The population dynamics of Cardiaspina albitextura (Psyllidae).

Abstract

During the last 16 years, the numbers of Cavdiaspina albitextura rose from low to high in many places, and either remained high or eventually resumed their original levels. In other places, the numbers of the psyllid remained low although they fluctuated considerably. The results of a population study which began in 1952 suggest that, in parts of the woodland colonized by C. albitextura, the stabilization of psyllid numbers occurred temporarily - in some areas at low mean levels of population density and in others at high mean levels. In other parts of the woodland, psyllid numbers changed progressively with time. The occurrence of numerical stabilization appeared to depend upon the extent of the mortality caused directly or indirectly by certain environmental agencies whose "deciding" or "conditioning" influence determined when and where stabilization was possible. At low psyllid densities, the principal conditioning agencies were probably the whole complex of species predacious on psyllid nymphs and eggs, and weather. At high psyllid densities, the conditioning agencies were the density of host trees, and wind. Providing that sufficient mortality was caused by the conditioning agencies, negative feed-back mechanisms were able to implement stabilization. At low psyllid densities, birds predacious on adult psyllids appeared to be the operative or "density-governing" agency. The intensity of their attacks, which tended to increase with increase in psyllid density, operated in a probabilistic manner to limit increase in population numbers. At high psyllid densities, food and space favourable for oviposition were the operative agencies. The limited availability of food and space brought into action a stabilizing mechanism which functioned automatically through forms of intraspecific competition.