Biogeographical Change of the New Zealand Endemic Fauna Following the Introduction of Rat Species
Writer’s comment: Through the Education Abroad Program at the University of California, Davis, I spent two years of my undergraduate career at the University of Edinburgh in Scotland. While a student there, I made a slight departure from my previous studies in biology and took several geology and geography classes. By doing so, I began to better appreciate the critical link between the evolution and ecology of organisms and their changing physical environments. In particular, I developed a keen interest in New Zealand, for its geological history and unique wildlife epitomized the power of these processes. But it was only upon my return to UCD and enrolling in EVE 147 that I was able to conduct thorough research into the subject with a biogeographical perspective. The following paper presents the results of this work, as well as how the findings relate to the urgent need for conservation.
Instructor’s comment: Biogeography (Evolution and Ecology 147) is a very synthetic course in which biology and geology are thoroughly integrated in order to explain spatial patterning in the distributions of organisms. A “long” term paper is required. Students select their own topics (a list of selected titles of previous papers is provided, only as an indication of the sorts of topics that might be appropriate), which must then be approved by me. Katie chose to write about human impacts on the New Zealand biota, focusing primarily on the adverse effects of rats (which arrived early, as ubiquitous human commensals). With concern rising steadily about the impact of introduced species on native biodiversity worldwide, her topic is very timely. In fact, UCD wildlife biologists have very recently shown that roof rats (Rattus rattus) are responsible for major declines in the reproductive success of arboreal songbirds right here in the California Central Valley!
—Arthur M. Shapiro, Evolution and Ecology
The fauna found today on the islands of New Zealand represent two groups of organisms that have been in conflict since humans arrived nearly a thousand years ago. Comprised of unique animals such as flightless birds, enormous insects, and rare reptiles, the struggling assemblage represents millions of years of isolated evolution and is like none other in the world. Unfortunately, a dominant introduced population of human commensals, including cats, dogs, weasels, ferrets, and pigs, has posed a serious threat to this fascinating endemic fauna. Arguably, three species of rat have had the most detrimental impact, significantly altering the biogeography of a wide range of native species. They have established themselves on the majority of islands in the archipelago, independently caused the extinction of some indigenous species, and reduced the distribution of many others. The birds, insects, and lizards of New Zealand have thus far been fighting a losing battle against the rats, but recent conservation and restoration has improved hopes that these organisms will not be lost.
Geohistory of New Zealand
The evolution of this unique set of endemic organisms, including moas, kiwi, tuataras, and wetas, was dependent on the distinctive history of the islands themselves (Hickson et al . 2000). A simplified overview of New Zealand’s substantial climatic and tectonic change in the geologic past is presented here. The islands of New Zealand are continental in origin, having been part of Gondwanaland 90 million years ago. They joined present-day Australia and East Antarctica through now submerged continental shelves, such as the Lord Howe rise and the Campbell Plateau, and were situated within the Antarctic Circle (Cooper and Millener 1993). Then, about 82 million years ago, the region began to drift away from the megacontinent as oceanic crust formed through plate spreading, creating the Tasman Sea and South Pacific Ocean in existence today. Thus, some organisms were isolated on New Zealand through this vicariance event (Hickson et al. 2000). Other species later crossed these water barriers and successfully dispersed to the islands, which are presently 1300 km from Australia (Cooper and Cooper 1995).
In the past 40 million years, New Zealand has experienced cycles of warming and cooling that have in turn influenced the evolutionary and biogeographical processes of both the flora and fauna. Up until the Oligocene, the region was virtually inactive tectonically, and denudation reduced the elevation of the landscape (Cooper and Millener 1993). Warmer climates also dominated at this time (Gill 1991). Then about 30 million years ago, nearly 80% of the land exposed today was submerged under the rising ocean. Mitochondrial DNA evidence from several bird species shows a clear restriction of genetic diversity at this time, an indication that terrestrial organisms experienced a genetic bottleneck (Cooper and Cooper 1995). During the Miocene and the Pliocene, the climate fluctuated while uplift along a transpressional fault created mountain ranges like the Southern Alps (Cooper and Millener 1995). Glacial expansions and regressions feature in the Pleistocene, a cycle that continues in the present.
