Differential Vulnerability in the New Zealand Vertebrate Fauna

Richard N. Holdaway, Palaecol Research

First, I'd like to thank the American Museum and the Center for Biodiversity for inviting another antipodean here to balance the Australian continent. I think New Zealand can balance it quite well, in that we have a unique system which can teach us quite a lot about the causes of extinction.

For those of you who are unfamiliar with South Pacific geography, New Zealand is 1,200 miles southeast of Australia. It's not far enough, but. . . . We took all the pretty terrain, and the mountains with us when we left -- New Zealand was the last major land mass on the planet to be permanently settled -- and that allows us to look very closely at what happened when people arrived, or when the influences of people arrived. People arrived via, apparently, eastern Polynesia, from the northwest; and there is some doubt as to whether anyone had made the trip from the northwest. I take New Zealand as the Keramadecs, including Norfolk, Chatham and the sub-Antarctic islands, so claim a large patch of the southwest Pacific.

New Zealand has the virtue, in this instance, compared to North America or Australia, in being a very diverse archipelago -- extending right from Norfolk down to the sub-Antarctic islands. The main islands straddle the west wind drift. The climate is, I like to think, warm temperate maritime. The winds deposit their rainfall on the western side of the islands which leaves a rain shadow in the east -- and particularly in the center of the South Island. So the original vegetation before humans arrived -- at least during the Holocene -- was dense podocarp and southern beech forests. The climate was very wet down the western side of the island, and over much of the North Island. On the east coast and in the interior, the vegetation was far more diverse floristically and had a simpler vertical structure. But it was a mosaic vegetation of shrubland and mixed forest, with very little grassland which was mainly confined to the high altitudes. The Southern Alps reach 12,000 feet and the rain shadow is dramatically shown by the change from a rainfall of 200-plus inches down to 40 inches within about 10 kilometers.

Around the main islands, particularly in the north and in Cook Strait, and the southern end around Stewart Island, there are many islands of varying size, all originally with forest vegetation. The Chathams group to the east consists of a main island and a group around Pitt Island. The Auckland Islands are to the south, with the largest land mass to the south of New Zealand. People arrived and settled throughout the main islands and on some of these offshore islands here. They reached the Kermadecs and Norfolk, and settled on the main Chatham Island and Pitt Island in the Chathams group.

The original fauna of New Zealand -- "original" meaning the Holocene fauna before human arrival -- consisted of something over 120 species of breeding birds, skinks, geckos, at least two species of tuatara, and three bats, excluding seals. By and large, the terrestrial ecosystems were based on birds.

New Zealand was and is extremely rich in oceanic birds. Although this talk is about birds, you should keep in mind that New Zealand also had a considerable herpetofauna (geckos and skinks). New Zealand also had some magnificent giant invertebrates -- which I won't say any more about except to show you the size of one of the large tree wetas, a giant flightless cricket about the size of a man's thumb.

The extinctions in New Zealand were dramatic. In the South Island, about a third of the breeding species before humans arrived became extinct -- and that's true whether you consider all species or terrestrial species. For the locally extinct species, it's the same order of magnitude. Globally extinct, the terrestrial species are, if you like, preferentially represented.

One of the main characteristics of the New Zealand fauna, in addition to its being a home of extinction, is that it's also an area where there were many extirpations and reductions in range and peripheralizations. The New Zealand fauna is now represented mainly on offshore islands. The species that are on the mainland -- and this is the species existing in the South Island. The breeding species of bird before Europeans arrived -- before humans arrived, rather -- broken down into uneven size classes, mass classes -- with red indicating the extinct taxa, blue the extant. And this group, which I've separated out, is relicts -- that is relicts when Europeans arrived.

These large animals, from 5 kilograms up to 200-plus kilograms, were the major herbivores -- the moas and the geese -- and the eagle which preyed on them. And their extinction -- which I won't elaborate on today, apart from saying it occurred between 400 and 600 years ago -- it is unequivocally related to human, direct human intervention by hunting.

The interesting species in New Zealand are among the microvertebrate fauna -- particularly those from 500 grams down to 10 grams. And this conflicts with what we've been hearing about today, large organisms going extinct. In New Zealand we had almost as many small organisms, including lizards and invertebrates.

Many of these relicts are in the, say, 100 grams to 5 kilogram size range. And in answer to one of the points raised this morning, one of these relict species -- a large rail called the takahe -- was found as late as 1948 to still be existing in the southwest of the South Island. The people from 90 to 120 miles away -- the Maori who were living there last century -- were surrounded by a smaller congeneric species in the swamps, which they didn't eat to any extent. But they made annual trips to Fiordland to crop the takahe population. The population became smaller and more difficult to hunt, but they went there specifically to remove those birds because they were traditional food.

One of the other interesting things about the New Zealand fauna, and the New Zealand extinctions, is that we have no Pleistocene-Holocene event at all -- no discernible one at all. This is just a selection of dates from a survey that Trevor Worthy and I started in 1991. These are the ones going back to 20,000 BP; these are acronyms for taxa -- the moas and geese, down to tuatara and bats. These are birds, tuatara, bat, and the Polynesian rat. These dates were done to test hypotheses of environmental change and movement of faunas.

