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Effectively conserving biodiversity with limited resources requires scientifically informed and efficient strategies. Guidance is particularly needed on how many living plants are necessary to conserve a threshold level of genetic diversity in ex situ collections. We investigated this question for 11 taxa across five genera. In this first study analysing and optimizing ex situ genetic diversity across multiple genera, we found that the percentage of extant genetic diversity currently conserved varies among taxa from 40% to 95%. Most taxa are well below genetic conservation targets. Resampling datasets showed that ideal collection sizes vary widely even within a genus: one taxon typically required at least 50% more individuals than another (though Quercus was an exception). Still, across taxa, the minimum collection size to achieve genetic conservation goals is within one order of magnitude. Current collections are also suboptimal: they could remain the same size yet capture twice the genetic diversity with an improved sampling design. We term this deficiency the ‘genetic conservation gap’. Lastly, we show that minimum collection sizes are influenced by collection priorities regarding the genetic diversity target. In summary, current collections are insufficient (not reaching targets) and suboptimal (not efficiently designed), and we show how improvements can be made.

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Maintaining a living plant collection is the most common method of ex situ conservation for plant species that cannot be seed banked (i.e., exceptional species). Viability of living collections, and their value for future conservation efforts, can be limited without coordinated efforts to track and manage individuals across institutions. Using a pedigree-focused approach, the zoological community has established an inter-institutional infrastructure to support long-term viability of captive animal populations. We assessed the ability of this coordinated metacollection infrastructure to support the conservation of 4 plant species curated in living collections at multiple botanic gardens around the world. Limitations in current practices include the inability to compile, share, and analyze plant collections data at the individual level, as well as difficulty in tracking original provenance of ex situ material. The coordinated metacollection framework used by zoos can be adopted by the botanical community to improve conservation outcomes by minimizing the loss of genetic diversity in collections. We suggest actions to improve ex situ conservation of exceptional plant species, including developing a central database to aggregate data and track unique individuals of priority threatened species among institutions and adapting a pedigree-based population management tool that incorporates life-history aspects unique to plants. If approached collaboratively across regional, national, and global scales, these actions could transform ex situ conservation of threatened plant species.

Conservation of imperiled plant species often requires ex situ (offsite) living collections. Protocols for developing these collections most often emphasize sampling depth, but little is known about the genetics of such collections. This study compares how well a single collecting protocol can capture the diversity in wild populations of two closely related species. We selected two exemplar species, bay rush (Zamia lucayana) and sinkhole cycad (Zamia decumbens), based on similarities and differences that allow for rigorous comparison, including geographic isolation and reproductive factors. For each species, we compared in situ plants to ex situ plants via the same panel of 10 microsatellite markers. Genetic distance analysis shows high fidelity of the ex situ collections to their in situ source populations and sub-populations. Structured resampling of allele capture from the in situ populations by the ex situ collections shows that allele capture increases as number of ex situ plants maintained increases, but with a diminishing rate of increase. Difference in the rate of allele capture between the two species was significant at the 123Biodivers Conserv (2017) 26:2951-2966 DOI 10.1007/s10531-017-1400-2 a = 0.1 level, (p = 0.097) but not at the a = 0.05 level. At larger collection sizes, the difference in rate of allele capture showed a high practical significance (d = 5.41). These data illustrate that a unified collecting protocol can achieve similar allele capture among related species, but also that geographic and reproductive factors can influence the rate of allele capture.

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