THE ENVIRONMENT 



335 



parasites; other plants are carnivorous, mostly upon 

 small insects; and animal distribution is related to the 

 distribution of plant communities. According to dis- 

 tribution, some animals are found with a single plant 

 species, vegetative form, or plant community; other 

 animals are wide-ranging, occurring in many habitats 

 and showing only a general alliance to plants. 



Animals rarely dominate their environment. In 

 fact, the usual role of animals in a habitat is that of a 

 dependent. However, there are many circumstances 

 in which animals partly modify or even alter their 

 habitat. For example, grazing or browsing animals 

 may overgraze or seriously aflfect vegetation, causing 

 certain plants to be replaced by other species and 

 thereby creating a new habitat. Effects similar to 

 those from overgrazing can develop when animals 

 feed upon seeds and seedlings. On the other hand, 

 some animals activities are definitely beneficial to 

 plants. For example, insects and birds are instrumen- 

 tal in pollination, and therefore reproduction, of 

 many plants; other animals are very efTective in 

 spreading plant seeds. 



Interrelations between animals assume complex 

 patterns. The major types of interanimal activities 

 were mentioned as symbiotic phenomena. 



COMPETITION 



Competition often is given a much broader inter- 

 pretation than that described earlier. In the more 

 inclusive sense, it involves direct and indirect rela- 

 tionships. Direct competition, also called interference, 

 is competition much as was defined, referring to di- 

 rect and antagonistic action by both of the competi- 

 tors whether they be members of the same species 

 fighting for food, shelter, or mates, or members of 

 different species actively contending for some ma- 

 terial. Indirect competition, or amensalism, occurs 

 when one organism either monopolizes materials 

 (e.g., food or shelter) required by another, or creates 

 conditions (e.g., shade or toxicity) detrimental or 

 lethal to another. 



Both direct and indirect competition are closely re- 

 lated in their effects within a species (intraspecific com- 

 petition) and between species {interspecific competition). 

 For this reason, unless otherwise stated, further men- 

 tion of competition refers to both competition and 

 amensalism. 



Antagonism among organisms need not be con- 

 stant. It might occur only at certain times of the day 



or year, at certain stages of the life cycle, or in cer- 

 tain habitats. However, its consequences are fairly 

 predictable. Within animal species it often is instru- 

 mental in creating social dominance, forming terri- 

 tories, and determining population size. Between 

 species it also influences population size, but its main 

 interspecific effects are to regulate the ecological role 

 of species and the process of speciation. 



ECOLOGICAL VARIATIONS 



The ecological factors discussed in this chapter are 

 in a sense individual physical or biological facets of 

 any environment. Although ecological variation, espe- 

 cially that of a cyclic nature, can be treated as an 

 environmental factor, such variation really is not an 

 individual factor. Rathet, variation is a synthesis of 

 ecological phenomena of two kinds, daily and annual. 



Daily variation, or diurnation, naturally is related to 

 the presence and absence of the sun during a twenty- 

 four hour cycle. However, the sun in providing a 

 source of energy to any locale affects light, tempera- 

 ture, moisture, wind, and life itself. Therefore, the 

 daily cycle of day and night, owing to progressive 

 change in solar influence, exhibits somewhat regular 

 changes in those factors affected by the sun. 



Annual variation often is examined from three 

 points of view: periodicity, phenology, and aspecta- 

 tion. Periodicity refers to the cyclic recurrence of par- 

 ticular life cycle phenomena, usually on an individual 

 species basis. For example, basic heredity as modi- 

 fied by environment and annual changes in environ- 

 ment causes an annual cycle of germination, vegeta- 

 tive growth, flowering, and fruiting in flowering 

 plants. The particular stage in a species' life cycle at 

 a particular time of the year is its periodicity. Phe- 

 nology is more of a calendar treatment of annual varia- 

 tion, referring to the date or season when a particular 

 periodicity is expressed by a species or when a par- 

 ticular over-all community appears. 



Aspectation pertains to the appearance of an entire 

 community during different seasons. Also, aspecta- 

 tion is the seasonal rhythm or cycle of the presence 

 and activities of conspicuous organisms within a 

 community. 



Aspectation is difficult to separate from a time 

 table, or phenology. For this reason, aspectation 

 normally includes the concept of phenology. In this 

 sense aspectation encompasses the periodic or sea- 



