CPC Best Plant Conservation Practices

to Support Species Survival in the Wild

Reintroduction & Translocations

Best Practices for Plant Reintroduction: Preparing Rare Plants

Key Messages about Preparing the Rare Plant Reintroduction
  • Careful planning of biological, ecological, political, and financial support for the reintroduction will help ensure success.
  • Designing a reintroduction entails linking genetic source to the recipient site characteristics.
  • Conducting reintroductions as experiments will result in a lesson learned and can help build plant reintroduction science.

Although it is impossible to say definitively, we believe that many failed plant reintroductions could have succeeded if appropriate preparation had been undertaken. Being prepared for a reintroduction requires a good strategy coupled with large investments of time and resources. 

This requires commodities that are often in short supply in our rapidly changing world—patience and persistence. It may not be possible to know all factors we describe below, but the more that is known the higher the likelihood of success, and practitioners should at least be aware of the gaps in their knowledge.

Reviewing your plant reintroduction plan by addressing the following questions will allow you to assess your degree of preparedness. This comprehensive list is designed to help practitioners identify gaps in their knowledge. 

Once knowledge gaps are identified, there is an opportunity to weigh whether there is adequate information to proceed. The risk of proceeding without the knowledge can be assessed along with the risk of taking no action and losing the species. 

We recommend that plant reintroductions be conducted as experiments precisely designed to fill in these knowledge gaps. In this way, each reintroduction can not only help future actions for the target species but may in turn help others doing plant reintroductions around the world.

Previous CPC publications have addressed detailed preparations for reintroductions with regard to demography, genetics, and horticultural practice (Falk and Holsinger 1991; Falk et al. 1996; Guerrant 1996). 

Specific guidance for ex situ collection and management is essential preparation for reintroductions (Guerrant et al. 2004). Our aim here is to provide guidance for establishing sustainable populations in the wild where they may have opportunities for adaptation, evolution and interactions within a natural ecosystem. 

Although it is necessary to describe the steps of the plan sequentially, often several steps are conducted simultaneously.  (See the “Questions to Ask When Planning a Reintroduction” below.)

Questions to Ask When Planning a Reintroduction

  1. Is the taxon already living at the recipient site, was it historically present there, or is this a completely new location?
  2. Have you considered legal issues, logistics, and land management? (McDonald 1996)
  3. Is the biology and ecology of the species understood? (Menges 2008; Maschinski, Albrecht et al. 2012)
  4. Are genetic studies needed? (Neale 2012)
  5. Have germination protocol and propagation methods been determined? (Guerrant 1996; Guerrant et al. 2004; Haskins and Pence 2012)
  6. Has a suitable recipient site been identified and are land managers supportive? (Fiedler and Laven 1996; Maschinski, Albrecht et al. 2012)
  7. Are pollinators known and present?
  8. Are plants susceptible to herbivory? Will they be protected?
  9. Have threats been reduced or eliminated?
  10. How many plants or seeds are available and how many are needed? (Guerrant 1996; Albrecht and Maschinski 2012; Knight 2012)
  11. What is the experimental design? (Falk et al. 1996)
  12. How will success be measured? (Pavlik 1996; Monks et al. 2012)
  13. What kind of aftercare for plant and site management will be needed and how frequently should it be performed?
  14. What is the involvement of the land manager/owner?
  15. What is the monitoring design and plan for reporting results?
  16. In what ways will you involve the public in your project? (Maschinski, Wright et al., 2012)
  17. Suitable habitat is not available, nor understood.

(Falk et al. 1996; Vallee et al. 2004; Maschinski, Albrecht et al. 2012)

Arctostaphylos glutinosa growing in Northern California

Making the Plan

Design your plant reintroduction as an experiment and seek peer review.

  • Identify the project leader and key collaborators, who will be responsible for planning, supporting, implementing, site management, monitoring and reporting findings of the project.
  • Identify areas of expertise needed to execute the reintroduction. If not represented in the collaborative group, then seek outside experts to join the team. For example, enlist the help of a scientist with experience in experimental design and statistical analysis to ensure that you have adequate replication to answer your research question. Or you may need to ask an experienced horticulturist to help you grow sufficient numbers of plants.
  • Plan the reintroduction based upon the best scientific information available. Rely on peers to review your reintroduction plan and provide feedback and alternative points of view. Finding peers to review your reintroduction plan may be easy or difficult depending upon where you reside. Rely on the global community to assist you. (See the “Potential Reviewers for Reintroduction Plans.’)
  • Train and adequately manage all personnel and volunteers that are involved.
  • Consider addressing theoretical questions in the reintroduction experiment/project to advance the field of reintroduction biology. (See “Questions to Consider When Designing Reintroduction Experiments.”)
  • Define goals of reintroduction related to the recovery of the species. Set objectives.
  • Develop methods, decide the plant and population attributes that will be measured, determine monitoring protocol, frequency and duration, and reference the analysis.

