232 SUBANTARCTIC ISLANDS OF NE\y ZEALAND. [Ecological Botany. 



8. Effect of Animals upon the Vegetation. 



(A.) SEALS. 



Before the coming of the white man, seals were extremely numerous. The 

 fur-seal (Arctocephnlus jorsteri), which is now almost extinct except on the Bounty- 

 Islands, existed in enormous numbers ; but, as its home was on exposed rocks, it 

 would have little eifect on the vegetation. The hair-seal (Arctocephalus hookeri), on 

 the contrary, is abundant even yet in the sheltered harbours, and may be frequently 

 met with at 50 m. or more inland, within the forests or the meadows, which it 

 reaches by means of well-beaten tracks. It is easy to see that where these animals 

 are numerous the undergrowth of the forest may be destroyed, as is the case on 

 Ewing Island, or the great herbaceous plants of a Pleurophyllum meadow flattened 

 to the ground. They frequently lie amongst the Poa foliosa tussocks, which latter 

 will undoubtedly benefit from the manure thus received. On Enderby Island the 

 hair-seals may in some small degree disturb the equilibrium of the dunes. According 

 to Hamilton, sea-elephants on Macquarie Island wallow amongst the tussocks near 

 the sea -beach. 



(B.) BIRDS.* 



Sea-birds play a very important part with regard to the vegetation of certain 

 parts of the islands. One class, the petrels, honeycomb the soft peat with their nesting- 

 burrows, and thus assist considerably in draining its surface. Also, the whole of 

 the birds — and these are in their millions, if the whole area be considered — supply 

 immense quantities of manure, not a product of the vegetation itself, but a 

 distinct addition to the soil. This fertilising, together with that from the seals 

 noted above, doubtless much favours the luxuriant growth of Poa foliosa, which is 

 such a marked characteristic of the shore vegetation. On the Snares all the streams 

 are veritable liquid manure — a fact which speaks volumes as to the fertilising action 

 of the birds on a peaty soil, whose abundant nitrogen cannot be utilised by the plants. 

 Excessive manuring may, indeed, lead to the presence of some special plant, as Senecio 

 antifodus of Antipodes Island and Cotvla Featherstonii of the Chathams 



But the chief effect of the birds, where they are present in vast numbers, is the 

 complete or partial destruction of the vegetation. On the Bounty Islands, beyond 

 a green alga on some of the rocks, there is no visible plant-life, the islands being 

 packed in the breeding season with millions of penguins and large numbers of moUy- 

 mawks. 



It is on the Snares especially that the effect of the penguins may be studied. 

 These colonies of birds, varying in numbers from a dozen or two to many thousands 

 crowded together in one place (fig. 17), exercise a profound influence upon the 

 vegetation. Not merely are the rookeries situated near the shore, but also within 

 the Olearia Lyallii forest and on the tussock meadows all over the island. Where a 

 rookery has existed for some time the ground is devoid of visible plant-life, becoming 

 a mass of mud and filth within an encircling wall of tussocks. Rookeries at all 

 stages of development may be seen, from those in which the tussocks are being 

 flattened down to those of bare ground as just described. Ultimately these 



* Ostenfeld ("The Land-vegetation of the Faeroes," p. 894, 1908) gives some interesting 

 details regarding the effect of sea-birds, especially how certain species determine the presence of 

 special plants. 



