On competition between different species of graminivorous insects

Abstract

The growth of pure populations of the beetles Rhizopertha dominica and Oryzaephilus surinamensis , and of the moth Sitotroga cerealella , was observed in a standard medium of wheat. This was maintained at a constant level by the periodic removal of ‘conditioned’ frass and the addition of fresh grains. The population of each species rose to a maximum and remained fluctuating about this value indefinitely. A comparison of the rates of oviposition, with the rates at which adults emerged, showed that in the maximum population there was an enormous mortality (always over 90%) in the immature stages. When pairs of species competed Rhizopertha eliminated Sitotroga because their larvae, between which most of the competition occurred, have the same needs and habits. But each of these species was able to survive with Oryzaephilus because this species occupies a different ‘ecological niche’. The Verhulst-Pearl ‘logistic’ equation (1), for the growth of population of a single species in a limited environment, and the Lotka-Volterra simultaneous equations (2), for the growth of population of two species competing for the same limited environment, were fitted to the census data from all the experiments. The biological assumptions on which they are based proved to be true for practical purposes for Rhizopertha and Sitotroga populations. These assumptions are that the value of the potential rate of increase remains statistically constant and that all the factors inhibiting increase are linearly related to population density. Further­ more, a single factor, larval competition, was represented by the single indices standing for interspecific inhibition. It follows that the maximum population (or equilibrium position) should be independent of the initial population, and this proved to be so for all species. Equations (2) did not always fit the observed points very well, but they were always success­ful in predicting the outcome of competition. It does not follow from this that these equa­tions have any general validity. Their basic assumptions are by no means universally true and, unless they are shown to be so for a particular species under known, environmental conditions, no biological deductions can be drawn from them. Where they do apply they describe the course of change of population of two competing organisms with an accuracy which depends on the constancy of the coefficients involved. Two kinds of organism will be able to survive together only if they differ in needs and habits, i. e. occupy different ecological niches. Populations living in a medium of unrenewed wheat rose to a maximum and then declined as the food became exhausted and ‘conditioning’ increased. The eventual extinction of the population was due, not to the cessation of oviposition, but to the failure of the larvae to survive. The longevity of Rhizopertha adults was lower in unrenewed than in renewed medium, and lower still when this species was competing with Sitotroga in unrenewed medium. The longevity of the other species, and the sex ratio of Sitotroga , were apparently unaffected by these conditions. The fecundity of Rhizopertha females decreased with time, and the length of Sitotroga adults of both sexes decreased in succeeding generations. The competitive relation­ship between both Sitotroga and Rhizopertha , and Oryzaephilus shifted slightly in favour of the former species in unrenewed as compared with renewed media. In a renewed medium this relationship probably depends chiefly on the destruction of eggs and pupae by adults and larvae, for which the more predaceous Oryzaephilus is better placed. In an unrenewed medium the ability of the larvae to make the best use of the limited food supply is the determining factor, and here the other two species have the advantage. The competitive relationship between Rhizopertha and Sitotroga remained the same in both media.