Potential Reviewers for Your Plant Reintroduction Plans

In some regions, there are panels of plant conservation experts who review reintroduction methods and plans as a part of ongoing legislative process. For example, the North Carolina Plant Conservation Program requests and evaluates reintroduction plans as part of the process of granting legal permission to proceed with a plant reintroduction in the state of North Carolina, US.

Experts operating in different areas of the world are also available. The Center for Plant Conservation provides a resource to learn about reintroductions that have been done and is a source for potential peer reviewers (info@saveplants.org).

The Re-introduction Specialist Group IUCN has a Re-introduction Practitioner’s Directory 1998 intended to facilitate communication between individuals and institutions undertaking animal and plant re-introductions.

The Global Restoration Network provides a web-based information hub linking research, projects, and practitioners.

Questions to ask When Designing Plant Reintroduction Experiments

  1. What additional knowledge is needed about the species biology or other factors? How can the reintroductions be planned as experiments to address these unknowns?
  2. What is the experimental design? How much replication is needed for adequate statistical power? How will the study be analyzed?
  3. Have you considered testing aspects of ecological theory, such as founder events, small population dynamics, establishment-phase competition, dispersal and disturbance ecology, succession, metapopulation dynamics, patch dynamics on population persistence, resilience and stability over time?
  4. Using the reintroduced population as a cohort, will you examine natural variation in survival, mortality, and recruitment and tie these to environmental factors?
  5. Will the reintroduction test key habitat gradients of light, moisture, elevation, or temperature?
  6. Will the underlying environmental drivers of population growth be measured? (Knight 2012)
  7. Will genetic factors be part of the experimental design?
  8. What traits will be monitored and how will they be analyzed?
  9. Will the reintroduction further our knowledge of key principles related to rare species’ ability to cope with climate change?
  10. Are you testing factors within a single site or across multiple sites?
  11. Has a monitoring plan been developed? How long will monitoring be conducted? Have you considered an adaptive monitoring plan? What will the duration of the experiment be?
  12. Have you developed a clear unambiguous datasheet to track reintroduced plant growth, reproduction and survival? If the monitoring persists for decades, will your successors be able to interpret the data you have collected?
  13. Will the data be housed within your institution or elsewhere so that your successors will be able to use it?
  14. How will the plants be mapped and marked/numbered?
  15. If plants are susceptible to herbivory, will their response be included in the design or should the plants be protected?

(Falk et al. 1996; Vallee et al. 2004; Maschinski, Albrecht et al. 2012)

The Law, the Land, and Funding

Obtaining legal permission to conduct the plant reintroduction.

  • Familiarize yourself with the laws and regulations associated with a reintroduction. Note that these may differ for augmentations, reintroductions, and/or introductions.
  • In some locations you may be required to obtain one or many permits before conducting a reintroduction (for example, from the landowner/ land manager, local, regional or national authorities).
  • Often a carefully written plan for the reintroduction is required for the permit.
  • Note expiration date of all permits involved and requirements for periodic reports or updates to permitting agency.
  • If reintroduction is done as a mitigation, it is critical that all preliminary planning steps be taken within legal parameters. (See Falk et al. 1996 for extensive discussion regarding mitigation.)

Involve landowners and land managers.

  • Ensure that landowners and land managers are involved and supportive of the project and can account for possible changes in the future.
  • Discuss the long-term support and management of the proposed recipient habitat with land managers. Lack of management can doom a restored population to fail.
  • Develop a written agreement outlining who will be responsible for what action and any special protocols that need to be followed by parties working on the site.
  • Set a schedule to meet with the recovery team periodically to assess the species’ condition and the status of the restored population.
  • If future changes require intervention, determine a process for evaluating impacts on the restored population. For some agencies, it may be necessary to develop a protocol or decision tree to trigger management action.
  • Develop a mechanism for handling any conflicts that may arise (for example, management for one species is detrimental to another species, etc.).

Secure adequate funding to support the plant reintroduction project.

  • Ideally, funding should be garnered for implementation and for several years, if not decades, following the installation. At the very least, parties proposing a species’ reintroduction should be committed to seek long-term funding support for the project. This requires that you have detailed the cost of implementing, monitoring, and management of the restored population. Committed partners, who are willing to provide in-kind services and/or volunteer citizens, who are willing to monitor the restored population will help make this aspect feasible.
  • Determining the outcome of a reintroduction takes time. Expect to devote >10 years to monitoring to determine whether a population is sustainable (Monks et al. 2012). There are few recorded reintroductions that have created sustainable populations in which multiple generations have been completed within 25 years (Dalrymple et al. 2012). Key life-cycle events such as next generation seedling recruitment and reproductive maturation can take years to decades in long-lived species. Thus, a few decades may be required before fates of reintroduction can be determined.

