EFFECTS OF RAIN, SHEEP AND TEPHRITID FLIES ON SEED PRODUCTION OF 2 ARID KAROO SHRUBS IN SOUTH-AFRICA

1. The density of Pteronia pallens (Compositae), a shrub toxic to shee p, increases in Karoo rangeland at the expense of a congeneric palatab le shrub (Pteronia empetrifolia). The effects of rainfall, herbivory b y sheep and tephritid fly damage on seed production of these shrubs we re compared over a 4-year period. 2. Flowering and number of seeds con taining embryos were positively correlated with annual rainfall, and b oth varied between treatments, habitats and years.3. The percentage of capitula damaged by Desmella anceps (Diptera: Tephritidae) larvae dif fered between the Pteronia species, but was greater in both in dry yea rs when flowers were scarce. 4. Browsing of P. empetrifolia by sheep d uring flower development reduced flowering by 80-90% and caused a 40% decrease in the number of potentially viable seeds per capitulum. Pter onia pallens was not browsed, and flowering did not differ between a s heep camp and an exclosure in any year. 5. In high rainfall years, ari d rangeland shrubs produce large crops of viable seeds. Protection of palatable shrubs such as P. empetrifolia from domestic livestock durin g flower development in wet years may prevent further decreases in the ir populations.