The population biology of the early spider orchid Ophrys sphegodes Mill. III. Demography over three decades

1. Compared to animals, long-term, large-scale demographic studies based on plants are scarce. A 32-year plant-based demographic study of the rare early spider orchid Ophrys sphegodes is presented, covering periods of management by cattle grazing (1975–1979) and sheep grazing (1980–2006). 2. Annual recruitment exceeded mortality under sheep grazing, but numbers of emergent plants did not increase for many years. Eventually, following rapid population increase, numbers fluctuated strongly, with high recruitment in 1 year followed by heavy mortality the next. 3. The population’s distribution between different life states varied considerably, even between consecutive years with identical management. On average, almost 30% of plants were dormant. Most dormant periods lasted <4 years (c. 78% were = 2 years), but dormancy of up to 8 years was recorded. 4. Most orchids had short lives from first to last appearance, but some lived for >20 years. Age-specific survivorship data yielded a half-life of 2.25 years. 5. Peak flowering date advanced by 0.5 day year-1 during the study. Flowering was earlier after warmer years and later after winters with more frosts. Inflorescence height and leaf number were positively correlated with rainfall during inflorescence extension, but negatively correlated with temperature and sunshine hours over up to a year before flowering. Higher temperature was associated with less of the population flowering and more being vegetative. The proportion in dormancy was unaffected by climatic variables. 6. Annual recruitment and mortality were positively correlated with temperature in the previous year, and annual recruitment was positively correlated with the number of flowering plants in each of the two previous years. In most years, <1 plant was recruited per flowering plant in each of the two previous years. 7. Despite a dramatic increase in the number of emergent plants c. 10 years after management changed to sheep grazing, and large numbers of emergent plants thereafter, mortality greatly exceeded recruitment over the last 10 years of this study. 8. Synthesis. Conservation of orchids like O. sphegodes, which have numerous ‘weedy’ life-history characteristics, is heavily reliant on appropriate management. Although previous management prescriptions for conservation remain valid, some site disturbance will be beneficial to recruitment.