The Effect of Intraspecies and Interspecies Competition on the Fecundity of Two Species of Flour Beetles

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Abstract
The study forms one unit of a long-range program on the quantitative analysis of competitive or selection pressures existing between two populations of flour beetles, T. confusum and T. castaneum. It is already clear from other findings that it is impossible for these two species to co-exist indefinitely in one population under environmental conditions similar to those in the present work. In this paper an anlysis is made of the effect of density (both intraspecific and interspecific) on the net fecundity of the beetles and its significance in interpreting the outcome of competition between the two species. Net fecundity (eggs laid minus eggs eaten) was assayed under constant conditions of temperature, humidity, and food supply, but maintained at different population densities of the single and mixed species. A. For single species populations the following results were established: (1) T. castaneum exhibited higher net fecundity than T. confusum at all densities. This clearly seems to be a species specific difference. (2) The net fecundity of both species was reduced proportionately by the same amount as the crowding increased. (3) When crowded with males only of the same species, the net fecundity of females was reduced much less than when crowded with males and females (sex ratio unity). There is evidence that the reduction in net fecundity of females crowded with males only up to a total density of 16 insects per 8 grams of medium could be accounted for by cannibalism alone. When, however, density was increased to the same extent with equal numbers of males and females, the reduction in net fecundity was much greater but cannibalism accounted for only 26% of this reduction. It is apparent that the major effect of crowding by females is a direct effect on real fecundity. No reason can at present be advanced for these interesting differences between the sexes. Although the total cannibalism increased with adult density (when the number of eggs available was kept constant) the rate of cannibalism (eggs eaten per insect) decreased with density for both species. This probably accounts for the reduced significance of cannibalism at higher densities. There were no striking differences in the rates of egg consumption of the two species. B. For the mixed species populations the following results were established: (1) The net fecundity of T. confusum is inhibited by T. castaneum (males) to the same extent as T. confusum inhibits itself, but T. confusum inhibits T. castaneum to a greater extent than T. castaneum inhibits itself. (2) T. castaneum inhibits the net fecundity of T. confusum to the same extent or a little more (depending on the density) as T. castaneum inhibits itself. T. confusum inhibits T. castaneum more than T. confusum inhibits itself. These findings fulfil the conditions appropriate for the Lotka-Volterra equations descriptive of the conditions in which one species would be expected to drive out the other in competition. (3) Whereas in single species populations the average ratio of net fecundity of T. confusum to T. castaneum over all densities studied was 1: 1.28, in populations in which each species is in the presence of 7 times as many members of the other species the ratio is reversed to 1.33:1. (4) The effect of competition on net fecundity could not be accounted for by cannibalism. Neither species exhibited any preference for eggs of one or other species. It is concluded that T. confusum has a direct inhibiting effect on the rate at which T. castaneum lays eggs. C. The general conclusion reached is that the net fecundity of either species is reduced more when crowded with females than with males. The effect of males is due primarily to cannibalism of eggs whereas the effect of females is due to an additional direct effect of crowding on real fecundity. In mixed species populations T. confusum inhibited the net fecundity of T. castaneum more than T. castaneum inhibited itself. This important effect cannot be explained as a result of differential cannibalism of eggs by either species, for they have no preferences. It is evidently related to a direct inhibiting effect of T. confusum on the real fecundity of T. castaneum. The consideration of crowding of one species on the net fecundity of the other would lead to the expectation that T. confusum would drive out T. castaneum in competition. This is what happens in competition under certain experimental conditions (Park, 1948). It is not concluded, however, that net fecundity reduction is the prime factor operating to determine the competitive relationship between the two species. This would be a premature conclusion, since crowding operates by increasing mortality rates as well as by reducing fecundity rates. The effect of differential crowding on mortality rates is a facet of the total problem of competition between these two species which has not yet been investigated. The present study illustrates the complex relationships which exist between competing species, even when only one aspect of the relationship, namely net fecundity, is considered. It is only, however, by detailed analysis of the influence of density on each component of population increase, that the real nature of competition will become elucidated.

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