Effect of Introduced Animals on the Plant-covering.*
As I have suggested when speaking of the aborigines, very little change would be wrought in the vegetation of the island by either Moriori or Maori. They would have little object in setting fire to large tracts of country; and, even in case of such being burnt, there would be neither the foreign plant nor animal factor to cause a marked change in the reproduced vegetation. Even at the present time it is almost impossible to discriminate between a piece of bog vegetation which has been burnt many years ago and a piece of the primitive formation. It may therefore be assumed that, to all intents and purposes, the plant-covering of the island at the time of the advent of the missionaries was precisely as it had been for
[Footnote] * In connection with this, and with the effect of burning on the vegetation of New Zealand, the interesting paper of Canon Walsh may be studied (53).
a very long period. But as the animals which were introduced increased in number and spread over the entire island a very different state of affairs arose for the indigenous plants. They too, like the Moriori, had been long isolated, and the various species had become finally differentiated without any regard to the attacks of grazing animals. Cattle and horses roaming over the sand-dunes would loosen the sand, and also feed in many instances on those plants which bound the sand together.
There are few balances more finely adjusted than that between sand blown inland and its fixation by plants. Once disturb that balance, no matter how slight the disturbance may be, the equilibrium will be destroyed, and the resistance of the sand-binding plants overcome. Thus the destruction of a few plants growing on the dry sand of the shore just above high-water mark, the great source from which all inland-blown sand arises, will increase the volume of wind-driven sand against which the dune plants have to contend.
In Chatham Island, as pointed out before, quite a number of plants grew on this upper strand, conspicuous amongst which was the majestic forget-me-not Myosotidium nobile. The leaves of this plant are much relished by sheep, and so, as the settlements of both white man and Maori are usually near the coast, this plant would very early on be attacked. Not only do sheep eat the leaves, but pigs dig up and feed on the great rhizomes, with the natural consequence that the endemic M. nobile, one of the most magnificent and interesting plants in the world, is now all but extinct in the wild state. A few plants still exist on the north coast of the island, notably near Matarakau and Waikauia, and there are a few in the neighbourhood of Red Bluff, while below Te Awatapu there is a bed of plants still in its virgin state; but the long line of this plant on the sea-shore, with its huge shining green leaves and great heads of blue flowers, is lost to the world for ever. Happily the plant is very amenable to cultivation in favourable localities, and almost every settler's garden contains some fine examples. Were a piece of ground fenced in from sheep, &c., the plant would again reappear, as in the case of an old Moriori grave-yard fenced in by its owner, Mr. H. Grennel, and described by Professor A. Dendy in a paper read at a recent meeting of our Institute (15). The sand, no longer held by the strand plants, blew inland, became piled up against the dune forests, and, gradually accumulating and advancing inland, it finally buried them, so that now, instead of a fringe of trees all round the sandy parts of the coast, there are high moving dunes in many places.
In the forest cattle and horses eat the foliage and bark of the trees, at the same time breaking down the undergrowth and
the lianes. I was much struck when first examining Chatham Island forests by their want of undergrowth and lianes, which did not accord at all with Travers's description when writing of the Moreroa Bush, thus: “The whole so interwoven with our old friend the supplejack as to be almost impenetrable (51, p. 176). However, when visiting the Te Awatapu Forest I saw clearly that the lowland forests were no longer in their primitive condition. Messrs. Chudleigh, Shand, and Cox have also all assured me that the forests were formerly very much denser and lianes more abundant than is now the case. In addition, at the present time the forest on the Horns is undergoing an early stage of destruction by cattle, which have only reached that part of the island recently. Almost everywhere are trees broken or, in the case of Piper excelsum, uprooted, while the ground is much trodden and seedling trees and young ferns destroyed.
