The effects of sheep stocking level on invertebrate abundance, biomass and energy utilization in a temperate, sown grassland.
Abstract
The results of the study are summarized as follows: (1) Sixteen groups of large invertebrates were sampled at 42-day intervals over 3 yr from six grassland sites. The sites were grazed continuously with ten, twenty and thirty sheep per ha. Invertebrate collections were made from the herbage and litter and soil cored to 25 cm. (2) Stocking intensity affected the abundance and biomass of all invertebrates. The Hymenoptera (ant) group increased in numbers and biomass with each increase in sheep stocking; all other invertebrates were reduced substantially at the highest stocking level. Scarabaeid larvae and large Oligochaeta showed peak values for abundance and biomass at the intermediate stocking level. The abundance and biomass of the remaining groups were reduced with each increase in sheep numbers. (3) Base levels of invertebrate energy expenditure were determined from field censuses of abundance and mean fresh biomass, site measurement of habitat temperature and published respirometry data. A value of 0.50 for net tissue growth efficiency was derived from published studies and used to calculate additional heat production associated with growth. Data on assimilation efficiency and ratios of production to respiration were taken from the literature. (4) Invertebrate energy expenditure was apportioned between the activities of herbivory, decomposition and predation. Increases in sheep numbers resulted in a comparative decline in the activity of small decomposers. Scarabaeid larvae were the main invertebrate herbivores and large Oligochaeta and Collembola were the major decomposers. (5) Sheep were the dominant herbivores. Changes in energy expenditure with increased stocking level indicated that the sheep flocks were better able to derive energy from grazing than the invertebrate herbivores. There was no evidence that invertebrates reduced the wool production or biomass of the sheep at the low and intermediate stocking levels. (6) Approximate energy budgets were calculated for the total invertebrate communities. Energy values for the net annual residues of invertebrate excreta and corpses declined with stocking levels but the values exceeded the excretal returns from the sheep at the low and intermediate rates.