accuratf if given in terms of carbon or nitrogen con- 

 tent, or calories. 



.Attempts have recently been made to compute 

 more significant biomasses of l)ird jwiiulations, using 

 physiological constants. A biomass comiiosed of few 

 but large individuals has a lower metabolic activity 

 than an equal biomass composed of a large number 

 of small individuals. In one study (Turcek 1956), 

 the importance of different species in the community 

 was evaluated in terms of the total body surface area 

 rather than total weight jiresented by each species, 

 using the formula .V • 10 • li'""''. where .V is the num- 

 ber of individuals and W the average weight of tiie 

 species. In another study (Salt 1957) the number of 

 individuals was multiplied by the mean weight of the 

 species raised to the 0.7 power (.V- [/'""). One gets 

 the best evaluation of the importance or influence of 

 a species ifi a community where metabolic activity 

 can be measured directly and expressed in terms of 

 calories per unit of time (Macfadven 1957. Teal 

 1957). 



Produrtivin 



.SLJCCE.SSION 



Communities are in ;i more or less continual 

 process of change (Clements 1910). These changes 

 result in part from the reactions and coactions of the 

 organisms themselves and in part from such external 

 forces as changing i)hysiogra|)liy, changing climate, 

 and organic evolution. The habitat is usually affected 

 as well as the community, and as the habitat changes, 

 new species invade it and become established, and old 

 species di.sappear. These changes are especially no- 

 ticeable in dominant s])ecies, since these species exert 

 a controlling role over the composition and structure 

 of the community as a whole. The replacement of one 

 comnnmity by another is succession, and succession 

 continues until a climax or final stage is reached. 



-Succession is a process. The series of steps or 

 communities comjirising a successional sequence lead- 

 ing to the climax is the sere. Seres are sometimes 

 classified according to the predominant force that is 

 bringing them about. These forces are biolic, cli- 

 matic, physiofnal^hic. and t/eoloc/ic and their resultant 

 seres are commonly called hioscres. clisercs, eoseres, 

 and geoscres. 



A characteristic of communities that has be- 

 come of considerable importance in modern ecological 

 research is productivity. The number of individuals 

 or biomass present in a community at any one time 

 is the standing crop. At the beginning of the year 

 or reproductive season the standing crop is usually 

 small, but as reproduction and growth take place 

 there is an increase in the amount of organic matter 

 making up the biomass of the community. The pro- 

 duction of organic matter per unit of time and area 

 is productivity. Productivity is commonly indicated 

 on a yearly basis, but it is also possible to measure 

 monthly, weekly, or daily production. Small standing 

 crops may have a high productivity and large stand- 

 ing crops a low productivity, hence average bio- 

 mass or standing crop differentials between different 

 communities is not comparative of the productivity 

 of the habitats in which these communities occur. 

 The largest standing crop which a habitat can sup- 

 port without deterioration, or the maximum num- 

 ber of biomass of animals that can survive least favor- 

 able yet tolerable environmental conditions during a 

 stated period of time, is the carrying capacity. Carry- 

 ing capacity is determined not just by the amount of 

 food available, but also by shelter, social tolerance, 

 and other factors (Edwards and Fowle 1955). A 

 variety of methods are being used to measure pro- 

 ductivity of different kinds of organisms and of dif- 

 ferent habitats. It is desirable to indicate productivity 

 as accurately as possible in descriptions or analyses 

 of community dynamics. 



Biotif 



suffi's.ston 



Biotic succession is brought about by forces 

 inherent within the community and in the activities 

 of the plants and animals themselves. The most im- 

 portant of these activities are the organismal reac- 

 tions and coactions that produce modifications in the 

 iiabitat and interrelations between species. Important 

 reactions involve filling in of ponds with plant and 

 animal remains, the addition of organic nutrients to 

 sterile soil, and the reduction in light intensity by 

 increasing density of plant growth. With progressive 

 improvement of the soil and changing light and mois- 

 ture conditions, a series of new dominants come into 

 the area. When invasion of new species occurs, in- 

 tense competition develops ; if the invaders are suc- 

 cessful, the old species disappear as a new community 

 replaces the old one. 



Contributing factors that may be involved are dif- 

 ferences in growth and dispersal rates, which are 

 different for different species. After a forest fire or 

 logging operation, herbaceous plant growth is im- 

 mediately stimulated ; since herbs grow rapidly, they 

 become dominant within a year or two. Shrubs begin 

 to spread, and tree suckers or seedlings also appear 

 quickly, but because they require a longer time to 

 reach maturity several years may elapse before they 

 gain control of the area. 



Succession is considerably influenced by the kinds 

 of propagules available in the vicinity. Seeds, spores, 

 and the like are dispersed more or less readily, de- 

 pending on form. Some kinds of animals roam more 



The biotic community 21 



