384 



COMMUNITY ECOLOGY: 



proportions and conditions be just right for transfer 

 some great distance. 



The last two hypotheses to explain intercontinental 

 dispersion are continental drift and shifting poles. 

 Continental drift proposes a one-time single land 

 mass composed of the materials of all the continents 

 (Figure 19.22). This large, single continent is sup- 

 posed to have separated and the segments to be the 

 present continents. Although there are certain merits 

 in this hypothesis, the general consensus of most 

 biologists is that a proposal of continental drift cre- 

 ates more problems than it solves. (Further details of 

 this hypothesis are given among the readings, es- 

 pecially Wulff.) 



The hypothesis of shifting poles was used to explain 

 what once appeared to be unbelievable, temperate 

 floras at the poles. Present evidence implies fluctua- 

 tions in past climates, hence denies the shifting poles 

 hypothesis. Also, the geological record would indi- 

 cate that the North Pole has not shifted since at least 

 the Late Cretaceous. If such shifting occurred before 

 the Cretaceous, it would not have been of primary 

 importance in the present distribution of life. 



Jurossic 



Cretaceous 



DISTRIBUTION SCHEMES 



How is one to summarize present-day distributions 

 of organisms? If the species constituting each com- 

 munity had exactly the same ecological amplitude 

 and distribution, and if neither of these were chang- 

 ing, a very simple mapping procedure could be ap- 

 plied to record the spatial relations of each kind of 

 community. However, this simple situation does not 

 prevail. Probably no two species have the same 

 range. When this simple fact of biogeography is ex- 

 tended to the investigation of communities, it be- 

 comes extremely difficult to define exact limits, be- 

 cause communities tend to merge over fairly broad 

 geographic areas. For this reason, even if one had 

 information on the precise area of each species, no 

 completely satisfactory summarization or scheme of 

 distribution would be possible. However, distribu- 

 tion is even more complex: the ranges of species and 

 communities are dynamic. Both individuals within 

 communities and communities as a whole are contin- 

 uously affected by their changing environment. As a 

 consequence, areas are always being changed. There- 

 fore, man's attempts to categorize them are found 

 to suffer from inaccuracy. 



Although completely accurate summaries of floras 

 and faunas are impossible, distribution schemes can 

 outline major trends in the spatial relations of life. 

 Moreover, these major tendencies — if it is remem- 

 bered that they are no more than tendencies — are a 

 great aid to human understanding of the general 

 groupings of life and of how such groupings in one 

 area compare with and are related to other areas. 

 For these reasons, some of the distribution schemes 

 will be examined. 



Eocene 



PI iocene 



Figure 19.22 Stages in the hypothesis of continental drift: The initial 

 stage supposedly was o single, huge land moss called Pangea and was 

 found during the Paleozoic Era. The intermediote stage, conjectured for 

 the Eocene, showed continental separations, the so-called jigsaw puzzle 

 effect of the continents, and the presence of two major continents (a 

 northern Laurosia and a southern Gonwonaland). The culminating 

 stage, starting with conditions much like now, is proposed for the 

 Pliocene. (After Wegner.) 



FAUNAL REGIONS 



One of the first distribution schemes was proposed 

 by P. L. Sclater in 1858. This scheme divided the 

 world into zoogeographic realms. Because the scheme is 

 a valuable general summary of animal distribution, it 

 has received further study and modification up to the 

 present time. 



Six faunal realms, or regions, are recognized (Fig- 

 ure 19.23). Each realm is a major part of the earth, 

 which had only minor internal barriers during the 

 Tertiary but was isolated from other realms during 

 some part of the Tertiary, However, this does not 

 mean that each realm contains a completely unique 

 fauna or flora. Only the total life of each realm dis- 

 tinguishes these major areas. 



