Influence on Reproduction 325 



of a successful breeding colony of the guano-producing cormorant 

 on islands off Peru (Hutchinson, 1950). Among mammals, popula- 

 tions of muskrats smaller than about 1 pair per linear mile of stream 

 or per 35 hectares of marshland appear not to breed successfully 

 (Errington, 1945). 



Degree of crowding can influence the sex and structure of some 

 species and the coordination of these features with the natural life 

 cycle. Crowding has been shown to influence the sex ratio in certain 

 cladocerans, in Bonellia, and in Crepidula. Moina, a common clado- 

 ceran inhabiting temporary ponds, reproduces parthenogenetically 

 under ordinary conditions with the production chiefly of females. If 

 the population becomes crowded, however, males begin to appear. 

 This reaction is obviously related to the reduction of space attending 

 the drying up of a pond and necessitating the production of resistant 

 eggs if the population is to survive. Since resistant eggs are produced 

 only sexually, males must be present in the population. The produc- 

 tion of males by crowding is thus nicely attuned to the needs of these 

 animals under the exigencies of life in temporary ponds. 



In the aphids, or "plant lice," which attack vegetation, the winter 

 eggs hatch into wingless females; these reproduce parthenogenetically 

 under ordinary circumstances, with the result that the population on 

 each plant grows rapidly. If the plant host becomes overcrowded 

 and consequently withers, the destruction of the whole group of 

 aphids is threatened. However, crowded conditions react on the 

 aphids in such a way as to bring about the production of winged 

 females. This extraordinarily neat adaptation makes possible the 

 migration of many of the aphids to other plants, relieving the con- 

 gestion at home and establishing new centers of population growth. 

 Later in the season males appear and sexual eggs are produced that 

 tide the population over the winter period and complete the cycle. 



Another effect of the size of breeding population is in relation to 

 genetic elasticity and, hence, the adaptability of the progeny of a 

 species to varied conditions. Most species of plants and animals in 

 nature are composed of a great many biotypes, that is, types of indi- 

 viduals that grow and react differently because of different genetic 

 constitutions. Owing to varying environmental conditions certain 

 biotype groups become established in different ecological regions of 

 the range of each species. These ecological subdivisions of the 

 species are known as ecotypes and are genetically distinct races. 

 Since the ecotypes are interfertile, they are placed in the same taxo- 

 nomic species. Ecotypes are sometimes recognized as subspecies, 

 but in other instances they are not sufficiently distinct morphologically 



