Thomas Park

Experimental Studies of Interspecies Competition II. Temperature, Humidity, and Competition in Two Species of Tribolium

T HE primary objective of this paper is to record in considerable detail a series of laboratory observations dealing with the phenomenon of competition and to array certain conclusions that emerge from these data. Before doing this, however, it is meaningful to introduce, and attempt partially to answer, two questions. The first: What logical steps should be followed in the study of interspecies competition? The second: Can such competition be defined in abstract terms? In respect of the first question it seems to me that a long-range program dedicated to the serious study of interspecies competition ideally should progress through the following five conceptual stages:

Natural Selection and the Outcome of Competition

The present study suggests that when two small populations compete for the same resource and one of them is eliminated after a few generations, natural selection resulting from competition between the two is not likely to have had appreciable effects in determining the outcome. This should not be interpreted as evidence that random genetic changes taking place during this period cannot affect the outcome of competition, not obviously, that natural selection resulting from interspecies competition may not be important in evolution.

THE EFFECT OF INTRASPECIES AND INTERSPECIES COMPETITION ON THE FECUNDITY OF TWO SPECIES OF FLOUR BEETLES

The flour beetles Tribolium confusum and Tribolium ccstaneum have been used in an extensive investigation of interspecies competition (Park, 1948). A finding of this study was that when no special precautions were takento eliminate the sporozoan parasite Adelina tribolii, T. confusum, as a single species, typically maintained populations about twenty to thirty per cent larger than did T. castaneim. No known differences in temperature, humidity, light, medium, or handling existed between these two series. In mixed-species populations, that is when there was competition between the two forms, T. castaneum was usually driven out by T. confusum. In the absence of the parasite, T. castaneum usually drove out T. confusum. There is some indication that in their natural distribution the two species vary in relative abundance according to latitude (Good, 1936) and to the breeding medium, whether wheat or flour (Miller, 1944). It is hoped that studies of competition may shed some light on such problems in the population ecology of the two species as well as on the general nature of interspecies competition itself. In many ways these beetles are excellent objects for the laboratory study of such phenomena. They inhabit the same ecological habitat from which they derive their entire food supply and in which their life-history, and that of successive generations. is passed. The DoDulations. includ-

Mortality Resulting from Interactions between Adult Flour Beetles in Laboratory Cultures

THE THE design for this experiment was suggested by the appearance of unbalanced sex ratios in competition cultures when Tribolium confusum was eliminated by T. castaneum (Park, 1957). The new data reveal that adults of these beetles are able to cause mortality among themselves in a way which hitherto was scarcely suspected, but the mechanism underlying the disturbance in sex ratios is still only partially understood.

Experimental Studies of Interspecies Competition. III Relation of Initial Species Proportion to Competitive Outcome in Populations of Tribolium

ADOLPH, E. F. 1929. The oxygen consumption of isolated frog skin under the influence of solutions. Jour. Exper. Zoal., 53:313-25. BOELL, E. J., and TAYLOR, A. B. 1933. Studies on the electromotive force in biological systems. I. The effect of homologous carbamates on the E.M.F. of frog skin. Jour. Cell. and Comp. Physiol., 3:355-83. BRINLEY, J. F. 1927. Studies on the physiological effects of hydrogen cyanide. Biol. Bull., 53: 365-89.

Mortality Patterns in Eight Strains of Flour Beetles

Male and female flour beetles are described in terms of one mortality characteristic: adult age at death. Four strains of Triboliurn ccnfusum and four strains of Tribolium castaneum were studied. The actual data are reported in two Appendix tables. Death rates were found to depend upon the species, strain, sex, and age of the individuals concerned. Differences between the mortality patterns of the two sexes were especially striking and seemed to have a common character for all strains and both species.

Flour beetles: responses to extracts of their own pupae.

Two species of flour beetles, Tribolium confusum and Tribolium castaneum, respond to neutral pith disks impregnated with aqueous extracts of their pupae in such a way that the adults and larvae tend to select the extract of the other species. These findings have implications for the population ecology of Tribolium.

The Laboratory Population as a Test of a Comprehensive Ecological System

HE time is ripe for a careful and critical scrutiny of certain aspects of the terminology issue in ecolA ogy. If judiciously done such scrutiny should furnish an over-view of basic ecological concepts and principles. However, this is not a simple task. It would be easy to hold that all ecological terminology is bad; that the field is not yet ready for a nomenclature of its own; that such a nomenclature can never be of value. It would be equally easy to hold that the ecologists have been unusually perspicacious in matters of terminology and that ecologic terms are both meaningful and empirical. Unfortunately, neither position appears to be sound. It must be granted immediately that any system of ecological terminology that is brief, skeletal yet general is much to be desired. It is apparent that, even if capable of development, such a system is yet to be discovered. There are good reasons for this. The primary ones seem to be (a) that the environmental relationships of organisms in nature are innately complex, and (b) that the evidence necessary to delineate these relationships is sadly lacking. The present treatment is in no sense a discourse on esoteric terminology. In fact that is a subject about which the author is both skeptical and critical (Park, I939a). However, I have been interested for over a decade in problems concerned with the experimental and statistical biology of populations and have seen the growth of a good deal of respectable information collected primarily by using insects and microdrganisms as experimental material. (For key references ee Allee, I93I; Bodenheimer, I938; Gause, I934; Park, I939; Pearl, i92w5; Thompson, I939; and Wright, I931.) It seems to me that much of this subject matter is ecological in character. This lends impetus to the present paper in which an attempt will be made to apply certain facts and concepts gleaned from population research to a general ecologic system intended to fit a diverse series of natural field communities. Although the laboratory population studies have been approached from several points of view they share, in the main, certain common properties. Some of these properties may be listed as follows: (i) There is an attempt in all these studies to express the results in quantitative terms. This is a desirable, though not always possible, end-product in any ecological venture. (X) There is essentially a complete absence of terminology in the studies. This may mean that the results are not too well integrated. It may also mean that this phase of ecology is free from the bias of prematurely coined terms and, by that token, poorly crystallized concepts.

The Laboratory Population as a Test of a Comprehensive Ecological System (Concluded)

Aggregation a d Coaction E ARLIER in this paper the point was made that aggregation occurs in nature, and, as a result, coactions are set up. These coactions are designated in terms of their end-effect on the population as co-operation, disoperation, and competition. It is our plan to illustrate these three forms of coaction by the data of experimental population studies. One clarification is necessary before proceeding. We shall be discussing Tribolium and Drosophila. Populations of these species are aggregations in the statistical sense that they are groups of organisms, and in the biological sense that they respond as a unit or population integrated by biological processes. They are not, however, as closely-knit aggregations as can be studied under laboratory conditions. Laboratory aggregations often occur without physical crowding within a habitat. This means that their reality as aggregations does not depend on spatial confinement. A few examples of the latter follow: aggregations of Paramecium, terrestrial isopods, schooling fish, all truly social insects, Asellus (isopod) and certain amphipods during the breeding season, many birds under night roosting conditions, earthworms when the soil loses moisture, tubifex worms, mice at night or when the temperature lowers, and so on. These aggregations also furnish interesting material for coaction analyses. They can not be studied here but the writings of Allee (I93I; I934; 1938) should be consulted for further information.