Exploring the hypothesis
The title of this paper asks a question which frames this discussion: can ex situ collections conserve different plant species equally well? An intuitive answer might be “no,” given the wide phyletic distances within the plant kingdom, potential great diversity in biology and geography of individual populations (Hoban and Schlarbaum
2014), life histories, threats, and circumstances among plants (Griffith et al.
2011), and the particular challenges in cultivating many species (Calonje et al.
2010). This study seeks a more data-driven answer to that question by isolating many of those variables, and carefully considering the differences that remain between these two empirical cases.
We hypothesize that for the same collection protocol, the rate of genetic capture can differ even between closely related species. For the two species examined here, the null hypothesis (no difference in rate of genetic capture) is not rejected at the 95% confidence level, but is rejected at the 90% confidence level (Fig.
3). Furthermore, the effect size value (
d = 5.41) shows a high practical significance at larger collection sizes. Thus, for this discussion, it is useful to consider how these two species behave similarly in ex situ collections, and also how they differ.
Similarities
Comparing the two
Zamia species shows that both exhibit a relationship of diminishing returns on genetic capture as a function of collection size, i.e. that the rate of increase in genetic capture decreases as the number of plants increases (Fig.
3). Both species show greatest variation in allele capture for collections composed of a single accession (i.e. half-sibling cohort). As the number of accessions increases, the % allele capture increases and the range of % allele capture decreases. This relationship of diminishing returns has been found in similar studies for unrelated plant species. Richards et al. (
2007), also using microsatellite data and similar simulated resamples of a collection of Texas wild rice (
Zizania texana), observed the same diminishing returns on allele capture as collection size increased. Volk et al. (
2005) show a similar pattern for wild apple (
Malus sieversii), also using microsatellite data. A study of Keys thatch palm (
Leucothrinax morrisii) also using structured resampling found the same general relationship, (Namoff et al.
2010). The similarities in allele capture between the
Zamia species in the current study, and among other species investigated in a similar manner, further validate the emphasis of current collecting protocols to sample for depth within a given population of any species (Guerrant et al.
2004,
2014; Haidet and Olwell,
2015).
Differences
The main difference is that for a given collection size, bay rush collections capture a greater amount of genetic diversity than collections of the sinkhole cycad. Conversely, to achieve a targeted level of allele capture, a greater number of sinkhole cycad collections than bay rush collections would be required, e.g., for 80% allele capture, 190 or 140 plants respectively (Fig.
3). For comparison, 125 plants of Texas wild rice achieved above 80% allele capture (Richards et al.
2007), and a core collection of 15-20 plants of wild apple captures 80% of allele diversity (Volk et al.
2005). In the Keys thatch palm example (using dominant ISSR markers), a collection of only 15 ex situ plants were required to reach 80% allele capture (Namoff et al.
2010). Thus, the idea that every species is different with regard to allele capture in ex situ collections (Griffith et al.
2015) is supported by these data, even between the two closely related
Zamia species studied here. By using an identical collecting protocol and genetic assay method, this leaves biological differences as the most likely cause of differences in genetic capture, but sampling error could also be considered (Hong and Ellis
1996). Either way, given the level of resource input required for ex situ collections (Pardey et al.
1998; Cibrian-Jaramillo et al.
2013), these differences have implications for feasibility and management. Thus, these data can inform planning for ex situ conservation collections, if the biological similarities and differences between cases are carefully considered.
Insights from this model system
By examining two closely related species which differ in geographic and reproductive factors, this study provides a comparison which can offer insight for ex situ collections protocols. In this way, we address some limitations expressed in Namoff et al. (
2010), which noted that a single species assay offers information, but it is not known how broadly such findings can be applied. By comparison, insights and recommendations can be further refined.
Differences in geographic structure are known to affect genetic structure (Gapare and Aitken
2005; Lopez-Gallego and O’Neil
2010). Thus, geographic structure is considered to affect allele capture for ex situ collections (Touchell et al.
1997; Hoban and Schlarbaum
2014; Hoban and Strand
2015). The case study presented here can empirically illustrate the need to consider geographic structure in collecting protocols by comparing two cycad species with different geographic structure. Bay rush exists as a single continuous population within 7 km, whereas sinkhole cycad is separated into two disjunct major populations 7 km apart. This geographic structure is mirrored in the multivariate analysis of genetic distance data for bay rush, which shows little separation by either axis (Fig.
2), while the same analysis for sinkhole cycad completely separates its two populations via a single axis (Griffith et al.
2015). For bay rush, a lack of clear differentiation by genetic distance justifies treating the in situ plants as a single population for management purposes (Calonje et al.
2013).
Can these differences in genetic and geographic structure inform collecting protocols? This can be answered by separating out the allele capture for selected cohorts within the bay rush ex situ collection, and comparing to sinkhole cycad (Table
3). These data show that collecting from only a single sub-population of bay rush can result in either close to expected or far less than expected genetic capture than from similarly-sized collection of plants from multiple sub-populations. Ex situ collections from Hamilton’s (n = 77), for example, can only capture up to 52% of the genetic diversity of the full in situ population of bay rush, far less than the 68% expected for a collection of 77 plants randomly chosen from all three subpopulations (Table
3). However, ex situ collections from either Buckley’s or Petty’s perform much nearer to expectations (Table
3). A similar, but even more variable result is seen for sinkhole cycad (Table
3); ex situ plants from a single population either capture much fewer (Sinkhole 1), or close to the expected amount of alleles. Thus, the current study exemplifies the need for consideration of geographic factors when developing ex situ collections, and lends support to the idea that every population should be considered separately for such work (Ceska et al.
