Seasonal Abundance of Soil Mites (Acarina) in Two Contrasting Environments

Abstract

The soils of a secondary regrowth forest plot and an adjoining cassava plot in southwest Nigeria were investigated over an annual cycle to show the seasonal variations in their mite (Acarina) populations. Data on rainfall, soil moisture content, and soil temperature were simultaneously collected. Microenvironmental differences in the two plots were reported to have influenced the densities, age structure, and fluctuation patterns of the mite populations. Like most other arthropods, mostly insects and spiders studied in other tropical environments, the soil mites showed within and between seasonal changes in abundance at both the forest and cassava plots. Two patterns of seasonal fluctuations characterized by a single maximum population density on the one hand and multiple peak populations on the other hand were shown by the mite groups in the forest and cassava plots. Unlike most mite groups whose patterns of fluctuation were similar in the forest and cassava plots, Haplozetes sp., Carabodes sp., Oppia sp., Tectocepheus sp., and Parasitidae were reported to have reacted to the difference in environmental attributes between the forest and the cassava plots by exhibiting different fluctuation patterns between the two plots. The population build-up potential of mites in the cassava plot, especially Haplozetes sp., is discussed. SOIL MITES (ACARINA) are the most abundant forest soil microarthropods (Strickland 1945, Belfield 1956, Lasebikan 1974, Badejo 1982). They have been reported by these workers to account for over 50 percent of the total microarthropod density of forest soils. Being mostly detritivores, they play a very important role in nutrient cycling by increasing the rate of litter breakdown and mixing breakdown products with organic matter in the soil (Edwards et al. 1970). Seasonal differences in the abundance of soil arthropods have been demonstrated by various workers (Salt 1952, Belfield 1956, Davis 1963, Block 1966, Erasmus & Ryke 1970, Niijima 1971, Lasebikan 1975, Usher 1975). These workers reported that microarthropods undergo enormous fluctuations in numbers, these being susceptible to small changes in microenvironment, and that water is the primary factor influencing population size. In the tropics, detailed studies on seasonality of arthropods have been based mostly on insect populations (Bigger 1976; Buskirk & Buskirk 1976; Janzen 1976; Wolda 1977, 1978). Unlike temperate countries, there is no study on fluctuation in mite populations lasting through an annual cycle. The aim of this study was to compare the seasonal changes in the densities of mites in a tropical forest and a cassava plot, with the hypothesis that microenvironmental variations in these contrasting environments influence the fluctuation in the numbers of soil mites. Age structure and fluctuations in mite densities were compared to water availability and temperature conditions in the soil. STUDY SITE AND METHODS This investigation was carried out in a 10 m x 10 m forest plot marked out of a regrowth forest and in an adjoining (separated by a 1 m strip) 10 m x 10 m cassava plot within the Teaching and Research Farm of the Obafemi Awolowo University, Ile-Ife, Oyo State, Nigeria (Lat. 7?29'N, Long. 4?34'E). This farm is about 5 km from the University central campus. This forest is a 20 year old fallow resulting from the traditional shifting cultivation system. The vegetation of the forest plot consisted mainly of trees including Albizia zygia (all plant names after Hutchinson & Dalziel 1954-1972), Cola acuminata, Manihot sp., and Morinda lucida. Prominent among the shrubs were Mimosa invisa and Cnestisferrugiana. The herbs included Chromoleana odorrata, Desmodium gangeticum, Sida acuta, Cassia hirsuta, Panicum maximum, and Mormordica charantia. Plant identification was performed at the Obafemi Awolowo University Herbarium (IFE). In February 1983, the adjoining plot to this forest plot was manually cleared, raked, and burned. Ten rows of heaps were made with each row being 1 m apart. Within each row the heaps were also 1 m apart. Subsequently, cuttings of a sweet variety I Received 30 July 1989, accepted 23 January 1990.