Australasian Science Magazine

Ancient DNA analysis of moa bones from this site on New Zealand’s South Island helped reveal that moa were very well adapted to deal with the effects of climate and habitat change.

By NIC RAWLENCE

A new study of ancient DNA has revealed that New Zealand’s giant flightless moa were well-adapted to deal with the effects of climate and habitat change, leaving only overhunting and habitat destruction by humans as reasons for their extinction

Most people have heard of sabre-tooth cats, mastodons, woolly mammoths, woolly rhinos and giant sloths, but Australia and New Zealand have had their share of megafauna as well. In Australia there were giant wombat-like creatures called diprotodons, and a large flightless bird closely related to ducks and geese colloquially known as the “demon duck of doom”. New Zealand had the giant moa and Haast’s eagle.

The skeletons of megafauna adorn museum palaeontology galleries and inspire young kids to learn about natural history. However, there is considerable debate among palaeontologists as to what caused the extinction of these megafauna, and many others worldwide. These extinctions have occurred over the past 50,000 years resulting in the loss of at least 97 genera worldwide.

The main contenders for these extinctions are thought to be human impact (over-hunting and habitat destruction), climate and associated habitat change, or a combination of these. In some cases, like on the Pacific Islands, there is overwhelming evidence for human impact. However, in many other parts of the world, finding the culprit or culprits is difficult because human arrival and impact was contemporaneous with major episodes of climate and habitat change and was relatively far back in time, allowing erosion to destroy the evidence.

What is needed is a location where the effects of climate change and human impact can be separated. New Zealand offers this unique opportunity to investigate how megafauna responded to major episodes of climate and habitat change in the absence of humans.

Humans, specifically Polynesians, arrived in New Zealand during the late 13th century at a time of relative climatic stability. This was very much later than in Eurasia and North America, which were impacted by humans millennia earlier.

New Zealand’s pre-human fauna was diverse and dominated by birds, including nine species of large flightless moa, a giant eagle, giant flightless geese and the adzebill, which continues to puzzle scientists today.

The moa were the largest and most diverse herbivores in the pre-human ecosystem, with a body size range of 20–250 kg. However, within 200 years one-quarter of New Zealand’s birds were extinct.

Early Victorian naturalists stumbled upon large areas of bleached white bones, suggesting that human hunting may have caused their extinction. Since then there has been strong archaeological and palaeoenvironmental evidence to suggest that overhunting and habitat destruction were the main culprits.

How Did Megafauna Respond to Climate Change?

Armed with the ideal location, my collaborators and I used radiocarbon dating, stable dietary isotope analysis and ancient DNA to study the effect of climate and habitat change on moa in the absence of humans.The study involved palaeo­ecologists, palaeontologists and geneticists from Australia, New Zealand and the USA, and the research has been published in the journal Quaternary Science Reviews.

We focused on two species of moa: the crested moa and the heavy-footed moa.

The crested moa is the rarest moa in New Zealand’s museum collections, and had the most restricted distribution. It lived in sub-alpine grassland–shrubland in the north-west of New Zealand’s South Island. There has been some debate about whether crested moa survived the pronounced climate and habitat change at the end of the last ice age or became extinct, as no verified Holocene remains had been found.

The heavy-footed moa has been described as a 40-gallon drum walking on children’s gumboots. It lived mostly in areas of montane grassland–shrubland and forest margin habitats in eastern and southern areas of the South Island.

For both crested and heavy-footed moa, the numerous caves and swamps around the South Island have preserved a large number of specimens dating back 50,000 years that we were able to analyse.

The first phase of the research involved a large amount of fieldwork, including caving expeditions to remote areas of New Zealand, palaeontological excavations of swamp deposits, and scouring New Zealand’s museums for specimens. The research was divided into three different lines of inquiry.

Climate and Habitat Change

First, we studied how climate and habitat change affected the distribution of crested and heavy-footed moa. To answer this question we radiocarbon-dated numerous crested and heavy-footed moa remains covering the geographical and temporal range of each species, and compared these dates to their geographical location and altitude.

For crested moa we showed that as the climate warmed since the last ice age the altitude of crested moa specimens increased from near sea level during the ice age to sub-alpine areas during the Holocene. Surprisingly, we also showed that crested moa did not go extinct at the end of the last ice age but survived into the Holocene until at least 115 years after Polynesians arrived in New Zealand. This research was recently published in the Journal of the Royal Society of New Zealand.

