Between 80 and 85 million years ago, Gondwanaland, a giant continent made up of what today is Africa, Antarctica, Australia, and South America, broke up, thus causing what is now New Zealand to become separated from the larger landmass. After the separation, any creature unable to cross a considerable distance of ocean could not migrate to New Zealand. Snakes and most mammals evolved after the separation. Thus there are no New Zealand snakes, and bats, which flew there, and seals, which swam there, were the only mammals on New Zealand when Polynesian settlers (the Maori) arrived there about a thousand years ago.
When the Maori arrived in New Zealand, they encountered birds that had been evolving for 80 million years without the presence of mammalian predators. The most striking of these animals must have been moa. Now extinct, moa were gigantic wingless birds that stood as much as 10 feet (3 meters) tall and weighed as much as 550 pounds (250 kilograms). They are known from a diverse array of remains including eggshells, eggs, a few mummified carcasses, vast numbers of bones, and some older fossilized bones. The species of moa that are currently recognized occupied ecological niches customarily filled elsewhere by large mammalian browsing herbivores. They may have had relatively low reproductive rates; apparently, they usually laid only one egg at a time.
It seems possible that when Captain James Cook first visited New Zealand in 1769, moa (or at least one of the moa species) may have still survived in the remote areas in the western part of New Zealand’s South Island. If so, these individuals would have been the last of their kind. Climatic conditions in New Zealand appear to have been relatively stable over the period during which moa became extinct. Different factors could have worked in concert to account for their abrupt disappearance.
Vegetation was considerably altered by the Maori occupation of New Zealand, a change not easily explained by climate variation or other possible factors. Forest and shrubland burning appears to have reduced the prime habitat of many moa species. However, the main forest burning started around 700 years ago, after what current archaeological evidence indicates was the most intensive stage of moa hunting. While there appears to have been extensive burning on the east side of New Zealand’s South Island, large forest tracts remained in the most southern part of the island. Because major habitat destruction seems to have occurred after moa populations already were depleted, and because some habitat that could have sheltered moa populations remained, it would seem that other factors were also at work in the extinction of these birds.
For South Island, human predation appears to have been a significant factor in the depletion of the population of moa. At one excavated Maori site, moa remains filled six railway cars. The density of Maori settlements and artifacts increased substantially at the time of the most intensive moa hunting (900 to 600 years ago). This period was followed by a time of decline in the Maori population and a societal transition to smaller, less numerous settlements. The apparent decline fits the pattern expected as a consequence of the Maori’s overexploitation of moa.
Finally, the Maori introduced the Polynesian rat and the dog to New Zealand. The actions of these potential nest predators could have reduced moa populations without leaving much direct evidence. The Maori may have also inadvertently brought pests and disease organisms in fowls, which could have crossed over to eradicate moa populations. The possibility of analyzing ancient DNA to identify past diseases of extinct animals is being explored. However, evidence of such diseases is difficult to determine directly from paleoecological or archaeological remains. For these reasons, it is hard to determine the likelihood that introduced disease organisms were a cause of the decline of moa, but they are potentially significant.
While the last of these possible causes remains speculative, definite clues exist for the action of the first two causes. The story of moa species and their demise raises ecological issues on the vulnerability of species to human-caused changes--including altered vegetative cover of the landscape, change in the physical environment, and modification of the flora and fauna of a region by eliminating some species and introducing others.
在8千万至8千5百万年前，冈瓦纳大陆——一片巨大的大陆，由今天的非洲、南极洲、澳大利亚和南美洲组成——分裂了，从而使得今天的新西兰与陆地分离。分离之后，如果无法跨越相当长距离的海洋，任何生物都不能迁徙到新西兰。蛇类和大部分哺乳类动物也在分离后发生了进化。因此，当波利尼西亚居民（毛利人）在1000年前到达新西兰时，新西兰没有蛇类，而飞到那里的蝙蝠和游到那里的海豹是唯一的哺乳动物。 毛利人到达新西兰时遇到了一些鸟，8千万年里它们在没有哺乳类动物天敌的环境下演变进化。这些动物里最显著的就是恐鸟。现在已经灭绝了的恐鸟，是一种巨大的无翼鸟类，站起来高达10英尺（3米），重达550磅（250公斤）。因为各种遗迹，它们为人所知，这些遗迹包括了蛋壳、蛋、少量的兽类干尸、大量骨头和一些更古老的骨头化石。最近才被确定的恐鸟这个物种所处的生态龛位在别的地方一般来说是被大型哺乳类食草动物占据。它们的繁殖率可能相对较低；看起来，它们通常一次只下一个蛋。 很有可能的是，当詹姆斯·库克船长1769年第一次来到新西兰时，恐鸟（至少是其中一种恐鸟）可能仍然生活在新西兰南岛西部的偏远地区。如果是这样，这些恐鸟可能是最后的恐鸟。新西兰的气候条件在恐鸟灭绝期间似乎是相对稳定的。多种不同因素一起导致了它们的突然消失。 在恐鸟占据新西兰的期间，植被变化相当大，这种变化不是简单地用气候变化或其它可能的因素就可以解释的。森林和灌木带的燃烧似乎减少了很多恐鸟物种的主要栖息地。但是，主要的森林火灾大约开始于700年前，之后就是现代考古学的证据所表明的猎杀恐鸟的最集中阶段。虽然在新西兰南岛的东边似乎有大面积的火灾，但在岛的最南边还有大片森林。因为主要的栖息地毁坏似乎发生在恐鸟数量已经所剩无几之后，也因为一些栖息地可以保护恐鸟使其种群延续，因此其它因素似乎也造成了这些鸟的灭绝。 在南岛，人类捕杀似乎是导致恐鸟数量急剧减少的一个重要因素。在一个开挖出来的毛利人遗址里，恐鸟残骸可以装满6节火车车厢。毛利人定居点的人口密度和手工艺品数量增加的时间大体上是在猎杀恐鸟的最集中阶段（900-600年前）。这个阶段之后，毛利人的人口数量下降，并经历了定居点更小、数量更少的社会转变。这一明显的下降与毛利人过度捕杀恐鸟带来的预期结果相符。 最终，毛利人将波利尼西亚鼠和狗引入了新西兰。这些潜在的巢穴捕食者的一些行为可能导致了恐鸟数量的减少，却没有留下太多直接证据。毛利人也无意地带来了宠物以及飞禽病原体，它们很有可能传播开来，从而导致恐鸟种群灭绝。通过分析古代DNA来确定过去灭绝动物的疾病的可行性正在研究当中。但是，有关这些疾病的证据很难直接通过古生物学或考古学遗迹来判定。出于这个原因，很难断定被引进的病原体是导致恐鸟减少的一个原因，但是它们的作用可能非常重要。 虽然这些可能的理由中的最后一个还是猜想，但是前两个理由是有确切线索的。恐鸟物种及其灭绝的故事提出了生态问题，即物种对由人类所造成的变化（包括地表植被变化、自然环境变化、通过消除一些物种和引进另一些物种而对一个区域的动植物群所造成的改变）的脆弱性。