Distribution of life cycle stages in a lithophytic and epiphytic orchid

Density-dependent processes may have multiple effects on populations, which among other things include the regulation of population abundance and of the relative distribution of life-cycle stages within populations. The epiphytic habitat is often characterized as highly ephemeral and therefore epiphytic orchid populations may never achieve density-dependent regulation. In this study, we investigated the potential for density-dependent regulation in epiphytic and lithophytic orchids by examining the association between seedlings, juvenile and adult life-history stages in the Caribbean endemic orchid,Lepanthes rupestris in a cross-sectional study of 179 populations surveyed in the Luquillo National Forest along a riparian area where it is locally abundant. Under density-dependent regulation we expected a negative association between the ratio of seedling/adults and juveniles/adults and total population density. Population density was in the range of 140 individuals per m², however patch sizes were small and mostly limited to less than 0.5 m² with a maximum of 3 m². We found no evidence of reduction of the ratio of seedlings or juveniles to adults as population size increased in either tree or boulder populations suggesting negative density dependence for population regulation inL. rupestris is either rare or occurs at even higher densities than those measured here. Moreover, we found positive (although weak) relationship between the ratio of seedlings and juveniles to adults and population size, suggesting that facilitation may be occurring.