S42 



POPULATIONS 



made of the fauna above and below this 

 source, thus furnishing a control area which 

 is not contaminated and an experimental 

 area which is. The experimental area typi- 

 cally exists as a gradient, characterized by 

 excessive contamination where the wastes 

 are discharged and followed by a progres- 

 sive decrease in contamination downstream. 

 This question has been rather well studied 

 for the Mississippi, IlUnois, and other 

 American rivers, and, although the Utera- 

 ture cannot be reviewed in any detail, cer- 

 tain points merit attention. 



In a general way, pollution has both a 

 direct and an indirect eflFect on the popula- 

 tions. Acting directly, it can drive forms 

 from their habitats; it frequently predisposes 

 the fauna to disease and infection; it may 

 increase disproportionately egg and juvenile 

 mortality; and it may actually kill adult 

 members of the group. Acting indirectly, it 

 reduces oxygen supply; it changes the 

 character of the bottom; it may harm or at 

 least alter the vegetation; and it may limit 

 both the number of habitat niches and the 

 food supply. It is also true, of course, that 

 some species such as members of the 

 Tubificidae thrive on pollution, so much so, 

 in fact, that they are frequently called 

 "pollution-index forms." 



A brief illustration of the effect of pollu- 

 tion on fresh-water lish populations is af- 

 forded by the studies of Thompson and 

 Hunt (1930) on the fishes found in the 

 west branch of the Salt Fork River (IlH- 

 nois) above and below a source of pollu- 

 tion. Collections in both localities were 

 made over a quadrat 330 yards square, 

 and the forms were segregated by species, 

 number, and size. In the clean-water quad- 

 rat twenty species were recorded, while in 

 the contaminated-water quadrat fifteen of 

 these were not present at all and four of 

 the five that were present were reduced in 

 number from 320 to one; twenty-two to 

 one; seventy-one to eighteen; and 126 to 

 two. There was also a suggestion (though 

 from meager data) that the five species 

 taken in the polluted sample were smaller 

 in size. 



Invertebrates as well as vertebrates are 

 subject to depletion by pollution. Galtsoff, 

 Chipman, Hasler, and Engle (1938) have 

 presented an excellent illustration of this for 

 oyster populations of the York River, Vir- 

 ginia. During the last twenty years or so 

 this population and the industry dependent 



upon it have been declining seriously, and 

 the authors set forth comprehensive evi- 

 dence that this decline is caused by Ubera- 

 tion of a tiade waste from a pulp-mill plant 

 located on the river. The various effects of 

 this waste on the oyster were studied, along 

 with an ecological survey of the stream 

 environment. The findings can be sum- 

 marized somewhat as follows: 



In the first place, the York River, apart 

 from pollution, was about as favorable for 

 the oysters as productive neighboring 

 streams. For example, studies of the cycles 

 of saUnity, temperature, and plankton pro- 

 ductivity showed these to be quite normal 

 and conducive to robust populations. Also, 

 the York River forms, when transplanted to 

 nearby locaUties, flourished, thus suggesting 

 that nothing was intrinsically wrong with 

 the oyster stock. 



This indicates that the observed popula- 

 tion decline actually resulted from the pol- 

 lution, certain effects of which on the phys- 

 iology of the oyster were assayed experi- 

 mentally. The pollutant brought about con- 

 traction of the adductor muscle with the 

 consequence that the valves remained 

 closed for an abnormal length of time. This 

 naturally interferes with feeding and res- 

 piration. The waste also inhibits ciliary 

 action in the gills and reduces the efficiency 

 of the complex pumping mechanism. The 

 last two influences cause the oysters to be- 

 come dwarfed, and they fail to store a nor- 

 mal amount of glycogen. Unless the forms 

 lived too long under polluted conditions 

 these physiological inhibitions could be re- 

 moved by returning the animals to clean 

 water. In clean water normal growth rates 

 were reestablished, glycogen was again 

 stored in adequate amounts, and lime was 

 deposited in the shells. 



Despite these adverse effects of pollu- 

 tion, the oyster population did not entirely 

 cease propagation. But the decline was 

 steady and caused the authors to make the 

 following justifiable recommendation: "That 

 the presence of pulp-mill waste in the 

 water is the cause of the failure of oysters 

 to grow and fatten in the upper part of the 

 river and that the elimination of pulp-mill 

 pollution is therefore a prerequisite for the 

 restoration of the oyster industry in the 

 York River" (p. 42). 



This is a first-rate illustration for our 

 present purposes. A single density-inde- 

 pendent environmental factor (pollution) 



