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 analyzing 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 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.

Study Species

We selected our target perennial trees and shrubs (Table 1, Supplemental Table 1) because: (1) They are “exceptional species” (Pence 2013) that present difficulties for seedbanking (low seed production, low seed viability, or recalcitrant seeds), and must be conserved ex situ as living plants. (2) They cover a broad phylogenetic range (Monocots, Rosids, Magnoliids, and Cycadophytes; see Supplemental Material), so our results can be applicable across plant groups. (3) They represent variation in geographic ranges or numbers of populations. (4) All are threatened taxa, and some are critically endangered.

We studied taxa from five genera. Two subspecies of Hibiscus waimeae, subsp. hannerae and subsp. waimeae, are endemic to the island of Kaua‘i in Hawai‘i. Both are threatened by habitat degradation, non-native herbivores, competition by invasive plants, seed predation by insects, and a possible decline or loss of native pollinators. Hibiscus waimeae subsp. hannerae is rarer- approximately 200 adult individuals from three valleys remain and little or no regeneration occurs.  Magnolia ashei, (Synonyms: Magnolia macrophylla ssp. ashei and Magnolia macrophylla var. ashei) is a small deciduous tree with large leaves and flowers that occurs in only 10 counties in Florida (dataset already presented in von Kohn et al. 2018). Magnolia pyramidata (Synonym: Magnolia fraseri var pyramidata) is a medium-sized deciduous tree reported from Texas to South Carolina, though with few occurrences in most states, except southern Alabama and northwestern Florida. Pseudophoenix ekmanii is restricted to a small area of the Dominican Republic and threatened by illegal destructive harvesting of sap. Pseudophoenix sargentii, the sister species of P. ekmanii, has the widest distribution of its genus, from Florida to Belize and east to Dominica, yet populations are isolated and locally imperiled. Quercus georgiana is a specialist on stone outcrops known from small, fragmented populations in Alabama, Georgia, and South Carolina. Quercus boyntonii is a small tree occurring on sandstone outcrops in a few counties in Alabama, making it one of the most range-restricted US oaks. Quercus oglethorpensis, formally described in 1940, occurs in sparse isolated populations across the southeast, also in granite outcrops. Zamia decumbens is a slow-growing infrequently reproducing cycad from southern Belize, with less than 500 extant plants, while Zamia lucayana, a fast-growing, frequently cloning cycad from the Bahamas, with one population of fewer than 1,000 plants on a single island (we expand on the dataset from Griffith et al. 2017). 

Sampling and genotyping

In situ populations were identified via Global Biodiversity Information Facility (GBIF) occurrence data and local collaborator networks. We attempted to sample as many wild populations as possible, to best represent the wild extant genetic diversity. Ex situ locations were identified via Botanic Gardens Conservation International’s PlantSearch (https://members.bgci.org/data_tools/plantsearch), Beckman et al. (2019), and personal contacts. Details of molecular markers, PCR conditions, field sampling, and genotyping are detailed in Supplemental Materials.

All data files as well as a readme are in the zip.