What goes up must come down – why high fecundity orchids challenge conservation beliefs

Abstract

The life history of orchids is relatively well understood qualitatively, but quantifying this model and extracting a conservation message remains challenging. Orchid conservation requires the means to assess a species' vulnerability, and to restore populations that have lost numbers or range. In this study, we examine orchid population dynamics and, by extension, a large number of similar organisms with Type-III survival strategy: high fecundity and high mortality at early stages of life. By assuming a balance between fecundity and mortality, we establish a theoretical dynamic population model for orchid species, which also runs under scenarios of high environmental variability for various levels of fecundity. Our findings show that population variability increases mainly through survival variability. High fecundity acts as an amplifier of environmental variability and can cause populations to reach levels where extinction is a threat. Thus, extinction probability can paradoxically rise with increasing fecundity. In addition, at high fecundities, the extinction probability of orchids and other organisms characterized by very high fecundity and high pre-reproductive mortality may be relatively insensitive to initial population size, compared with organisms with low fecundity and mortality. The high fecundities of orchids do not shield them from extinction but may cause them to be more vulnerable to extinction. Changes in the environment have even greater impact than low initial population sizes.