Tuatara: Volume 13, Issue 1, April 1965
Some Discontinuous Distributions of Plants Within New Zealand and their Ecological Significance — Part II: Disjunctions between otago-southland and nelson-marlborough and related distribution patterns
Some Discontinuous Distributions of Plants Within New Zealand and their Ecological Significance
Part II: Disjunctions between otago-southland and nelson-marlborough and related distribution patterns
An Earlier Account by Wardle (1963) presented an hypothesis of Pleistocene extinction of plants in the central South Island, and the likelihood of the presence of Pleistocene refugia in the extremities of the Island, among other places. The present writer, independently, had come to similar conclusions from study of distributions of species in the South Island. This paper was written in the belief that it could add considerable detail to the chain of evidence presented by Wardle and elucidate some of the problems of distribution.
Two groups of plants are disjunct** over wide geographical distances in New Zealand. The fact that numbers of species are disjunct together makes it most probable that the causes of separation are common for all of these species. The first disjunction is between Auckland† and Nelson and will be considered later.
The second striking series of disjunct species consists of plants with a conspicuous gap in their known distribution in the central South Island. Wardle listed a few of these and drew on the presence of a relatively large number of endemic plants in each of Otago-Southland and Nelson-Marlborough, with correspondingly few endemics in the central South Island, as support for his hypothesis of Pleistocene extinction in the latter area. An implication of the hypothesis is that Nelson, Marlborough, Banks Peninsula, coastal Southland. Stewart Island and offshore islands were refugia where large numbers of species including some forest plants survived the glacial maxima of the Pleistocene.
* E. L. Hellaby Research Fellow.
** For plant geographical terminology see Cain (1944).
In examining, the present distribution patterns of species disjunct between the northern and southern parts of the South Island, and of endemics in both these areas, the present writer came to the conclusion that a simple statement of Pleistocene extinction in the central South Island did not explain all the facts of distribution. The detailed knowledge of distribution of plants and vegetation, together with information from non-botanical fields, allows elaboration of the hypothesis as formulated by Wardle. In this article evidence is brought forward in support of a modified hypothesis.
It is proposed here to deal with species fully disjunct between Nelson-Marlborough (and/or North Island) and Otago-Southland (and/or Stewart Island); with closely related (vicarious) species, whose distributions do not meet in the central South Island; with species which are abundant to north and south of the South Island and less common between; with the patterns of distribution of species endemic to the north and south of the South Island; with some distributions which indicate that the populations of these plants have undergone recent contraction: and with some disjunct animal distributions.
|Nelson-Marlborough and/or north island||Otago-Southland and/or stewart island|
|Celmisia dallii||C. holosericea|
|C. rupestris||C. hectori|
|Dracophyllum densum||D. politum|
|Euphrasia cheesemanii||E. australis|
|Gentiana sp. aff. lineata||G. lineata|
|G. sp. aff. montana||G. montana|
|Pimelea sericeo-villosa||P. pulvinaris|
|Raoulia rubra||R. buchananii|
|Senecio adamsii||S. revolutus|
|Chionochloa juncea||Ch. acicularis|
‘Partially’ Disjunct Species
Endemics of Nelson-Marlborough and Otago-Southland