Understanding Species Biology

Knowing the biology and ecology of your taxon will benefit the plant reintroduction plan and experimental design. We advise gathering information from the literature on your target taxon and closely related congeners. If possible, conduct experiments if there are gaps in your knowledge.

CPC Best Reintroduction Practice Guidelines: Astragalus bibullatus Case Study

CPC Best Reintroduction Practice Guidelines: Astragalus bibullatus Case Study Matthew Albrecht, Missouri Botanical Garden (SePPCon 2016) Reintroduction is a critical component of rare species conservation with... Read More

Know the species’ biology and ecology.

  • Knowing the mating system will determine whether source material should come from a single population or from mixed populations and the spatial pattern of outplanting. For example, due to remnant populations lacking compatible alleles for successful reproduction, reintroductions done with Florida ziziphus required carefully selecting compatible individuals from more than one location to achieve reproductive success (Weekley et al. 1999; Weekley et al. 2002). In contrast, Schiedea obovata, which is capable of selfing or outcrossing, requires keeping all outplantings separate (Kawelo et al. 2012). Highly inbreeding taxa are more likely to form ecotypes than outcrossing species.
  • If a species is dioecious, the spatial design of plantings should place male and female plants in close proximity (for example, Zanthoxylum coriaceum in Maschinski et al. 2010).
  • Species or conditions that may require special techniques for growing and implementing a reintroduction include: edaphic endemics, species with specialist pollinators, and species requiring symbionts for germination and growth.
    • Edaphic endemics: In Astragalus bibullatus an edaphic specialist of limestone cedar glades, translocations are only successful when conducted on a specific type of limestone even though multiple types of limestone occur in the historic range of the species. The species can only be propagated in very well-drained soil and must be watered from below to prevent disease (Albrecht, Missouri Botanical Garden, personal observation).
    • Specialist pollinators: Using pollinator baiting techniques at potential reintroduction sites can ensure pollinators are present before the outplanting occurs (Reiter et al. 2016). Lack of pollinators limits orchid distributions, thus knowing pollinators are present before conducting a reintroduction is advised (Phillips et al. 2014).
    • Mutualists: Providing inoculated soils containing mycorrhizal fungi may help establish the outplanted population (Haskins and Keel 2012). Because some taxa require symbionts to germinate or grow (Ogura-Tsujita and Yukawa 2008; Janes 2009; Haskins and Pence 2012) knowing whether there are obligate mutualists will influence reintroduction success. Attempting to germinate or grow such species without their obligate mutualists will fail. Providing inoculated soils containing mycorrhizal fungi may help establish the outplanted population (Haskins and Keel 2012).

How Pollination Ecology Can Improve Recovery of the Southeast’s Threatened Flowering Plants

Gavin Shotts, Department of Biological Sciences, Auburn University Bashira Chowdhury, Department of Entomology & Plant Pathology, Auburn University Producing seeds is critical to maintaining sustainable... Read More

Site Selection

Choosing a recipient site should be done with great care and intention. Several conditions influence a species’ ability to colonize a new site including functional ecosystem processes, appropriate associated species, and ongoing management to remove threats and maintain ecosystem health.

In general, seek a recipient site with great similarity to the place where the rare species is thriving. Knowing the site history may help explain existing conditions. Although it is impossible to know with certainty what a site will become in the future, as much as possible practitioners should try to imagine the future conditions the reintroduced population will face. Ongoing management and threat abatement are essential for maintaining conditions conducive to population sustainability.

In addition, it is important to think about any recipient site in the context of the species’ whole distribution. Because corridors may facilitate migration and dispersal between patches, especially with the onset of climate change (Noss 2001), a reintroduced population can serve an important function of connecting existing populations either by forming a stepping-stone between patches or by expanding the size of existing patches. Connecting 15 or more patches will improve chances for the entire metapopulation capacity (see Hanski and Ovaskainen 2000). (See the “Questions to Ask about the Recipient Site” below.)

Questions to Ask About the Recipient Site

  1. Have you researched the history of the recipient site?
  2. Have you incorporated species-specific factors related to optimal population growth to assess suitable recipient sites for your taxon?
  3. Have you identified species-specific environmental and community factors in occupied versus unoccupied patches?
  4. Have you ranked several potential suitable recipient sites to determine the best place for the reintroduction to occur?
  5. Is there still suitable habitat left within the species’ range? (See Falk et al. 1996 for discussion of range.)
  6. Are recipient sites of sufficient quality and with sufficient long-term protection to ensure the long-term security of the reintroduced population?
  7. Are threats absent or adequately managed at the site?
  8. What were the previous threats that may have caused the species to become extirpated from site?
  9. What is the potential for future threats?
  10. Is current and future land use of the recipient site and surrounding sites compatible with sustainability of the reintroduced population?
  11. Are potentially hybridizing congeners present at the recipient site? Which ones? Are they present at nearby sites? Are they present within the target species’ range?
  12. Is the recipient site within the species’ climate envelope now? Are there models suggesting the location will be safely within the climate envelope in the future?
  13. What site preparation is required before the plants can be installed (for example, canopy thinning, invasive removal, etc.)? Will habitat manipulation continue after plants are installed?
  14. Does the species require habitat conditions that no longer exist on site (for example, fire, periodic inundation, etc.)? Can those conditions be mimicked?

