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Cultural landscapes as biogeographical experiments: a New Zealand perspective

2000; Wiley; Volume: 27; Issue: 1 Linguagem: Inglês

10.1046/j.1365-2699.2000.00373.x

1365-2699

Peter Holland,

Rangeland and Wildlife Management

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 temperate countries, Sorghum halepense ( Fig. 1) has been growing in New Zealand since the 1970s. As one of the world’s more aggressive and difficult to eradicate herbaceous weeds ( 13) Johnson grass could affect the country’s pastoral sector, possibly even compromise the survival of some native species, if it were to become widely established. Sorghum halepense, or Johnson grass: (A) basal part of a plant, showing rootstock, (B) panicle, (C) segment of fruiting branch, and (D) seeds (source14). Towards the end of the Mesozoic the New Zealand archipelago was on the southeastern flank of Gondwana. Today it lies close to the centre of the ocean hemisphere. Its biota, distinguished by a substantial majority of endemic species, evolved in virtual isolation until about 1000 years ago when the first settlers came out of the north. Together with agricultural techniques developed elsewhere in the Pacific basin, Maori brought a few subsistence plant and animal species. Sweet potato (Ipomoea batatas) could be cropped in milder parts of the North and South Islands, and in favourable years as far south as the Opihi River, but it was not until introduction of the common potato (Solanum tuberosum) in the late 18th century that intensive gardening become an option throughout the country. During their exclusive occupation of the land, Maori learned to manage populations of valued native plants and animals to ensure sustainable harvests ( 17). For example, the thick main stem and below ground parts of Cordyline australis were probably the third most significant food source in the south, after eel (Anguilla dieffenbachii) and mutton bird (Puffinus griseus), and a major component of ritual food exchanges. Its tops were commonly transplanted into grassy areas on limestone for harvesting four or five years later. Second growth shrubland was periodically burned to encourage the spread and growth of Pteridium esculentum, the rhizomes of which were an important source of dietary carbohydrate. Coriaria arborea, Corynocarpus laevigatus and Solanum aviculare were valued 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 southern Maori were observing traditions and applying knowledge accumulated over forty generations to ensure regular 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 forests and shrublands from 85 to 53% on the eve of European settlement, with induced grass and shrub communities expanding to fill the gap ( 16). Populations of the introduced Polynesian dog (Canis familiaris) and Polynesian rat (Rattus exulans) became established outside settled areas. The large ratites ( 1) and about twenty species of carinates ( 11) disappeared, driven to extinction by the combination of hunting pressure and habitat modification, and ecosystem functioning throughout the country was permanently affected. Unlike Maori, European settlers had ready access to an immense body of theoretical and practical knowledge, large numbers of economically important plant and animal species, and an array of sophisticated 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 of Canterbury during the 1870s, sheep farming was an eminently experimental activity. The successful landholder 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 survive browzing pressure, and who appreciated which tree and shrub species might provide shelter from strong winds for people, plants and animals. Although native trees were valued for fuel and lumber, and large numbers of forest plants were collected by colonists for propagating in homestead and cottage gardens, indigenous species rarely figured in the long term plans of the first three generations of European settlers living outside the tussock grasslands. And while colonists and their families sought recreation by hunting native birds and catching fish, they speedily imposed new food chains throughout the lowlands and low hill country by means of clearance or cultivation and the many plant and animal species they introduced. The settlers who pioneered farms in grassy, swampy or forested areas initiated their particular 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 and rabbits, even possums and wallabies, were released to foster a diversified biota, but little thought was given to the consequences for native species. Perhaps settler society was responding to Charles Darwin’s belief that European plants and animals have the competitive edge over those native to New Zealand and will inevitably dominate. During the 19th century it was widely felt that the country’s native plants and animals were on the high road to extinction. As one early botanist put it, ‘there can, I think, be no doubt whatever that the native vegetation will eventually be, if not entirely, exterminated and the floral features of the country altogether changed through the introduction of these foreign weeds’ ( 2: 334). He might not have reached that bleak conclusion, however, had he experienced the extensive closed canopy forests on the western flanks of the Southern Alps. Another reason for the pessimism of many 19th century biologists might be the apparent strangeness to European eyes of New Zealand’s bird and insect 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 growing in ways that could not have been predicted in their native territories. Other introductions thrived in the habitats where they had been released, grew numerous, then spread. And the country’s weed flora swelled 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 start of European settlement, Rumex acetosella had become a serious weed in the inland grasslands of Canterbury and remained so until extensive topdressing with lime for pasture improvement brought it under 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 occupying their farms, European settlers were reporting rampant weed growth on exposed sod banks where gorse (Ulex europaeus) seed had been planted for hedging. Within a few