Enhancement of Monarto mintbush populations in South Australia by translocations

Summary

Monarto mintbush (Prostanthera eurybioides), a Critically Endangered South Australian shrub confined to two disjunct populations totaling roughly 630 wild plants near Monarto and Mount Monster, was the focus of a series of translocation trials by the Botanic Gardens of Adelaide testing microsite, weed control, herbivore exclusion and soil-additive variables to guide larger-scale population enhancement. A microsite trial found open, rocky terrain far outperformed sheltered or densely vegetated sites, with 80% first-year survival versus 20% and 0% at the other two settings; a separate trial found water-storage hydrogel crystals made no significant difference to survival or growth at either site. Weed control proved essential, since all transplants in weedy plots died within a year versus 60% survival in hand-weeded plots, and herbivore-exclusion fencing was similarly decisive, with fenced plots reaching 96% survival after 11 years at Mount Monster compared to 67% in unfenced, heavily grazed plots roughly a third the size. Natural seedling recruitment first appeared in fenced plots ten years after translocation and has continued annually since, always within 1-2 meters of parent plants. Full-scale translocations enhanced the Monarto population with 126 seedlings in 1997, reaching 24% survival after 12 years with annual flowering but no observed recruitment, while 1998 plantings at two sites near Mount Monster reached 88% and 57% survival after a decade, with 147 seedling recruits counted at one site after eleven years. Major difficulties included grazing, weeds, rocky terrain complicating fencing, and drought-driven mortality among larger, ungrazed plants. The project rates itself Successful, having achieved all four original goals despite Monarto's continued lack of natural recruitment.