220 CHAPTER II 



purely theoretical assumptions but very few direct observations of interspecific 

 competition as a major factor governing distribution. Personally, I feel especially 

 suspicious of the slogan: "competition between near related species", because, 

 in this connection, the ecological and not the taxonomical relationship must be 

 decisive. 



Some of the most reliable observations on competition were made on earthworms 

 (fam. Lumbricidae), implying that "peregrine", i.e. the anthropochorous, more or 

 less cosmopolitan species, after introduction into a new country, intrude upon the 

 endemic species of the original fauna or even completely exterminate them over 

 large areas (Michaelsen, 1903, p. 24; Smith, 1928, p. 347 a.f.). It seems also natural 

 that the rather permanent burrow system of an earthworm should represent a 

 sort of individual "territory", avoided by other individuals of the same or ecolog- 

 ically related species and that a competition about space takes place, similar to 

 that between sedentary organisms. 



Klots (1951, p. 201), Ferguson (1955, p. 188), and others remarked that the 

 indigenous Pieris napi L. (Green-veined White) of North America has noticeably 

 decreased in places where the introduced Pieris rapae L. (Small or Cabbage 

 White) had become established (since i860). It is difficult to imagine a com- 

 petition for food in this case as napi appears to prefer wild, thinleaved Cruciferous 

 plants, but rapae the thickleaved, usually cultivated forms (Brassica, &c.) (Petersen, 

 1954a, p. 195). It seems more reasonable to suggest that the normal courtship 

 and other sexual activity of the "weaker" species (napi) has been disturbed by 

 the presence of the very similar rapae; the upper side of the male is almost identical. 

 This explanation may have relevance for other diurnal insects of great vagility 

 and guided by optical sense (the scent of the male in the two Pieris species seems 

 to be quite different; Ford, 1946, p. 97). For nocturnal Lepidoptera the olfactory 

 sense governing sexual behaviour may be similarly deceived by the intrusion of 

 a related species with similar smell of the female. Dr. Petersen {in lit.) suggests 

 that, in the case of the two Pieris mentioned, a higher resistance against parasites 

 in rapae may be an important factor for its higher viability as compared with 

 napi. It would perhaps be worth while to investigate whether the purposely in- 

 troduced Braconid wasp Apanteles glomeratus L. (p. 222) has played a role there. 



From my own experience on Carabid beetles, I have already (Lindroth, 1949, 

 p. 554-560) given at some length the reasons for my deeply felt doubts as to the 

 validity of "competition" (in the limited sense, i.e. exclusive of predacy and 

 parasitism) as a major factor determining the abundance or even the existence of 

 a species in a certain habitat. In the lawns of Halifax, Nova Scotia, the introduced 

 Amara aenea DeG. was so exceedingly abundant that the population density widely 

 surpassed that of all Amara species together in any similar spot studied by me 



