– Reduced fecundity and genetic diversity in small populations of rewarding versus deceptive orchid species: a meta-analysis

Abstract

Background and aims -Understanding how plant traits mediate the relationship between plant population size, fitness and genetic diversity is fundamental to conservation. Many orchid species are threatened, possibly due to their susceptibility to the consequences of habitat fragmentation and the associated decreasing population size. One third of all orchids do not produce nectar to entice pollinators, and this is believed to promote gene flow and outcrossing. Therefore, we hypothesised that deceptive orchids are less susceptible to reductions in population size than rewarding species. Methods -We reviewed the available literature and used traditional and phylogenetically independent meta-analyses to explore relationships between population size, fitness (fruit set) and genetic diversity. Key results -Results showed that orchids in general are highly susceptible to the consequences of small population size. Fruit set significantly decreased in small populations of rewarding, but not in deceptive species. Deceptive orchids were, however, as likely to lose genetic diversity in small populations, even though significantly less genetic differentiation (Fst) was present between populations of deceptive orchids. Finally, we found that homozygosity did not relate with population size, which may indicate a selection process against less fit homozygotes in small orchid populations. Conclusions -We conclude that reductions in population size most likely resulting from habitat loss and fragmentation represent major threats to the population viability of orchids. Fitness of rewarding orchids declines through ecological mechanisms, whereas deceptive orchids may be more susceptible to genetic loss as they are more susceptible to decreased gene flow.

Low population viability in small endangered orchid populations: Genetic variation, seedling recruitment and stochasticity

“…Additionally, no indications for a lower survival due 502 to inbreeding were found using the mark-recapture analyses. A similar lack of a relationship 503 between the inbreeding coefficient and population size has been observed in several meta-analyses 504 in orchids (Gijbels et al 2015) and other plants (Aguilar et al 2008; Honnay and Jacquemyn 2007; 505 Leimu et al 2006). The lack of a correlation in this study may be explained by historically low 506 variation and purging of deleterious alleles in these populations or by the low number of markers 507 and low levels of polymorphism in our study populations.…”

“…These processes may 447 include failure to attract pollinators or problems in plant-pollinator interactions, resulting in low 448 seed production (Ågren 1996;Sih and Baltus 1987). Orchids are particularly vulnerable to pollen 449 limitation due to their pollinator specialisation and decreased fruit set has indeed been observed in 450 several small orchid populations (Abeli et al 2013;Gijbels et al 2015). However, we observed 451 capsules also in the small populations, suggesting that flowers did not fail to set fruits due to 452 pollinator deficit.…”

Factors influencing IUCN threat levels to orchids across Europe on the basis of national red lists

“…(Robbirt et al 2014). Furthermore, fragmentation has increased the isolation of populations, with deceiving species even more sensitive to decreased gene flow than nectariferous species (Gijbels et al 2015). Intriguingly, although these patterns are now clear with regard to pollination, the link between mycorrhizal specialization and rarity is far less clear, with some of the most strongly specialized orchids often being the least rare (Shefferson et al 2005;Phillips et al 2011).…”

The mating consequences of rewarding vs. deceptive pollination systems: Is there a quantity–quality trade‐off?

“…, Gijbels et al. ) may offset the considerable quantitative disadvantages resulting from limited attractiveness.…”

“…By comparison, the more limited export by N− Disa species typically involved matings with non-neighbors, which are likely less related (other orchid species: Chung et al 1998, Forrest et al 2003, Chung and Chung 2008, Jacquemyn et al 2009), reducing the chance of inbreeding depression among resulting offspring. To the extent that N− species mate with unrelated plants more than N+ species, the superior genetic quality of such outcrossmatings (other orchid species: Alexandersson and Ågren 2000, Jacquemyn et al 2007, Gijbels et al 2015 may offset the considerable quantitative disadvantages resulting from limited attractiveness.…”