Cultural landscapes as biogeographical experiments: a New Zealand perspective

Journal of Biogeography,

27

, 39–43

© 2000 Blackwell Science Ltd

MILLENNIUM GUEST EDITORIAL NUMBER 8

Blackwell Science, LtdOxford, UKJBIJournal of Biogeography0305-0270Blackwell Science Ltd, 200027no issue no.2000372Millennium guest editorial number 8Millennium guest editorial number 8Millennium Guest Editorial Number 8100Graphicraft Limited, Hong Kong

Cultural landscapes as biogeographical experiments: a New Zealand perspective

All land-based economic activities involve risk: for people, native species, and the environment.

A presumed native of the Mediterranean basin and an aggressive adventive in many warm temperatecountries,

Sorghum halepense

(Fig. 1) has been growing in New Zealand since the 1970s. As one of theworld’s more aggressive and difficult to eradicate herbaceous weeds (Holm

et al

., 1977) Johnson grasscould affect the country’s pastoral sector, possibly even compromise the survival of some native species,if it were to become widely established.

MANAGING ISLAND ENVIRONMENTS

Towards the end of the Mesozoic the New Zealand archipelago was on the southeastern flankof Gondwana. Today it lies close to the centre of the ocean hemisphere. Its biota, distinguished by asubstantial majority of endemic species, evolved in virtual isolation until about 1000 years ago whenthe first settlers came out of the north.

Together with agricultural techniques developed elsewhere in the Pacific basin, Maori brought a fewsubsistence plant and animal species. Sweet potato (

Ipomoea batatas

) could be cropped in milder partsof the North and South Islands, and in favourable years as far south as the Opihi River, but it was notuntil introduction of the common potato (

Solanum tuberosum

) in the late 18th century that intensivegardening become an option throughout the country.

Figure 1 Sorghum halepense, or Johnson grass: (A) basal part of a plant, showing rootstock, (B) panicle, (C) segment of fruiting branch, and (D) seeds (source Muenscher, 1960).

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40 Millennium guest editorial number 8

© Blackwell Science Ltd 2000,

Journal of Biogeography

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During their exclusive occupation of the land, Maori learned to manage populations of valuednative plants and animals to ensure sustainable harvests (Urlich Cloher, 1996). For example, the thickmain stem and below ground parts of

Cordyline australis

were probably the third most significant foodsource in the south, after eel (

Anguilla dieffenbachii

) and mutton bird (

Puffinus griseus

), and a majorcomponent of ritual food exchanges. Its tops were commonly transplanted into grassy areas on lime-stone for harvesting four or five years later. Second growth shrubland was periodically burned toencourage the spread and growth of

Pteridium esculentum

, the rhizomes of which were an importantsource of dietary carbohydrate.

Coriaria arborea

,

Corynocarpus laevigatus

and

Solanum aviculare

werevalued for their edible fruits, and seeds were deliberately spread to suitable sites as was the edible seaweed

Porphyra columbina

. At the time of European contact in the latter half of the 18th century, the southernMaori were observing traditions and applying knowledge accumulated over forty generations to ensureregular harvests of valued indigenous, and a smaller number of introduced, plant and animal species.

Maori also burned tracts of wooded and grassy land, precipitating a decline in closed canopy forestsand shrublands from 85 to 53% on the eve of European settlement, with induced grass and shrubcommunities expanding to fill the gap (Taylor & Smith, 1997). Populations of the introduced Poly-nesian dog (

Canis familiaris

) and Polynesian rat (

Rattus exulans

) became established outside settledareas. The large ratites (Anderson, 1989) and about twenty species of carinates (Fleming, 1979) dis-appeared, driven to extinction by the combination of hunting pressure and habitat modification, andecosystem functioning throughout the country was permanently affected.

Unlike Maori, European settlers had ready access to an immense body of theoretical and practicalknowledge, large numbers of economically important plant and animal species, and an array of sophis-ticated tools. They, too, changed the New Zealand environment, but in even more profound ways.

To Samuel Butler, who worked a pastoral property in the interior hill and mountain country ofCanterbury during the 1870s, sheep farming was an eminently experimental activity. The successfullandholder was the person who had learned to match sheep breeds to local environmental conditions,who had discovered which native and introduced plants were nutritious to stock and could survivebrowzing pressure, and who appreciated which tree and shrub species might provide shelter fromstrong winds for people, plants and animals. Although native trees were valued for fuel and lumber, andlarge numbers of forest plants were collected by colonists for propagating in homestead and cottagegardens, indigenous species rarely figured in the long term plans of the first three generations of Europeansettlers living outside the tussock grasslands. And while colonists and their families sought recreationby hunting native birds and catching fish, they speedily imposed new food chains throughout the low-lands and low hill country by means of clearance or cultivation and the many plant and animal speciesthey introduced. The settlers who pioneered farms in grassy, swampy or forested areas initiated theirparticular forms of environmental transformation, the impact of which is still being felt.