(Falk et al. 1996; Vallee et al. 2004; Maschinski, Albrecht et al. 2012)

Choose a suitable recipient site.

  • Determine the cause of declines in wild populations. Ensure that threats can be ameliorated in the recipient reintroduction site (Dalrymple et al. 2012; Knight 2012).
  • Evaluate potential reintroduction sites using a recipient site assessment or quantitative assessment (Maschinski, Falk et al. 2012). Base your evaluation on the natural habitat where a population has positive (or at least stable) growth rate (Dalrymple et al. 2012; Knight 2012).
  • To choose among several potential sites, rank reintroduction sites incorporating logistics or ease of implementation, quality of habitat, and management influencing the species’ ability to persist at a site (Maschinski, Falk et al. 2012; see Figure 5.1).
  • To account for uncertainty, incorporate heterogeneity into the reintroduction plan. Use multiple sites and multiple microsites (even outside of our expectations) to test heterogeneity of conditions needed for optimal growth for all life stages of a species (Dalrymple et al. 2012; Maschinski, Falk et al. 2012; Maschinski, Albrecht et al. 2012).
  • Because the fine-scale requirements for individual plant growth and optimal population growth are often unknown, using microsite as an experimental factor is good practice. Measure abiotic conditions (for example, soil, precipitation, temperature) and biotic conditions (for example, predators, mutualists, invasive species) at the reintroduction site that are associated with plant performance and population growth (Knight 2012; Maschinski, Falk et al. 2012). Ensure that there are adequate areas for population expansion (that is, microsites within the recipient site and adjacent suitable habitat outside of the recipient site).
  • Realize that if environments conducive to positive population growth are rare or non-existent, additional reintroduction activities, beyond simply reintroducing propagules, will be necessary (Knight 2012; Maschinski, Falk et al. 2012).
  • Note that using existing populations and their habitat conditions as reference points for reintroductions will not always be appropriate if the species does not have positive growth rate at these locations (Possley et al. 2009; Dalrymple et al. 2012; Knight 2012; Maschinski, Falk et al. 2012; Maschinski, Wright et al., 2012).
  • It will be essential to use an experimental context to determine key factors necessary for positive population growth.
  • Reference points may not be available within core habitat under climate change conditions (Dalrymple et al. 2012). Similarly, geographic distribution may not be a good reference for fundamental niche space. For this reason, known historic range may not necessarily be the only guide to assess optimal habitats for successful reintroduction (Maschinski, Falk et al. 2012; Maschinski, Wright et al., 2012).

Figure 5.2 – Recipient site assessment based upon ranking criteria related to logistics and habitat quality. The assessment can be used to score a single or to prioritize among multiple sites. Scores ≥ 27 ≤ 54 are acceptable reintroduction sites. When choosing among multiple sites, the best site will have the lowest total score and no single criterion scoring 3. (Adapted from Maschinski, Falk et al. 2012.)

Questions to Ask About Habitat or Landscape Level Considerations

  1. Does the recipient site contribute to natural patterns of heterogeneity in the species’ distribution?
  2. Have you considered habitat connectivity? Is healthy suitable habitat nearby that will allow for the restored population to expand in area and number of individuals? Is adjacent property suitable habitat? Is adjacent property protected?
  3. Are there metapopulation possibilities? Have you accounted for between site factors as well as within site factors? Is the site located in close proximity to extant populations or other reintroduced populations?
  4. What are the distances between the proposed reintroduction and nearby wild populations?
  5. What benefits or detriments do the nearby sites give the restored population?

(Falk et al. 1996; Vallee et al. 2004; Maschinski, Albrecht et al. 2012)

Create a sustainable population.

  • Improve the probability of creating a sustainable population by considering the metapopulation context of your reintroduction site.
  • Choose recipient sites that have connectivity and increase the probability of dispersal (Maschinski, Falk et al. 2012).
  • Consider landscape level phenomena. (See the “Questions to Ask About Habitat or Landscape Level Considerations” above.)

Genetics Considerations

Ideally, the genetic composition of the source material needs to be a balance between representing the local gene pool and creating a new broadly genetically diverse population

Reasons to consider genetic studies in a reintroduction plan include helping to make decisions about appropriate location(s) for collecting source material, confirming whether hybridization may be a potential problem, confirming the species taxonomy, or determining whether to use mixed or single population source material (Falk and Holsinger 1991; Falk et al. 1996; Neale 2012). 

