Population decline in the epiphytic orchid Aspasia principissa

The population dynamics of the epiphytic orchid Aspasia principissa, growing in the moist tropical forest of Barro Colorado Island, Panama, were studied from 1997 until 2004. Using growth analysis, projection matrix analysis, elasticity analysis, and different types of simulations, we identified the components of the life cycle with the strongest effect on population growth rate (λ), and related differences in vital rates to environmental variation, mainly in precipitation. Such information, which is almost completely lacking for tropical orchids, is essential for the efficient conservation of these frequently rare and endangered plants. Elasticity analysis indicated that the population growth rate (λ), which averaged 0.92, was primarily affected by survival, and much less affected by growth or by sexual reproduction. Simulations, which included different levels of pollinator limitation, showed that complete pollination would raise λ to such an extent as to allow long-term persistence of the population. Pollinator limitation per se, however, is presumably not responsible for the currently observed population decline. Instead, we discuss a possible link between low λ and (a) a long-term decline in precipitation and (b) recent increases in forest dynamics: variation in annual rainfall significantly affected both recruitment and growth rates of smaller orchid individuals, while the hypothesised increase in the rates of branch and tree falls would increase mortality rates in this epiphyte.