37 



to 3 or none in 2820 feeto Variations from a straight 

 descending line graph may result partly from imperfect 

 sample sizes, or they may reflect the presence of a few 

 larger predaceous individuals or other patterns of 

 patchinesso 



Indicator Species 



An indicator species is one which can provide 

 precise information in geography, ecology and distribution, 

 or one which can indicate characteristics of physical and 

 chemical signif icancCo Such species, to be of value, 

 have certain requireraentSo They should be specifically 

 identified, they should be endemic to an area, not 

 cosmopolitan in range, and their horizontal and ecological 

 limits should be known. They should be easily recognized 

 so that their identity cannot be confused with nearly 

 related ones. In some recognizable form of their life 

 history they should occur in all seasons, or at least 

 not fluctuate. For those undergoing changes in develop- 

 ment, their stages should be known and recognizable. 

 They should also occur in sufficient numbers to represent 

 stable, not stray, populations (Sverdrup, Johnson, and 

 Fleming, 1942). Their tolerances to varying amounts of 

 salinities, to changes in temperature, to varying concen- 

 trations of silt, to possible toxic substances, or 

 degrees of acclimitization of these factors should be 

 known. Thus, sedentary or attached species have greater 