For example, you may wish to pursue genetic studies if you suspect there are hybridization problems, if the species looks different in different locations, if one or more populations of the species has distinct ecology from the majority of populations, or if it is difficult to distinguish this species from a congener. 

You may also wish to conduct genetic studies if you know or suspect that your species has variable ploidy levels across populations (Kramer et al. 2018). Conducting a molecular genetics study can help elucidate the mating system; the degree of natural inbreeding; the level of genetic divergence among collection sites or subpopulations; area of seed and pollen dispersal; the degree of genetic relationship or co-ancestry between adult plants in natural conditions; and the neighborhood within which adults are genetically related (Crossa and Venkovsky 2011). (See CPC Best Practice Chapter, “Genetic Guidelines for Acquiring, Maintaining, and Using a Conservation  Collection.”) 

Often, it will be necessary to work with local geneticists at botanic gardens, universities and/or government facilities to do the genetic studies. Although costs for genetic analysis are becoming more reasonable with technological advances, be aware that adequate funding will be required for proper genetic work. 

Complementary to genetic studies are hand-pollination studies, common garden experiments, or reciprocal transplant studies. The latter will allow researchers to understand the performance of the species for a particular source in a new setting. Each has advantages and disadvantages.

When are genetic studies needed?

  • Ascertain whether genetic studies are needed before conducting the reintroduction and, if possible, conduct studies to measure genetic structure of the focal species (Neale 2012).
  • A genetic assessment of wild populations is advised before conducting a reintroduction if the species meets any of the following criteria in the box “Questions to Ask about When are Genetic Studies Needed
  • Once genetic data is available, review compatible management options (Ottewell et al. 2016).
  • In the absence of genetic data, it is valuable to utilize information on species life-history traits, such as habit and breeding system, to inform reintroduction decisions (Neale 2012).

Use a genetically diverse founding population.

  • Use a large genetically diverse founding population to improve chances of establishing a self-sustaining population (Guerrant 1996).
  • To compensate for propagule losses due to mortality during reintroduction, start with an estimate of desired numbers of individuals surviving to reproduction in a new founding population. Then, account for expected losses during establishment. Some of these calculated losses can be mitigated by maintaining backup clonal material.
  • When growing the material for purposes of a reintroduction or other reintroductions, keep in mind the reproductive biology of the species. (See CPC Best Practices Chapter, “Genetic Guidelines for Acquiring, Maintaining, and Using a Conservation  Collection.”) For example, obtaining 10 female plants of a dioecious species may require planting twice as many seeds as the expected germinant count if the sex ratio is 50:50.

Questions to Ask about When are Genetic Studies Needed?

Assessing the genetic diversity of wild populations can reveal insights about the biology of the species, however genetic studies can be expensive and may not always be necessary. They can include either molecular work (genotyping, sequencing, genome or ploidy analysis) or common garden studies. These types of studies are advisable before collecting a rare species or before conducting a reintroduction if the wild populations have any of the following characteristics:

Within-population issues

  1. Population has fewer than 50 individuals flowering and setting fruit.
  2. The species is clonal.
  3. Little or no viable seed is being set.
  4. There are potential taxonomic concerns (taxonomic ambiguity, potential hybrids, or variation in ploidy).

Issues across the species’ range

  1. The species is declining and little is known about the biology or life history of the species.
  2. The species has highly fragmented and isolated populations.
  3. The species looks different in different locations.
  4. One or more populations of the species has distinct ecology from the majority of populations.

(Maschinski, Albrecht et al. 2012)

Use founders with evenly represented family lines.

  • Collect and maintain seeds from each maternal line separately. In this way, it is possible to know and intentionally control even representation of the different founders.
  • Minimize “unconscious’ or artificial selection during seed increases or augmentation of natural populations. Note that variation in germination and growth of maternal lines should be expected. Resist the temptation to over-represent the winners—those abundantly available, vigorously growing maternal lines that may skew the diversity of the population—but rather consciously maintain even family line representation (Guerrant et al. 2004; McKay et al. 2005).

Questions to Ask About Wild Populations

  1. What is the genetic structure of the wild populations?
  2. What is the dispersal capability of the species?
  3. If hybridization is a concern, what are the ploidy levels of the wild populations (McKay et al. 2005)?
  4. Does the species suffer symptoms of inbreeding depression?
  5. Is there evidence of outbreeding depression?
  6. Based upon special ecology, unique morphology (that is, ecotypes) or spatial disconnection from other populations, do you suspect that a population has local adaptation?
  7. Based upon the presence of a congener in the wild population and/or variable morphology, do you suspect that the species is hybridizing with a congener?

