Kiwi Bird Population Decline
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Published: Mon, 21 May 2018
The kiwi (Apteryx) is a flightless bird endemic to New Zealand. Widely known as the elusive national symbol of New Zealand, the kiwi is an endangered species occupying areas of both the north and south islands of this South Pacific nation. Comparable to the size of a domestic chicken, this unique species belongs to an ancient group of birds called the ratites, commonly known for their lack of ability to fly. The flightless nature of the kiwi is especially notable since scientists have been unable to discover how the bird reached New Zealand’s remote geographical position in the first place. Similar to many of New Zealand’s endemic species, the kiwi are mostly nocturnal. Although they are the smallest species within the ratite family, the kiwi lay remarkably large eggs in comparison with their body size. Moreover, though the kiwi is classified as a bird, it has many mammalian tendencies that make it unique from any other bird species on earth. Its hair-like feathers, nostrils on its beak and sophisticated senses of touch and smell have led scientists to occasionally describe the kiwi as an ‘honorary mammal.’ These distinctive characteristics are most likely due to the lack of mammals acting as predators in New Zealand during the evolution of the kiwi thousands of years ago. This in turn allowed it to develop in such an uncommon manner.
There are five acknowledged kiwi species in New Zealand: the Great Spotted Kiwi, the Little Spotted Kiwi, the Okarito Brown Kiwi, the Brown Kiwi and the North Island Brown Kiwi. The three species of the Brown Kiwi have been determined through DNA analysis, geographic and ecological distribution and behavior. The Little Spotted Kiwi no longer exists on either of the two main islands, with only small populations remaining on predator free areas offshore. Each of the five accepted kiwi species are listed as endangered with a conservation statuses of either ‘nationally critical’ or ‘seriously declining’ in New Zealand. The estimated overall population of this bird has declined from 100,000 in 1998 to less than 70,000 in 2008. Despite numerous conservation efforts since the kiwi was deemed endangered, the species’ continued survival remains fragile.
Reasons for Decline: Predators
It is widely accepted that the arrival of European settlers in New Zealand inevitably led to the demise of the kiwi. Upon examination of the reviewed sources, it is seen that this was the most powerful impetus in the ultimate decline of kiwi species. With the arrival of European settlers in New Zealand in the 17th and 18th centuries came foreign, exotic species; namely weasels, cats, pigs, dogs, ferrets and other mammals that the kiwi was unable to coexist with (Sales, 2005). These animals were brought over by Europeans attempting to cultivate farms in areas of the main islands with substantial kiwi populations. The colonization of New Zealand brought about such pests that would attack kiwis, destroy their eggs and take over reproductive zones (Robertson, Colbourne 2001). Pryde and Cocklin et. al. (1998) explains the negative effects of introduced mammals on many endemic species in New Zealand. Since the remote island nation had a lack of native mammalian predators to begin with, flightless birds such as the kiwi could nest in ground communities without imminent predator danger. However, as Pryde and Cocklin et. al. (1998) explains, the flightless kiwi would become an easy victim to introduced ground predators. They cite the European stoat, originally meant to eliminate rabbits, as a ground predator that wreaked havoc among the kiwi population.
Basse and McLennan et. al. (1997) conducted a study evaluating the predation thresholds for survival of all five kiwi species. A particular species is subject to heavy predation at some point during their life cycle, where evolved reproductive and behavioral strategies may counteract or compensate for such predation losses. However, looking into the history of the kiwi and the factors surrounding its decline, Basse and McLennan et. al. (1997) concludes that sudden environmental changes in New Zealand led to a dramatic increase in predation rates. They state, “The endemic northern brown kiwi evolved for about 70 million years in the absence of mammalian predators, but it now coexists with up to seven carnivores introduced by humans over the last eight-hundred years” (Basse and McLennan, 1997, p. 241). Subsequently, the abundance, range and survival of all five kiwi species continue to be severely comprised on the mainland of New Zealand. In the same study, 42 dependent kiwi chicks were examined for three weeks in order to determine the affects of predation. Predators killed 10 of these chicks while 12 died of natural causes. After the three-week examination period, it was discovered that of the 20 remaining chicks, predators killed 17 while 2 died of natural causes (Basse and McLennan 1997). It is apparent from this study that introduced predators caused major problems in the birth and dependency of newborn kiwi chicks.