There are sufficient endemic vascular plant species in the South Island confined within the areas north of a line from about Greymouth to Motunau and the south of a line from about Bruce Bay to the Waitaki River mouth to enable the northern and southern floras to be regarded as distinct from the rest of the South Island flora (see Wardle, 1963a; approximately 110 endemic species in Nelson-Marlborough and 90 in Otago-Southland excluding the Foveaux Strait species). The central South Island proper, however, is characterised by relative poverty in endemic species, with only about 14 such species being confined approximately within the boundaries described above, four of them on Banks Peninsula. Some of these central South Island endemics are plants of rupestral habitats and others favour periglacial habitats up to about 9000 feet in the high Central Alps (Myosotis explanata, Ranunculus godleyanus, R. grahamii). Rupestral, scree and high altitude species of somewhat wider distribution will be discussed later. Individual distribution patterns of the endemics which have been grouped here into Nelson-Marlborough and Otago-Southland floras resolve the endemics into minor groups centred on focal areas within both northern and southern South Island. Thus there is a group centred on northwest Nelson and a smaller group in eastern Nelson, and Marlborough including the Kaikoura ranges. Similarly in the south a group centres on Fiordland and another on Central Otago. These may be designated ‘western’ and ‘eastern’ facies respectively, but individual species transgress any boundaries which may be drawn. The fully disjunct species and vicarious pairs of species have similar distributions. It is not intended to enumerate the distributions of these endemic species in detail. The generalised patterns of distribution will be described with the aid of a map (Fig. 7). From Fig. 7 it is seen that northern patterns may be grouped into at least nine ‘types’. These are designated only for convenience in description. Individual distribution patterns show great complexity, with each species tending to behave independently. Each ‘type’ is named here according to the absolute southern limit of the species within it and is characterised by the presence of at least three species.page 19
|1.||Tasman Mountains: Poranthera alpina, Hebe ochracea, Pseudowintera traversii, Astelia spp. (several undescribed), Pittosporum dallii.|
|2.||Buller Valley-Clarence River headwaters: Hebe coarctata, Anisotome deltoidea, Nototlaspi australe.|
|3.||Paparoa Range: Celmisia dallii, C. lateralis, C. parva, Euphrasia townsonii, Dracophyllum townsonii, Senecio hectori.|
|4.||Taramakau-Waimakariri Valleys: Chionochloa australis, Parahebe cheesemanii, Hebe decumbens.|
|5.||South Westland: Rubus parvus. Euphrasia disperma, E. cockayniana, Hebe gracillima.|
|6.||Wairau Valley-Eastern Nelson: Celmisia rutlandii, C. cordatifolia, Hebe rigidula, Scutellaria novae-zelandiae, Pimelea suteri.|
|7.||Kaikoura Ranges: Ewartia sinclairii, Wahlenbergia matthewsii, Celmisia cockayniana.|
|8.||Mount Terako-Lower Waiau Valley: Olearia coriacea, Haastia pulvinaris, Helichrysum coralloides. Pachystegia insignis, Hebe hulkeana. Senecio monroi.|
|9.||Middle Waiau Valley: Helichrysum microphyllum, Pimelea concinna, Schizeilema roughii. The southern patterns fall into at least nine ‘types’,* each named here according to its northern limit.|
|10.||Fiordland: Olearia oporina, O. crosby-smithiana, Euphrasia integrifolia. Chionochloa teretifolia, Dracophyllum fiordense, Senecio bifistulosus.|
|11.||Mt. Earnslaw-Mt. Aspiring: Aciphylla congesta, Celmisia argentea, C. bonplandii, Anisotome capillifolia.|
|12.||South Otago: Celmisia lindsayi, Pimelea lyallii, Centrolepis strigosa (not endemic but restricted to this area). Euphrasia repens.|
|13.||Central Otago: Mysotis pulvinaris, M. albosericea, Celmisia prorepens, Aciphylla simplex, Pygmea myosotoides. Lepidium kirkii.|
|14.||Waitaki Valley: Pachycladon novae-zelandiae, Anisotome brevistylis, Dracophyllum muscoides.|
|15.||Kirkliston Range-Southern Hunters Hills: Parahebe tetragona, Ourisia glandulosa, Celmisia ramulosa.|
|16.||Mt. Cook-Northern Mackenzie Country: Celmisia densiflora, C. brevifolia, Carmichaelia petriei, Pimelea pulvinaris.|
|17.||Rangitata Valley: Aciphylla dobsoni, Dracophyllum prostratum, Hebe hectori.|
|18.||Hokitika River-Arthurs Pass-Rakaia River: Lobelia linnaeoides, Raoulia hectori, Bulbinella gibbsii var. balanifera.|
In all the patterns so far described the species involved are relatively abundant and continuously distributed within their area. Later in this account there are also recorded a few cases of species from northern and southern New Zealand which show considerable disjunction between continuous populations and other known localised occurrences. This is probably to be related to contraction of areas.
