Structure and dynamics of a cave population of the Guano mite, Uroobovella coprophila (Womersley)

Abstract

The biology and ecology of U. coprophila was studied in mite cultures in the laboratory and from analyses of samples taken from a heap of bat guano in a small cave chamber. The guano heap was formed over many years of cyclical roosting in the chamber by several thousand bent-winged bats, whose faeces and urine form the source of energy for the permanent community of organisms in the heap. U, coprophila is a major component of this guano community because of its exceedingly high numbers (up to 33.7 x l0*6 per square metre). Freshly defaecated bat guano was collected from the cave and used as a substrate for laboratory cultures of mites. The mites feed and mature on fungi. There are five stages in the life cycle: eggs, larvae, protonymphs, deutonymphs, and adults of both sexes. No phoretic nymphs were found. The mean duration in days of each of these stages was determined at 14,20, and 24°C. There was a positive linear relationship between temperature and the rate of development of larvae and protonymphs. Age-specific fertility and mortality were determined. Laboratory experiments supported the hypothesis that a regular supply of fresh bat guano was necessary for continuous growth and reproduction of U. coprophila. When fresh dung was not available, females ceased laying eggs and growth of the immature stages ceased. Nymphs and adults became quiescent. Annual changes in the cave population of mites and their vertical distribution in the guano heap were determined from quantitative samples of bat guano collected at regular intervals from August 1966 to July 1970. A sieve-sampling method for guano deposits is described; mites were extracted by a modified Tullgren funnel. Mites feed primarily in the surface layers of guano and move into the heap to moult. Sex ratios were found to be significantly biased towards males at most times of the year. Several generations were produced each year. These overlapped and became indistinguishable. A graphical method was used to determine instar-specific mortality of the mites. Recruitment to the population occurred when bats were present in the chamber and fresh dung was being deposited. When bats were not present, the fresh guano was used up rapidly and growth and reproduction of the mite population ceased. It is suggested that the mite population progresses through an annual cycle of four phases keyed with the annual roosting cycle of the bats and that limitation of the mite numbers is by a complex interaction between fly larvae, food, and temperature of the guano until an absolute shortage of food occurs and mites become quiescent.