In a similar study, Potter and McLennan et. al. (1996) examines the role of predation on adult species of kiwi in New Zealand. Using radio-tracking devices, 209 adult kiwis of mixed species were monitored with 14 being killed during the extent of the study. The report also found that mortality rates of the overall population did not differ between adult species or sexes and that dogs and ferrets were the main predators of the adult kiwi species. The Potter and McLennan et. al. (1996) study of the adult kiwi mortality rate reveals an important notion regarding its endangerment. The mortality rate of adult kiwis due to predation is only 5-16%, which is not necessarily a startling figure. However, this study goes further in depth in examining the role of predation among kiwi chicks and juveniles. Potter and McLennan et. al. (1996) estimates that predators killed 8% of chicks, 45% of juveniles and as many as 60% of young kiwi species during the study. Models indicate that kiwi populations experiencing such predation will decline at rates of over 6% annually without conservation efforts. It can be concluded from the Potter and McLennan et. al. (1996) study that predators sustain the most damage on newborn and young kiwi population to a much greater degree than adult species. After kiwi chicks enter the juvenile stages, acquiring a greater range of movement, they become increasing vulnerable to predators.
This hypothesis is supported by McLennan and Dew et. al. (2004). In this study, the predation risk of North Island Brown Kiwi was investigated. Over 10-year duration from 1992-2002, 53 adult and 126 young kiwi were radio-tagged at Lake Waikaremoana and observed in order to determine causes of death, survival rates, and rates of growth. As expected, McLennan and Dew et. al. (2004) found that predation rates were lowest among adults, but severe for young kiwi chicks. The study states, “Young kiwi suffered intense predation from stoats during the first four months of life, but thereafter became too large (>800 g) for stoats to kill” (McLennan, Dew, 2004, 241). The Potter and McLennan et. al. (1996) and McLennan and Dew et. al. (2004) studies reveal that predation among kiwi species is most imminent during the juvenile stages of the bird’s lifetime. The kiwi is most vulnerable to stoats and ferrets as predators in the time period between birth and adulthood. McLennan and Dew conclude, “Predation is limited largely to young kiwi and it is losses at this stage of life that are causing populations to decline” (McLennan and Dew, 2004, 242).
The introduction of foreign mammalian predators in New Zealand has indubitably led to a major decline in the kiwi population. However, the introduction of exotic mammals would have never been possible without the human colonization of the islands hundreds of years ago. The South Pacific nation’s remote island position, combined with a relatively low population density would lead one to believe that environmental problems would be rare. However, extensive anthropogenic modification over hundreds of years has inevitably led to forest, land and water degradation (Glasby, 1986). This widespread anthropogenic manipulation of New Zealand’s natural environment is yet another crucial reason for the sharp decline in kiwi species. Exploitation of natural resources, including deforestation, has destroyed much of the kiwi’s natural habitat.
Beattie et. al. (2008) describes New Zealand’s environmental deterioration. Over a 700 year period, native forest cover has been plummeted from covering 80% of the country to only 25% of the land area. This has been extremely detrimental to the kiwi, which depends on lowland forests for survival and reproduction. Taborsky et. al. (1995) also notes that deforestation and environmental exploitation in New Zealand has antagonized the fierce territorial nature of kiwi species. Destruction of such kiwi habitat has inevitably led to increased competition for breeding areas, food and living territory (Taborsky, 1995). Furthermore, destruction of the kiwi’s natural forest habitat has pushed the bird into farmlands where it is increasingly vulnerable to mammalian predators (Taborsky et. al. 1995).
North and McLennan et. al. (1997) examines the negative affect of habitat loss on declining kiwi populations. This study, conducted at Lake Waikaremoana on New Zealand’s North Island, compared the incidence of predation on Brown Spotted Kiwi species after the construction of a dam. Lake Waikaremoana is an area known for its abundance of Brown Spotted Kiwi that is currently declining at high rates. North and McLennan et. al. (1997) relates such a drop in kiwi population with lowered lake levels after the construction of the dam in the late 1920’s. It is noted that since the original dam was built, three separate power stations have been constructed along Lake Waikaremoana, requiring moderate forest clearing. Since the construction of the dam, North and McLennan et. al. (1997) states that lowering the lake has exposed nearly 300 hectares of lakebed and beaches. This has led to the creation of large expanses of soft sediment uncovered in areas of rivers and streams where colonization of introduced tall weeds and grasses has been witnessed. North and McLennan et. al. (1997) states that colonization of introduced and native grasses in unnatural land expanses has created new types of habitats that would not exist if the Lake had stayed at its natural levels. It states, “The grasslands differ fundamentally from forest, and introduce a pastoral element into an otherwise native ecosystem. The area covered by grasses is small, but the habitat is distributed widely around the lakeshore, forming a network of pockets and ribbons” (North and McLennan, 1997, 12).
