question of "fitness", for when rabbits were later introduced, they multiplied 

 rapidly among the native marsupials, and actually became a pest. 



The most effective barriers or obstacles to migration may turn out to be a 

 relatively small feature of the soil or the population. Acres of seemingly good 

 earth may remain sterile for want of a chemical element which plants use in 

 very small quantities at most, such as magnesium. On the other hand, early 

 settlers may obstruct migration, either by pre-empting all the available space 

 or by being actively antagonistic. Human wanderers have frequently been 

 stopped by micro-organisms producing tropical diseases rather than by wild 

 beasts or by previous settlers. 



Types of Community To most of us a forest is the most familiar natural 

 community. The inhabitants of a desert make up quite as distinct a com- 

 munity, but most of us would have to be shown, since we commonly think of a 

 desert as having no life at all. A swamp has its characteristic plants and ani- 

 mals, as has a scrub or a sand-dune. 



The inhabitants of the ocean differ from those of fresh-water lakes, but 

 there are many types of communities in the former and also in the latter. 

 Tidewater plants and animals differ in many ways from those that occupy the 

 bottom offshore, as well as from those that live at or near the surface. And 

 deep-sea forms differ from both (see illustration opposite). Brook life and pond 

 life differ from each other, and they differ also from the forms living in larger 

 streams and in large lakes. Among the most important life communities in 

 this country are the grasslands — of two types, the semiarid plains and the 

 moister prairies — which have played a great role in the development of our 

 food resources (see illustrations, pp. 89, 569, 643, 646). 



How Are Communities Formed? 



The First Settlers To a barren spot containing no organisms whatever, 

 the winds would ordinarily bring thousands of seeds and spores representing 

 dozens of species. Which species could take hold would depend upon the 

 amount of moisture, the temperature, the sunlight, and the chemical condi- 

 tion of the soil. To establish themselves in such a barren situation, plants 

 must be able to endure the winds, glaring sunlight, extreme fluctuations in 

 temperature and in moisture, and extreme combinations of soil chemicals. 

 Most of the plants with which we are familiar could not endure so much pull- 

 ing and pushing. Early settlers have to be tough. 



The pleuroccus cell might hold on to the rough surface of a rock. If there 

 is enough moisture in the air, it may grow and multiply. But then, it cannot 

 stand sunlight. The gemmule of a lichen might do better, since the fungus 

 partner can absorb enough moisture from the atmosphere to supply both it- 

 self and the algal partner (see Appendix A). The excretions of the lichen grad- 

 ually dissolve some of the rock's surface and so contribute to the making of soil, 



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