226 



The Evolution of Communities 



50 •♦0 3ij^- 



STORM RAINFALL. JULY 1. 1953 



8.00 



7.50 



8.00 



JULY 1953 RAINFALL 



347 



/••^ 



3 25-^. 



400 ^_ 

 4.25 



^•25- — ^^' 3.75- 

 4.00 



WE'EKLY RAINFALL. JULY 1-7. 1953 



1049^ 



12.5 



135 



JUNE-AU6. 1953 RAINFALL 



Fig. 97. Variation in rainfall on 100 square miles in Illinois during a day, 

 week, month, and summer. Numbers in italics are rain measures at indix'idual 

 stations; numbers in arabic style are calculated isohyetal contours. (After Huff 

 and Neill.) 



explained at length their conviction that inimical weather plus the 

 action of predators and parasites usually reduce populations of 

 animals below the levels of competition for space and food. This 

 thesis has an immediate application to communities. 



Based on outbreak data concerning economic insects, Glen ( 1954) 

 suggested as a general maxim that in a simple biotic system such 

 as the few insect species found on a pure stand of an agricultural 

 crop the crop-feeding insect species could increase to outbreak 

 proportions readily and rapidly but that in a complex community 

 this was not so likely to happen. The simple system would have 

 few predator species which would depress the prey populations, 

 but a complex community would have a large assemblage of pred- 



