Selecting source and recipient sites

The geographic distribution of the species should be adequately understood before selecting source populations for the translocation (Section 2.3.2). The selection of source populations will depend upon the purpose of the translocation, the location of recipient sites and the amount of genetic diversity within the source populations. Some knowledge of the species’ genetic diversity and structure is helpful in designing seed collection strategies (see Section 3.3, Case Study 4.2). Unless there is a specific reason for genetic analysis to be conducted (e.g. disjunct distribution, disrupted breeding systems), general guidelines can be followed.

Much consideration has been given to seed collection strategies in recent years. Previously there was a strong focus on collecting seeds from local plants or populations (local provenance), based on assumptions that local genotypes are best adapted to local environmental conditions (Leimu and Fischer 2008; McKayet al. 2005). More recently, provenance strategies have concentrated on situations where populations are small and isolated, and have identified that focussing on local populations may lead to seed collections with limited genetic diversty or from plants subject to inbreeding (Breed et al. 2013; Broadhurst et al. 2008a; Sgro et al. 2011; Weeks et al. 2011). It is well accepted that maximising genetic diversity is important for long term persistence of populations, and this can be achieved by sourcing seeds (or cuttings – see below) from multiple populations.

Currently, provenance strategies give consideration to the factors that might influence the persistence of populations in the face of changing climate conditions. These strategies recommend maximizing genetic diversity as well as including genes that may enhance adaptation to changing climate. A number of different provenancing strategies have been proposed. A composite provenance strategy uses seeds from multiple populations with a focus on populations close to the translocation site with decreasing contributions from populations further away designed to mimic patterns of gene flow (Broadhurst et al. 2008a) whereas admixture provenancing advocates using seeds from multiple populations in equal proportions (Breed et al. 2013). Predictive provenance strategies aim to identify source populations that have the same current climatic conditions as the recipient populations in the future (Sgro et al. 2011). These strategies require species distribution modelling in relation to current and future population sites under future climate conditions (see Section 3.5), but does not take into account the large amount of uncertainty associated with future climate predictions and species distribution modelling. Climate adjusted provenance strategies advocate sourcing seeds from multiple populations in the direction of projected climate change (without explicit site/climate matching) to include adaptive genes from across the climate gradient (Prober et al. 2016). This strategy is based on accumulating evidence for presence of adaptive genes within species across climate gradients (Jordan et al. 2017; McLean et al. 2014; Steane et al. 2017; Steane et al. 2014).

Provenance strategies that use mixed seed sources are appropriate for species whose distributions do not include large disjunctions or habitat variability. Geographic disjunctions in species distributions may indicate that the species has divergent lineages where substantial genetic or phenotypic differences have accumulated among two or more groups of populations over time. These populations may have been historically isolated or may even represent cryptic species (i.e. different species that cannot interbreed but contain individuals that are morphologically identical (see Section 3.3.2); and (see Byrne et al. 1999; Llorens et al. 2015 for examples of species with historical disjunctions)). Consequently, unless there is genetic evidence that the populations across the disjunction are not genetically differentiated, it is recommended to keep seed sources from these types of populations separate. See Chapter 3a Section 3.3.2 for reference to genetic mixing considerations including when to mix and when not to mix.