Long-term demographic studies involving repeated censuses produce invaluable data for understanding how and why natural populations change in size and structure. There have been numerous studies of this type in animal ecology (e.g. Murie 1944; Sinclair 1977; Clutton-Brock, Guiness & Albon 1982; Clutton-Brock, Major & Guinness 1985; Messier 1994; Stenseth et al. 1997), and many of them are celebrated for the insight they provide, and repeatedly described in ecology textbooks, and mined for data to address new research questions. With few exceptions (e.g. Lauenroth & Adler 2008; Roach, Ridley & Dudycha 2009), plant ecology lacks comparable studies. This article describes a demographic study of the early spider orchid, Ophrys sphegodes Mill., that is similar in duration and scale to these classic studies on animals. It is based on over 30 annual censuses, often involving several hundred plants, making it virtually unique in duration and in size among plant population studies. Altogether, data were obtained from 3681 plants of O. sphegodes during the study, and full life histories were recorded for 2499 of these.
Tamm (1948, 1972; Inghe & Tamm 1988) pioneered the demographic study of terrestrial orchids. The Orchidaceae has now attracted more demographic research than any other plant family; Kull (2002) lists 42 studies on 30 orchid species in 18 different genera, and several more have been published since (Whigham & Willems 2003). However, many early studies only reported counts, often just of flowering plants, in different years (Farrell 1991; Vanhecke 1991; Silvertown et al. 1994; Øien & Moen 2002), and few studies involve censuses, from which detailed information about recruitment, mortality, life spans and other population characteristics can be obtained. Moreover, although there are exceptions (e.g. Wells 1967; Hutchings 1987b; Jacquemyn et al. 2007), many of the census-based studies gathered data from few plants in few years. Consequently, despite the effort invested in the study of orchids, and notwithstanding the contributions of recent publications (e.g. Kéry & Gregg 2003, 2004; Shefferson et al. 2003; Shefferson, Kull & Tali 2005; Gregg & Kéry 2006; Pfeifer et al. 2006; Shefferson 2006; Jacquemyn et al. 2007; Shefferson & Simms 2007; Shefferson & Tali 2007), many aspects of orchid population dynamics are still poorly understood (Kull 2002; Whigham & Willems 2003).
Like many other orchid species, O. sphegodes has declined significantly in range and local abundance in recent decades (Hutchings 1987a, 1989a; Dixon et al. 2003; Kull & Hutchings 2006), with habitat destruction, isolation of remaining populations, unsuitable management and eutrophication among the prime causes. Ophrys sphegodes is now rare in the UK, and appropriate management (Hutchings 1987a) and an understanding of the demographic effects of different management regimes are vital for its conservation. This study examines the demographic behaviour of O. sphegodes under different forms of management and its impact on conservation of the species. The investigation focussed on the largest remaining population of O. sphegodes in the UK. The study began in 1975, when the habitat was managed by cattle grazing. This was replaced by sheep grazing in 1980. From 1981 onwards, grazing animals were removed each year, from c. 1 month before O. sphegodes flowered until after seed dispersal (April to early September). Earlier analyses of the first 10 years of the study (Hutchings 1987a,b; Waite & Hutchings 1991) showed that annual recruitment fell steadily under cattle grazing, and annual mortality was high, resulting in a progressive decline in the number of plants in the population. After sheep grazing was introduced, recruitment exceeded mortality every year. Despite these beneficial effects of the change in management, counts of emergent plants gave little evidence of an improvement in the size of the population, or in its condition.
This study examines several aspects of the demography of O. sphegodes. First, population dynamics before and after the change in management are analysed in detail. Secondly, the influence of prior climate on flowering behaviour is examined, and evidence of a shift in flowering phenology is sought. Thirdly, the effects of prior climate upon the proportion of plants in different life states (vegetative, flowering, dormant), on plant performance, and on annual recruitment and mortality, are examined. Fourthly, demographic information obtained after 32 years is compared with results reported after 10 years, to determine whether the additional period of study altered understanding. Finally, previous management recommendations for conserving O. sphegodes are reviewed and reassessed.