Competition 397 



history, behavior, and requirements, but subtle differences exist in the 

 responses to the conditioning of the medium, in the effects of crowd- 

 ing on natahty, mortaHty, and rate of development, or in the rates at 

 which the beetles eat each other's eggs. Under certain conditions 

 T. confusum was found to inhibit the net fecundity of T. castaneum 

 more than the latter species inhibits itself, with the result that T. 

 confusum drove out T. castaneum in competition. Under slightly 

 different circumstances T. castaneum gained the upper hand, but 

 never would the two species coexist permanently in the same unit of 

 flour (Birch, Park, and Frank, 1951). 



In similar tests involving mixed cultures of two species of clado- 

 cerans, Daphnia pulicaria always caused the extinction of Simo- 

 cephalus vetulus. Since these species do not directly attack one an- 

 other at any stage, the displacement of one species must be due to 

 difference in toleration for shortage of food, chemical conditioning, 

 oxygen lack, or physical effects of crowding, but precisely what is the 

 crucial aspect of the competition awaits further investigation (Frank, 

 1952). 



From both laboratory and field studies we learn that two species 

 having essentially the same requirements from their immediate en- 

 vironment do not usually form mixed steady-state populations. The 

 most closely related species of a genus generally have different geo- 

 graphical ranges. If they live in the same region, they inhabit dif- 

 ferent types of habitat or they obtain their food and other necessities 

 in a slightly different way— in other words, they occupy different 

 habitat niches, as discussed more fully in Chapter 13. 



When two closely competing species are unable to continue living 

 in the same habitat, the determination of the species destined to 

 survive depends upon which species is favored by the existing en- 

 vironmental conditions. If conditions change, as they do regularly 

 with the seasons in temperate regions, for example, a different species 

 may come to be favored. When conditions are not optimal in re- 

 spect to one ecological factor, the range of tolerance is often reduced 

 with respect to other factors. These general relations help to explain 

 the fact that different species succeed one another during the season, 

 as is seen in phytoplankton populations, with no one species holding 

 numerical superiority throughout the year. 



The type of competition between species that vie with one another 

 by direct aggression has been given mathematical formulation by 

 Volterra (1931) and Lotka (1934) for various cases in which one or 

 more species feeds upon another species. When the population of 

 one species grows at the expense of another, as in the predator-prey 



