THE VARIABILITY OF ORCHID POPULATION-SIZE

Orchid population size has long been believed to be highly variable in time and space. Here we present the first quantitative assessment of the variability of orchid population sizes and compare them with plant and animal population data from the literature. Three measures of tem poral variation were used (standard deviation of log(10) (population s ize), Hurst exponent and variance in the intrinsic rate of increase). Eight unfertilized plots of Orchis morio in a factorial fertilizer exp eriment were assessed for variability in population size over a 26 yea r period. The Hurst exponent across the eight plots gave measures cons istent with pink noise well into black noise (0.86 to 1.87). The varia tion in the intrinsic rate of increase suggested that the expected tim e to extinction in the plots differed by an order of magnitude. Standa rd deviation of log(10) (population size) across plots was greater tha n the corresponding temporal variation within plots. Standard deviatio n of log(10) (population size) reached an asymptote between 10 and 20 years. The range of temporal variation between plots in this and other orchid populations was as great as the mean values reported for birds , insects and mammals. These results have highlighted the importance o f spatial variation: until we know the relative importance of spatial and temporal variation for more plant species we will be unable to ass ess whether orchid populations are more variable in size than other sp ecies. (C) 1998 The Linnean Society of London.