Egg number varies with population density: a study of three oribatid mite species in orchards in Egypt.

This study was done to investigate the relationship between the reproductive potential of oribati (estimated by egg number) and their population density in different orchard habitats. We hypothesized that species with narrow feeding niches would be more susceptible to intraspecific competition and would show a decrease in egg number with increasing density, while general feeders would show egg numbers independent of density. To investigate these predictions, soils were sampled four times a year in orange, mango and date palm orchards at each of three sites in Al- Gharbia Governorate, Egypt. The number of mites extracted, the total number of eggs contained in their bodies and the gut contents were evaluated. The results show a significant difference between species in the way egg number varies with density. In Scheloribates laevigatus, a positive correlation between population density and egg number per individual is present. For Xylobates lophotrichus, however, a negative correlation was observed, while in the case of Zygoribatula undulata, no significant correlation between population density and egg number could be demonstrated. Gut contents showed that S. laevigatus has the widest trophic breadth, while X. lophotrichus is the most specialized feeder. We argue that the relationship between egg number and population density is shaped by differences in feeding habits: a specialized feeding habit is associated with a negative relationship between egg number and population density, whereas a broad trophic niche seems to be associated with a positive relationship between egg number and population density. This suggests that feeding habits have important population dynamic consequences in these soil-living invertebrates.