Another change which concerns the proportional representation of species in a forest-formation is destruction of certain trees through animals eating the bark. Pseudopanax chathamicus, Plagianthus chathamicus,* and Coprosma chathamica very often suffer through this cause. Such trees being destroyed, others easily and rapidly produced from seed take their places, and so are now more numerous than formerly. Mr. W. Jacobs tells me that, owing to this cause, the constituent trees of the Moreroa Forest no longer exist in the same proportion as they did twenty years ago. Certain plants have been almost eradicated by grazing and uprooting. The case of Myosotidium nobile has been already referred to. Aciphylla traversii, another very characteristic Chatham Island plant, is now very scarce. Its leaves are greedily eaten by sheep, and its thick tap-root is devoured by pigs. At the present time isolated plants may be found in boggy ground in all parts of the island, but large areas may be traversed without encountering a plant. Even in the fairly primeval tableland district it only exists in any quantity on the steep banks of some of the small lakes which are not easily reached by stock. Veronica dieffenbachin and the other forms of Veronica closely allied to that species are, according to Mr. Cox, greedily eaten by sheep. These Veromcas, in consequence, are now confined to rocks and banks of creeks or lakes; doubtless at one time they were much more plentiful. It is probable also that the great sowthistle, Sonchus grandifolius, found now chiefly in places inaccessible to stock, was at one time much more abundant on the sand-dunes; but I have no proof as to
[Footnote] * This plant is also destroyed by the settlers, who at times use the bark for making hats. The same remark also applies to the palm. Other trees, especially Olearia traversii and Pseudopanax chathamica, are cut down for fencing-posts and firewood.
whether it has been reduced through being eaten by stock or through instability of the dunes. At any rate, either cause can be traced to the advent of domestic animals.
Besides changing the vegetation through feeding upon it, horses and cattle have also had a very great influence on the water-content of the soil. Wandering over the swampy and boggy ground in search of the food which was originally very plentiful there, they gradually consolidated the surface of the ground. By this means many of the swamps of the island have been turned into rich grazing land. The racecourse at Waitangi is an excellent example, and such “reclamation” of ground by grazing animals can be observed in every state of progress from quite dry meadow to almost primeval swamp. In this particular instance, too, the effect of the final close cropping of the herbage by sheep may be observed, and the change wrought by this on the vegetation estimated. Certain plants which formerly did not form any large percentage of the original vegetation, or which were altogether absent, now make up the meadow land of the racecourse and the ground between the low forest and sand-dunes from Waitangi to Te One. On this piece of ground sheep in very large numbers are constantly grazing, and yet the present vegetation manages easily to hold its own, and has entirely replaced that of the original swamp, which must in large measure have been similar to that described under the heading “Swamp Formation.” The plants consist almost entirely of those which possess a far-reaching and rapidly growing prostrate or subterranean stem-system, which, through the great power of vegetative increase which it gives, the abundant food-supply which it contains, and its being secure from damage, enables its possessors easily to resist the attacks of grazing animals. At present the Waitangi Racecourse is a flat meadow marked with many small hillocks or unevennesses, which proclaim the presence of former clumps of swamp vegetation. Everywhere the ground is covered with a thick turf. This is composed of Cranizia lineata in very large quantities; also Pratia arenaria, the introduced Poa pratensis, a small species of Juncus, Potentilla anserina, all in large quantities; Hydrocotyle asiatica abundant, but hardly so much so as the preceding; a variety of Epilobium cœspitosum, Myriophyllum pedunculatum, Lagenophora forsters, Elecoharis gracillima, and Gnaphalium collinum, also fairly plentiful. On the driest portions of the hillocks is abundance of Gnaphalium filicaule. The whole of these plants have stems which are either underground or creep close to the surface, and several, as we have seen, have great powers of adaptation either to wet or dry conditions of soil. Also, some may not be much eaten by sheep, but it is significant that the two which are especially
abundant, Grantzia lineata and Pratia arenaria, are considered by Chatham Island sheep-farmers* most valuable pasture plants; and there seems little doubt that these two plants at any rate, owing to former plant adversaries having been removed, partly through changed edaphic conditions and partly through close grazing by animals, have become very much more abundant in Chatham Island generally than was formerly the case—helped, of course, by the great vegetative increase of their stems rendering them safe from the attacks of sheep. But if a plant be isolated and there is no other food for the grazing animals it may not be able to hold its own. Thus Poa chathamica, notwithstanding its strong wire-like rhizome, cannot resist close grazing on the tableland bogs, where no other food is present for the hungry animals, and it may be eradicated for a time.
Besides domestic animals, certain European birds have been brought over to the island, and others, strange to say, amongst which are the sparrow and blackbird, are said to have made their way from New Zealand unaided. Such birds play a much more prominent part with regard to the introduced than to the indigenous vegetation, doing a great deal of damage to the crops and gardens of the farmer. They also carry and distribute the seeds of both native and introduced plants; in this case, as pointed out before, doing the work of the former indigenous birds, which now for the most part have become very limited in number. Perhaps the greatest work such birds are performing is that of spreading the blackberry all over the island, but this matter concerns the next section.
As for the effect of introduced insects, I procured no information; probably the hive bee plays an important part in fertilising certain of the introduced plants, and so causing their spread.
[Footnote] * Mr. E. R. Chudleigh, for instance.