The Effects of Sheep Numbers on Associations of Collembola in Sown Pastures

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Abstract
Species diversity, richness and evenness of Collembola populations were estimated for seasonal samplings of pastures that were grazed continuously with sheep at 10, 20 and 30 ha-1. The sampling zone consisted of herbage to 5 cm height, the litter mat and soil from 0-5 cm depth. The number of Collembola species was reduced from 21 to 15 over the range of stocking treatments. The evenness component of the Brillouin Diversity Index increased with stocking rate; opposite trends in species richness and evenness resulted in no consistent change in the index of diversity. Increases in evenness were attributed to a relatively greater effect of increased sheep numbers on the abundance of the more common hemiedaphic species. The epigaeic species were the least aggregated and the euedaphic group the most aggregated overall. Aggregation was increased by increased stocking; the euedaphic Collembola being the least affected. In subsidiary samplings to a soil depth of 30 cm the pattern of vertical distribution of the hemiedaphic species was greatly changed with increased sheep numbers; the distribution of the euedaphic Collembola was largely unaffected.