GARDEN PESTS IN NEW ZEALAND 



the establishment of organisms; the latter reduce these plant residues 

 to humus, and during this process of decomposition produce food 

 materials of an organic origin suitable for the nutrition of the sequential 

 plant covering. So the process proceeds until a soil is formed of 

 sufficient extent and quality for the support of a more extensive and 

 increasingly complex vegetation; thus, in the cycle of life and decay, 

 stores of organic compounds are elaborated by plants and returned to 

 the soil, which they enrich, and where they are decomposed by 

 organisms, and so maintain the supplies of food materials suitable for 

 the maintenance of vegetation. 



These phenomena of plant establishment and succession, correlated 

 with soil formation, were clearly demonstrated by the re-establishment 

 of vegetation after the soil and plant life had been) destroyed by the 

 historic eruption in 1883 of Krakatoa, a volcanic island in the Straits 

 of Sunda, between Java and Sumatra. The first plants to be established 

 on the volcanic deposits were species of terrestrial alga?, which gradually 

 spread and built up soil suitable for the development of soil organisms 

 and for the growth of seeds brought to the island by birds and ocean 

 currents. So rapid were the changes brought about by these influences, 

 that within a period of twenty years after the eruption the barren 

 ground was reclothed by a dense and varied plant covering. 



Organisms that form part of the organic complex of the soil range 

 from the more conspicuous species, such as slugs and snails, insects, 

 spiders, wood lice, millepedes, earthworms and eelworms, to such micro- 

 scopic forms as protozoa, fungi, algae and bacteria, the last three being 

 members of the plant kingdom. These organisms may be grouped as 

 follows : 



( 1 ) Temporary inhabitants that .enter the soil for shelter, or to 

 feed as scavengers on decaying organic matter, or both e.g., 

 slugs, snails, wood lice, certain insects and some eelworms. 



(2) Permanent inhabitants that are dependent on the soil for their 

 development and supplies of food, either throughout or during 

 most of their lives e.g.., certain insects and spiders, millepedes, 

 earthworms, eelworms, protozoa, fungi, algae and bacteria. 



The organisms in the first group play a comparatively minor part 

 in soil development, and influence its fertility to an almost negligible 

 extent, the temporary scavengers, perhaps, being of more importance 

 since they aid in the reduction of vegetable residues. The forms in the 

 second group, however, are invaluable as soil-making agents and in the 

 production of plant food materials, the least important among them 

 being the insects, spiders and millepedes. Many are merely scavengers, 

 but some insects, such as grass-grubs and the caterpillars of certain 

 moths, and millepedes, feed upon living plants and so add organic 

 matter to the soil in their excreta, which also contains quantities of soil 

 swallowed with the food, this latter mechanical action aiding in the 

 pulverising and opening up of the soil; certain eelworms, too, that 

 attack living plants play a somewhat similar part, .in that they are 

 primary causative agents in the decay of healthy tissues. Other forms 

 of insects, together with spiders and some eelworms, are predaceous 

 upon their fellows, the remains of the latter being added to the soil 



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