Ophrys sphegodes is a short-lived tuberous orchid of chalk and limestone grassland. It produces a rosette of leaves in September–October and flowers in the following year from late April to the end of May. Every year, some plants are dormant, and some emergent plants are vegetative. Few inflorescences exceed 15 cm in height in the population studied, and most bear four or fewer leaves and flowers. Ophrys sphegodes can self-pollinate, but pollination is also carried out by males of the rare solitary bee Andrena nigroaenea (Schiestl et al. 1997). In most years, few (6–18%) seed capsules produce seeds (Summerhayes 1951; Lang 1980). Plants that do not flower or produce seeds die back quickly, whereas those with fruits persist until after seed dispersal in mid- to late August.
Ophrys sphegodes is a species of southern and central Europe, reaching its northern range limit in southern England. Between 1930 and 1975, its UK range contracted by c. 80% towards the south-east (Hutchings 1987a). Removing or damaging plants is now prohibited by law.
The study was carried out in Castle Hill National Nature Reserve, East Sussex, UK, an area of 47 ha of ancient calcareous grassland and restored grassland on former arable land, between 100 and 190 m a.s.l. on Upper Chalk bedrock. The vegetation is primarily mesotrophic grassland of community type MG4 (Rodwell 1992), which developed during many decades of close grazing by sheep. More recently, cattle have been used for grazing. Further details of the soils and vegetation can be found in Gay, Grubb & Hudson (1982) and Hutchings (1983, 1987a).
The population of O. sphegodes is spread over several hectares of a south-west facing hill slope with an inclination of c. 16°. It consists of many thousands of plants. Because of the area it covers and the number of plants it contains, data were collected from a small proportion of the population within a permanently marked plot. Photographs and floristic records since 1980, when sheep grazing began to be used again as a management regime, show that there has been little change in the grassland, or in its composition or standing crop, during this time.
Methods of data collection have been consistent since the study began in 1975, enabling the demography of O. sphegodes over more than 30 years to be directly compared with behaviour over the 10-year period reported in Hutchings (1987a,b). Although repeating censuses within years (Wells & Cox 1991; Sanger & Waite 1998) and the use of mark–recapture analysis (e.g. Kéry & Gregg 2003, 2004, Kéry et al. 2005) might have yielded additional information, logistics, practicality and time constraints limited data collection to one census per annum. The large number of plants censused in most years and the duration of the study, combined with the short life span of O. sphegodes, are presumed to compensate for any limitations in this approach.
In 1975, a 20 × 20 m plot was permanently marked out by driving brass rods into the bedrock until flush with the soil surface. From 1975 until 2006, the plot was systematically and meticulously searched at the peak of the flowering season for all flowering, grazed and vegetative plants of O. sphegodes. Thirty-one censuses were carried out in 32 years. Access to the countryside was banned by the UK Government in 2001 because of an outbreak of foot-and-mouth disease, preventing data collection in that year.
The position of every emergent plant was recorded each year by triangulation from two of the corners of the plot, to an accuracy of 0.5 cm, and rectangular coordinates calculated. The life-history stage of each plant (flowering or vegetative), its condition (intact, inflorescence grazed) and number of leaves were recorded. Flower number and height of inflorescences were also recorded. The life histories of all recorded orchids were then reconstructed by examining the maps and status of plants between years.
Some orchid species pass through a subterranean phase between germination and first appearance above-ground that can last for several years (Wells 1981). This phase appears to be short (two years or less) in at least some species of Ophrys (Ziegenspeck 1936). Because of this, recruits can only be registered from their first appearance in the emergent population, which may not correspond with the year of germination. Therefore, ages and life spans were calculated from first appearance. Previously recorded plants can also become dormant for one or more years, after which they may re-emerge, flower and set seed again. Several studies have assumed that orchids that fail to appear above-ground for three consecutive years are dead (Hutchings 1987a; Kull 2002; Pfeifer et al. 2006). However, longer dormancy can occur in some species, including O. sphegodes, although it may be unusual (Shetterson 2009). In this study, as in Hutchings (1987a,b), orchids that did not appear for three consecutive years were assumed dead unless later records contradicted this assumption. It is therefore not possible to be certain whether orchids that were absent for 1 or 2 years at the end of the study were dead or dormant. Similarly, the first plants to be treated as new recruits were those recorded for the first time in the fourth year of the study. Retrospectively, for each year of the study, plants that were found to have been dormant were assigned to one of three categories (dormant 1, dormant 2, dormant >2) depending on how many consecutive years they had been dormant.
Population size was determined, together with the numbers and proportions of plants in the emergent and dormant fractions of the population and in the different states (flowering, grazed, vegetative, dormant 1, dormant 2 and dormant >2). Annual and accumulated recruitment and mortality, and the accumulated balance between the two were estimated. The frequency distribution of life spans and lengths of episodes of dormancy were analysed, and the half-lives of each annual cohort of newly recruited plants, and the proportions of each cohort that flowered in the first year above-ground, were also calculated.
The data were examined to determine whether the proportions of plants in the vegetative, flowering and year 1 dormancy states in a given year were correlated with the proportions in each of these states in the next year. In addition, relationships were sought between the proportions of vegetative, flowering and dormant plants in each year and mean number of leaves, inflorescence height and number of flowers per inflorescence in the same year. Bonferroni adjustments for the level of significance were made in each of these three sets of analyses, with the critical value for P raised to 0.005 (there were nine tests in each case).
Relationships between population performance and selected climate variables were examined using data from the nearest Meteorological Office weather station, at Eastbourne, East Sussex UK, 21 km east of the study site and 7 m a.s.l. The climate data used were the number of air frosts in the winter preceding flowering, and summed monthly rainfall totals, mean monthly temperature and summed sunshine hours for 12 (May–April), 8 (September–April), 4 (January–April) and 2 (March–April) month periods prior to flowering each year. These periods were selected to represent, respectively, climatic conditions (i) for the whole year between two flowering periods, (ii) from leaf emergence until flowering, (iii) during the period of temperature increase before spring flowering and (iv) during inflorescence extension. Correlations were sought between each climate variable and the percentage of the whole population that was vegetative, in flower, or dormant, the mean number of leaves borne by emergent plants, and mean inflorescence height. As O. sphegodes produces a new tuber every year, it was predicted that, unlike the situation in some other orchid species (Leeson, Haynes & Wells 1991), there would be no significant effects of climate further in advance of flowering on the performance of established plants. To test this prediction, analyses were also carried out using the same climate metrics for 12–24, 12–19, 12–15 and 12–13 month periods prior to flowering. In addition, correlations were sought between climate variables during the 12 months from May to April of years t – 1 and t – 2 and the number of new recruits observed in year t, to allow for the possibilities of recruitment to the emergent population both within 12 months of the climatic cues, and after a delay of 1 year. The effects of climate in year t – 1 on mortality in year t were also examined. As climate metrics for periods of different duration were correlated, relationships between climate and performance were assessed separately for the 12-, 8-, 4- and 2-month periods prior to flowering. For each climate variable and for each time period considered, a total of five tests were carried out. Bonferroni adjustments for significance were made in each set of analyses, with the critical value for P raised to 0.01. Finally, the effects of the selected climate variables on peak time of flowering, as reflected by the date on which each census began, were also examined. All statistical analyses were carried out with systat 10.2 (Systat Software Inc., Richmond, CA, USA).