452 Succession and Fluctuation 



of the population are staggered so that httle change is apparent for 

 the community as a whole. For example, in certain tropical regions 

 where individual trees shed their leaves at different times, the vegeta- 

 tion as a whole remains green throughout the year. Similarly, in 

 tropical waters individual species of plankton in small numbers wax 

 and wane, but the complexion of the community may continue essen- 

 tially the same for long periods. At higher latitudes where climatic 

 conditions change markedly during the year, the abundance of popu- 

 lations and the composition of the community vary widely with the 

 season. Yet even here the community may in some instances come 

 around each season to essentially the same composition as in the 

 previous year, resulting in a rather faithful repetition of the seasonal 

 cycle. 



On the other hand, constancy within a commimity is the exception 

 rather than the rule, even within the same board climax condition. 

 Every species in the community fluctuates to some extent in respect 

 to rate of vegetative growth or of reproduction, and sometimes the 

 variations are very large. In earlier chapters we have mentioned the 

 various types of fluctuation of populations of individual species and 

 also the reciprocal oscillations of certain populations of species ex- 

 ploiting one another. We shall now consider fluctuations in com- 

 ponent species against the background of the community as a whole. 

 All but the most minor changes in the growth or reproduction of one 

 species of a community is bound to have repercussions among other 

 members of the biocenose. Sometimes the fluctuations are obvious 

 and spectacular, as in a plague of mice or of locusts; on other occa- 

 sions changes in the community originate in fluctuations among the 

 unseen microorganisms of the soil or of the water, or among patho- 

 genic bacteria or viruses. In either case the effects on the natural 

 community may be serious, and they often have a drastic direct or 

 indirect consequence for man. 



It is exceedingly difficult to ascertain the magnitude of fluctuations 

 in the community in any precise terms. Attempts to take a census of 

 wild populations have been made for a relatively small number of 

 biotopes. Even in those instances in which a systematic count has 

 been undertaken, the reliability of the census methods are hard to 

 evaluate, and rarely have population measurements been continued 

 sufficiently long to reveal significant relationships. The best available 

 census data for extended periods are for species that are of concern 

 to man either because they cause damage or because they provide a 

 useful product. Records of abundance in wild populations have 

 been kept for various disease organisms and insect pests, and for 