Such exposed areas of land have allowed introduced mammalian species to occupy these expanses on the shores of Lake Waikaremoana. North and McLennan et. al. (1997) remarks that this is leading to the destruction of kiwi species in this area. Trapping data from the conducted study shows that weasels, ferrets and cats, all major known kiwi predators, mainly live on the developed and exposed lakebed areas. Interestingly, North and McLennan et. al. (1997) found that this did not affect the mortality rates of young kiwi due to predation, but were associated with increased death rates among adult kiwi. The average mortality rate among adult kiwi is higher in Waikaremoana because of the increased prevalence of ferrets acting as predators. If the adult kiwis killed by ferrets are removed from the study’s sample, annual kiwi death rates drop from 13.7% of the population to 5.5% (North and McLennan, 1997, 13). The study concludes that the construction of the dam at Lake Waikaremoana created perpetually increasing land expanses, allowing unnatural habitats of introduced mammals to develop. These exposed habitats, which support predator ecosystems, have undoubtedly led to the demise of the Brown Spotted Kiwi species in this region of New Zealand.
Examining the data thus far gives a clearer look into the downfall of kiwi species in New Zealand. It appears that anthropogenic influence on the physical environment over the past 500 to 800 years has ultimately led to the decline of the kiwi. This is a dynamic and complex case of human exploitation leading to the endangerment of a species. With the arrival of Europeans in New Zealand came massive efforts of deforestation and natural habitat destruction in order to facilitate the development of human civilization. This includes the formation of farms, where the clearing of forests, compounded by the introduction of exotic species such as dogs, stoats and cats led to the demise of kiwi species. The Europeans introduced of a breadth of mammalian species that flourished in New Zealand’s natural environment that had an availability of vulnerable endemic prey such as the kiwi.
Elimination of Predators & Conservation Efforts
The demise of the kiwi has been described as an ‘ecological disaster’ and a ‘national embarrassment’ in New Zealand. The flightless bird’s unique qualities, endemic nature and cultural status have propelled the development of numerous conservation efforts such as the Kiwi Recovery Programme established in 1991 (Sales, 2005). Over the past three decades, protected kiwi areas have been designated throughout New Zealand. Many of these areas include offshore islands free of mammalian predators. Although isolation of kiwis on offshore islands has proven to be an effective method of conservation, natural regeneration of the species relies on the development of kiwi communities on the mainland.
Basse and McLennan et. al. (2003) describes the development of such kiwi reserves on the mainland of New Zealand. The preservation of the kiwi relies solely on the elimination of introduced mammalian predators in such kiwi reserves. This immediate goal of establishing kiwi reserves on New Zealand’s main lands entails the elimination of stoats, ferrets and other introduced predators. Unfortunately, controlling pests has proven to be costly and difficult with mixed results as a method of kiwi conservation. Furthermore, Basse and McLennan et. al. (2003) states that kiwi species have large area requirements and that an ideal kiwi reserve would require nearly 11,000 hectares of land. It states, “Protected areas for them in continuous tracts of forest probably need to be in order of hundreds rather than tens of square kilometers” (Basse and McClennan, 2003, 102). The widespread elimination of predators throughout these vast land reserves would be a difficult and costly method of kiwi conservation.
Besides predator control, Sales (2005) describes alternative methods of conservation such as raising juvenile kiwis in captivity and then releasing them at their adult size, and the establishment of zoo-like kiwi sanctuaries. These methods require extensive human interaction and influence with kiwi species. Many of these approaches towards kiwi conservation are experimental and have yet to be proven as effective methods (Sales, 2005). This comes at a crucial time when numbers of kiwi species continue to drop at alarming rates. With the arrival of Europeans in New Zealand came the introduction of foreign mammals that would wreak havoc among the country’s kiwi population. The preservation of the endemic kiwi, a cultural centerpiece in New Zealand, depends on the continued elimination of these introduced mammalian predators. This notion has been supported by nearly ever piece of literature examined regarding the endangered status of the kiwi in New Zealand.
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