The population biology of the early spider orchid, Ophrys sphegodes Mill I. A demographic study from 1975 to 1984

(1) The results of a ten-year (1975-84) demographic study of a population of Ophrys sphegodes, a rare orchid with a declining distribution in the British Isles, are presented, and the methodological and interpretational difficulties in conducting demographic studies on orchids are discussed. (2) Individual plants of 0. sphegodes show irregular patterns of appearance above ground between years. Plants which fail to appear for three consecutive years can usually be considered to have died ( > 0 95 probability). (3) The number of plants emerging in different years varied over a twofold range but failed to show the dramatic yearly fluctuations said to be characteristic of some orchid species. Each year between 65 and 970% (mean 840%) of recorded plants flowered. It was estimated that no more than half of the total population was above ground when flowering occurred in May, although a higher proportion of the population may have emerged in the previous autumn. (4) The species is heavily grazed by sheep. Invertebrate grazing is low, affecting less than 50% of the emergent population each year. (5) Mean flower spike height was correlated with summed rainfall during the period of emergence (October-May), but no other measures of population performance, or recruitment, showed convincing correlations with climatic variables. (6) Few 0. sphegodes plants emerge above ground, or flower, for more than three consecutive years. Most emerge for only one year before entering a dormant phase or dying. (7) A few plants achieve life spans after first emergence of at least ten years. No information is available about the length of the subterranean phase of development prior to first emergence. Depletion and survivorship curves conform to Deevey Type III with a slightly decreasing mortality risk through time. However, half-lives, which assume a constant mortality risk, calculated for uneven-aged sets of plants from the beginning of the study, were significant (P< 0 01 to P< 0 001) and ranged from 1 5 to 2 3 years from the year of first emergence. Almost all recruitment of new plants is by seed. Vegetatively produced rosettes of 0. sphegodes were estimated to form less than 5% of the emergent population in any year. (8) Annual recruitment fell considerably, and annual mortality was high, from 1975 to 1980. Since 1980, annual recruitment has exceeded annual mortality. Over the ten years of the study there has been a net gain of thirty-two plants to the population, although from 1975 to 1980 there was a net deficit of ninety-seven plants. This recent improvement in the status of the population is believed to be directly linked to current management practice which involves winter sheep grazing to produce a short turf, and protection from grazing during flowering, seed maturation and seed dispersal.