(McKay et al. 2005; Neale 2012)

Choose founders from a single source or mixed populations.

  • Sometimes it may be appropriate to use a single-source population, while other times it may be appropriate to mix populations for the founders.
  • The decision of whether to mix source populations or keep them separate should consider several factors: condition and context of the wild population(s), mating system, dispersal mode, ploidy level, and genetic structure. (See box “Questions to Ask Related to Wild Populations,” Fig. 3.2, “Summary of Collecting Recommendations for Numbers of Populations to Sample,” and Fig. 3.3, “Summary of Collecting Recommendations for Numbers of Individuals to Sample within a Population.”)
  • Traditionally, it is recommended to use founders from only a single wild population that is ecologically similar to the recipient site in order to preserve locally adapted genes. For example, if the species is an obligate outcrosser and is locally adapted to a site at very fine scale, then mixing populations may cause outbreeding depression (Neale 2012). This is especially true if there are known genetic differences between existing populations or if populations have more than 100 individuals, have distinct ecology, and have been separated for more than 20 generations (Frankham et al. 2011).
  • Mixing source material may be necessary if there is no appropriate ecological recipient site that matches extant population site, if the available source material is limited, or if there is evidence of low genetic diversity or inbreeding depression in the source population (Dalrymple et al. 2012; Neale 2012). We recommend mixing source material if the taxon has extant populations of less than 100 individuals with no chromosomal differences, no distinct ecological differences, and if populations have been separated less than 500 years (Frankham et al. 2011).
  • If mixing sources, keep track of each individual source through collection, production, and reintroduction to allow for rapid response should any issues arise.

Consider genetic rescue.

  • When the wild or reintroduced population has low genetic diversity and signs of inbreeding depression, consider genetic rescue (Frankham 2015).
  • Infusing new genetic stock into a wild or reintroduced population (genetic rescue) may be necessary to overcome detrimental effects of inbreeding (Frankham 2015). Introducing new individuals or genes (from pollen) could increase genetic diversity and fitness of a small, inbred population (DeMauro 1993; White et al. 2018).
  • Aim to release equal numbers of individuals from each source population early in the reintroduction to promote balanced admixture in the descendant population (Havens et al. 2004; White et al. 2018).
  • For species critically imperiled by threats that are genetically linked, genetic rescue may also comprise insertion of advantageous genes as is being done in crop development (Rinaldo and Ayliffe 2015).

Source Material & Horticulture

The source material used for any reintroduction may determine its fate. To give the new population the best chance to persist against future stochastic or catastrophic events, it is important to use plants that are adapted to site conditions, have adequate genetic diversity and good health (Falk et al. 1996; Guerrant 1996; USFWS 2000; Guerrant et al. 2004; Neale 2012).

Review and account for genetic concerns of source material from collection through propagation in the nursery to outplanting in field. This requires that you simultaneously evaluate and match recipient site characteristics (see “Choose a suitable recipient site” and “Create a sustainable population”) to genetic stock available for the reintroduction.

Review the US Fish and Wildlife Service Policies.

Plan the source material.

  • Review and plan the source material that will be appropriate to introduce to a particular site (Basey et al. 2015).
  • Identify the potential source material(s) available for reintroduction. Note collection site ecological conditions, community structure, and proximity to the proposed recipient site (Maschinski, Falk et al. 2012).
  • Collect or retrieve from a seed bank the source material whose location has similar climatic and environmental conditions to the recipient site(s). This is particularly important if the species has distinctly different appearance (or ecotypes) within wild population sites. Detailed information recorded on accession forms at time of the collection is essential for this evaluation.
  • The extent of gene flow between populations varies by species. Some may have isolated, locally adapted patches within a small area, whereas others may have great gene flow over great distances. Therefore, there isn’t a simple relationship between distance and genetic relatedness (Richards et al. 2016).
  • Use genetically heterogeneous founders to improve the ability to cope with varying conditions (Falk et al. 1996; Guerrant et al. 2004; Neale 2012). Theoretically, high levels of genetic diversity will equip the new population with adaptive potential needed to withstand stochastic and deterministic events including climate change, and can defend against potential genetic pitfalls of small populations such as founders effect and inbreeding depression. (See “Using a single-source versus mixing populations.”)
  • Genetic rescues may use targeted genotypes to restore fitness at a recipient site rather than focus specifically on maximizing genetic diversity in the founder population. (See “Consider genetic  rescue.”)

Questions to Ask Regarding the Genetics of Source Material

  1. From which wild population(s) should the material be collected for use in the reintroduction?
  2. What is the basis for collecting source material from a particular location?
  3. How will the source material be sampled?
  4. What is the genetic composition of the material reintroduced?
  5. Should material come from an ex situ source, only one wild source population, or mixed population sources?

