THE COXTIXENTAL FLORA OF SOUTH SWEDEN 243 



CHAI'TEK III. 



The ecology of continental species and the Physiography of conti- 

 nental regions. 



In an enquiry into the causes of the distribution of species the problem may 

 properly be stated thus: Hoic far does the distribution depend u/>on the ecological 

 demands of species r and hoi u have other factors, and what factors, prevented species 

 from attainifig the distribution allowed by their demands? 



As, of course, scarcely any species can be expected to be able to reach all 

 suitable localities if we concern ourselves with very extensive areas, this problem 

 can only refer to encjuiries within comparatively limited regions. 



An enquiry into the causes of the distribution of species, however, meets with 

 great difficulties. The ([ualities of the habitat that are of importance to the eco- 

 logy of species act in combination or interfere in each other's range of activity. 

 The ecological demands of species must further be very difficult to determine. 

 Each species has naturally a certain ecological amplitude, but its occurrence in 

 nature may be more limited than might be expected from the amplitude. In 

 addition to the dispersal power of the species, we should here have to think of 

 the rivalry for space with other species. 



The connection between the distribution of species and the quantity of lime 

 in the ground might probably partly be looked upon as the result of such a rivalrj- 

 (see further about this later on). Probably the same holds good with regard to 

 the relation between the common spruce and the beech at the western limit of 

 the former in Central Europe, as that limit pretty certainly depends to some 

 extent on the ecological demands of the spruce, i. e. its requirements in the matter 

 of climate. I^ut, cultivated and sheltered by man in its rivalry against the beech, 

 the spruce may grow up, regenerate and thrive fairly well, even far outside this 

 limit (compare Dengler, 19 12. See further, for instance. Warming, 190Q, p. 71). 



However, we know very little about the rivalry between species and the im- 

 portance of the part it may play in the distribution of species. The importance 

 of the rivalry would seem for the ]:)resent to remain a highly theoretical problem. 

 Hence we have scanty information about the source of error appertaining to an 

 enquiry into the ecology of species, founded on their mode of occurrence. 



The only way in which it is at present possible to determine the causes of 

 the distribution of species is in the first place to determine the distribution mi- 

 nutely and study the mode of occurrence of species. In so doing one should 

 try to learn to know as intimately as possible not only the climatic and edaphic 

 conditions of the habitats, but also the vegetation, the plant-community, the living 

 environment. It is in its capacity as a member of community, and consequentK- 

 subject to the laws tor the organization of the community, that relations of the 