* Since this article was written further field work reveals that another group of southern species has its northern limit in the Haast Pass-Lake Ohau region. The species include Dracophyllum menziesii, Coprosma astonii, Ranunculus buchananii.
Disjunctions Caused by Recent Contractions
A further situation complicates the position with respect to the hypothesis of Pleistocene extinction in the central South Island. Outside the main areas of distribution of numbers of species there are found distant, isolated occurrences. The ‘partial’ disjuncts described previously are of this type, but many others also exist which are not dicentric. Again, in these monocentric patterns the tendency is for a distribution to tail-off into the central South Island. The gaps between populations seem too great to have been bridged by long distance dispersal. In most cases, too, the apparent lines of dispersal lie athwart prevailing wind gradients or likely pathways such as river valleys and mountain ranges. Contractions of area are believed to be the explanation for these distributions. Species which appear to have contracted northward are: Celmisia allanii, continuous to the Hurunui River, one occurrence in the Godley River: Hebe cheesemanii, continuous to Hurunui River, occurrences at Mt Peel, South Canterbury, Kirkliston Range and Mt Alta, Otago; Ranunuculus insignis (‘monroi’ type), continuous to Waimakariri River; occurrences at Mt Hutt, Mt Peel and Four Peaks; Leucogenes leontopodium, local but not uncommon on North Island and Wairau Mountains, isolated populations at Hurunui River and Mt Peel; Hoheria sexstylosa, common to about 40 deg. 30 min. south latitude, occurrences on Banks Peninsula (possibly now extinct) and near Gore; Pittosporum patulum, local but not uncommon north-west Nelson to middle Clarence Valley, occurrences near Lake Ohau. Species which appear to have contracted toward the south are Euphrasia dyeri, not uncommon in bogs in Stewart Island, Fiordland, Southland and South Otago, occurrences at Mt Kyeburn and Mt Somers; Coprosma intertexta, common in Southland-Otago, occurrences at Cass, Waiau River, Rahu Saddle, north-west Nelson; Olearia moschata, common along the main divide to about Mt Cook, less common in Rangitata and Rakaia Rivers, rare at Arthurs Pass, represented by hybrids with O. page 22 avicenniaefolia (= O. haastii) in the Waiau River and probably by hybrids with O. nummularifolia at Boulder Lake in north-west Nelson. It may be noted here that the dicentric Nothofagus fusca which also has outlying populations in the central South Island, is sometimes represented there by hybrids with N. s. cliffortioides where no pure individuals of the former species are now found. Wind-blown pollen could account for this in some cases.
It seems probable that these monocentric and some or all of the ‘partially disjunct’ dicentrically distributed species have undergone contraction later than the separation of the species regarded as full disjuncts. Some of the latter may also have had their areas contracted.
Some animal distribution patterns fit into the general patterns of plant distributions outlined above. Powell (1957) described the following marine animals with disjunctions in their distributions between North Island and South Island: the mollusc Amphidesma ventricosum (toheroa) and at least one other mollusc and a species of brachiopod. Fleming (1950) described a molluscan fauna from the coast of Fiordland containing three genera and 16 species otherwise known only from North Auckland or North Island. Mr P. Johns (pers. comm.) states that although there are no species known by him to be disjunct between Nelson-Marlborough and Otago-Southland, certain groups of species of the terrestrial animals, ground beetles, millipedes and cockroaches are closely related in the two areas, separated by a gap in the central South Island. They are probably equivalent to the pairs of vicarious plant species. Similarly, the distributions of individual species extending into Canterbury from north and south are parallel in many cases to plant distributions. Lee (1956, 1959) recorded that one genus of earthworms in New Zealand was represented only in Fiordland and Auckland Province, and three species of Taranaki-Wanganui are most closely related to species of the south-western South Island and subantarctic islands. Comparatively unspecialised faunas consisting of widespread species are present in the eastern South Island. Westland is likewise unspecialised with respect to earthworms but there are some endemic species and the area had been colonised from both north and south. Banks Peninsula is also an area with a small number of endemic earthworms. The Wellington-Nelson area is regarded as a centre of dispersal for these animals.
