176 AGGREGATION, COMPETITION AND CYCLES 



sion, while annuation, aspection (including hibernation and estiva- 

 tion), and diurnation represent a descending scale of effects. All 

 these processes are concerned with numbers in varying form or degree, 

 though this is particularly true of annuation, which comprises the 

 fluctuations from year to year. With respect to the plant matrix, the 

 primary influence is one of structure, and hence these topics are mostly 

 considered in a later chapter (Beveridge, 1921; cf. Taylor, 1934; 

 IMcAtee, 1936) . By contrast, cycles of animal reproduction and migra- 

 tion are characterized by outstanding changes in population, often 

 directed by competition, and are properly treated as more or less 

 correlated results of the same or similar causes. Closely associated 

 with them is the problem of dynamic balance or equilibrium in the 

 biome, a condition that has usually been regarded as a more or less 

 static norm. 



Since land plants are stationary, fluctuations in their numbers are 

 less dramatic than with animals, but they are of the same order and, 

 as food and shelter, often assume a prior role in the causal sequence. 

 Such phenomena are most evident in annuals, as these are more sus- 

 ceptible to the climatic features of a single year. The most striking 

 instances are afforded by winter annuals in the Southwest; these may 

 be present by billions in arid grassland or in the Colorado and Mohave 

 deserts one year, and all but totally absent the next when the rainfall 

 is seriously deficient. Similar "flushes" are exhibited by phytoplank- 

 ton in small or shallow lakes, particularly by the blue-green algae; 

 fleshy fungi often display great variation also and many parasitic 

 species as well. 



With perennial forbs and grasses, the departures are naturally 

 much less striking, the annual effect being expressed largely in num- 

 ber and height of shoots and in seed production, since the competition 

 of parents precludes the ecesis of most of the offspring. The produc- 

 tion of dry material, as well as of seed, may vary severalfold, and 

 this is directly reflected in the coactions of grazing animals in par- 

 ticular. With woody plants, the increment of each year is diffused 

 over all or most of the plant in the form of an annual ring and as 

 short twigs, and hence is hardly to be noted. However, more notice- 

 able fluctuations are recorded in the seed crop, especially of conifers, 

 oaks, hickories, etc., and reflected in the ecesis, as well as in the coac- 

 tions of seed-eating animals. All these growth responses bear a more 

 or less definite relation to the sunspot cycle, and it is an interesting 

 fact that this relation was first seen in the annual rings of trees 

 (Douglass, 1909) and was later extended to vegetation in general 

 (Clements, 1916). 



