Whilst a loss of species diversity can be seen across the globe, some of the most prominent examples are on the many islands of the South Pacific, where Polynesian expansion across the pacific led to the extinction of many animal species (Pimm et al, 2006). Island populations are especially vulnerable to extinction, as their populations are often small, and may be confined to a minimal area of land. Their isolation also often results in the loss of adaptability to pathogens, predators and competitors that may be introduced, and to the rapid environment change often brought about by humans (Grayson, 2001).
The extinction of endemic flightless birds alongside plants and mammals in Australia coincides with the arrival of humans (Braje & Erlandson, 2013), which may have been visiting the country as early as 22,000 years ago (Flannery & Roberts, 1999), as suggest by a rise in charcoal and a change in tree pollen in the sedimentary record. This has been contributed to anthropogenic deforestation through fire, and not down to climate change (Flannery & Roberts, 1999).
The same applies to New Zealand, which wasn't colonised until much later in the 13th Century (Irwin & Walrond, 2012). Prior to humans living in New Zealand, it was home to many species of endemic birds that are now extinct. 21 species of small landbird are no longer with us today, and alongside them were 11 species of Moas (Braje & Erlandson, 2013) (Grayson, 2001). The only birds known to be totally wingless, the enormous Moa were a family of herbivorous flightless birds. The two largest species, Dinornis robustus and Dinornis novaezelandiae weighed over 200kg, and could reach a height of 3.6m (12ft) if they stood with their neck outstretched (Wikipedia, 2013).
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| Published in 1879; a photograph of Richard Owen, the director of London's Natural History Museum, standing beside a reconstructed skeleton of Dinornis novaezelandiae - Image via Wikipedia Commons |
The only natural predator of the Moa was the Haast's Eagle which was also driven to extinction when the Moa were wiped out completely in just over a century since the Maori populated the islands. Their extinction is believed to be a result of multiple factors, but all are attributed to human activities (BBC, 2009) (Grayson, 2001). Before the arrival of our species it is estimated that up to 90% of New Zealand was covered in forested, yet within a few hundred years almost all of the lowland forest had been destroyed, and with it vanished the habitats of the Moa and other animals (Grayson, 2001). Human predation, evidence of which has been gained from many archaeological sites, was another large factor in the Moas disappearance (Holdaway & Jacomb, 2000). Lastly, the unintentional introduction of Polynesian rats (Rattus exulans) is believed to have played a large role in the extinction of birds not only in New Zealand, but across the whole of the South Pacific (Pimm et al, 2006). Whilst it's doubted by many archaeologists that there was direct predation of Moa eggs by the rats, the rodents are likely to have caused competition for food and impacted the vegetation on which the Moa and other landbirds depended (Grayson, 2001).
Madagascar suffered similar losses following human colonisation roughly 2300 years ago; it too was home to two genera of large flightless bird (Mullerornis and Aepyornis) that are often referred to as Elephant Birds which went extinct between 1600 and 1700 AD (Braje & Erlandson, 2013). Aepyornis was the larger of two, and is believed to have reached weights of up 400kg and a height of 3m (10ft). The causes of extinction are less clear, but signs of butchery from archaeological sites suggest humans played a primary role (Burney & Flannery, 2005).
The birds of other remote islands didn't fare much better. Grayson (2001) suggests that, alongside anthropogenic predation, predation by invasive dogs, pigs and Polynesian rats brought to the islands by humans may have been largely responsible for the disappearance of endemic Hawaiian species. Of 17 species of landbird known from archaeological deposits, only four remain on Hawaii today (Braje & Erlandson, 2013). Rats are also believed to be partly responsible for the loss of 6 species of landbird and several seabird colonies, alongside much of the vegetation (through eating nuts and seeds and thus impairing the ability of the plants to reproduce) on Easter island, which in turn contributed to the breakdown of human society on the island (BBC, 2009)(Braje & Erlandson, 2013).
Grayson (2001) raises the possibility that anthropogenic activities may have, in some instances, created habitats for some subsets of bird species which would not be present were it not for such changes. Despite this, the general consensus is that the increased colonisation of Oceania by humans correlates with an increase in avian extinctions. The past extinction of island fauna paints a grim picture; it shows how easy it is for anthropogenic activities to lead to a loss of biodiversity. Unless we try to minimise our impacts and attempt to preserve and conserve the species that have so far survived, many more species of bird may join the mighty Moa.
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| New Zealand by Steve Taylor, on Flickr |
Cited:
- Extinctions in Near Time: Causes, Contexts and Consequences - Chapter 10: Late Quaternary Extinctions in Australia (T.F. Flannery & R.G. Roberts, 1999)