Like crested moa, the distribution of heavy-footed moa waxed and waned with changing climate. During glacial periods they expanded their range throughout widespread open grassland–shrubland. However, during interglacial periods, like the Holocene, their grassland–shrubland habitat fragmented with the return of widespread forests, and their distribution shifted to include previously glaciated areas.

Habitat Tracking in Response to Climate Change

The second part of the study used stable dietary isotopes to examine whether crested and heavy-footed moa shifted habitat when their ranges contracted and expanded in response to climate change. With isotopes, “you are what you eat”, so isotopes have been used to reconstruct the diet of extinct species and as a proxy for habitat.

First, though, some background about stable dietary isotopes using carbon (C) as an example. Carbon, like the other elements in the periodic table, comes in different forms called isotopes that are separated by their atomic weight. In the atmosphere, 99% of carbon is 12C, ~1% is 13C, and considerably less than 1% is 14C. 12C and 13C are stable, while 14C is unstable and decays at a known rate, and this allowed the radiocarbon dating of moa bones in our study.

In the atmosphere these ratios remain constant, with the majority of C in the form of carbon dioxide (CO2). However, in plants the ratio of 12C to 13C is different depending on the amount of sunlight plants are exposed to. This is due to photosynthesis, whereby plants take in CO2 and water and convert them to sugars and oxygen, which is released.

Plants in sunny open areas, such as the open tussock grassland–shrubland preferred by heavy-footed moa, will have less 12C to 13C compared with closed canopy forests. Once this ratio is locked in the plant material, it remains constant throughout the food chain. As moa are herbivores, we can determine their isotopic ratios from bones and determine which types of plants moa ate and the environment moa lived in during their lifetime.

In our study we were specifically interested in the isotopes of carbon and nitrogen. Carbon can be used to infer relative forest cover, like open versus closed canopies. In contrast, nitrogen can be used to infer moisture and rainfall.

The isotopes of crested moa showed that as their range contracted to higher altitudes they tracked their subalpine habitat. The isotopes also showed that heavy-footed moa tracked their open grassland–shrubland habitat as its range reduced during the Holocene.

DNA Clues to Population Sizes

The final part of the study used ancient DNA sequences to estimate the population sizes of crested and heavy-footed moa and how this was affected by the changing distribution of each species as shown by the radiocarbon dates and isotopes. This work was done in specialist ancient DNA facilities at the University of Adelaide’s Australian Centre for Ancient DNA.

When estimating population size from DNA sequences, the general rule of thumb is the more genetic variation within a population, the larger the population size. Using ancient DNA, radiocarbon dates and the appropriate statistical methods, we can reconstruct the population size through time.

For crested and heavy-footed moa we found that the estimated population sizes were closely related to the amount of available habitat, with changes in population size corresponding to major habitat changes. However, compared with other megafauna like bison and musk oxen, the population sizes of crested and heavy-footed moa were overall very stable over the past 50,000 years.

The waxing, waning and fragmentation of heavy-footed moa habitat left another signature in the DNA. The heavy-footed moa was split into two genetic lineages that were separated during glacial periods and came together during interglacial periods like the Holocene, with repeated glacial–interglacial cycles reinforcing this pattern.

Taking the radiocarbon, isotope and ancient DNA results together we concluded that climate and habitat change in the absence of humans did not have a major impact on crested and heavy-footed moa. Both species, and probably moa in general, were able to respond to the challenges that climate and habitat change produced by tracking the vegetation communities they favoured. Combined with increases in body size to counteract cold temperatures during the ice age, and dietary plasticity, moa were very well adapted to deal with the effects of climate and habitat change.

Compared with the palaeontological record in the northern hemisphere and Australia, megafauna had varied responses to climate and environmental change. The waxing and waning of population sizes suggests that population fluctuations and range shifts are a natural response to climate and habitat change and do not necessarily lead to extinctions.

So What Caused the Extinction of the Moa?

Would moa and the other avian megafauna of New Zealand still be around today if humans had not arrived in New Zealand? The answer is undoubtedly yes. This study and the mountains of archaeological and palaeoenvironmental evidence show that human hunting and habitat destruction, not climate change, were to blame for the extinction of more than one-quarter of New Zealand’s bird species. The last surviving moa populations were probably in isolated mountainous areas of New Zealand, and would have gone extinct by 1500 AD.