The Effects of Population Density upon Growth Reproduction, and Survival of Hyalella azteca

Abstract

T -HE problems of the biotic relationships of like organisms have attracted much interest. Naturalists have reported with enthusiasm noteworthy aggregations, sexual behavior, and social manifestations. This work continues but with a more critical attitude. The less obvious mass relations, seen only by resultant effects, led to a dichotomy of interest, which in the future and to some extent at present (Park, 1939) will coalesce. One approach is that of the study of the whole population, its growth, maintenance, and decline or evolution (Chapman, 1928; Wright, 1931; Gause, 1934; Bodenheimer, 1938). The other is the study of physiological or behavioristic effects of population density upon the constituent organisms (Allee, 1931, 1934, 1938). The physiological aspect is considered in the present study,' which attempts a demonstration of the effects of increasing the density of the population upon three fundamental processes: growth, reproduction, and survival. As an assay animal the common freshwater amphipod Hyalella azteca Saussure2 (H. knickerbockeri Bate), Orchestiidae, was chosen. In the use of an aquatic arthropod the study is unique among researches comparable in nature to those of Pearl and of others to be cited later. The conditions of the experiments were variable, but equal volumes of food and water were used in all, irrespective of numbers of animals present. The following brief review of the literature is necessarily closely confined to the question studied and deals with recent studies on arthropods. The references given in the first paragraph include summaries of results on population studies of other forms. Earlier work by the same or different authors may be found by consultation of the literature referred to below. Growth.-Body size (Warren, 1900oo) and rate of development (Banta, 1937) were retarded in Cladocera by crowding or its concomitant factors. Metamorphosis in the beetle Tribolium confusum was delayed, and body weight was reduced (Park, 1938) in relation to the density of the population and more directly to the degree of conditioning of the medium. On the other hand, evidence is present for an augmentative effect of increasing numbers of organisms. Development is accelerated in Periplaneta orientalis after the ninth instar, and the effect is apparent even in a density as low as 2 animals. However, the longest and heaviest animals were found among the isolated individuals, an effect attributed to increased disturbance by collision in higher densities (Landowski, 1938). Meal worms (Tenebrio molitor) raised 50 together in 60 cc. of meal consistently weighed more than did isolated larvae; even 150 animals in a similar volume maintained a higher weight than that of isolated ones until the sixtieth day (Michal, 1931). This was