The demography of terrestrial orchids: life history, population dynamics and conservation

Abstract

Abstract Terrestrial orchid life-cycles are complex and dependent on pollinators and mycorrhizal associates. Worldwide, orchid populations are declining because of urbanization, atmospheric nitrogen deposition and climate change. To advance understanding of the factors determining orchid population viability, we review knowledge about orchid demography, life histories and population dynamics. Orchids can produce thousands of seeds, although few survive to reach maturity, with mortality rates declining from juvenile to adult life states. Flowering and fruiting rates vary widely between years, and many populations, especially of deceptive species, are pollen- and seed-limited. Many species have long lifespans and periods of vegetative dormancy and exhibit costs associated with reproduction, sprouting, vegetative dormancy, growth and size. Population growth rates range from 0.50–2.92 (mean: 0.983 ± 0.026). Although vital rates can fluctuate widely between years and be strongly correlated, these correlations have little impact on population dynamics. Variation in spatial density of fungi and microsite quality, limited dispersal and competition generate density dependence in vital rates. Future research should elucidate the roles of biotic and abiotic factors on population dynamics to underpin effective management for conservation. Understanding the impact of idiosyncratic individual plant behaviour on population dynamics will also improve demographic parameter estimation, including population growth rate and net reproductive rate.