The Evolution of Communities 225 



There is a suggestion here that some selective action may be 

 involved, producing the beginnings of a differential specialization 

 in feeding sites between hijalimis and westwoodi. Also larch is the 

 one tree species harboring these three potential competitors, and 

 this suggests some sort of competitive interaction which might have 

 been the basis for past selection pressures resulting in the evolution 

 of E. mclachlani into a specialized lichen feeder. However, it is 

 equally plausible that the lichen-feeding arose from spontaneous 

 mutations that endowed ancestral E. mclachlani with the ability 

 to digest lichens, that the lichen-feeding individuals reproduced in 

 greater quantities than the others, and that they thus established 

 selection pressures leading to an associated behavior pattern. 



Direct Competition 



In modern communities examples of direct competition are legion. 

 In some cases the competition is passive, as in most plants, but in 

 others competition is accompanied by combative actions. In Hawaii, 

 for example, the Mediterranean fruit fly Ceratitis capitata is para- 

 sitized by four small hymenopterous wasps belonging to the genera 

 Opius and Tetrastichus. All lay their eggs in the fly larvae. The 

 newly hatched larvae of Opius have long, sharp mandibles. If more 

 than one parasite egg (including an Opius) is laid in a fly larva, 

 the Opius larvae thrash about and kill one another until only one 

 is left. They usually kill most of the much smaller Tetrastichus 

 larvae as well (Willard and Mason, 1937). 



The elemental mechanisms of competitive balance and ecological 

 oscillations probably account for many observed existing examples 

 of direct competition. In many examples, circumstantial evidence 

 certainly points to ecological oscillations. Of the six Illinois species 

 of Erythroneiira leaf hoppers living in direct competition on the 

 eastern sycamore Platanus occidentalis, E. lawsoni is the most abun- 

 dant in most of the localities in Illinois, but in certain localities 

 any one of four other species may predominate (Fig. 47). Further, 

 the proportionate rankings of the six species may change on the 

 same tree from year to year. In Illinois, temperatures fluctuate 

 annually in a moderately uniform fashion over the whole state, but 

 rainfall and humidity fluctuate greatly and in bizarre kaleidoscopic 

 fashion from locality to locality in the same year (Fig. 97). This 

 varying structure of the weather, correlated with slight physiologi- 

 cal differences in the leaf hopper species, could well explain the 

 competitive coexistence of five of these six species. 



Unrealized competition is another mechanism that comes into 

 play with increased number of species and more complex food 

 chains in modern communities. Andrewartha and Birch (1954) 



