Further experiments on insect competition

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Abstract
The growth of populations of the beetles Tribolium confusum and Oryzaephilus surinamensis was observed in media of wheat, coarse wholemeal flour and fine wholemeal flour, respectively. These were maintained at a constant level by the periodic transference of the insects to equal amounts of fresh media. Population growth was best observed in fine flour, from which all stages could be sifted out and counted. In populations of each species beginning with two adult males and two females, maxima for eggs, larvae and pupae succeeded each other and finally adults emerged and themselves rapidly rose to a maximum. The adult populations remained steady at the maximum, while egg, larval and pupal populations fluctuated round mean values. The rate of population growth was determined by the rates of oviposition and development on the one hand, and of cannibalism on the other. Such cannibalistic eating of eggs and, more important, of pupae by adults and larvae, rather than the limitation of food, also determined the maximum population size. A comparison of the rates of oviposition with the rates at which adults emerged showed that in such populations the mortality in the immature stages was over 99%. In competition with Oryzaephilus, which depended entirely upon mutual predation, Tribolium had the advantage because of its greater voracity. Oryzaephilus was driven out of the flour media which did not protect its pupae. But when the flour media contained glass tubing of such bore as to allow its larvae to enter it and pupate, but to exclude Tribolium adults and large larvae, Oryzaephilus survived together with Tribolium just as it did in wheat. In wheat, the beetle Rhizopertha dominica survived together with Tribolium. The three species Rhizopertha, Tribolium and Oryzaephilus also survived together in this medium. The results of these competitions support the contention that species with the same needs and habits are unable to survive together in the same environment while species which differ in needs or habits may do so. The position at which equilibrium between any two of the competing species was reached was independent of the initial density of each species. When constants of the Lotka-Volterra simultaneous equations for the population growth of two species competing for the same limited environment were calculated from the experimental data, they led to inequalities corresponding to definite equilibrium positions for these equations. The actual equilibrium positions reached by the populations were in every experiment the same as those reached by the equations. The biological assumptions on which these equations are based are not, however, strictly true for Tribolium and Oryzaephilus. Populations living in unrenewed flour rose to a maximum and then declined as the food became exhausted and 'conditioning' increased. As time passed, the age composition shifted from a majority of young stages to a majority of adults. The extinction of the Tribolium population was due to the failure of the larvae to develop and pupate, and of the Oryzaephilus population to this as well as to the cessation of oviposition. The adults, having failed to reproduce themselves, eventually died.