Moa had minor role in evolution of twiggy native shrubs – Stuff.co.nz

Chris Lusk/Supplied

These plants are closely related. On the right, a divaricate called Coprosma propinqua, or mingimingi. On the left, a broadleaved relative called coprosma robusta, or karamu. How did these evolve so differently?

Perhaps the longest-running debate about the evolution of New Zealand plants may have been resolved thanks to new research among their genes.

Divaricates are mostly shrubs or low trees, with small or tiny leaves, and densely interlaced and wiry stems. They are immensely twiggy and overseas the habit is called cage architecture. They are popular in many New Zealand gardens for their distinctive shapes and colours, and for attracting native insects, lizards and birds.

About 13 per cent of New Zealand woody plant species are divaricates, a much higher percentage than anywhere else.

The distinctive nature and prevalence of these NZ plants was noted in the late 1800s by European scientists and two competing theories arose.

READ MORE:* 23-million-year-old leaves hint trees might 'breathe' easier in high-carbon air* Ancient Northland kauri tree reveals secrets of Earth's polar reversal* Save these natives - plant them in your garden* Pitpat trees do it tough thanks to being high on the menu

The first held that divaricating was an evolutionary response to browsing by moa birds. In short, the modest amount of nutritious leaf material combined with the low nutrition twigs meant moa were better off eating other plants. That encouraged these plants to keep evolving as divaricates.

The second theory held that as divaricates most often occur in frosty and droughty regions, such as the east coast of the South Island, the habit is probably an adaptation to climate. Its thought that small leaves survive cold and dry conditions better than big, fleshy leaves.

In recent times, a synthesis theory has emerged that asked, Why not both?.

Enter the genetics research done by Kevin Maurin in his PhD thesis in biological sciences at the University of Waikato. He showed that divaricating started to emerge in New Zealand about 5 million years before present. In evolutionary terms, thats not very long.

But it probably matches up with a time when the continent Zealandia was undergoing a colder and drier period, suggesting climate played a strong role.

And it has a flip side as well. Moa ancestors were browsing Zealandias shrubs and small trees long before 5m years ago, and if moa had an evolutionary impact on a habit this widespread, then science would expect much older evidence of divaricates.

It is therefore now very difficult to argue that avian browsing alone was responsible for its over-representation in the New Zealand flora, concluded Maurin. Instead, this age is consistent with a strong influence of Pliocene-Pleistocene climates, although it does not exclude a role of avian browsing.

Indeed, the effect of browsing by moa was also probably involved, Maurin wrote.

Other insights arose out of the research by Maurin and his thesis supervisor, associate professor Chris Lusk, including that most divaricates probably werent of Gondwana origin a reference to the ancient super continent from which Zealandia eventually split.

Rather their ancestors probably arose in Australia and dispersed eastwards to these islands before encountering a cold, dry climate and moa.

They also noted that divaricates are browsed these days by wild deer, but that pressure probably wont lead to widespread loss because deer, like moa, get better nutrition from other plants.

But they were less certain about the impact of climate change. Thats expected to lead to drier zones, such as on the east side of the South Island, and its not clear how the divaricates will adapt to even drier conditions.

Meanwhile, having adapted to cold conditions, they face an increase in temperatures that may threaten their survival unless their caged architecture can somehow help them adapt further.

Read more from the original source:

Moa had minor role in evolution of twiggy native shrubs - Stuff.co.nz