660 



ECOLOGY AND EVOLUTION 



the mountain hare, L. timidus, on high 

 ground. A form of the mountain hare, L. 

 anglicus, was abundant on low ground in 

 Britain during the Pleistocene, but became 

 extinct when the common hare became 

 established. In Ireland, where the common 

 hare is absent, a form of the mountain hare 

 still occupies low ground. The hobby, Falco 

 subbuteo, and the kestrel, F. tinminculus, 

 overlap in region and habitat, but dijffer in 

 their food. Among Hawaiian birds, Phaeor- 

 nis ohscurus and P. palmeri (Turdidae) are 

 both found on Kauai, but one feeds mainly 

 on fruits and berries, and the other is 

 chiefly insectivorous. Four species of Ant- 

 arctic seals occupy the same habitat, but 



lated and competing forms (Mayr, 1948). 

 Hutchinson (1948) mentions two possible 

 exceptions to this rule: (1) where an ex- 

 ternal factor may act to rarefy the mixed 

 population, so that the environmental pos- 

 sibilities are not completely exploited; (2) 

 where continual chance oscillations of the 

 environmental variables may continually 

 reverse the direction of competition, so 

 that no equilibrium can be estabhshed. 



Competition of a specific nature may ac- 

 count for cases of geographic incompat- 

 ibility of closely related organisms. Davis 

 (1932) reported the probable invasion of 

 the large and aggressive plains garter snake 

 (Thamnophis radix) to the Illinois shores 



Fig. 241. Distribution of Kalotermes, the most primitive genus of living dry-wood termites 

 ( Kalotermitidae ) . Note the occurrence in peripheral geographical regions (temperate zones) 

 and peripheral ecological regions ( continental edges ) , as compared to the derived genera, 

 Neotermes and Glyptotermes, in Figures 242 and 243. 



have food diflFerences. A number of cases 

 are reported in which two related species 

 occupy the same area and habitat, but 

 diEFer in size and presumably in their types 

 of food. These include passerine birds, 

 woodpeckers, ducks, grebes, terns, gulls, 

 shrews, and weasels. 



There remain a few examples of closely 

 related species of birds of similar size that 

 seem to overlap in area and ecology. Lack, 

 in his detailed study of the cormorant, 

 Phalacrocorax carbo, and the shag, P. 

 aristotelis, discloses distinct diflFerences in 

 both nesting habitat and food. Cause's 

 thesis (Cause, 1934a) that two species 

 with identical ecology cannot persist to- 

 gether in the same area seems to be sub- 

 stantiated by the data on birds and many 

 other animals, and indicates coaction of re- 



of Lake Michigan, thus separating the 

 populations of Butler's garter snake (T. 

 butleri), which ranges from western New 

 York to Wisconsin (Fig. 240) (see also 

 Conant, Thomas, and Rausch, 1945). 



An instance of competing species of 

 fishes is discussed by Meek (1930, p. 147). 

 He reports that Peterson transplanted 

 young plaice from near the mouth to the 

 head of Limfjord, where they were natur- 

 ally rare or absent, although proper food 

 was plentiful. The viviparous blennies that 

 live at the head of the fjord competed with 

 the transplanted plaice for food, and the 

 success of the plaice was found to depend 

 upon the presence of adult cod that feed 

 upon the blenny. 



The eflFect of competition upon the spe- 

 cies constitution of communities is indicated 