(Falk et al. 1996; Vallee et al. 2004; Maschinski, Albrecht et al. 2012)

Propagating adequate numbers of plants is an important prerequisite to rare plant reintroduction. Amorpha georgiana (Georgia indigobush) seedlings are growing at North Carolina Botanical Garden to support a reintroduction on Fort Bragg, a Department of Defense Army Installation in NC. Photo credit: Mike Kunz.

Use ex situ source material.

  • CPC recommends using ex situ source material before collecting new material from the wild (Guerrant et al. 2004).
  • Using ex situ propagules will minimize adverse impacts on wild populations (Guerrant et al. 2004). Over several years it may be beneficial to add fresh stock to increase diversity and age structure (Guerrant et al. 2004) and improve the chances for successful establishment of the reintroduced population (Duquesnel et al. 2017).
  • Compelling reasons not to use ex situ propagules include: a) the collection site is ecologically very different from the recipient site, b) there is a more appropriate source population that can withstand collection, or c) the ex situ propagules you have available are not genetically diverse.
  • Bulking up ex situ collections through vegetative reproduction or seed bulking is often very feasible. When producing propagules for reintroduction, be aware of potential selection and genetic bottlenecks that may occur (Basey et al. 2015).
  • If ex situ material is not available, collect no more than 10% of seed produced in any year from wild populations to avoid harm to the wild populations with >50 plants. Collect from all individuals within the population if there are < 50 plants. Capturing broad genetic diversity may require collecting in different years and across the range of the fruiting season. (See Guerrant et al. (2004) for specific guidance regarding ex situ collection and management and CPC Best Practice Section, “Collecting Seeds from Wild Rare Plant Populations.”)

Choose the best propagule type.

  • Choose the best propagule type and size of founders based upon the species’ life history, recipient site characteristics, and logistics.
  • It is possible to use seeds or whole plants for any reintroduction, however there are advantages and disadvantages of each (Table 5.1).
  • Seeds may be easily collected, bulked in the nursery setting, and provide a rich source of genetic diversity for use in reintroductions. If seeds are orthodox, they are relatively inexpensive, easily and compactly stored prior to use. When seeds germinate at the recipient site, they are a bioindicator that germination is possible there. However, typically only a small percentage of seeds (1–10%) germinate in wild conditions and a small percentage of reintroductions have established with seeds (Albrecht and Maschinski 2012, Dalrymple et al. 2012, Guerrant 2012). Therefore, founder population size for seeds will require thousands to tens of thousands of seeds. Further, the time required to mature to reproductive stage from a seed varies with species’ life history. For most species, the most vulnerable life stages to mortality factors are the seed and seedling stages (Grubb 1977). The longer the seed or seedling stage remains in the wild, the more mortality should be expected.
  • To improve the likelihood of success of a seed reintroduction, use thousands, employ dormancy-breaking treatments if appropriate, protect seeds and seedlings from herbivory, and irrigate for months as is the practice for perennial whole plants (Bainbridge 2007, Maschinski et al. 2017).
  • For annual and short-lived species, seeds are often the best choice. Transplanting annual plants as seedlings or adults to the field is fraught with perils, as plants would not survive well or would require extreme care and watering on a daily basis if natural rainfall did not occur daily.
  • For species with intermediate lifespans (usually herbaceous perennials), whole plants have been shown to be most successful. Grow plants as large as is feasible to manage for transport to the reintroduction site and planting. Using physically large founders increases the likelihood of establishing a persistent population (Guerrant et al. 2004; Albrecht and Maschinski 2012). An exception to this is if habitats, such as rock outcrops, do not allow digging or transplanting whole plants. If this is the case, then seeds would be the best choice.
  • If the species is long-lived, reintroduce plants of varying size and life-stage to account for variable success of stages in different microsites (Albrecht and Maschinski 2012). Using different-stage plants will result in a more diverse population structure in the present and future and will increase the probability of finding the optimal conditions for the whole population to grow. For example, use juveniles and reproductive plants in the reintroduction. Sometimes, the two will have different microsite requirements (Wendelberger and Maschinski 2016). Generally, the largest plants one can manage to transplant will have greatest survival, as was the case with Amorpha herbacea var. crenulata (Wendelberger et al. 2008).
  • To improve the likelihood of success of a whole plant reintroduction, use large numbers, protect new transplants from herbivory, and provide irrigation aftercare for months.
  • For many trees, foresters have found the best survival and most cost effective size for transplanting thousands of trees is a long-rooted seedling (in a container that forces deep root growth). The best timing for planting is when trees are dormant for temperate species (North Carolina Division of Forest Resources 2009), while for tropical species planting in the rainy season is advised. Tree roots are best established from the seedling or small container size, as they tend to get root-bound and suffer from circular root patterns in containers. Palm trees are an exception. Large Pseudophoenix sargentii juveniles in 3–10 gallon containers reintroduced to the Florida Keys had higher survival than seedlings (Maschinski and Duquesnel 2007).

