The viability of two populations of the terrestrial orchid Cyclopogon luteoalbus in a fragmented tropical mountain cloud forest: Dormancy delays extinction

Abstract

Habitat fragmentation threatens the viability of increasingly isolated populations, particularly in species, such as orchids, with very specific requirements (e.g., pollination and germination). Many orchids undergo dormancy when conditions deteriorate and, if dormancy decreases the variability of population growth, it would be expected to buffer the negative impacts of disturbance/fragmentation. We investigated the effect of vegetative dormancy in delaying the effect of habitat fragmentation in two populations of the orchid Cyclopogon luteoalbus. Dormancy kept stochastic growth rate significantly higher than it would be if plants entering dormancy were to die instead. Time to 50% probability of quasi-extinction (95% reduction in population size) was 76–100y when dormancy was taken into account, but 17–43y when dormancy equalled death. The sign of the relative contribution (elasticity) that dormancy made to population growth differed between plants going dormant and plants re-sprouting, and between juveniles and adults. Consequently, the relationships between the elasticities of going dormant or re-sprouting and survival followed opposite trends, and the value of survival at which the two curves intercepted was higher for the population with the largest effect of dormancy on population growth. Conclusions: (i) dormancy delays the contribution that individuals make to population growth rate, but also the time to extinction, and (ii) both negative and positive effects of dormancy on population growth rate must be quantified to understand its overall contribution to fitness. Accurate quantification and management of vegetative dormancy is crucial for the conservation of plants possessing it.