Effect of Introduced Plants.†
With regard to the influence of introduced plants on the vegetation of Chatham Island I can say little, the time at my disposal not permitting me to collect examples, or even make a list of the species. Mr. T. Kirk published a list of those introduced plants which Travers collected (35), but it contains only twenty-eight species. Probably there were even then
[Footnote] † On the importance of this subject Mr. Hemsley, speaking of the work of the late Mr. T. Kirk (38), writes in Nature (27): “He”—Mr. Kirk—“has put on record facts connected with the introduction and colonisation of exotic plants in New Zealand that positively throw a new light and suggest new ideas on the present distribution of plants in cultivated countries generally.”
a great many more, and most certainly others must have put in an appearance during a period of more than thirty years. All the same, speaking generally, I do not think introduced plants have taken possession of the soil to anything like the same extent as in both Islands of New Zealand. When Chatham Island vegetation is destroyed by fire or cultivation, thus making way for introduced plants, it is certain indigenous plants which have become weeds rather than those which are introduced. For example, Acœna novœ-zelandiœ now abounds everywhere, becoming an actual torment to the pedestrian during certain seasons of the year, one's lower garments becoming completely concealed in a few minutes with a dense brown mass of its barbed fruits Again, the extremely wet character of the soil is antagonistic to the spread of many of those plants which have replaced the vegetation of New Zealand; while, on the other hand, the shade of the forest demands special adaptations from those plants which seek to get a foothold.* Certain plants, however, have spread very considerably. of such the blackberry (Rubus fruticosus) seems to be the only one which is a menace to any large proportion of indigenous plants. At first it was planted for hedges; but these hedges have now exceeded all bounds and are hedges no longer, but dense thickets. Were this all little harm would accrue, but through the agency of introduced birds the plant is spreading all over the island, especially within the forest areas. I noticed seedling plants in many places, even in the partly primitive tableland forests. On the banks of the Waitangi River are enormous thickets which hang right down into the water; indeed, in certain places considerable areas are occupied by this plant, and the original vegetation is entirely replaced. It is possible, if the spread of this plant is not checked in some manner or another, it may destroy the forest undergrowth entirely, as well as seize on large areas of open ground.
Poa pratensis is much valued in Chatham Island as a pasture grass; it has spread considerably in many places, and has even taken possession of certain stable sand-dunes, covering them with a turf. In wet meadows, such as the
[Footnote] * Introduced plants spread especially where the indigenous vegetation has been more or less disturbed. Where the plant-covering of a region is in its virgin condition, and there is nothing to bring any introduced plants except the wind, they often fail to become established. Thus Mr. T. F. Cheeseman saw only two naturalised species on the summit of Pirongia Mountain (5, p. 321), and these, he writes, “had in all probability been accidentally brought by the surveyors.” At the source of the River Poulter, in Canterbury, South Island of New Zealand, I saw no introduced plants of any kind in places where man, sheep, or fires had never been, although such country was fully exposed to the north-west wind, which must bring many light “seeds” from Westland (12).
racecourse before described, although it has become a distinct component of that recent plant-formation, it is no more dominant than some of its indigenous competitors. On dry slopes, where fires are constantly opening up room for the advent of introduced xerophytes, none have yet arrived which can make the slightest headway against Pteris esculenta.
Certain other causes distinctly operate in checking the spread of introduced plants, amongst which may be mentioned—the small area of cultivated land; the absence usually of roads, there being merely horse-tracks over the greater part of the island; the small amount of traffic with other countries; and, finally, the large number of sheep which graze on such land as introduced plants could best establish themselves on.