Prior to human settlement, the islands of New Zealand supported no mammalian predators (Atkinson 2001). The first to arrive was Rattus exulans, also known as the Pacific rat or Kiore rat in reference to its known origins in Polynesia (Atkinson 1973). This event occurred between 1200 and 1400 AD, dates estimated by studying anthropological fire evidence (McGlone and Wilmshurst 1999). It is believed that when the Maori people colonized New Zealand via watercraft, they cleared large stretches of dense, long-lived forest through burns (Roberts 1991). Before this time, natural fires were extremely rare in the moist environments covering the majority of the archipelago. This ecological shift consequently appeared in carbon and pollen records, which were both analyzed to determine this date of colonization and rat introduction.
The Pacific rat many have been the first to be introduced to New Zealand by humans, but it was not the last. In the late 18th century and coinciding with Captain Cook’s voyages to New Zealand, Norway rats (Rattus norvegicus) appeared on the islands. These brown rats or water rats may have also come to the region aboard sealing and whaling vessels that stopped at the archipelago or shipwrecked there (Atkinson 1973). Finally, in the mid-19th century, ship rats (Rattus rattus) also arrived via steamships that better suited their need for a warmer transportation environment (Atkinson 1973). The ship rat is also known as the roof rat, black rat, and bush rat.
These three species of rat now comprise a large portion of the predatory force found in New Zealand. They consume everything from beetles, weta, weevils, and spiders to lizards, eggs, and chicks, eliminating many organisms directly (Moors et al. 1992). In addition, this typical rat diet overlaps with the diets of many indigenous species, thus indirectly reducing their numbers through competition (Cree et al. 1995). Furthermore, there is evidence that the rats have affected vegetative species as well by eating bark, seeds, and seedlings (Towns et al. 1997). Acting as keystone species, R. norvegicus and R. rattus have disrupted entire ecosystems by respectively roaming the ground floor and taking to the trees (Gibbs 1998). R. exulans has also done extensive damage to the endemic fauna. While these rats have altered the biogeography of virtually all the species in New Zealand, this paper will focus on a small selection of birds, reptiles, and insects.
The presence of rats in New Zealand has been a key factor in reducing the population sizes of numerous bird species, and in the extinction of others. For instance, 32 species were lost after the Maori introduced the Pacific rat. This included 11 species of moa, the only entirely wingless bird (Gill 1991). Another 9 bird species disappeared with the introduction of Norway and ship rats by the Europeans (Dowding and Murphy 2001). While many factors, such as habitat destruction, hunting, disease, climate change, and other introduced species are partially to blame, the presence of rats seems to have been one of the most destructive elements (Dowding and Murphy 2001, Cree et al. 1995).
Before the three species established themselves, native birds were abundant in a wide range of habitats on New Zealand, from the forests to the coast. In part, this situation arose because different species have different evolutionary backgrounds; some are descendants of ancestral birds of Gondwana, while others arrived via later “sweepstakes” dispersal events (Cooper and Millener 1993). The broad distribution of birds has also been attributed to the lack of mammalian predators on the islands for millions of years (Atkinson 2001). When they finally did arrive, most of the avifauna were highly vulnerable for one reason or another, so bird populations shrank (Moors et al. 1992). Although the rats have affected a wide array of avian species, this section will focus on a forest bird, a scrubland bird, and shorebirds.
