Tuatara: Volume 17, Issue 1, May 1969
Extinction and Endemism in New Zealand Land Birds
Extinction and Endemism in New Zealand Land Birds
* Present address: Fisheries Research Division, Box 19062, Wellington.
In Both Pre-Human Times, and since the arrival of man about 1,000 years ago, many New Zealand land birds have become endangered or extinct. Some controversy has developed about the importance of man, and his manifold influences on the environment in causing these changes (Myers, 1923, Greenway, 1958, Williams, 1962, 1964, Fleming, 1962a, b). One aspect of this problem, which needs study, is ‘why did certain species become extinct or endangered while others persist?’ We need to know whether extinctions have occurred randomly, or whether the extinct species have something in common which may indicate possible reasons for the extinctions. If some such common characteristic can be found, then I think we have begun to understand the extinctions.
Data and Analysis
* Present address: Fisheries Research Division, Box 19062, Wellington.
Williams (1962) listed the birds in New Zealand which are extinct, or which, in his opinion, are in serious danger of extinction; I have adopted Williams' listing, and dealt with the following orders:— Dinornitbiformes, Apterygiformes, Podicipitiformes, Anseriformes, Psittaciformes, Galliformes, Gruiformes, Strigiformes, Columbiformes and Passeriformes. I also followed Williams in examining separately, the status of the species in the North, South, Stewart and Chatham Islands. To some extent, at least, each island constitutes a separate faunal area within the New Zealand region, since there are endemic species or subspecies in each and extinctions have occurred independently in each.
Oliver (1955) showed that most if not all, of the extinct land birds have been found as subfossil remains, or in moa excavations, and Fleming (1962a) reached a similar conclusion. If these extinct species are regarded as having survived until even the last two thousand years, it seems reasonable to regard their extinctions as contemporary in the geological time scale. On this basis I am taking the extinctions recorded by Oliver, Williams and Fleming, as having occurred in a contemporaneous bird fauna.
In analysis, I have listed separately the bird faunas of the North, South, Stewart and Chatham Islands. From this list, for each area, I determined the level at which each species is endemic to the New Zealand region, i.e. whether it belongs to an endemic order, family, genus, species, subspecies, or to a nonendemic subspecies; and I also noted whether each species has been recorded as either extinct or endangered, or is not so recorded. Then within the list for each island, I determined the percentage of species, endemic at each of the above taxonomic levels, which is in the extinct or endangered category (table 1).
Fleming (1962a) estimated approximate arrival times in New Zealand of the ancestors of the endemic orders, families, genera etc, as follows:— endemic orders in the late Cretaceous, more than 70 million years ago; families 25 to 70 million years ago; genera one to 25 million years ago; species 15,000 to one million years ago; subspecies 1,000 to 15,000 years ago; nonendemic subspecies in the last 1,000 years, some since the European settlement of New Zealand, about 120 years ago. I have taken the midpoints of these time intervals, such that the times for each are, say, 85 million years (order), 47½ million years (family), 12 million years (genus), 507,000 years (species), 8,000 years (subspecies) and 500 years (nonendémic subspecies). In fig 1, these levels were inserted along a log scale, such that the proximity of the taxa to each other is in accord with their time of arrival in the New Zealand region, as projected by Fleming (1962a).page 3
|Level of endemism||North Island||South Island||Stewart Island||Chatham Islands||Cumulative Total|
|Order **||Species endemic at this level||16||19||2||–||36|
|Species endangered or extinct||15||93.8||16||84.2||1||50.0||–||31||86.1|
|Family ***||Species endemic at this level||6||5||3||–||14|
|Species endangered or extinct||5||84.2||4||80.0||2||66.7||–||11||78.6|
|Genus||Species endemic at this level||17||21||10||13||61|
|Species endangered or extinct||9||52.9||11||52.4||3||30.0||13||100.0||36||59.0|
|Species||Species endemic at this level||11||11||7||7||36|
|Species endangered or extinct||3||27.3||4||36.4||1||14.3||6||85.7||14||38.9|
|Subspecies||Species endemic at this level||8||8||4||3||23|
|Species endangered or extinct||1||12.5||1||12.5||0||0.0||2||66.7||4||17.4|
|Nonendemic||Species endemic at this level||6||6||6||6||24|
|Subspecies||Species endangered or extinct||0||0.0||0||0.0||0||0.0||0||0.0||0||0.0|
* percentage of species endemic at each level
** Apterygiformes, Dinornithiformes
*** Callaeidae, Turnagridae, Xenicidae
The New Zealand Bird Fauna
The New Zealand land bird fauna is small, comprising only about 97 species. Fleming (1962a) postulated that since the late Cretaceous (at which time he considered that the moas and kiwis — orders Dinornithiformes and Apterygiformes — were in New Zealand), the fauna has been built up by immigration. The geographic relationships of the fauna were analysed by Falla (1953). The migratory and sea birds have extremely diverse relationships with southern temperate, Antarctic, Indo-Pacific, Holarctic and other, widespread species. The more recently arrived land birds are almost exclusively Australian, but the relationships of the older, endemic orders and families of birds are presently unclear. How the first, flightless birds arrived in New Zealand is a perplexing page 5 question of great interest and considerable debate. Fleming (1963) thought that the moas (and kiwis?) may have walked to New Zealand; Mayr (1963) and De Beer (1956) suggested that they may have evolved flightlessness after flying there. Apart from the moas and kiwis, there is no reason to presume that birds have arrived by any means other than by flying across the sea.
