ECOLOGICAL STUDY OF BRUSH LAKE. 1 63 



GENERAL CONSIDERATIONS. 



That the plants around the lake are arranged in rather defin- 

 ite zones appears to be due to two general causes : first, because 

 of the gradual change in the character of the substratum and 

 other factors from one condition to another ; second, because 

 certain plants are especially adapted to certain conditions and are 

 thus able to hold their ground against invaiders from the outside. 

 In an ordinary pond the successive modifications of the sub- 

 stratum occur in concentric belts and hence the succession of 

 more or less perfect zones or belts of vegetation. Between each 

 two contiguous zones there is a tension line where the struggle 

 for existence among the opposing individuals becomes very 

 severe. Each zone, therefore, has a fighting line on its inner 

 and outer edge, while in the central part the struggle is only 

 between plants of the community, both old and young, and such 

 chance strangers which may be able to gain a foothold in the less 

 densely populated areas. That many plants almost entirely 

 restricted to a single zone can hold their own, either farther in 

 or out, if once established is readily seen where normal conditions 

 have been disturbed. There may thus be considerable difference 

 in the result, depending on whether plants have had the oppor- 

 tunity of occupying a bare soil or whether they must contend for 

 the soil already occupied by others. Certain species might hold 

 their own indefinitely in a certain environment if once firmly 

 established, while they might not be able to gain a foothold if 

 the soil is already occupied by others. 



The seven zones of Brush Lake are slowly traveling inward, 

 as the lake is filling up with the wash from the surrounding hills. 

 Each zone follows up its ideal environment and is crowded out 

 on the outer margin by its next neighbor. The filling must have 

 been quite slow and gradual in former times, but at present it is 

 rather cataclysmic and will continue to be so in the future. There 

 are therefore unusual disturbances in the progression of the zonal 

 societies which have been alluded to above. The physiographic 

 changes determine the changes in the zones. If these are gradual 

 and slow in their progression the same will be true of the plant 

 societies, but if the filling is sudden and large additions are made 