The Mohua, a Forest Bird. One bird whose distribution has been reduced by rats is the mohua or yellowhead (Mohoua ochrocephala). A small passerine found only on the South Island of New Zealand, it inhabits the forests and feeds primarily on insects (O’Donnell 1996). Records from the past 150 years or so show that mohua were previously present in almost all of the forested areas of the island, but now can only be found in less than 25% of these regions. The declining numbers were recognized in the late 19th century, but despite this, many small populations have gone extinct in the past 20 years. Specifically, rats have been able to easily prey on this species because the birds build their nests in holes with a single entryway. This provides rats with the accessibility and the ability to consume incubating adult females as well as the chicks and eggs. In addition, mohua chicks are easy for rats to locate because of their loud vocalizations, and because they stay in susceptible nests longer than most other passerines (O’Donnell 1996).
The Kakapo, a Scrubland Bird. Surely one of the most bizarre birds in the world is the kakapo (Strigops habroptilus), and its very existence is highly threatened by introduced rat populations. This solitary parrot is nocturnal and flightless and is the only parrot that engages in lek displays to find mates (Moorhouse and Powlesland 1991). They feed on many forms of vegetation found in forests, scrublands, and subalpine areas, regions kakapos originally inhabited all across New Zealand (Clout and Craig 1995). With the arrival of rats, their distribution diminished until only Fiordland and Stewart Island supported small populations (Clout and Merton 1998). Today, the parrot is extinct from these areas and only 57 translocated individuals survive on offshore islands with primarily scrubland habitat, including Maud Island, Codfish Island, and Little Barrier Island.
The extent of rat predation on kakapos was only recognized with the attempts at conservation through translocation. On two of the three islands that populations were moved to, Pacific rats were identified as consumers of nestlings (Clout and Merton 1998). Given this observation, previous attacks by rats seem likely. Furthermore, the kakapo exhibits several traits that indicate its vulnerability to rat predation. First, the females build nests on the ground. Second, the young stay in the nest for ten weeks before gaining independence. Further increasing their susceptibility to rats, the nestlings are often left unattended while the mother forages at night for hours at a time (Moorhouse and Powlesland 1991). Finally, males may attract rats through their loud and flashy displays during leks, although this has not been specifically investigated.
Shorebirds. Another group of birds challenged by the three rat species is the suborder Charadii, which is composed of 21 species and subspecies of shorebird. Three of these are now extinct. Of the extant members, the oystercatcher Haematopus chathamensis, the stilt Himantopus novaezelandiae, the plover Charadrius obscurus, and the snipe Coencorypha aucklandica “Jacquemart Island” are all highly endangered with populations numbering about 100 individuals each (Dowding and Murphy 2001). They are all now each confined to single offshore islands or island groups despite once being found widely along New Zealand’s coastlines. In particular, Norway rats have been a significant factor in the creation of this disjunct distribution, as they are predators of many shorebirds. For example, some plover and stilt populations have suffered up to 70% losses in the presence of this rat (Dowding and Murphy 2001). Ship rats have clearly done damage as well, having caused the extinction of a snipe species within years of its arrival on Big South Cape Island in 1962 (Towns et al. 1997).
The traits underlying shorebirds’ susceptibility to rat predation are quite similar to those of other New Zealand species. They are larger in size, slow to reproduce, ground-nesting, and slow to mature (Moors et al. 1992). Also, some species exhibit a deadly naivety in regards to potential predators; they lack behavior to effectively repel rats or fail to build nests in protected environments (Dowding and Murphy 2001). On the other hand, some characteristics of shorebirds have helped them avoid complete eradication. Some have camouflaged eggs, which evolved to protect them from endemic predators like the southern black-backed gull and the Australasian harrier. Also, many species feign injury or create distractions, probably reducing predation by rats even further (Dowding and Murphy 2001).