1997; Krishnan et al.
2013; BGCI
2014). Thus, we recommend carefully considering geographic differences among populations when implementing ex situ conservation actions.
Table 3
Allele capture by cohort
Bay rush (Zamia lucayana) |
All ex situ plantsa
| 244 | 89.91 | 91.04b
|
Buckley’s only | 101 | 70.59 | 73.63 |
Hamilton’s only | 77 | 52.10 | 68.28 |
Petty’s only | 66 | 63.87 | 65.24 |
Sinkhole cycad (Zamia decumbens) |
All ex situ plantsc
| 205 | 77.63 | 79.29d
|
Sinkhole 1 only | 94 | 36.84 | 65.68 |
Sinkhole 2 only | 111 | 69.73 | 68.58 |
Bay rush and sinkhole cycad also differ in reproductive phenology and life history. As in all cycads, both species are dioecious and pollinated by specialist insects (
Rhopalotria dimidiata and
R. calonjei, respectively; O’Brien and Tang
2015). Bay rush is observed to have numerous in situ female plants showing multiple cones with complete seed set (Calonje et al.
2013), whereas in a recent observation sinkhole cycad shows much less reproductive frequency, with only 7 plants out of 375 bearing mature seed, and most of these bearing single cones (Griffith et al.
2015). These differences persist in a common garden setting. Bay rush collections begin to produce male cones in 2 years from seed, and female cones in 3 years; sinkhole cycad collections have only produced one male cone in collections 6 years from seed, and no cones in another cohort of collections 8 years from seed. Based on this limited information, the sinkhole cycad has a minimum absolute generation time at least three times as long as bay rush, and wild populations of sinkhole cycad exhibit considerably lower gene flow each year. This correlates with a greater number of seed collections needed to capture sufficient allele diversity for sinkhole cycad (Fig.
3).
Schoen and Brown (
1991) as well as Hoban and Strand (
2015) simulated the effects of selfing and limited dispersal on seed collection and also found that much greater sampling is needed when these reproductive factors reduce gene flow. While dioecious
Zamia are obligate outcrossers, very limited seed dispersal is observed for either species (Calonje
2010). However, the longer generation times and more limited coning of sinkhole cycad would also act to reduce gene flow in a similar manner (cf. Kremer et al.
2012). The greater collection size of sinkhole cycad required for a fixed level of allele capture correlates with these reproductive biology factors. As a contrast to each
Zamia in this study, the Keys thatch palm example examined an anemophilous, monoecious, panmictic species (Namoff et al.
2010; Griffith et al.
2011), and the much fewer numbers of ex situ plants required for high levels of genetic capture may correspond to these reproductive factors. Informed by these comparisons, we recommend that ex situ conservation plans include careful consideration of reproductive biology in sampling protocols.
Moving forward
By utilizing a comparison between two closely related species which differ in geographic and reproductive factors, we address some limitations about how broadly such findings can be applied. In this way, this study offers insight for ex situ collections protocols. Further parallel assays of other species, structured to include deliberate comparisons of rarity, reproductive isolation, dispersal, and generation times, would allow further testing of generalizations, and further refinement of protocols (Griffith and Husby
2010).
Adequate genetic capture is necessary for sustainable ex situ conservation collections, but sampling guidelines based on studies such as the current one must also take into account planned redundancy to mitigate against losses through reduced seed viability (Kay et al.
2011) and other losses (Griffith et al.
2008). A recent thread in the literature discusses ways in which collecting protocols insure against loss or drift (Guja et al.
2015; Guerrant et al.
2015; Hoban et al.
2015). Genetic capture via ex situ collections is only one portion of an integrated strategy for plant conservation (BGCI
2016), which can include basic research on plant diversity (Lorenzi et al.
2010), methods of sustainable use (Salomé-Castañeda et al.
2015), and involvement of stakeholders (García-Llorente et al.
2016).
Ex situ efforts currently maintain many plant species that would otherwise be extinct (Dhar
1996; Maunder et al.
2000; Sharrock
2011; Cousins et al.
2013). Despite these important cases, debate in the literature often questions the need for ex situ conservation (Hamilton
1994). Some root of this criticism is based on the perceived danger of authorities disregarding in situ conservation in favor of more politically feasible measures (Heywood
2009), or on inadequate attention to ecologically related issues (cf. Moir et al.
2012). Sometimes the critique of ex situ work is explicitly rooted in philosophical opinion (Rolston
2004). Careful review does note cases in which the benefits of ex situ conservation do not justify the costs (Clement et al.
2009). The conservation value of botanic garden ex situ collections can sometimes be overstated (Aplin
2008), most often due to insufficient data (Maunder et al.
2001).
We advance that the remedy for insufficient data is targeted data which allow rigorous assessment of conservation value. Studies such as the current work, which use targeted genetic data to assess the conservation value of ex situ collections, can help raise the understanding of exactly how well living collections can contribute to integrated efforts. Finally, cultivation of imperiled plants perfectly leverages the skills and assets of the botanic garden field to contribute to species survival. Regardless of the finer debate on the relative merits of ex situ botanic garden collections, the established feasibility of protective horticulture makes it an essential component in restoration efforts (Li and Pritchard
2009; Seaton et al.
2010; Vitt et al.
2010). Future world conditions may make ex situ work even more vital (Bridgewater
2016).