How the Ice Ages Boosted the Evolution of New Zealand’s Weird and Nervous Native Plants

Chris Lusk is Associate Professor of Ecology at the University of Waikato.

ANALYSIS: Recent genetic research has shed new light on the longstanding debate over the evolutionary origins of some of New Zealand’s quirkiest plants.

More than one in ten native trees and shrubs have small, widely spaced leaves on intertwining threadlike branches, often growing in a zigzag pattern. Once the preserve of botanists, some of these plants have recently gained popularity as ornamental plants.

Nowhere else on Earth has this form of “divariceous” growth arisen independently in so many plant families.

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This is a spectacular case of convergent evolution in response to environmental pressures. But what environmental pressures? The answer could help us decide how to manage New Zealand’s ecosystems.

Climate or moa?

The 19th century German botanist Ludwig Diels noted that small-leaved shrubs are typical of dry climates. He thought the divaricate form may have arisen in response to cold and dry conditions during ice ages.

In the 1970s, competitionmoa navigationA hypothesis has emerged arguing that the dvaricate form is a now anachronistic defense against browsing by the large flightless birds that went extinct soon after Polynesian colonization.

Experiences have since supported the navigation hypothesis. Yet the concentration of divaricate plants in frigid and dry regions suggests climate is also somehow involved.

The same goes for the proof that the small leaves of the divaricates are less vulnerable to cold only big leaves. But the climate does not seem to explain the unusual tenacity branches of divaricate plants.

A synthetic hypothesis

Molecular dating shows that most divaricate plant species arose within the last five million years. But fossils and genetic evidence show that moa has been here much longer than that. This means that moa navigation alone does not explain the evolution of divariate forms in so many plant families.

The evidence seems more consistent with a new synthetic hypothesis that moa browsing had more impact when the plants were exposed to a new combination of circumstances: global cooling, the development of frigid, dry climates downwind of the recently uplifted Southern Alps, and new fertile soils derived from the Ice Age . fading.

More than one in ten native trees and shrubs have small, widely spaced leaves on intertwining threadlike branches, often growing in a zigzag pattern.

wikimedia commons

More than one in ten native trees and shrubs have small, widely spaced leaves on intertwining threadlike branches, often growing in a zigzag pattern.

The frigid and dry climates posed direct physiological challenges to plants, but they also left them more exposed to browsing by preventing them from growing rapidly beyond the reach of moa. Climatic restrictions on growth therefore likely made anti-browsing defenses more important for plant survival.

Support for this hypothesis comes from a recent study experiencewho found that climate influenced the impact of deer browsing on the competition between divaricate plants and their broad-leaved relatives growing in gaps in fallen trees.

In addition, the new fertile soils created by the leaching of the glaciers would have improved the nutrient content of plant tissues, probably leading to an increase in grazing pressure. Studies of African savannahs show that divaricate-like spines and growth forms are typical of fertile soils with many grazing mammals.

Do deer act as moa substitutes?

For several centuries after the extinction of the moa, there were no great navigators in New Zealand, until European settlers introduced deer and other hoofed animals. Although valued as game and a food source, deer are also considered pests due to their impact on native vegetation.

Feeding experiments showed both birds and ungulate herbivores are not enthusiastic about eating divaricate plants if alternatives with large, soft leaves are available. The spacing of the small leaves far apart along the filiform branches reduces bite size and makes it difficult for mariners to meet their nutritional needs.

Scientists have studied the ancient moa diets by identifying pollen grains in fossilized poop (coprolites). Data interpretation is hampered by our inability to identify pollen to the species level in plant groups that include both divaricate and broadleaf species. But it would seem likely that divaricate plants present similar nutritional challenges to moa.

Analyse of moa coprolites suggests that the forest understory a millennium ago was more diverse than what we see today, after more than 150 years of deer browsing. This suggests that moa had less impact on vegetation than deer do today.

Factors limiting the impact of moa on vegetation

Unlike deer in contemporary New Zealand, the moa faced a deadly predator across the country: the now-extinct deer Haast’s Eagle. Although moa can graze safely under the forest canopy, they would have been endangered at waterholes and in open areas.

In contrast, although deer are under heavy hunting pressure in some areas, recreational hunting has little impact in remote and rugged areas such as the Kaweka Ranges, where uncontrolled deer populations sika deer threaten the regeneration of even relatively undesirable trees like the mountain beech.

Artist's impression of a Harpagornis (Haast's eagle) attacking moa.


Artist’s impression of a Harpagornis (Haast’s eagle) attacking moa.

Fast growing appetizing shrubs and small trees like karamu, skull and mahoe have probably had their best chance of escaping moa browsing when fallen trees let in enough light to allow them to grow quickly out of reach, at least in warmer neighborhoods where these plants can grow over a meter in one growing season.

The tree fall gaps were to provide two other benefits for appetizing plants. The remains of fallen trees can hinder access by large herbivores, and openings in the canopy would have exposed the moa to attack by Haast’s eagle.

Moa were probably less able to exploit vegetation on steep slopes than deer and goats are today. The impact of moa on New Zealand landscapes would therefore probably have been less widespread than the current impact of sabotaged browsers.

Finally, moa probably had a slower metabolism than comparably sized browser mammals, implying lower energy requirements and therefore lower feeding rates. Close living relatives of moa (kiwis and emus) burn less energy than herbivorous mammals of similar body weight or large flying birds such as swans and geese.

The future of deer in New Zealand

Deer could act as imperfect substitutes for the moa, but only if they are brought under effective control across the country.

1080 airdrops to control rats, stoats and opossums also usually kill deer, although the mortality rate varies widely. This is one way to keep deer populations at acceptable levels in remote and rugged areas, where recreational hunting pressure is insignificant. Aerial slaughter by shooting also showed potential.

Commercial hunting cannot be relied upon to control deer, due to the vagaries of the market. When the price of venison drops, there is little incentive to hunt deer. Aerial 1080 or aerial culling therefore currently appear to be the only realistic means of limiting the impact of deer in remote and rugged areas.

This article was originally published on The conversation. Read it original article.