Seasonal fluctuations in populations of Hypogastrura rossi (Salmon) and Isotomina thermophila (Axelson) in a Taita pasture

Abstract

A 15-month field survey of the population fluctuations of two dominant species of Collembola in a pasture soil at Taita was made, and these fluctuations related to temperature and moisture. The species studied were Hypogastrura rossi (Salmon) 1941 and Isotomina thermophila (Axelson) 1900. The work also involved controlled laboratory studies on the influence of temperature and humidity on the life-histories of H. rossi and I. thermophila. From field data, frequency, dominance and density values were calculated for H. rossi and I. thermophila. Both species are aggregated in the soil - individuals of H. rossi being more highly aggregated than individuals of I. thermophila. Numbers of these species, which fluctuate seasonally, reached maximum abundance in spring - H. rossi in September, I. thermophila in November. Field data suggested that when compared with I.thermophila H. rossi prefers cooler and moister conditions. The age distributions of the populations of H. rossi and I. thermophila were found to differ, prereproductives forming the highest proportion of the H. rossi population, reproductives the highest proportion of the I. thermophila population. Growth of H. rossi and I. thermophila conforms to Dyar's Rule (Dyar, 1890). The rate of embryonic development of H. rossi is faster than that of I. thermophila at lower temperatures (7-14°C) but is slower at higher temperatures (15-25°C). At all humidities tested, specimens of H. rossi survived for greater lengths of time without food than specimens of I. thermophila.