The number of invasions which has contributed to the present New Zealand bird fauna can be estimated by determining the number of phylogenetic stocks that has evolved in the New Zealand region — the number of species in each genus and the distribution patterns of the genera and species, e.g. nonendemic species and nonendemic genera with only one species in New Zealand have clearly dispersed to New Zealand and have their closest relatives elsewhere; nonendemic genera with several endemic or nonendemic species could have dispersed one or more times. Fleming (1962a) considered that the moas and kiwis may represent two invasions, and each of the endemic families of passerines seems likely to represent a separate, single invasion. With increasing endemicity and age, these estimations become increasingly difficult and less reliable, but the figures do provide some insight into the manner in which the fauna has been derived. In table 2 I have listed estimates of the maximum and minimum number of invasions which could have built the fauna. Compared with the total number of species (97), the number of invasions (44-59) seems to be very high; the ratio of invasions to species is around 0.5:1.0. And if we eliminate the speciose moas from consideration, there are about 70 species produced from between 43 and 58 invasions, and a ratio of 0.61-0.84:1.0, i.e. the figure for invasions is only a little lower than that for species in the fauna.
Apart from the moas (27 species), the bird groups invading New Zealand have not radiated or diversified to the extent seen in the birds of other islands and island archipelagoes (like the page 6 Galapagos, Hawaiian and Madagascar bird faunas). The kiwis, for which Fleming (1962a) projected an arrival date of late Cretaceous, are known from only three species. The Callaeidae, Xenicidae and Turnagridae, which may have arrived as early as the beginning of the Tertiary, though probably more recently, are represented by only three, three and one species respectively. There are no fossils, so that we have no idea how large the fauna was in former times, but the families rarely consist of highly diverse genera, no two families appear to have a common ancestry in New Zealand (except perhaps the two groups of ratites), and there is little evidence that the present fauna is a remnant of a formerly larger and more diverse fauna. Amongst the recent invaders, there is little speciation. The parrot genus Cyanorhamphus has four New Zealand species, Petroica has three and these are most diverse genera in the fauna. Thus it seems that the fauna has grown almost entirely by periodic invasion from Australia; a few of the invaders have split into several endemic species, mostly, it seems, by isolation on islands, and otherwise the endemic species have evolved phyletically.
Falla (1953) reported that the fauna is disharmonious. Although one tends to think of birds as easily able to overcome barriers to dispersal, Mayr (1964) suggested that birds differ drastically in their ability to cross water gaps, and found that large birds like ducks, herons and hawks cross water gaps easily, but that members of other families, especially small, forest-inhabiting land birds may ‘respect zoogeographic barriers extremely well’. Mayr (1965) showed that though there are 60 families of birds in New Guinea, only 16 of these make significant contributions to the bird fauna of Polynesia. Such differences in dispersal ability from group to group, are supported by an examination of the structure of the New Zealand bird fauna. It is especially true that the forest inhabiting species have diversified to produce subspecies on the various islands of the New Zealand region, suggesting little dispersal, even within the region. And comparison of the compositions of the bird faunas of Australia (donor) and New Zealand (recipient), suggests that differences in dispersal ability have profoundly affected the structure of the New Zealand fauna. In table 3, I have listed (from Keast, 1961) the number of species in the Australian fauna in each of the orders of birds which have representatives in the New Zealand fauna. Comparison of the percentages which each order contributes to the New Zealand and Australian faunas shows that the New Zealand fauna is by no means a random sample of the Australian, but that certain orders are over-represented, e.g. the Anseriniformes, while others are very much under-represented, e.g. Passeriformes. While it is likely that there are ecological reasons for some of the differences, it seems likely that variations in dispersal ability have also made important contributions. In addition to the small amount of page 7 speciation (as distinct from phyletic evolution) that has occurred in New Zealand, the structure of the bird fauna has been controlled first by the structure of the contributing Australian fauna and second by the ability of these species ‘available’ for dispersal, to disperse across the Tasman Sea. Although the New Zealand fauna represents a fragment of the Australian, it is certainly not a randomly derived fragment.
