488 Dynamics of the Ecosystem 



TABLE 21 



General Decrease in Abundance of Species with Increasing Latitude 



IN North America As Indicated by REPRESENTATrvE Data from 



Various Sources 



Approximate Number of Species 

 Florida Massachusetts Labrador Baffin Land 



Ferns and clubmosses — 70 31 11 



numbers of each, whereas in northern waters many fewer kinds of fish 

 are found but some of them exist in huge populations. It is no acci- 

 dent that most of the large commercial fisheries are located in high 

 latitudes. Enough cod are found among the Lofoten Islands off Nor- 

 way during the early spring to keep 4000 fishing vessels busy, and 100 

 tons of cod may be taken with one cast of the purse seine. Many 

 other illustrations of this generality may be found in faunal and floral 

 lists for regions of contrasting latitude, such as those of Russell 

 (1935). 



Although this typical difference between tiopical and non-tropical 

 populations has been recognized for a long time, opinion differs as to 

 its cause. One explanation lies in the fact that, since climatic condi- 

 tions are more favorable at low latitudes, many different kinds of 

 species can survive; and, because the total quantity of life in an area 

 is limited, each species can be represented by only a few individuals. 

 Furthermore, at high temperatures, with higher rates of reproduction, 

 mutations probably occur more frequently so that in the course of 

 evolution a larger variety of species has originated and more mutants 

 would be expected to survive under the favorable conditions of the 

 tropics. At high latitudes the reverse situation exists, and in addition 

 the repeated expansion of the polar climate may have reduced the 

 number of species as well as the time available for evolution. The 

 relatively few species that do survive in colder regions find little com- 

 petition and hence can develop large populations that exploit the en- 

 vironment to the limit. 



In addition to this general reciprocal relationship between abun- 

 dance of species and individuals, the standing crop in marine environ- 



i 