TABLE 5.1. Advantages and Disadvantages of Using Seeds or Whole Plants for a Reintroduction

Confirm that successful propagation is possible.

  • Confirm that the species can be successfully propagated and that adequate numbers of high quality, healthy, genetically diverse source material is available.
  • A critical step to accomplish prior to reintroduction is mastering the art of propagating large quantities of the species, acclimatizing them, and growing them ex situ. A declining species that has not been propagated such that large numbers exist in ex situ nursery stock is simply not a good candidate for reintroduction.
  • Acknowledge that you are not ready to proceed if you have not mastered this step.

Allow enough time to generate the source material.

  • Allow enough time to generate adequate numbers of source material prior to initiating the reintroduction. Depending on the species, this may take several months to several years.

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Keep detailed documentation on all source material used.

  • Keep detailed documentation on all source material used to restore populations. This documentation should be linked to permanent plant labels/ID tags attached to the reintroduced plants. Store these data in multiple locations.

Don’t use all material for the reintroduction.

  • Keep some material in reserve.
  • Genetically diverse source material should be safely backed up in an ex situ location so that regardless of whether reintroduction succeeds or fails there is still germplasm conserved.

Use good horticultural practice.

  • Acclimate plants to novel conditions (Haskins and Pence 2012). Transitions from culture medium to soil and from greenhouse to outdoors will require a period of adjustment to reduce the chance of shock.
  • Take phytosanitary precautions to insure that diseases will not be inadvertently transmitted.
  • Use native soils from the wild site (if possible) during nursery production. Native soils may require augmentation with sterile perlite or vermiculite to achieve consistency necessary to be container-grown. The benefit of native soils is that they potentially contain beneficial microbes; however, pathogens may also be transferred with native soil. Follow good nursery hygiene practices accordingly. We advise separating plants with native soil from the rest of the nursery in quarantine.
  • Weed pots containing plants destined for the reintroduction to reduce the chance of introducing weeds to the reintroduction site.
  • If using propagules that were derived from tissue culture, acclimatization will be important. We recommend gradually decreasing humidity, while subjecting cultures to ventilation or air exchanges before transfer to soil. Alternatively, methods could include increasing ambient CO2, decreasing sugar levels in the cultures, or treating with growth regulators to increase stress tolerance. (Haskins and Pence 2012)
  • Grow in controlled nursery conditions to maximize plant health and grow to appropriate size prior to moving to the reintroduction site.

Questions to Ask Related to Planning for Population Growth

  1. What founder population size will be used?
  2. What size and stage structure of plants will be used?
  3. How will the founding population be spatially configured to favor demographic persistence and stability?
  4. What is known about population growth, recruitment, and survivorship in wild habitats and what environmental or community factors are correlated with population growth rates?
  5. How will population growth, recruitment, and survivorship be monitored in the reintroduced population? And by whom?

(Falk et al. 1996; Vallee et al. 2004; Maschinski, Albrecht et al. 2012)

Planning For Population Growth

Use as many founding individuals as is feasible.

  • Use as many founding individuals as is feasible (50+ individual plants or thousands of seeds) to bolster population growth (Guerrant 1996; Albrecht and Maschinski 2012). Increasing the numbers of reproductive adults early in population establishment increases the chances of next generation recruitment (Albrecht et al. 2018).
  • Develop a demographic model for the species to determine the optimum founder size (Knight 2012).
  • When working with perennial herbs and sites in highly competitive environments like grasslands, founder population sizes will need to be larger than 50. Introduce enough individuals (seeds or juveniles) to be able to break through demographic and environmental stochasticity of low populations to achieve a viable population (Knight 2012). Planting higher numbers of individuals increases the probability that the population will persist and perhaps spread (Reichard et al. 2012). This may occur because of higher numbers of total seeds produced or perhaps because, even with some mortality after planting, sufficient numbers of individuals remain to reproduce.
  • Multiple outplantings over many years may be required to build up a population structure and size that sustains population growth over the long-term.
  • Use seed bulking at the nursery to generate enough seed for a reintroduction. This provides an opportunity to document F1 characteristics, such as variation in timing of germination, which can be compared to the wild population.

Create experimental conditions to improve germination.

  • Seek or create conditions experimentally with the intention of improving germination and the establishment and survival of next generation seedlings (Albrecht and Maschinski 2012).
  • Although used in large-scale restoration projects, to date there have been few published or reported reintroductions using seeds that have incorporated experimental designs with techniques to improve success of field seed germination and establishment, such as microcatchments (for example, Bainbridge 2007). Similarly, there are few reports of manipulating site conditions for the next generation seedlings. As this is a critical part of establishing a sustainable population, more attention should be placed on this step in the reintroduction process.
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