|Order||No. species||%||No. species||%|
Although there is bound to be some debate about how critical the state of some species is, many New Zealand birds are extinct, or considered to be in danger of extinction. The 27 or so species of moa are all extinct, but otherwise the extinct or endangered species are scattered throughout the various orders. The quite well represented rails and ducks have been seriously affected, also some families of passerines. On a regional basis, much extinction has occurred in the Chatham Island fauna and comparatively little in Stewart Island. The faunas of the more substantial land areas in the North and South Islands have extinction rates intermediate between those for the Chatham Islands and Stewart Island (see fig. 1). The reasons for these differences are not obvious, although for the Chathams, the high rate may be related to fragmentation and flooding of a formerly more extensive land area (see Fleming, 1962c). The isolation of the Chathams is such that extinctions are not easily remedied by invasions from mainland New Zealand. This is not the case in Stewart Island, where species or subspecies not endemic to the island can reinvade easily from the South Island.
* Not listed by Keast, 1961.
The value of the data at the ordinal level is of doubtful significance. When compared with the ratite birds anywhere else, New Zealand has an extraordinarily large number; but perhaps New Zealand acted as a centre for the radiation of large grazing birds, in the absence of grazing mammals and of mammalian and reptilian predators. And the land mass involved may have been significantly larger in the Tertiary than now. Sedentary, flightless birds may speciate more freely than their flighted relatives, but we can only conjecture here.
The foregoing results show that extinction has not occurred randomly in the New Zealand bird fauna, but that it is related in some way to evolutionary age. Species which have had a long evolutionary history in New Zealand seem now to be susceptible to extinction. This suggests some peculiarity in the evolutionary process in the New Zealand bird fauna, which in a time related manner affects the present viability of the species. It is acknowledged by several writers (e.g. Simpson, 1953, Darlington, 1948, Brown, 1957) that island evolved species, for a variety of reasons, tend to have ‘limited evolutionary potential’ (Brown, 1957), and this seems pertinent to the problem of island extinctions. MacArthur and Wilson (1967) have postulated that extinction may occur rapidly enough on isolated islands, to impose upper limits on the number of species present. Mayr (1965) noted that the bird page 9 fauna of New Caledonia is recent, with only one old endemic; this seems to be due to extinction, and replacement from the Pacific and Australia. It is not clear, though, whether there is any causal relationship between extinction and immigration. It seems unlikely that random extinction sufficed to eliminate all but one of the old elements in the fauna of New Caledonia, and perhaps reasonable to suppose that invasion of some species has at least accelerated the demise of others, if not actually initiating it. Such a supposition would seem to receive support from a comparison of the situations in New Zealand and New Caledonia. Both islands date from the Mesozoic, so have long been ‘available’ for the acquisition and development of bird faunas. But New Zealand is much larger and more isolated. If extinction is related to invasion, and is operating in a non-random manner with a bias towards older elements, one might expect that the greater isolation of New Zealand would produce a reduced invasion rate, thus less extinction and an older bird fauna. On the other hand, the greater area of New Zealand would operate to increase invasion and, in part, reduce extinction. New Zealand certainly has an older bird fauna. The fact that there is only one old endemic bird in New Caledonia suggests to me that extinction of a non-random nature may have occurred there, as in New Zealand, but that the process has been more rapid, because of New Caledonia's smaller size and its proximity to the Australian-Pacific bird faunas.
