, windy or frosty climate, especially on the South Island where most divaricates evolved.
A recent study tested two new ideas. In one, researchers looked at remote islands where moa never occurred, places like the Chathams, the Sub-antarctics and the Kermadecs. These places were too far away for swimming and moa were flightless.
If moa helped evolve divaricates, then scientists would expect to find no divaricates on islands always without moa.
And that’s exactly what they found. ‘‘There are no endemic divaricate species on any of the remote islands’’. This was ‘‘strong’’ evidence, wrote lead author Jarden Howard and colleagues from the University of Auckland and the Auckland War Memorial Museum. (There are a few divaricates on the
Chathams, but not true endemic ones.)
Divaricates also interest biologists because they often have very closely related relatives that did not evolve the divaricated habit. These near relatives are called ‘‘counterparts’’ or ‘‘congeners’’.
Lots of these close cousins are on the remote islands. It’s probably unlikely that congeners spread as far as the Sub-antarctics, for example, and their closely related divaricating brothers and sisters didn’t, the researchers said.
A second aspect of the study tested ‘‘tensile strength’’ – how strong the wood is.
Even the smaller species of Moa had large, strong jaw muscles. If some New Zealand plants evolved in reaction to being browsed by moa, then strong wood was hypothesised.
This has been tested before but never with an ‘‘engineering-grade tensile-testing machine’’ that clamped and pulled mature wood until ‘‘stem failure’’. It was meant to replicate the action of the powerful moa jaw and beak.
They caused stem failure with divaricates and their closely related counterparts. They found ‘‘21 of the 22 divaricates had higher tensile strength than their non-divaricate [counterparts]’’.
Moreover, ‘‘divaricate species mean tensile strength approached twice that of their non-divaricate congeners’’.
Together the remote islands and tensile strength findings ‘‘suggest that there may have been a co-evolutionary interaction between these large flightless browsers and the New Zealand flora that may have contributed to the. . . divaricate form’’, the scientists concluded.
There’s DNA evidence showing divaricating plants started to emerge about 5 million years before present – a time when the continent Zealandia was undergoing a cold and dry period, suggesting climate played a strong role.