[Read before the Wellington Philosophical Society, 12th January, 1878.]
One of the most interesting branches of scientific investigation is the displacement or replacement of plants and animals which we know is now in progress over nearly all parts of the earth's surface. In islands and continents where man has not taken up his permanent abode, the process is slow but none the less certain; seeds of plants from other latitudes, wafted by the waves, germinate on the shore, and finding a suitable habitat, are gradually diffused through the interior; other seeds, or possibly fragments of plants themselves, are borne by birds, even by insects, or in some rare cases carried by winds; seeds of plants from more distant regions may be accidentally thrown overboard from passing ships, or the sailor landing to inter his dead shipmate, leaves behind him the northern chickweed, or the broad-leaved plantain, which so habitually follows the track of the pioneers of civilization that the North American Indians have poetically termed it the “footstep of the whites.” It is easy to realize how by these and similar noiseless agencies, material changes may be produced in the aspect of the flora of an uninhabited country in the course of centuries. But with the advent of man other forces acting in the same direction are brought into operation partly by design and partly by accident, so that for a time these changes are accelerated in a constantly increasing ratio, and the work of centuries is compressed into a decade. The forest is destroyed, the vegetation of the plain is changed, or at least so intermixed with exotic plants that its aspect is entirely new. Foreign weedy plants spread through the land, destroying by their superior vigour much of the original vegetation. In more distant situations sheep and cattle feeding closely upon the herbs, or on the tender shoots of shrubs,
speedily destroy the undergrowth, admitting the light and air. These in their turn act unfavourably on the larger vegetation which has attained its growth in a dark damp atmosphere, and the injurious agency gradually extends over a constantly widening area. But in this colony it rarely happens that the process of displacement passes into complete replacement; it rarely or never results in the extirpation of indigenous species, although it greatly reduces the number of individuals. The admission of air and light, while unfavourable to certain plants, tends to increase the vigour of others, which exhibit a luxuriant growth they had never before displayed, and at length a turning-point is reached, the invaders lose a portion of their vigour and become less encroaching, while the indigenous plants find the struggle less severe and gradually recover a portion of their lost ground, the result being the gradual amalgamation of those kinds best adapted to hold their own in the struggle for existence with the introduced forms, and the restriction of those less favourably adapted to habitats which afford them special advantages. This, in brief, is a statement of the phenomena now in progress throughout the colony; but at present we are not in a position fully to appreciate several of its bearings.
It can scarcely be expected that those who were familiar with the general features of the vegetation of New Zealand before they were modified or changed by the progress of settlement will at once accept the statement I have given as correct. They have witnessed the steady onset of axe and fire, the unceasing advance of cattle and sheep, and they have been so impressed with the almost total extinction of many striking plants over areas where they were formerly abundant, as to have lost sight of the tenacity with which plants in general maintain their existence even under unfavourable conditions, of the surprising power of adaptation which they often exhibit under changed circumstances, and are led to the conclusion that sooner or later the majority of our native plants must inevitably become extinct. I can only share in this fear to a limited extent, and could almost count upon my fingers the particular plants for which such a danger is most to be feared. In no part of the world has agriculture been carried to a higher pitch of perfection than in the British Islands; in no part have the open lands been more completely brought under cultivation; yet we know that under these adverse circumstances not more than two or three species, at most, have become extinct, although many have become extremely rare, and only maintain themselves in situations offering peculiar advantages. The Killarney fern, Trichomanes radicans, Sw., has often been reported as extinct, yet scarcely a year passes without some new station being discovered, or some of the old stations proving reproductive. Asplenium
germanicum, Weiss, affords another instance of the tenacity with which plants maintain their existence under the most unfavourable conditions. Adiantum capillus-veneris, L., Woodsia ilvensis, Br., W. hyperborea, Br., Nephrodium cristatum, Rich., are similar examples. Not only have the plants here named to endure the changed conditions brought about by agricultural and pastoral occupations, but they have suffered largely from the ravages of vulgar curiosity-hunters, who value a thing only for its rarity, and sometimes strive to render the habitat of a rare plant unproductive, in order to enhance the value of the specimens in their possession; and from the mania for fern-collecting, which for years past has been a fashionable pursuit in Britain, as well as from the more legitimate but far less destructive indents of botanists, they may therefore be taken as extreme cases.
Numerous flowering plants exemplifying the same tenacity of existence under unfavourable conditions, and the power of adapting themselves to changed circumstances, might be named, but I will only state that during a detailed examination of the flora of the Auckland Isthmus and North Shore, extending over ten years, I failed to obtain the slightest evidence that a single species had become extinct, yet the district sustains a population of about 25,000 souls, on an area of 43,000 acres, less than half the extent of many sheep runs in the South Island. It is one of the oldest settled districts in the colony, and agriculture is in a more advanced state than in many other places; the only remains of forest are the few small patches of bush at the mouths of gullies between Takapuna and Lucas Creek, all traversed by cattle, while the open lands not actually under cultivation have been subjected to repeated burnings. Yet under these unfavourable conditions this small area, no part of which is above 650 feet in altitude, contains 440 species of phænogamic plants and ferns, representing 78 natural orders out of 91 under which these plants are arranged thoughout the colony, and giving an average of six species to the square mile. Moreover, this area is shared by 300 naturalized species, of which nearly two-thirds are as much at home as the natives of the soil. It is needless to offer further statements in support of my conclusion.
This paper is intended simply as a contribution to our knowledge of the distribution of naturalized plants in the colony and comprises an enumeration of the species observed by the writer in the Wellington district, with a few others for which the authority is stated in each case. It must not, however, be taken as exhaustive, since, without doubt, the list would be considerably increased by a careful examination of the more distant parts of the Wairarapa, the Upper Rangitikei, the country between Marton and Wanganui, and between Wanganui and the southern boundary of Taranaki, with all of which I am personally unacquainted.
In a future paper I purpose discussing in detail the position of naturalized plants with regard to the indigenous flora and their general effect on the progress of the colony, but for the present confine myself to a few remarks on certain species which exhibit features of special interest in this district.
I may, however, point out that the gradual decrease in the number of species as we travel southwards, which to a certain extent characterizes the indigenous flora, is exhibited also by our naturalized flora. Comparing the naturalized plants of this district with those of Auckland, we find the proportion to be less than 1.25 to 2, Auckland having fully 400 naturalized species, Wellington under 250. Making a fair estimate for the number of species yet to be collected in the unvisited portions of this district, it can scarcely be expected that the total will exceed 300; and' it may be added that the decrease is more strongly marked as we go further south.
It does not appear that this increasing paucity of species is solely due to a lower temperature. The peach ripens its fruit as thoroughly about Wellington as in any part of Auckland; yet while a constant succession of young trees is produced in the northern district, they are so few about Wellington that, except in peculiarly favourable situations, the plant does not increase when left to itself. The potato exhibits the same difference in a still higher degree: it would stand a much better chance of becoming permanently naturalized in Auckland than in Wellington; while the fig, which never flourishes here except under cultivation, in Auckland, even when utterly neglected, holds its ground and increases by suckers, although rarely by seeds, which in all probability are seldom formed owing to the absence of insects capable of effecting its fertilization. Similar remarks apply to the vine, the Cape gooseberry, and other garden plants, whether producing edible fruits or otherwise; but, on the other hand, the Kentish cherry and garden gooseberry increase with great rapidity when left undisturbed—the cherry both by suckers and seeds, the gooseberry by seeds and the rooting of the lower branches—so that a single wild plant sometimes forms a bush several feet in diameter.