Effects of rain, sheep and tephritid flies on seed production of two arid Karoo shrubs in South Africa.
The density of Pteronia pallens (Asteraceae), a shrub toxic to sheep, increased in Karoo rangeland at the expense of a congeneric palatable shrub, P. empetrifolia. The effects of rainfall, herbivory by sheep and tephritid fly damage on seed production of these shrubs were compared over a 4-year period. Flowering and number of seeds containing embryos were positively correlated with annual rainfall, and both varied between treatments, habitats and years. The percentage of capitula damaged by Desmella anceps (Diptera: Tephritidae) larvae differed between the Pteronia species, but was greater in both in dry years when flowers were scarce. Browsing of P. empetrifolia by sheep during flower development reduced flowering by 80-90% and caused a 40% decrease in the number of potentially viable seeds per capitulum. P. pallens was not browsed, and flowering did not differ between a sheep camp and an exclosure in any year. In high rainfall years, arid rangeland shrubs produce large crops of viable seeds. It is suggested that protection of palatable shrubs such as P. empetrifolia from domestic livestock during flower development in wet years may prevent further decreases in their populations.