Along with invertebrates of all kinds, breeding stocks of birds, deer, fish, goats, hares, mustelids andrabbits, even possums and wallabies, were released to foster a diversified biota, but little thoughtwas given to the consequences for native species. Perhaps settler society was responding to CharlesDarwin’s belief that European plants and animals have the competitive edge over those native to NewZealand and will inevitably dominate.

During the 19th century it was widely felt that the country’s native plants and animals were on thehigh road to extinction. As one early botanist put it, ‘there can, I think, be no doubt whatever that thenative vegetation will eventually be, if not entirely, exterminated and the floral features of the countryaltogether changed through the introduction of these foreign weeds’ (Armstrong, 1879: 334). He mightnot have reached that bleak conclusion, however, had he experienced the extensive closed canopyforests on the western flanks of the Southern Alps. Another reason for the pessimism of many 19thcentury biologists might be the apparent strangeness to European eyes of New Zealand’s bird andinsect dominated ecosystems.

Whatever the reason, many plant and animal species were purposefully or accidentally imported.Several economic and decorative plants escaped from cultivation to become pests, a few of them grow-ing in ways that could not have been predicted in their native territories. Other introductions thrived inthe habitats where they had been released, grew numerous, then spread. And the country’s weed floraswelled by the year, with a few species becoming established in relatively unmodified ecosystems.

How long, if at all, does it take an introduced species to naturalise? Within two decades of the startof European settlement,

Rumex acetosella

had become a serious weed in the inland grasslands ofCanterbury and remained so until extensive topdressing with lime for pasture improvement brought itunder control. It is still abundant but has since been overtaken as a serious pest by other, more aggressive,weeds. Thistles, especially

Cirsium vulgare

, caused problems from the earliest days. And a decade after


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occupying their farms, European settlers were reporting rampant weed growth on exposed sod bankswhere gorse (

Ulex europaeus

) seed had been planted for hedging. Within a few years gorse was tobecome a serious weed in its own right, as did two other common hedge plants, broom (

Cytisusscoparius

) and hawthorn (

Crataegus monogyna

). Nor did towns and cities escape the adverse effects ofnaturalized plant and animal species, as the publications of Esler (1987a,b,c, 1988a,b,c) make clear formetropolitan Auckland.

EXPERIMENTING WITH THE ENVIRONMENT

Three types of environmental experiment were initiated by Maori and European settlers. The firstinvolved large scale modification of the land and its vegetation cover by people. The second involvedselective removal of native animals from indigenous ecosystems. And the third involved purposefully aswell as accidentally introduced plant and animal species. The three types of experiment, all of themlacking controls, continue to this day.

Most adventive plants thrive in disturbed habitats, notably those created by people and their animals(Fig. 2). With the historically recent expansion in the colonial and trading activities of European nations,borders between faunistic and floristic realms have become highly permeable and the geographicranges of many species have expanded. In most settled areas, weedy plants have become widespreadand their increasing dominance is viewed with alarm (Vitousek

et al

., 1996).Two decades after the start of organised European settlement in Canterbury, Armstrong (1872)

reported an adventive flora of some 170 species, ‘being nearly one-fourth of the total number offlowering plants (naturalised and native) found in the province’. Seven years later, his son’s surveys(Armstrong, 1879) brought the total to 250 species. By the 1960s, there were more than 1000 speciesof adventive flowering plant (Healey, 1969) and their impact was felt throughout the territory. ‘Thereare now few truly primitive plant communities in Canterbury, if absence of adventives is a criterion,and we find over extensive areas adventive plants competing for space with indigenous plants, withother adventive plants, and with members of the cultivated plant flora’ (Healey, 1969: 265). A decadeearlier Smith (1958) had noted the initiation of ecologically novel plant communities composed ofindigenous and adventive species. By then, numerous biogeographic and ecological experiments werewell underway, especially in the low herb layer of many local plant communities.

Early in the 20th century, Leonard Cockayne argued that if New Zealand could be cleared ofpeople and their animals then many adventive species would disappear. Since then, documented instancesof naturalisation have multiplied and it is not evident that all adventives occupy habitats directly orindirectly created and sustained by people and their animals.

Berberis

spp, seem here for the long haulas do

Clematis vitalba

,

Hieracium

spp,

Ribes sanguineum

,

Rubus fruticosus

and

Stipa trichotoma

, to

Figure 2 Spontaneous establishment of exotic conifers on depleted tussock grassland in the Waitaki valley.