The present (i.e. last one or two thousand years) extinction rate for New Zealand birds must be higher than in former times. The existing rate of 45 recent extinctions (Williams, 1962) in a known fauna of 97 species (47%), with many more species endangered, and the significant bias towards the decline and extinction of the old, and survival of the recent elements, would have produced a much more recent fauna than we now have, had this rate been in effect for long. Clearly then, extinction has accelerated with the arrival of man in New Zealand, and perhaps because of his presence.
Fleming (1962a) postulated that there was much bird colonisation in the late Tertiary and little in the Pleistocene, but that the present invasion rate is high. Reasons for much invasion in the late Tertiary are not readily apparent, though a reduction during the Pleistocene is understandable. Falla's (1953) analysis showed that the fauna is largely of Australian origin, and we should expect few species to have been invading colder New Zealand from warmer Australia at a time when climates were deteriorating. Increased post-Pleistocene invasion may be related to rising temperatures, which would increase the favourability of New Zealand habitats for Australian birds.
In historic times there has been a remarkable amount of invasion. Fleming (1962a) recorded 12 species in the late Holocene prior to the arrival of the white man, and seven more established in the last page 10 100 years. Obviously this too is a higher invasion rate than could have been sustained through a long period — or the species which became established were transient occupants and quickly became extinct. However, there is no question that some of the historic arrivals are amongst the most abundant, aggressive and successful bird species in the fauna today, e.g. Zosterops australis, Notophoyx novaehollandiae. If the observed rate had been occurring long, saturation would have been achieved long ago, and the percentage of old endemics would probably be much lower than it is. Again, I think we are observing a high rate of colonisation as an ‘artifact’ of man's presence; this is due to the opening up of new habitats and increasing the niche diversity, as a by-product of agriculture and forestry, the maintenance of large areas of marginal (Wilson, 1965) or transient habitats and the establishment of new forest types. Examination of the record of stragglers to New Zealand, in Notornis (the journal of the New Zealand Ornithological Society), shows that a veritable stream of strange birds has been arriving in New Zealand from Australia and the Pacific, some species arriving repeatedly. Most of these have been solitary birds and few appear to have stayed long, but it has apparently only taken the provision of new habitats by man, to allow the species, which may have been straggling to New Zealand for centuries, to become established. The landscape of New Zealand has changed dramatically during the last 100 years, and an examination of the habitats of the recently established species will show the extent that man's activities have affected the rate of successful invasion. The welcome swallow (Hirundo neoxena) which is recently established in New Zealand, is reported breeding mostly in association with man; nests have been found mostly under bridges, but also in buildings, vehicles and boats (Edgar, 1966, Wagener, 1966, Turbott, 1965). How much the niches left vacant by earlier extinctions have been occupied by newcomers is not known.
Extinction rates on islands may have been high always, as a result of random genetic effects and lethal fluctuations in population size. The process of extinction in New Zealand has apparently been accelerated by the arrival of man, by his own predation, the introduction of other predators (mustelids, cats, dogs, rats, hedgehogs), the destruction and modification of existing habitats (deforestation and river disturbance), forest modification by browsing herbivores (many species of deer, plus pigs, goats and Australian opossums), and the introduction of many new bird species, with their diseases and parasites. All of these have probably played a role in initiating or accelerating the decline and extinction of New Zealand bird species. If extinction is primarily a product of the presence of man and his associated biota, then the extinction of the old endemics, which tend to be less active ground feeders, and weak fliers becomes more easily understood.
During the past one or two thousand years, there has been substantial decline and extinction of New Zealand land birds. The small land bird fauna is derived principally from Australia. Immigration, mostly if not entirely across the sea, began in late Mesozoic times and the fauna has been built up by immigration, followed mostly by phyletic evolution, with little speciation. The fauna is a fragment of the Australian fauna, selected by ability to cross oceanic barriers between Australia and New Zealand.
Analysis of extinction and decline of species, and of extinction and immigration rates shows that there has been much more extinction amongst the old endemics than amongst most recent species, and that both the extinction and immigration rates must be higher now than in pre-human times in New Zealand.
I am grateful to the following for reading and commenting on draft of the manuscripts:— Drs. K. J. Boss, P. J. Darlington, Allen Keast, Ernst Mayr, Giles W. Mead, G. R. Williams. I especially appreciate the helpful comments of Dr. Charles A. Fleming. The financial support of a New Zealand National Research Fellowship is acknowledged.
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