I'll just point out one here, Puffinus spelaeus. There's a range of dates for this small petrel, going right through to 600 BP. These are all gelatin dates done by accelerator mass spectrometry. I should point out that much of this work, the early part of this work, was done in collaboration with Trevor. I wish he could be here, but the conclusion that we both reached was that there is no Pleistocene-Holocene event. The faunas we can trace across that boundary involve shifts in distribution -- particularly in the South Island -- in concert with shifts in the vegetation, as shown by the pollen record. And we have a very good, rich record in the South Island.

And the faunas do change. On the west coast, for example, there were Haast's eagles there preying on the very largest moas 18,000 years ago, at the height of the Otiran glacial, whereas closer than 10,000 years ago -- the eagles and those moas were confined to the east.

Much of what I'm going to say next is contingent on accepting gelatin AMS dates -- and this has been made brutally apparent to me by the reaction of the archeological community to something I said a year or so ago. So I'll just put in a few points in support of my chronology. This is a site I'm excavating in the North Island. It involves about two meters of sediment under a rock overhang. This is post-European arrival, with sheep manure. This is a pre-European sediment down to this gray layer, and that is about up to 600 or 700 millimeters thick. That is a single volcanic event. That's the last Taupo ignimbrite at 1,800 years BP. This site is 50 nautical miles downwind from the vent, and this is on the downwind side of the hill. The terrain is blanketed with this ash, and it provides a very convenient time marker.

At the bottom, under the plastic bags, there is an ash dated at 3,300 years BP, and this sediment is fortunately fossiliferous. . . . Here, -- just between the Waimihia Tephra and the pre-Taupo surface, we have moa material -- that's a moa femur, a moa rib -- a surface which can be traced throughout the deposit. These are in-filled rabbit burrows -- the rabbit was introduced last century. The rabbit burrows do not intrude into the pre-Taupo surface.

We can take gelatin from these individuals, which are time-constrained. We can also take material, such as this moa eggshell, which is just below the ash. In this site I have an egg associated with the moa, so I've done gelatin dates versus eggshell dates; eggshell dates versus the ash, and the chronology so far stands up pretty well.

So what we see is the general picture. For New Zealand, there was no extinction event at of the Pleistocene-Holocene. All the extinctions are within the past 2,000 years. If we take this as the number of species, breeding species of bird in the South Island during the Holocene, and we've got that tagged within one or two species . . . we're pretty confident on that. If we follow the dotted line along here, that number of species is presumed to have occurred right through until the time of arrival of the Polynesian settlers. This is calendar years before present. I take my time line from 2,000 years. This is well-established. Polynesians settled in New Zealand about 800 years ago; and even my opponents in the chronology accept this as a firm date. So human influence in New Zealand per se dates from this time, and this bar indicates the large-scale deforestation by fire which occurred. Fifty percent of the South Island forests -- mainly those in the east -- were removed by fire within 200 or 300 years of human settlement. And if you believe the hypothesis that Rattus exulans -- the so-called "Pacific rat" -- arrived with the settlers, then its influence starts around 800 or 900 years ago.

Since then, a range of other predators has been introduced, including European man. This second horizontal bar shows the period of European-induced fires, which removed another 40% of the forests. At point 3, Rattus novegicus, the large Norway rat, is assumed to have come ashore and colonized. At point 4, which is around about the middle of last century, or early last century, feral pigs and cats arrived, and late last century mustelids -- the stoat, ferret and weasel -- were introduced to remove the rabbits, which the Europeans had already brought here. And Rattus rattus did not arrive in the South Island till around 1890.

Now, several things did not get here. There is no evidence that the Polynesian pig arrived in New Zealand, nor that the chicken arrived -- so there was no known instance of an introduced bird at that stage. So disease carried by birds to other birds can be, I think, effectively eliminated from the system.

The dates that I have obtained on Rattus exulans material from natural sites, such as the one that I showed, give initial dates for its presence in New Zealand at least 1,800 years ago, with the oldest date on other evidence being just before the Taupo eruption. Now, this, of course, implies human contact with New Zealand before human settlement, but that's not a problem. The people were moving around at that time. But it also gives us, if we recognize this date -- and even if it's up to about 1,500 BP, if there's 400 or 500 years slop in the system -- we are then talking about a time when we can study the effects of a generalist omnivorous predator on the New Zealand ecosystem without the presence of humans to confound it. So it's opened up a new window for us to examine what happened with these small species, which human hunting cannot be expected to explain. No one's going to hunt out a 10-gram acanthisittid wren for food -- it's just not energetically viable.

We have a good handle on the number and timing of extinctions in this period. We know from the species composition of the late period, pre-European middens, that there was roughly that number of species. Whether you imply a period of stasis before European arrival or a consistent rate of decline, there is still a major decline between the time of Rattus exulans arriving and European contact. This secondary event has a lot of structure, which we can discern from the dates of arrival, but I won't go into it today.

