Current issue: 51(5)
Severe climatic conditions promote gregariousness, because the number of species capable of surviving is small. Thus, it its more common in cold or arid regions. For instance, sands and gravels that are poor in nutrients, and subject to drying, are often characterized by pure crops of pine, whose seedlings are comparatively drought resistant. One example of gregariousness due to favorable conditions of seedlings, seed-dispersal and germination is birch. It produces seed abundantly from an early age, and the light seeds are dispersed widely by the wind. Gregariousness can also be induced by conditions which destroy all species except those specially adapted to resist extermination. One example of this is annual fires. A species may be gregarious in some conditions and sporadic in others.