General.

In Part II of this book an attempt has been made to paint a picture both of the primeval plant-covering of New Zealand and of those communities as yet but slightly modified by the action of man. This Part, on the contrary, treats of that greatly altered or entirely new vegetation which now occupies so much of the land, and some of the fundamental causes are discussed which have operated in bringing about the vast difference between the New Zealand of 1769 and that of 1927. On this intensely interesting subject a good deal has been written, some of which contains important data and luminous suggestions, but, on the other hand, erroneous statements are even yet widely accepted as truths and misconceptions have crept into authoritative scientific writings¹.

Statistical.

The number of exotic species more or less firmly established in New Zealand is about 514³, including 3 confined to the Ker-page 354madecs, (pteridophytes 2, gymnosperms 3 — but probably more — monocotyledons 113 and dicotyledons 396). They belong to 75 families and 279 genera, the largest, together with the number of species for each, being the following: -— (families) Gramineae 83, Compositae 75, Leguminosae 46 Cruciferae 30, Caryophyllaceae 23, Labiatae 16, Scrophulariaceae 15, Rosaceae 14, and Polygoonaceae and Ranunculaceae 13 each; (genera) Trifolium 16 Ranunculus 13, and Bromus 9. Twenty-four families and 219 genera are represented which are wanting in the New Zealand flora.

As for the source of this large exotic element, by far the greater part is European, no fewer than 375 species belonging to this class. Australian species number only 30, notwithstanding constant traffic for many years with Australia, including the importation of living plants, seeds and agricultural products. North American, South American and South African species number respectively 26, 18 and 12, and the remainder are mostly subtropical or tropical of wide distribution.

Coming now to the distribution within the Region of the exotic species it is as follows: — North Island 469 species, 111 of which are restricted thereto; South Island 399, 41 of which are restricted thereto; Stewart Island 72 (cockayne: 1909) but there must be more, yet so much of the vegetation is virgin and introduced plants cannot become established; Kermadec Islands, as recorded by Oliver, 51; Chatham Islands (list supplied by E. N. Northcroft) 128; and Subantarctic Islands 25, including Phormium tenax and Acaena Sanguisorbae var. pusilla of New Zealand proper, but Campbell Island having now being used for sheep-farming for more than 24 years, there should be considerably more. With regard to their vertical distribution, the greater part is confined to the lowland belt of which some 12 are restricted to the coast-line or there abouts, and 50, at a high estimate, gain the montane and lower subalpine belts, but none are restricted to the high mountains.

Regarding the lifeforms of the species, 23 are trees, 28 shrubs, 94 of the grass-form, 5 of the rush-form, 344 herbs or semi-woody plants, 6 lianes, 3 parasites and 10 water-plants.

The species themselves differ greatly in their relative abundance and there is a gradual decrease from those of the widest distribution, and with ample individuals, to such as are only recorded so far from one or two localities, where they just hold their own. Certain species, too, are present in abundance, but they are confined to a definite habitat of perhaps limited extent and others, again, are restricted to the cultivated areas, or to waste ground, and really have little or nothing to do with modifying the primitive associations. It is then a most difficult matter to decide as to relative abundance or importance, and I am far from satisfied with the figures about to be given, since species belonging to different categories are counted as equal. About 64 species may be considered very common, 65 common, 105 neither common nor rare, 54 of local occurrence, 100 rare and 126 — the page 355largest class — very rare; indeed, if the species of the last two classes he taken together, it is plain that 44 per cent of the exotic flora is of no moment. On the other hand, the very common and common species form together only 22 per cent of the aliens. Perhaps, on the whole, the most wide-spread species are Rumex Acetosella and Hypochoeris radicata followed, but at some distance, by Holcus lanatus and Trifolium repens. But none of the commonest species, nor, indeed, any of the exotic plants, are nearly so "aggressive" or have spread so widely on uncultivated ground, under the influence of settlement, as the indigenous Pteridium esculentum, Leptospermum scoparium or perhaps Danthonia pilosa in its various forms. Various forms of Acaena novae-zelandiae and A. Sanguisorbae are also almost the equal in aggression of any exotic alien. Without giving a full list of the 64 very common species the following may give some idea of their character: — Anthoxanthum odoratum, Agrostis alba, Holcus lanatus, Dactylis glomerata, Poa pratensis, Ranunculus repens, Rubus fruticosus (in a wide sense), Ulex europaeus, Cytisus scoparius, Trifolium repens, Centaurium umbellatum, Prunella vulgaris, Plantago major, Erigeron canadensis, Chrysanthemum Leucanthemum, Cirsium arvense and Hypochoeris radicata.

In order to ascertain more fully the position the exotic plants hold in the vegetation it is necessary to consider their habitats. In this regard waste ground far and away stands first in the number of its alien plant-population. Ground of this character is always in process of being made in a "new" country and it occurs on roadsides, on little used roads themselves, on railway embankments and the like, on cuttings, on unused sections in towns and villages and, indeed, on bare ground generally. Obviously, far more than one habitat is included under the above term, but in all there is open ground ready for occupation by plants and all kinds of growing-places are provided in regard to sun, shade and moisture-content of the soil. If to the species of waste ground be added those of cultivated land in its widest sense, by far the greater part of the exotic flora is accounted for and 100 species is perhaps too wide an estimate for those aliens which really come into competition with the indigenous species.