Agents altering Plant-distribution.
The primitive plant covering, then, has largely disappeared, and has been replaced by introduced species, particularly by grasses; the balance in the remaining plant associations has been altered, and the conditions under which many of the species live are quite changed. Probably the grasslands of the Lyttelton Hills have been less altered than the forests of the peninsula, for the tussock form still dominates; yet here certainly many changes have taken place and others are still in progress. The chief destructive agents at work where cultivation has not been employed are continual tussock-fires, bush-fires, drought, sheep, cattle, on the Lyttelton Hills rabbits, and on Banks Peninsula hares.
On the grasslands the tussock-fires make for the extinction of the ferns, for the reduction in the number of the species, for the replacement of certain grasses by others, for the destruction of isolated shrubs, and for the multiplication of a few plants that can to some extent withstand the action of fire—e.g., Coriaria sarmentosa.
On the other hand, bush country when burned and allowed to restock itself reproduces the usual fire weeds; but these are seldom or never allowed to remain, being either destroyed by the hand of man or replaced in old burns by Pteridium esculentum or species of Leptospermum. These changes however, will be considered in greater detail under the various plant formations.
It may, however, be noted here that in all the moister bays of the peninsula—that is to say, from Port Levy eastward and southward to Akaroa—cocksfoot (Dactylis glomerata) was sown on the burnt areas, and the result has been the development of an artificial plant association. After the first sowing this grass has replaced itself in a remarkable way, and though reaped for forty or fifty years has continued to produce crops of almost undiminished vigour. Largely owing to the scarcity of casual labour the cocksfoot harvest is now becoming a thing of the past, and dairy-farming is to a large extent replacing it.
The cattle are grazed on the cocksfoot, which is the chief ingredient of the pasture lands, though rye-grass (Lolium perenne), timothy (Phleum pratense), and crested dogstail (Cynosurus cristatus) are also to be met with in smaller quantity. Sheep are to be found chiefly on the tussock areas, though even on the cleared forest areas they are in many places being stocked in increasing numbers, owing to the smaller amount of labour involved in looking after them. Further discussion of these economic matters would, however, be out of place here.
The changes due to the action of animals on the vegetation, though extensive, are not so wide-reaching as those of fire. Sheep crop certain grasses, and thus prevent them seeding. They also attack other species of plants, such as Angelica montana, Carmichaelia subulata, and even the young spinous leaves of Aciphylla squarrosa. On the other hand, their function in the spreading of plants by means of seeds can hardly be overestimated. The large increase in danthonia (Danthonia pilosa) referred to farther on may be cited in this regard. Other species spread by the same means are Urtica urens, Acaena novae-zelandiae, and Marrubium vulgare (white horehound). Cattle, again, are probably entirely destructive. Phormium soon disappears in their presence, as also does Marrubium. In Pigeon Bay, for example, the upper portions of the hills are in many places grey with horehound, whilst the cattle-country immediately below is perfectly clear. Unfortunately, cattle will not run in the rocky, broken country on
the hill-crests, and here Marrubium has become the worst pest of the district. It remains to be seen whether with time it will work itself out. Probably in the absence of sheep it is but little distributed. Cattle also, as is well known, rapidly destroy the undergrowth in bush country. Rabbits, also, on the area dealt with are probably almost entirely destructive in their action. They reach many places which are either inaccessible to sheep or at least not usually grazed by them. On some of the rocky points of the Lyttelton Hills, which would otherwise be plant-sanctuaries, scarcely a plant is left untouched by them. Shrubs and grasses are all grazed, and only the inaccessible chasmophytes escape. Hares apparently are much less destructive than rabbits, because they are in smaller numbers, do not burrow, and do not frequent cliffs and rocks. The tendency of these agents is not only to reduce the number of indigenous species, but also, as a rule, to limit the numbers of the individuals in the remaining species.
Certain species, however, are apparently on the increase as the result of changed conditions. Such are Danthonia pilosa, D. semiannularis, Acaena novae-zelandiae, Coriaria sarmentosa, Cotula squalida, Dichondra brevisepala, and Oxalis cormculata. The two last-mentioned invade even cultivated ground, and I have seen O. corniculata growing about the steps of the public buildings of Christchurch.
During a succession of dry seasons the grasses mentioned become common in garden lawns on the hills, and replace temporarily or permanently the shallow-rooted lawn-grasses. In dry seasons, too, rabbits attack the bark of trees, and thus assist the drought in its attack upon the vegetation Melicytus ramiflorus, Schefflera digitata, and Nothopanax arboreum are amongst the first to suffer, particularly when standing in the open.