Understandably nicknamed “demon grasshoppers,” weta are large flightless insects that range from 4 g to 42 g in weight (Gibbs 1998). These stenopelmatid orthopterans fall into three categories with varying numbers of species: 7 species of tree weta, 11 species of giant weta, and 2 species of tusked weta. Between these, they exhibit a wide variety of lifestyles and inhabit many environments (Morgan-Richards and Gibbs 2001). In general, weta are believed to have once been found primarily distributed in various mixed forests of hardwoods and podocarps, but in recent times have shown a preference for introduced gorse (Sherley 1998). This plant appears to provide protection for the weta from predators, including rats. Records or evidence of the former distributions of weta do not exist or are unreliable (Gibbs 1998). The most recent analysis of both morphological and genetic data suggests that weta populations found today arose through multiple invasions of New Zealand (Morgan-Richards and Gibb 2001).
The pronounced decline in numbers of weta is the direct result of rat presence in New Zealand. On the islands today, weta tend to only be found where mammalian predators are not (Morgan-Richards and Gibb 2001). Large, flightless, slow-moving, and nocturnal, wetas are especially vulnerable targets for both Norway and ship rats. Additionally, their dependence on pheromone communication apparently makes them quite “smelly” to rats and reveals their location (Gibbs 1998). Their high fecundity fails to boost their numbers as it is counteracted by an exceptionally slow life cycle of 18 to 36 months to adulthood (Gibbs 1998). Weta such as Deinacrida rugosa and D. heteracantha have suffered rat predation for all these reasons. Furthermore, the threatened species Hemidiena crassideus exemplifies how nomadic behavior and failure to remain protected in a single shelter can lead to rat predation. Alternatively, some species are not as vulnerable, living in areas where rats are not found (Gibbs 1998). For instance, D. parva of the Kaikowra Ranges lives 1200 to 1600 m out of the altitudinal range of rats, and the bluff weta (D. n. sp. 2) protects itself by living in narrow rock crevices.
Tuatara. The two extant species of tuatara, Sphenodon punctatus and S. guntheri, are burrowing, nocturnal reptiles endemic to New Zealand. Although no adequate fossil evidence has been found antedating the Pleistocene, they are considered to have originated on Gondwana (Towns and Daugherty 1994). Having most likely dispersed from Antarctica, they have inhabited many islands of New Zealand since the Jurassic (Bell et al. 1985). Their numbers reached their height during the Mesozoic but have been on the decline (Cree et al. 1995). Today they are disjunctly distributed on mainly offshore islands (Newman and McFadden 1990). Originally, it was assumed that the species’ most recent reductions in distribution stemmed from the presence of Europeans. Because the Maori considered tuataras spiritually important, they clearly did not purposely direct harm towards the reptiles. Unfortunately, the rats they inadvertently introduced did (Cree et al. 1995).
The Pacific rat devastated entire populations of tuatara on the islands, thus taking great responsibility for the losses. The numbers of tuatara found on islands inhabited by these rats are typically less than on ones that are not (Cree et al. 1995). Although this is circumstantial evidence, the correlation cannot be ignored and provides significant evidence that the Pacific rat influences tuatara population dynamics. More direct evidence of this species’ influence has been observed on the mainland where rates of recruitment have fallen in its presence (Newman and McFadden 1990). On the other hand, Pacific rats have been known to coexist with these reptiles, but the exact reasons remain unclear (Cree et al. 1995). Perhaps this can only occur on larger islands, or in regions where a greater number of species interact.
Obviously the Pacific rat has been destructive to tuataras, but later rat introductions have done their fair share of damage as well. In 1982, for instance, Norway rats were introduced to Whenuakura Island and in a matter of two years eliminated an entire population of 130 individuals (Towns et al. 1997). Also, the diet of the tuatara is restricted to beetles, spiders, weta, insect larvae, and other reptiles, so the tuatara must compete with rats for food (Ussher 1999). Finally, rats are known to consume shorebirds and their eggs (see above), indirectly undermining the tuataras, which are dependent on several species of petrels and shearwaters (Bell et al. 1985). These birds provide burrows and food for these reptiles.