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name six taxa. With a naturalised angiosperm flora of 2000 species, a little smaller than the total ofnative vascular plants, New Zealand appears to have gained a substantial number of resident species injust 150 years. At the same time, however, there is no evidence for major extinction of native plants.

CONCERNS AND PROSPECTS

How many resident species can an island nation like New Zealand support? Which of its ecosystemsare most likely to become home to introduced species, and what functions will those plants serve in thenew ecological fabric? Will some ecosystems prove especially vulnerable to invasive species? Might anewcomer share its genes with a closely related native species, and what long term effect would thathave on ecosystem functioning? Does it now make sense to think of ‘natural’ vegetation when ourfootsteps are everywhere?

Models like that of Bazzazz (1983)—which involves the arrival of a propagule, its possible establish-ment, then population growth, spread, dominance or displacement, and integration through niche shiftor character displacement—can be expected to aid in untangling what is happening around us andpoint to useful management routines. Others, like Equilibrium Island Biogeography (EIB) theory, raisefundamental questions. A tenet of the EIB model is that once the biota of an island reaches equilib-rium, a new arrival can persist only if an established species is displaced. What is not clear is how dis-placement is effected or even how long that might take. Did the flora of New Zealand fall short ofequilibrium biodiversity on the eve of European settlement? The palaeoenvironmental literature pointsto major losses in vertebrate and higher plant species during the Pleistocene, leaving the country withan impoverished biota. Is the rich adventive flora of lowland New Zealand mainly explained by eco-logical opportunities in its substantially transformed vegetation cover for newcomers that have beenseparated from their co-evolved predators? And what about adventives growing in primarily nativeecosystems; will they suppress native species, possibly even bringing about their extinction, thrivealongside them or disappear? We are almost certainly witnesses to an early stage in the developmentof new functional ensembles of native and exotic species. How long will integration take, which species’populations will suffer decline or growth in numbers, and will the new ensembles of native and exoticspecies persist? Answers to those questions require information from carefully designed and managedprogrammes of field observation and experimentation.

Most field experiments involve controlled manipulation of just one or two factors, then pro-grammes of observation extending over months or years. Alternatively, it might be possible to modelinteractions and shorten observation time by running a computer simulation. The experiments Irefer to lack those advantages and raise substantial practical, logistic and funding issues. Ensembles ofplants and animals are usually highly buffered, have long lag times, and tend to switch betweenmetastable states. Their path after perturbation is difficult to predict. For example, if the soil seed bankhas been depleted then vegetation change will reflect what remains on site and in nearby reserves ofpropagules. And if pollination or dispersal should depend upon just one animal species then its dis-appearance from the area could take from years to decades to show in the plant population. It followsthat programmes of observation and experimental manipulation on ensembles of native and naturalizedspecies should run for many years.

I am not advocating a passive stance towards invasive species, and am certainly not proposing westand by and do nothing to control importation of pest plants and animals or eradicate what is alreadyestablished. Rather, I am arguing that new functional ensembles, comprising native and adventiveplant and animal species, are apparently developing in our cultural landscapes and more can beexpected. We need to understand what is happening in them to facilitate the survival of native species.

PETER HOLLAND

University of Otago, P O BOX 56, Dunedin, New Zealand, [email protected]

REFERENCES

Anderson, A. (1989)

Prodigious birds: moas and moa-hunting in New Zealand

. Cambridge University Press,Cambridge.

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Armstrong, J. B. (1879) A short sketch of the flora of the province of Canterbury, with catalogue of species.

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, 325–353.Armstrong, J. F. (1872) On the naturalised plants of the province of Canterbury.

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, 284–290.Bazzazz, F. A. (1983) Characteristics of populations in relation to disturbance in natural and man-modified

ecosystems.

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(ed. by H. A. Mooney and M. Godron),pp. 259–275. Springer-Verlag, Berlin.

Esler, A. E. (1987a) The naturalisation of plants in urban Auckland, New Zealand: 1. the introduction andspread of alien plants.

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, 511–522.Esler, A. E. (1987b) 2. Records of introduction and naturalisation.


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Esler, A. E. (1987c) 3. Catalogue of naturalised species.


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, 539–558.Esler, A. E. (1988a) 4. The nature of the naturalised species.


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, 345–385.Esler, A. E. (1988b) 5. Success of the alien species.


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, 565–584.Esler, A. E. (1988c) 6. Alien plants as weeds.


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Holm, L., Plucknett, D. L., Pancho, J. V. & Herberger, J. P. (1977)

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Cultural landscapes as biogeographical experiments: a New Zealand perspective

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