Distributions of many plants and some animal groups suggest that there is a real floristic-faunistic gap in the central South page 23 Island. The explanations for the interrupted patterns of distribution as described above cannot be simple, however, and various factors seem to have been involved. Glaciation during the Pleistocene seems to have been a primary cause, but glaciation was multiple and there were long interglacial periods with mild climate so that the effects of ice maxima and glacial recessions on plant distributions must have been repeated again and again.
A brief consideration will now be made of the role of glaciation and post-glacial climatic changes in bringing about discontinuous distributions. A more detailed account of these changes, together with interpretations of the available pollen analyses will be published elsewhere. References for the statements made below are included in Burrows (1964 M.S.).
* B.P — before 1950 A.D
The explanation of this apparent anomaly is thought to be two subsequent events. The first of these was a renewed ice advance, less extreme than the Poulter advance. By analogy with events in the northern hemisphere and in South America (Flint 1961, Auer 1956, 1958) this short-period stadial, known in New Zealand as the Birch Hill advance, occurred about 10,000-11,000 B.P. The moraines left by it are most evident in the central South Island, and there were probably only cirque glaciers in Nelson and Fiordland. Soon after the Birch Hill ice advance there began a period of cool (upland) to warm (lowland) moist climate known as the ‘climatic optimum’. This effectively prevented further beech advance at the time. Cranwell and von Post (1936) and others have demonstrated how podocarp-broadleaved forest during this time largely superseded beech forest. Beech was, however, present at all times and must have been restricted to higher altitudes and various somewhat extreme habitats. At the fronts with mixed forest in central Westland, the Taramakau and Paringa Rivers, beech species remain limited to the present day by efficient competition chiefly from broadleaved trees such as Quintinia acutifolia and Weinmannia racemosa. The high rainfall occurring in central Westland is an important controlling factor. In headwaters of Canterbury rivers from Mt Cook to the Wilberforce tributary of the Rakaia the upland podocarp forest was maintained in the same way and beech immigration is probably of recent origin. The beech forest front with mixed forest in these valleys is approximately at the position of moraines of presumed Birch Hill page 25 age. Other forest types appear, thus, to have been maintained in the central South Island, at the expense of beech, by high rainfall in the period from about 9,000 B.P. to about 5,000 B.P. (and in some cases to the present). N. menziesii forest, which dominates in Fiordland, supplanted mixed forest (Harris, 1963), probably because of its presence in many places during the ‘climatic optimum’ and because of the local features of cool climate and extremely wet soils to which it is well suited. A renewed glacial advance, probably about 6,500 B.P., may have had some bearing on the subsequent vegetational changes. By analogy with these known situations, other species including some alpine plants could have been restricted by climatic factors.
Some of the discontinuities in populations of ‘partial disjuncts’ and monocentric species which have undergone contraction may have been brought about by the same causes. It is difficult to conceive of any phenomenon so profound that it could cause extinction in the central South Island on the scale described in the opening part of this article later than the last main series of glacial advances of the Pleistocene. The Birch Hill and later advances do not seem to have been intense enough to cause primary disjunction. It seems probable however that post-Poulter climatic changes have inhibited plant migrations from refugia and have also in more recent times brought about contractions in areas. It is probable that subsequent to the period of moist climate with mild temperatures between about 9,000 B.P. and 5,000 B.P. (for dating see Deevy and Flint, 1957) climates began to become at first warmer and then increasingly cooler and drier. Some specific data on distributions of certain plant species support this theory of climatic deterioration probably since about 2,500 B.P. A peat sample from the Grey River valley dated at about 8,300 B.P. (Bowen, ex Grant Taylor and Rafter, 1962) contained Metrosideros robusta pollen (a little south of its present range) and a small amount of Agathis australis (Dr W. F. Harris pers. comm.). Cranwell and von Post (1936) mention the occurrence of Alseuosmia pollen in a sample from Southland. This sample is presumably from a time equivalent to the above date — during the ‘climatic optimum’. Agathis is not now found south of the Waikato, and Alseuosmia not south of Nelson so that major contractions of area are evident. Some time after about 5,000 B.P. beech pollens begin to supplant podocarp pollens in the peat columns, apparently in a warm, dry period (Cranwell and von Post 1936, Harris 1963). This process was accelerated after about 2,500 B.P. (Deevey and Flint 1957, Harris 1963). Since this time there must have been a decline in temperature and also in rainfall. Changing climate may well be the most important cause of the contractions of many species. Dry, even arid climates seem to be involved in the limitation of species such as Nothofagus menziesii. page 26 Biologists and others should look for evidences of a period of aridity in post-glacial times. The arid period may have occurred within the last few thousand years.
