ANIMAL AGGREGATIONS 



399 



killed by wolves or by coyotes (Leopold, 

 1933). 



One final illustration will introduce a 

 still more complicated aspect of surface- 

 mass relations, this time at a psycho-social 

 level. Among human populations, interest- 

 ing examples of the working of this prin- 

 ciple are furnished by various kinds of cul- 

 tural "islands" surrounded by people of 

 different beliefs or social patterns. Mennon- 

 ite communities in rural North America 

 provide one such test. In earlier decades, 

 when travel was difficult and communica- 

 tion with other colonies was limited, rural 

 Mennonite colonies needed a minimum of 

 about fifty families to maintain a socially 

 secure community with a reasonably cer- 

 tain future. A Mennonite population of this 

 size permits the basic community services 

 —those furnished by shoeshop, barbershop, 

 general store, as well as the common 

 church and school— all to be in Mennonite 

 hands. Under favorable pioneer conditions, 

 forty families could keep such a commu- 

 nity going, but the smaller group was usu- 

 ally more vulnerable. Below about forty 

 families, inbreeding on the one hand, and 

 more numerous "outside" contacts, includ- 

 ing marriages, with non-Mennonites on the 

 other, became more and more disruptive 

 with decreasing numbers. In a way this is 

 a sociological application of the group-sur- 

 face to group-mass relationship that we 

 have seen previously in simpler patterns. 

 An upper safe population size also exists 

 in Mennonite colonies above which their 

 system of congregational organization and 

 lay ministry does not function well; also in- 

 tracolony rivalries are more likely to pro- 

 duce disruption when the groups are larg- 

 er. Even under present day conditions of 

 travel and communication, oversmall Men- 

 nonite colonies are vulnerable, although 

 congregations of twenty to twenty-five 

 famifies can survive, provided they main- 

 tain close contacts with their coreligionists 

 in other communities." 



In brief general summary: Many of the 

 protective values furnished by animal ag- 

 gregations depend on the reduced amount 

 of surface in relation to total mass that 

 characterizes aggregated animals as con- 

 trasted with a similar number of scattered, 

 isolated individuals. The mass protection 



• Personal communication from Mr. P. C. 

 Hiebert and Mr. J. W. Fretz of the Mennonite 

 Central Committee. 



ranges from relatively simple physico- 

 chemical relations, protection from bac- 

 teria, or from predators, to human situa- 

 tions in which the aggregated colony has 

 fewer contacts with outsiders who repre- 

 sent a different and a dominant culture. 

 Presocial and social homoeostasis (p. 672) 

 grows in part from fairly simple relations 

 between the surface and the mass of the 

 units under consideration. Even so, aspects 

 other than surface-mass relationships are 

 frequently involved. 



MINIMAL AND OPTIMAL POPULATIONS 



As with Mennonite colonies, populations 

 of nonhuman organisms show phenomena 

 associated wdth population size and den- 

 sity more or less closely comparable to 

 those already reported for macerated 

 sponges. Although supporting instances are 

 known for widely diverse species, ranging 

 from bacteria to elephants (of. Alice, 

 1938), critical data as to the exact level 

 to which a local population can fall with- 

 out danger of extinction are hard to find. 

 Conditions vary within the species in the 

 same habitat, from habitat to habitat, and 

 from species to species. 



All too often there is no authenic record 

 of population densities in the years preced- 

 ing extinction. The decline in numbers of 

 the heath hen of New England, a relative 

 of the prairie chicken, is an exception (p. 

 328). The books by Allen (1942) and Har- 

 per (1945), surveying the recently extinct 

 and threatened species of mammals of the 

 world, should be consulted in this general 

 connection. Among the species treated, the 

 history and present status of the wisent, or 

 Lithuanian bison (Bison bonasiis bonasus), 

 as reviewed by Harper, present aspects of 

 interest. These large mammals, closely re- 

 lated to the American bison, stand 6 feet 

 high at the shoulders. They were once 

 abundant throughout Europe. Caesar re- 

 cords them as being plentiful in the forests 

 of Germany and Belgium; they were appar- 

 ently common throughout central Europe 

 in the sixteenth century. The date of ex- 

 tinction in different localities is generally 

 unknown, but the last bison was killed in 

 East Prussia by a poacher in 1755. Rem- 

 nants of two herds representing the Lith- 

 uanian and Caucasian subspecies, respec- 

 tively, were still ahve up to the outbreak 

 of World War II in 1939. 



