POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



341 



of the snail Neritina virginea. He and 

 others observed ofiF and on for many years 

 a "salt pond" in the vicinity of Kingston, 

 Jamaica, which at certain times was con- 

 nected with the ocean, while at other times 

 the pond-ocean junction was effectively 

 blocked by accumulated sand. During the 

 connected intervals the sahnity of the pond 

 water was lower and much Uke that of the 

 sea; during the isolated periods evapora- 

 tion went on regularly, there was scant 

 rainfall and little drainage into the pond 

 from the land, and the sahnity increased. 

 This sequence of events set up a natural 

 experiment from which it can be concluded 

 that after extended periods of high sahnity 

 the Neritina population is larger in terms 

 of number of forms, but composed of 

 dwarfed individuals; after extended periods 

 of low sahnity the snails are larger in size, 

 but less numerous. Andrews attributes 

 these differences primarily to the action of 

 salt concentration operating essentially in 

 a density-independent manner. It appears 

 that this finding is not limited to gastero- 

 pods, since certain other moUusks behave 

 somewhat similarly. 



It seems reasonable to conclude that 

 salinity does not ordinarily function as a 

 hmiting factor for animal populations. 

 StenohaUne organisms capable of surviving 

 only narrow changes in salt concentration 

 are usually found in environments in which 

 the salinity is relatively constant. Euryha- 

 hne organisms capable of tolerating wider 

 changes in salt concentration are found 

 when considerable variability in salinity is 

 hkely. This, of course, follows as an ob- 

 vious point. It can be argued that there is 

 a greater abundance of species and individ- 

 uals of marine organisms in coastal regions 

 because of the lower sahnity existing there. 

 This is a dangerous, and possibly specious, 

 argument a priori, however, since the coast- 

 al waters may be more favorable in other 

 ways also; e.g., greater food supply. As 

 seems true for pH, it is easy to reach the 

 ad hoc conclusion that salinity is an effec- 

 tive factor, but such a conclusion is hard to 

 prove without recourse to experimentation. 



Cowles (1930) supports this point in 

 discussing the ecological distribution of dia- 

 toms. "It is well estabhshed from a study 

 of geographical distribution that certain 

 diatoms (oceanic) are characteristic of 

 waters of high salinity, such as that of the 

 open ocean; that others (neritic) are char- 



acteristic of waters of lower sahnity found 

 along the sea coast and in estuaries; and 

 that still different ones frequent the fresh 

 waters or rivers emptying into the ocean. 

 But, also, it is well known that many of 

 these diatoms are able to stand a large 

 range of salinities and that oceanic as well 

 as neritic diatoms are often found in estu- 

 aries where the sahnity is very low" (p. 

 317). 



Calcium 



Certain aspects of calcium and magne- 

 sium ions as elements of the aquatic envi- 

 ronment were discussed in Section II (p. 

 203). An extension of this subject at the 

 species population level is afforded by the 

 observations and experiments of Jewell 

 (1939), which are concerned with the re- 

 lation of calcium bicarbonate content of 

 fresh waters to the distribution of sponges 

 (SpongiUidae) in northern Wisconsin. Jew- 

 ell showed that some sponges are sensitive 

 to calcium bicarbonate concentration and 

 appear to be limited in their distribution by 

 this factor. Other sponges have a wide 

 range of toleration. For example, Ephydatia 

 mulleri is absent from waters both high and 

 low in calcium, but is found in the inter- 

 mediate range of 5.6 to 16.3 mgm. of cal- 

 cium per hter. Populations of this species, 

 however, are largest in the middle and 

 lower parts of this range. There is thus a 

 spectrum of possible toleration within 

 which lies a more restricted optimum. On 

 the other hand, Spongilla ingloviformis is 

 extremely sensitive to calcium tension and 

 is not found when a concentration of 3.16 

 mgm. per hter is exceeded, despite the fact 

 that sturdy populations are present in habi- 

 tats of lower tensions. The other extreme is 

 exemphfied by Heteromyenia repens, which 

 tolerates the impressive range of 2.66 to 

 53.4, a range beyond which fresh waters 

 in Wisconsin rarely go. Jewell also con- 

 cluded that calcium and magnesium in or- 

 ganic combinations are not important 

 agents in sponge distribution. 



Pollution 



The pollution of rivers by sewage and 

 trade wastes offers an interesting oppor- 

 tunity to study this factor as it affects popu- 

 lation abundance, distribution, and succes- 

 sion. It is frequently possible to trace the 

 source of pollution to a particular region, 

 and observations and measurements can be 



