RELATIONS AMONG SPECIES 



359 



twining tips grow in a rotating movement. Tendril 

 climbers have various structures modified into thin, 

 elongate tendrils that encircle or in some way "grasp" 

 any object they contact. Root climbers periodically 

 grow roots that attach the plants in their upward 

 growth. Leaners are almost capable of independent 

 support, but gain their upward growth by leaning on 

 nearby plants. Leaners have no specialized means of 

 support to their upward growth. Climbers are also 

 called lianas. 



Epiphytes live upon other plants, but' for support 

 only. 



Nongreen Planis. These plants, because they lack 

 chlorophyll, do not carry on photosynthesis. There- 

 fore, they are dependent upon other creatures for 

 their food. 



Saprophytes obtain their food from dead organic 

 matter. 



Parasites usually live upon and receive their food 

 from living creatures. 



A few to all of the above layers may be present in a 

 given habitat. For example, a well-developed forest 

 may have all strata present, but a grassland may have 

 only an herb stratum. When many layers are present, 

 each possesses unique environmental factors and each 

 progressively lower stratum is composed of plants 

 that are more independent mechanically as well as 

 being shorter. In addition, each layer that is present 

 has a few characteristic plants, the dominant species; 

 however, the entire habitat tends to hinge upon the 

 highest stratum. For example, if the highest domi- 

 nant trees of a forest habitat are destroyed, many 

 species of the lower strata will be lost; but lower 

 strata might be removed without pronounced changes 

 in the trees or any other unaffected layer. 



Any type of habitat might show stratification. 

 Shrub habitats often have many lower strata present. 

 Even a herbaceous habitat can have two or more 

 layers, each herb stratum created by many species of 

 the same general height. 



Significance. Progressively lower strata usually 

 have less light, lower daytime temperatures, higher 

 night temperatures, a moister atmosphere, and 

 lower wind velocity; the taller plants have greater 

 influence on the environment. In general, the lower 

 the stratum the less the environmental extremes. 

 Also, stratification frequently is an indication of the 

 developmental status of a community. The climax, 

 because of its stability, regularly has well-organized 



strata, with climax species adapted to the environ- 

 ment. In contrast, serai stages normally have poor 

 strata formation; most of the plants must germinate 

 and grow in more open situations, because shade is a 

 limiting factor for them. In addition, each layer com- 

 prises a single unit of vegetation, or layer society. 



SUCCESSIONAL STATUS 



Succession is the somewhat orderly process of 

 progressive replacement of organisms in a restricted 

 area over a long period of time. This replacement is a 

 continuous thing; the plants and animals of a com- 

 munity develop their environment to the point where 

 new species can invade the area, replace the previous 

 species, and develop a new community. Each of the 

 communities on the way to a stable situation is called 

 a serai stage. The first species or group of species 

 invading an area constitute the pioneer or primary 

 serai stage. From this, various intermediate serai stages 

 are present on the way to the final stable habitat, the 

 climax. A single successional sequence from pioneer 

 stage through climax is a sere. 



A climax might be somewhat difficult to diagnose 

 in nature. However, any climax possesses a single 

 characteristic which aids in its recognition. This 

 characteristic, or phenomenon, is a state of dynamic 

 equilibrium. Climax organisms generally modify 

 neither one another nor their physical environment 

 sufficiently to create surroundings more favorable 

 for potential invading species or less favorable for 

 the species already in the habitat. Therefore, the 

 climax is a phenomenon in which environmental fac- 

 tors are relatively stable and species composition is 

 relatively fixed. 



DYNAMIC EQUILIBRIUM 



This picture of balance in a climax can cause mis- 

 understanding. In spite of the fact that a climax is a 

 condition of equilibrium, it is not one of static 

 equilibrium. Although species composition of the 

 characteristic dominant species at least is somewhat 

 fixed, some fluctuation in numbers and even spatial 

 relations is not uncommon. This fluctuation partly 

 exists because daily, seasonal, and annual variation 

 in ecological factors is the rule, and factor variation 

 causes some response by organisms. Therefore, any 

 kind of equilibrium that is present within such varia- 



