102 



the head of Main Creek. Sample 6 is clearly affected considerably by the 

 evaporation of sea water. It has a high salt content, sodium chloride 308 per 

 cent., but is poor in humus. 38 per cent., and also in nitrogen, 07 per cent. 



All these granitic soils, except that from the Casnarina woodland, are 

 neutral or on the alkaline side of neutrality. 



III. Bjotic. 



Biotic factors in the environment do not have so obvious an effect upon 

 the flora of the Pearsons as they have upon the Franklins. '"'^ 'J'he soil is unsuit- 

 able for the burrowing activities of either the mutton birds or penguins, which 

 were so abundant there. Mutton birds are absent and the penguins have to 

 nest under rocks. The only grazing animals are the indigenous wallabies 

 (Pctrogalc pearsoni), which are frequently seen on Northern Island. Their 

 chief food plant appears to be Atriplcx pahidosum, but their feeding does not 

 damage the bushes appreciably. One strange feature is the remarkable abund- 

 ance of the droppings of some insect. These occur thickly on the surface of 

 many soils, especially in the Olearia shrubland and the lower slopes of the 

 Casuarina woodland. The faeces are hard, black pellets, roughly cylindrical, 

 2 to 3 mm. long by 1 mm, in diameter. They are extraordinarily frequent in 

 some places and can be swept up off the soil by the handful. Mr. A. M. Lea, 

 Entomologist of the S.A. Museum, kindly informs me that they resemble the 

 dung of a cockroach. A large species of this insect is generally found under 

 stones on the island. Judging by the amount of dung, they must occur in very 

 large numbers at some seasons. Presumably they feed on vegetable debris, 

 which they return to the soil in a form that is not readily removed by wind or 

 leeching. The deposit is so abundant as to affect considerably the percentage 

 of nitrogen in the soil anlayses. 



Human interference with the vegetation is limited to the results of one or 

 two fires that have been started on the islands by visiting vandals. The eft'ect 

 of these has been almost to destroy the Casuarinas south of Main Creek ; else- 

 where the burnt-out flora has regenerated well. Several years ago a number 

 of sheep were landed upon the Northern Pearson. There are none there now, 

 and the vegetation appears uninfluenced by them, unless the grass Fcstiica 

 hromoides was brought to the island by this means. Fcstitca hromoidcs was the 

 only non-indigenous Australian plant collected, except the variety littoralis of 

 Soiichiis aspcr. 



VEGETATION. 



Three main vegetation types occur on the Pearsons, exclusive of various 

 communities on cliff faces, creek beds, blown sand, etc., which are not easily 

 placed in relation to the others. They are: — (1) The woodland and scrub 

 series on granitic soil with a climax of Casuarina stricta woodland; (2) saltbush 

 consisting of Atriplcx pahidosum dwarf shrubland, occurring chiefly on granitic 

 soil; and (3) the various communities on travertine limestone. The first 

 two types may be considered as formations, and the ecotone lines are usually 

 clearly distinguishable. An exception occurs on the right, or south, bank of 

 Main Creek. There the blurring of the line appears to have been aff'ected to 

 some degree by fire. The last series, as explained below, is formed merely as 

 a matter of convenience. 



Woodland and Scrub Series. 

 I. Casuarina stricta woodland. 



A woodland, or "forest," of Casuarina stricta occurs on the three large 

 hills of Northern Pearson. The trees grow to a height of 20 to 30 feet (pi. v., 

 fig. 1). The length of the green assimilating branches varies considerably with 



t-5) Osborn, T. G. B., loc. cit., p. 203, 1922. 