years gorse was to become a serious weed in its own right, as did two other common hedge plants, broom (Cytisus scoparius) and hawthorn (Crataegus monogyna). Nor did towns and cities escape the adverse effects of naturalized plant and animal species, as the publications of 5-10 make clear for metropolitan Auckland. Three types of environmental experiment were initiated by Maori and European settlers. The first involved large scale modification of the land and its vegetation cover by people. The second involved selective removal of native animals from indigenous ecosystems. And the third involved purposefully as well as accidentally introduced plant and animal species. The three types of experiment, all of them lacking 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 geographic ranges of many species have expanded. In most settled areas, weedy plants have become widespread and their increasing dominance is viewed with alarm ( 18). Spontaneous establishment of exotic conifers on depleted tussock grassland in the Waitaki valley. Two decades after the start of organised European settlement in Canterbury, 3 reported an adventive flora of some 170 species, ‘being nearly one-fourth of the total number of flowering plants (naturalised and native) found in the province’. Seven years later, his son’s surveys ( 2) brought the total to 250 species. By the 1960s, there were more than 1000 species of adventive flowering plant ( 12) and their impact was felt throughout the territory. ‘There are 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, with other adventive plants, and with members of the cultivated plant flora’ ( 12: 265). A decade earlier 15had noted the initiation of ecologically novel plant communities composed of indigenous and adventive species. By then, numerous biogeographic and ecological experiments were well 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 of people and their animals then many adventive species would disappear. Since then, documented instances of naturalisation have multiplied and it is not evident that all adventives occupy habitats directly or indirectly created and sustained by people and their animals. Berberis spp, seem here for the long haul as do Clematis vitalba, Hieracium spp, Ribes sanguineum, Rubus fruticosus and Stipa trichotoma, to name six taxa. With a naturalised angiosperm flora of 2000 species, a little smaller than the total of native vascular plants, New Zealand appears to have gained a substantial number of resident species in just 150 years. At the same time, however, there is no evidence for major extinction of native plants. How many resident species can an island nation like New Zealand support? Which of its ecosystems are most likely to become home to introduced species, and what functions will those plants serve in the new ecological fabric? Will some ecosystems prove especially vulnerable to invasive species? Might a newcomer share its genes with a closely related native species, and what long term effect would that have on ecosystem functioning? Does it now make sense to think of ‘natural’ vegetation when our footsteps are everywhere? Models like that of 4—which involves the arrival of a propagule, its possible establishment, then population growth, spread, dominance or displacement, and integration through niche shift or character displacement—can be expected to aid in untangling what is happening around us and point to useful management routines. Others, like Equilibrium Island Biogeography (EIB) theory, raise fundamental questions. A tenet of the EIB model is that once the biota of an island reaches equilibrium, a new arrival can persist only if an established species is displaced. What is not clear is how displacement is effected or even how long that might take. Did the flora of New Zealand fall short of equilibrium biodiversity on the eve of European settlement? The palaeoenvironmental literature points to major losses in vertebrate and higher plant species during the Pleistocene, leaving the country with an impoverished biota. Is the rich adventive flora of lowland New Zealand mainly explained by ecological opportunities in its substantially transformed vegetation cover for newcomers that have been separated from their co-evolved predators? And what about adventives growing in primarily native ecosystems; will they suppress native species, possibly even bringing about their extinction, thrive alongside them or disappear? We are almost certainly witnesses to an early stage in the development of 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 exotic species persist? Answers to those questions require information from carefully designed and managed programmes of field observation and experimentation. Most field experiments involve controlled manipulation of just one or two factors, then programmes of observation extending over months or years. Alternatively, it might be possible to model interactions and shorten observation time by running a computer simulation. The experiments I refer to lack those advantages and raise substantial practical, logistic and funding issues. Ensembles of plants and animals are usually highly buffered, have long lag times, and tend to switch between metastable states. Their path after perturbation is difficult to predict. For example, if the soil seed bank has been depleted then vegetation change will reflect what remains on site and in nearby reserves of propagules. And if pollination or dispersal should depend upon just one animal species then its disappearance from the area could take from years to decades to show in the plant population. It follows that programmes of observation and experimental manipulation on ensembles of native and naturalized species should run for many years. I am not advocating a passive stance towards invasive species, and am certainly not proposing we stand by and do nothing to control importation of pest plants and animals or eradicate what is already established. Rather, I am arguing that new functional ensembles, comprising native and adventive plant and animal species, are apparently developing in our cultural landscapes and more can be expected. We need to understand what is happening in them to facilitate the survival of native species.