30 



UNDERWATER GUIDE TO MARINE LIFE 



DISTRIBUTION AND DISPERSAL 



In any community, a number of species is found, each of which has its own 

 distribution Cfig. 6). Some species might be found in only one community— the 

 parrot fish in the coral reef— and some might be found over a wide range of 

 habitats— the common eel, ranging from the Sargasso Sea to fresh-water streams 

 at different stages of its life. 



U.S. COAST 

 H SPECICS OR 1.3% 



TROPICAL, SUB-TROPICAL 

 AND TfMPERATE AMERICA 

 5 SPf CIES OR 0.9 % 



WCST INDIES AND 

 TROPICAL PACIFIC 

 8 SPECieS OR 2.5% 



TROPICAL AND SUB-TROPICAL 

 AMERICA-2IO SPECIES OR 6b% 



PELAGIC -v3 9 Spec IE S OR (2.3% 



WORLD WIDE NEAR SHORE 

 IH SPECIES OR 'i.'i% 



WEST INDIES , 

 EAST ATLANTIC AMD EUROPE 

 8 SPECIES OR 2.5% 



SARGASSUM-4SPeCIES OR l-SX 



Fig. 6. An example of geographical distrihtition— species of Bermuda shore fishes 

 (^adapted from Beehe and Tee-Van, 1933^. 



The distributions of the species of a community are far from static, and the 

 numbers of individuals of any one species in a community are subject to con- 

 stant variation. Some communities are even compound, that is, there may be 

 several animal communities present in any one zone. For instance, the coral reef 

 has strikingly different day and night communities, and the temperate zones 

 have very different seasonal communities. It even sometimes happens that the 

 fluctuation in numbers of one form of life basic to a community mav change 

 the whole complexion of the environment. This is what has happened in con- 

 nection with the decrease in eelgrass in the 1940's. 



Fluctuations like these have a great deal to do with factors aff^ecting distribu- 

 tion and dispersal— the questions of why animals are found where they are and 

 how they get there. Such fluctuations complicate the studv of communities 

 greatly, because one is never sure just which species are community members 

 and which are not. 



The answer to why animals are found in one place and not in another, lies 

 in understanding the principle of ecological valence. Briefly, valence describes 

 a range of tolerance which an animal can stand in relation to any factor of 

 the environment. The valence of coral reef formation with respect to temperature 

 ranges from a low of 68° Fahrenheit to well over 100°. The valence of whales 

 in relation to food depends upon the supply of the planktonic crustaceans on 

 which they feed. Both of these valences are small and limiting in value, but 

 valences can be large. For example, the valence of killifishes with relation to 

 salinity ranges all the way from pure fresh water to pure sea water. The valence 