When Europeans arrived, the Pacific rat was found discontinuously throughout New Zealand. These arrows indicate major island groups -- or major within the New Zealand context -- which held Pacific rats. Correspondingly, these are the weights of males, females and the range for present populations of Pacific rats. And you can see that there's a decline in mass; the ones in the south are much lighter compared to these ones. These are almost twice the size of Pacific rats found elsewhere in the Pacific, and you can see they reach a mass of around about 180 grams, which is quite an important observation.

I mentioned that one of the virtues of New Zealand as a study area is that it is an archipelago, which means that we have patterns of presence and absence for both predators and the prey. So in these we can look at which species survived on islands like Big South Cape, or Little Mangere, or Aorangi, or Little Barrier. This is a pictorial model of the spread of the Pacific rat through New Zealand, with calendar years from left to right. Each red bar indicates the presence of the rat, but not necessarily of people -- on the North Island, South Island, Stewart Island, and the important group of the Chathams, and some of the smaller islands.

Now, this should be looked at in the context of the present view of the distribution of Pacific rats in New Zealand -- that they went everywhere instantaneously about here in time. The distribution of predators and prey in New Zealand has been known for a very long time. It's been commented on, published on -- but the time element has been ignored. Until the perception of different arrival times is injected, you cannot get much of a picture from there, because there are too many exceptions. For example, there are Pacific rats on Little Barrier Island, but there are also tuatara there. And the same goes for the Hen and Chickens Islands nearby. And tuatara are known to be susceptible to predation up to 150 millimeters snout-vent length. So the idea that these were still present was inexplicable in terms of a holus bolus arrival around here -- but is imminently explicable in terms of this model.

From these we can make predictions. These are the known extinctions, or latest occurrence of a couple of important species -- the Coenocorypha snipe, and Pycroft's petrel. On Norfolk Island, the snipe is extinct when European's arrived about 200 years ago, and Pycroft's petrel was very rare. And the importance of these two species is that they have different life-history strategies. Snipe breed at two years. Pycroft's petrels, which are about the same size, do not breed until seven or eight years, and they have a very high adult survivorship.

Now, that is very important when you consider that the predator there had an upper size range on the prey it could take. So if you're cropping the young of an animal which has a high adult survivorship, and waits a long time before breeding, then it is going to survive longer -- has a much longer relaxation time than one which starts breeding much younger, and you're taking the chicks. And, in addition to that, assuming that the rat can take animals up to its maximum size, then we get the impression that anything above that is going to be released from predation. This is the Southern snipe which is extremely vulnerable. That was the distribution when Europeans arrived, basically. One island off Stewart Island, the Chathams, and one animal was shot on Little Barrier Island in 1870. They used to be throughout the rest of the archipelago.

This is a diving petrel. It's also within the size range of the rat as a predator of adults. But it starts breeding at two years old, so it's an intermediate case. The is a fairy prion. It has a mutually exclusive breeding distribution to that of the Pacific rat. The egg is vulnerable, the chicks are vulnerable, and the adults. Fairy prions are now found only on a few offshore islands, but used to be on the mainland.

The implication is that there is an upper size limit for predation at the reproductive stage and for the adult. And this is a mottled petrel whose adult stage is about 200-plus grams and is above -- to 300 grams -- it is above direct predation. The egg is just within the vulnerability range.

Similarly, for the shearwaters -- there's an extinct shearwater on the west coast which survived to 600 years ago. These are fossil distributions for the gavia and huttoni here. Huttoni is now found only on tops of mountains, and gavia on offshore islands.

In a nutshell -- for predation by Pacific rat, there is a maximum egg length which is vulnerable to rat direct predation, and we know these rats take eggs; this was demonstrated this past year. And we know that they depress breeding up to 60% in some bird populations. There is also a body mass limit which is around about the maximum size of the rat itself. So these two lines -- assuming that's the maximum body mass and that's the maximum egg size -- which proscribe those species which are vulnerable to extinction, those which are relatively invulnerable and had to wait the cascade effect of the other mammals to arrive, and these interesting ones in the middle -- where you get a pattern, depending on the rat size and distribution.

Those are the clusters of petrels. The smallest Pycroft's petrel could not coexist with rats. These are the two snipe which could not coexist. Down here we have saddleback and wrens; and these are the storm petrels. None of those exist on the mainland. This group has relicts on the offshore islands; this group has relicts on the mainland, in areas where the larger predators arrived.

So we can ascribe not only the extinction of some species in this area to the Pacific rat, but we can also point out that some longer lead-time species which have high adult survival and eggs and chicks in the lower-size range will take much longer to exterminate. Those are the ones which carried over and blurred the area in here.


We have time for a quick question.

From audience:

Can you rule out this early occurrence of Rattus exulans to some kind of early human contact with New Zealand? Do you think it really got in there without any human assistance?


I'm sure it arrived with human assistance. I am equally sure that the humans did not stay or arrive to colonize for another thousand years.