Plants held in ex situ collections such as botanic gardens and arboreta can provide material for ecological restoration while safeguarding species against extinction in the case of habitat loss. For long term species survival, it is necessary to ensure that ex situ collections maintain a high percentage of the total genetic diversity of each threatened species. While seed banking can aid in this goal, many seeds, particularly from the tropics, do not remain viable under the dehydration necessary for banking. Because living collections have space and funding limitations, it is valuable to know how many individuals of a given taxon need to be collected to efficiently reach genetic diversity goals, as well as knowing whether closely-related species, such as those in the same genus, can be assumed to have similar requirements, decreasing the amount of genetic study necessary.
In new research published in Proceedings of the Royal Society B, lead author Sean Hoban of the Morton Arboretum and colleagues investigated the question of how many individuals are required to safeguard a given percentage of genetic diversity in ex situ collections. The researchers studied 11 taxa of woody perennials across five genera, all of which are threatened and impossible to seedbank. They first determined how much genetic diversity is presently held in collections and then resampled wild population datasets to estimate how much diversity could be captured for a given collection size. From these results, the authors were able to calculate the minimum collection size that would allow the capture of 70% and 95% of the total genetic diversity of the species.
The authors found that between 40% and 95% of existing genetic diversity, excluding the rarest alleles, was conserved in current collections of each taxon, with most falling below genetic conservation targets. Collection size wasn’t a perfect predictor: some were more efficient than others in capturing diversity. Surprisingly, the collection sizes required to meet targets varied widely even within a given genus. For two species in genus Zamia, for example, capturing 95% of alleles required 82 individuals of one species, but only 44 of the other.
The data revealed that current collections are not as efficient as they could be. In some cases, with improved sampling design, twice the genetic diversity could be captured without increasing the overall number of specimens. Conversely, a smaller number of total plants, sampled efficiently, could provide the same amount of genetic diversity presently held. The fact that congeneric species cannot be assumed to have the same minimum collection size points to a great need for genetic study. However, in cases where information on a species’ genetics and demography are lacking, the authors estimate that between 30 and 200 individuals will be needed if sampled randomly.
“[F]or centuries, gardens simply tried to get as many species as possible, with only one or a few specimens per species. They often obtained specimens via seed from another garden, which did not conserve additional new genetic diversity,” explains Hoban*. “There now exist some basic rules of best practice for conserving genetic diversity, [for example,] sample seed from 50 plants throughout a population or sample from at least five populations, as advised by the Center for Plant Conservation.”
Still, most collections face constraints on how many individual plants they can support. “[F]or individual gardens, space is a big concern. Most gardens acting alone would find it challenging to fully enact recommendations of 100+ trees for a single species, let alone multiple ones,” says Hoban, explaining that a garden might have to choose between keeping large numbers of just a few species versus just a few specimens of many different species. “This is one driver behind our work—efficient use of space to maximize conservation value in botanic garden collections. This constraint at the individual garden level emphasizes the need for coordination among gardens to ensure that as a collective we are keeping most species with large numbers of specimens. There are over 3000 botanic gardens globally, and together we have the space and expertise to meet the recommendations.”
This coordination among gardens, what Hoban refers to as the metacollection, is a key safeguard against losses of individual specimens. “Distributing risk across sites is essential. Many gardens have seen damage from hurricanes, fires, floods, and new pests in recent years, in addition to normal loss due to age and accidents. Although many people think of trees in a garden as “permanent,” they are in fact quite ephemeral. And, metacollections quickly overcome the space limitations of a single landsite. Metacollections capture genetic diversity better than single-garden collections. Gardens need to work together!” says Hoban. “We are in a transition time, with more awareness and action around best practices for genetic conservation in gardens now than there was 10 to 20 years ago. Still, the transition is ongoing both within gardens and among gardens working together in a coordinated way to safeguard biodiversity.”
*These quotes also received input by co-authors Seana Walsh (National Tropical Botanic Garden), Patrick Griffith (Montgomery Botanical Center), Murphy Westwood (Morton Arboretum), Matt Lobdell (Morton Arboretum), and Michael Dosmann (Arnold Arboretum, Harvard).