Geckos and Skinks. On New Zealand, the extant lizard fauna is comprised of 30 skinks (Scincidae) and 29 geckos (Gekkonidae) while 3 species appear to be extinct (Towns and Daugherty 1994). As with other hepetofauna of New Zealand, their prehuman distributions are difficult to determine due to the lack of fossil evidence. Although it was once speculated that the lizards arrived about five million years ago, a recent genetic study of skink phylogeny suggests that they actually began diverging over 20 million years ago (Towns et al. 1985, Hickson et al. 2000). The hypothesis suggests that at this time, sea levels transgressed, many islands formed, and allopatric speciation of skinks occurred. Either way, it is likely that in the Miocene both skinks and geckos dispersed to New Zealand via a land bridge from New Caledonia (Towns et al. 1985). Today, these lizards are primarily disjunctly distributed across the archipelago (Towns and Ferreira 2001).
In comparison with the previously presented animals, the biogeography of these lizards in New Zealand does not seem as drastically influenced by the presence of rats alone. Despite this, the threat of rat populations to endangered skinks and geckos should not be overlooked. On several islands, the eradication of Pacific rats has facilitated the expansion of distributions of multiple species, including eight geckos and twelve skinks (Towns et al. 2001, Towns and Ferreira 2001). Although this is again circumstantial evidence that rats predate on these lizards, the pattern should not be overlooked. Also, the arrival of Norway rats on Motuhorpapa Island in the 1950’s was quickly followed by the disappearance of all lizards once found there (Towns and Daugherty 1994). Finally, Pacific rats are believed to have eliminated the skinks Cyclodina alani and C. macgregori from the mainland. These examples all show that rats may be involved in extinction events of New Zealand lizards.
Conservation and Restoration
An obvious theme running through the length of this paper that has yet to be directly addressed is the matter of conservation. All of the animals mentioned above, as well as endless others, are threatened on New Zealand. Only in the past 25 years have significant measures been taken to protect endemic species, starting with the Reserves Act of 1977. This legislative action created many island nature preserves throughout New Zealand, hopefully protecting them from further ecological damage (Moors et al. 1992). More recently, the Biosecurity Act of 1993 and the Hazardous Substances and New Organisms Act of 1996 have been implemented to halt introductions of dangerous foreign species (Vitousek et al. 1997). Finally, the Department of Conservation has made noticeable strides, from prioritizing endangered species to initiating a series of restoration programs (Cree et al; 1995, Dowding and Murphy 2001).
In regards to rats specifically, extensive measures have been taken to stop their negative impact on the endemic fauna. Although concrete evidence of their influence is often lacking, the indication of rats’ predatory and competitive behaviors towards native species is strong enough to justify these actions (Cree et al. 1995). First, rats have been entirely eradicated from nearly 60 islands through poisoning and trapping (Towns and Ferreira 2001). This is often in preparation for ambitious translocations of endangered populations to rat-free islands. The source of organisms comes from either natural populations or captive breeding programs. Some species, such as kakapo, tuatara, lizards, and weta, have responded positively to this artificial restoration (Towns et al. 2001, Sherley 1998, Clout and Craig 1995). For these improvements to persist into the future, both native and introduced species need to be carefully monitored as well as more thoroughly researched. Most importantly, the reintroduction of rats must be prevented (Dowding and Murphy 2001).
Although introduced rats are far from the only factor affecting the biogeography of the endemic fauna of New Zealand, they have been shown to be an important one. Unfortunately, the understanding of the rats’ role in reducing animal distributions often relies on indirect evidence. Clearly, further field research on the three species could identify more unambiguous cases of rat competition and predation, rectifying this shortcoming. Furthermore, the ecology and evolution of the native fauna need further investigation in order to more accurately judge the impact rats have on their populations. Also, continued genetic studies using modern techniques could eventually clarify their biogeographical histories, consequently improving conservation and restoration efforts. Through these efforts, the goal of expanding the distribution of some threatened endemic species may indeed be realized.
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