214 



The Evolution of Communities 



Fig. 90. Consequences of interspecific competition between two species of 

 Tribolium flour beetles at six combinations of temperature and humidity. H, T, 

 C, indicate hot, temperate and cold, respectively; W, D, indicate wet and dry 

 respectively. (From Park.) 



interfered with the production of D. pseudoohscura more than the 

 reverse, nevertheless even at extremely high population densities 

 both species survived. 



In both of these instances, if the food supply was periodically 

 replenished (as it is normally in nature), both species of each pair 

 would coexist indefinitely. In the Tribolium species the usual "loser" 

 would exist as small local populations; in the Drosophila species 

 both would exist side by side. 



ECOLOGICAL OSCILLATIONS 



In his Tribolium castaneum versus T. confusum experiments, T. 

 Park (1954) found that at certain combinations of temperature 

 and humidity one species was invariably the sole survivor, whereas 

 at other combinations the second species was the sole survivor 

 (Fig. 90). The extreme "weather" conditions in these experiments 

 were 30° C. and 70 per cent R.H. versus 24° C. and 30 per cent R.H. 

 In the hot moist cultures, T. castaneum eliminated T. confusum 

 only after coexistence for more than a year, and in the cool dry 

 cultures T. confusum eliminated T. castaneum only after coexist- 

 ence for more than half a year. If the two species coexisted in a 

 climate having annual fluctuations ranging between these extremes, 

 first one species then the other would tend to predominate, result- 

 ing in an oscillating relative abundance of the two and continued 

 survival of both. 



In experiments with two parasites of the azuki bean weevil Cal- 

 losobruchus chinensis, Utida ( 1957 ) found that the system of two 



