iCS^fe 



and forest. The species composition of each stage 

 varies with the chemical nature of the rock, the pre- 

 vailing climate, and the locality. 



In the first stage, various kinds of lichens com- 

 pete for a foothold, but crustose types usually pre- 

 cede foliaceous types. Mosses and such fruticose 

 lichens as Cladonia follow foliaceous lichens; or may 

 initiate the sere, telescoping the earlier lichen stages 

 (Keever, Costing, and Anderson 1951). 



FIG. 8-6 (a) a tardigrade, Echiniscu: 

 oibounae, occurring in moss, possessing 

 long filaments (Heinis 1910). (b) 

 ant-lion adult (Shelford 1913). 



"f 



Lichens and mosses soak up moisture in wet 

 weather. They derive mineral nutrients from the un- 

 derlying rock. Carbon dioxide secreted from the rhi- 

 zoids forms a weak acid with water and dissolves the 

 binding material of the small rock particles. Rhizoids 

 may penetrate rock for several millimeters. These 

 plants trap windblown dust and obtain nitrogen from 

 organic compounds in it. When the plants die, they 

 become an addition to the accumulation of organic 

 matter. Herbs, grasses, ferns and later stages invade 

 to continue the crumbling of the rock and buildup of 

 soil. Freezing and thawing of water may crack the 

 rock, and in these cracks wind- and water-borne soil 

 lodges and supports plants. Once shrub and tree 

 roots get started in crevices, their growth exerts a 

 powerful force further splitting and crumbling the 

 rock. 



Animal life 



Animal life in the pioneer plant stages on rock 

 is scanty. Ants and spiders roam over the bare rock, 

 and insects of various sorts may stop there, tempo- 

 rarily. Spiders may construct webs and nests in rock 

 crannies or amongst the lichens. Some tardigrades 

 find preferred niches in lichens. Mosses offer a some- 

 what more substantial microhabitat, but only those 

 animals that can tolerate great extremes of flooding, 

 dessication, heat, and cold can survive. Such forms 

 are found in the rhizopod protozoans, nematodes, 

 bdelloidid rotifers, tardigrades, copepods, small in- 

 sects, and mites (Heinis 1910). They often have spe- 



04 



Habitats, communities, succession 