Scattered discontinuous distributions of various other widespread species throughout the South Island may have resulted from the same causes of contraction as the examples described above. One cause of many such discontinuities in the eastern South Island, however, undoubtedly is fire during the Maori era (see e.g. Molloy et al. 1963). Subfossil remains demonstrate that forest has been extensively fragmented by fire within the last thousand years. Species dependent on a forest environment are limited by this (see Burrows 1961). Fire is not the only likely cause of discontinuity, nor even the primary one since the evidence for climatic deterioration in the last few thousand years is quite clear. Climatic fluctuations seem to have continued up to the present (Holloway 1954, Wardle 1963b).
Brief consideration of the Auckland-Nelson disjunction (Wardle 1963a) may now be made. During the early Otira Glaciation the southern and central North Island apparently were subjected to severer climatic conditions than was north-west Nelson. The latter area was at the western side of an extensive plain exposed by low sea levels (Fleming 1962). General extinction of forest occurred in the southern half of the North Island although scrub and grassland were present at lower altitudes. During the Poulter stadial beech forest was probably extensive. The question as to whether the tender kauri associate species survived the whole Otira Glaciation in north-west Nelson is still an open one. The influences of Birch Hill cooling have not yet been recognised in this area nor in the North Island. The ‘climatic optimum’ climates would have enabled expansion southward of the kauri associates (and probably kauri itself) so that disjunction of these species may be subsequent to it. Vulcanism in the central North Island (see e.g. Taylor 1953) is almost certainly an important contributor to limitation of many species north of the 38th parallel. Widespread ash showers have probably been as efficient in causing extinction in the centre of the North Island as glaciation in the centre of the South Island, but their invocation as a cause of complete extinction in the whole southern North Island poses various problems. Why did some of the species concerned not survive in the western North Island or near Wellington?
The detailed information presented above supports the general hypothesis of Wardle (1963a) that the Pleistocene glaciation caused extinctions of plants (and animals) in the central South Island and contractions into refugia. In the north, the refugia are likely to have been the coastline and hills of north-west Nelson to the Paparoa range, the coastline of Marlborough and, by virtue of a few plants endemic there, Banks Peninsula. In the south, refugia page 27 are likely to have been Southern Fiordland, the coast of Southland to South Otago, and Stewart Island. The latter area and its extended coastline is likely to have been of primary importance as a refuge. A group of specialised endemic species in Central Otago, another in northeast Nelson and eastern Marlborough and another found throughout the South Island mountains suggests that these areas also carried some vegetation at least throughout the latter part of the Otiran Glaciation. Some distribution patterns suggest comparatively recent contractions of the area of a considerable number of plant species.
Grateful acknowledgment is made for permission to consult specimens from those responsible for the herbaria of the Canterbury Museum and the Botany Division, D.S.I.R. I have also to thank Mr A. P. Druce and Mr N. Elder for information about existing North Island distributions of Pimelea aridula. Several of my colleagues have offered useful criticisms of the script and I express my thanks to them. I am especially grateful to Dr W. F. Harris for permission to use some pollen analytic information, to Dr R. P. Suggate for permission to use carbon dates and, with Dr M. Gage, for their willingness and generosity in discussing problems of glacial geology.
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