This page summarises how conservation practitioners re-establish species in the wild, and ARC’s approach to using these methods to help amphibians and reptiles in the UK. Reintroduction is a complex area of conservation practice, and this page attempts to briefly highlight the key issues. For those wishing to explore further, references are given in links in the text and are listed in full at the end of the article.

The terms reintroduction, translocation and captive breeding are sometimes used interchangeably but in fact have distinct meanings (see in particular IUCN SSC, 2013). Reintroduction means “the intentional movement and release of an organism inside its indigenous range from which it has disappeared.” Translocation means “the human-mediated movement of living organisms from one area, with release in another”; translocation can therefore be a way to implement a reintroduction, and can also refer to moving animals outside their indigenous range. Captive breeding is the generation of new individuals from animals held in captivity; captive breeding can therefore be a way to source animals to be used in a reintroduction or translocation.

Why reintroduce species?


So, as well as improving and recreating habitats, we have to consider alternative means of ensuring that animals can populate unoccupied patches of habitat, where that is appropriate. Reintroductions are achieved through the release of animals from captive populations or by moving animals from one area of the wild to another. In the UK, reintroductions have played an important role in the conservation of amphibians and reptiles, and will likely do so in future (see Carter et al 2017.)

ARC and its predecessor The Herpetological Conservation Trust have pioneered the reintroduction of amphibians and reptiles in the UK, working in partnership with landowners, volunteers, professional conservation organisations, captive breeders, scientists and government agencies. These efforts have focused on the sand lizard, smooth snake, natterjack toad and pool frog, and we are currently developing protocols for the adder. Our work has resulted in significant gains, for example re-establishing the range of the sand lizard (returning it to 7 counties from which it was lost). We have reversed national level extinction for pool frog, which went extinct from Britain in the 1990s, and for natterjack toad and sand lizard in Wales. Reintroduced animals now constitute around a fifth of the total UK natterjack toad population. Over the years, ARC has improved its reintroduction methods through experience, reference to emerging evidence, and liaison with others working in this field.

When should reintroduction be undertaken?

This depends on a complex set of considerations. Essentially, reintroduction is indicated when it’s the most effective way to deliver a nationally agreed recovery goal for a species, where the potential harms (see below) are outweighed by the benefits, and - where appropriate - it will complement other conservation action for that species. Importantly, a reintroduction is not indicated solely because it is technically feasible to obtain or breed animals and release them - project leaders should assess whether it is also appropriate and desirable to do so. A key principle of reintroduction is that the reasons for the original loss of the species should be reversed. For amphibians and reptiles, this often relates to the creation and maintenance of appropriate habitat conditions in a way that is consistent with other conservation objectives.

What standards should reintroductions work to?

There is a range of evidence, guidance, good practice and legislation that together give practitioners a clear basis for planning, implementing, monitoring and reporting on reintroductions. The best single reference is the global guidance produced by the IUCN, entitled “Guidelines for Reintroductions and Other Conservation Translocations”. Each country has its own set of regulatory and advisory arrangements too. Complying with standards is a significant consideration when planning reintroductions. It is good practice to produce carefully evidenced rationale for a particular reintroduction long before any animals are secured or other practical measures are put in place, not least because peer review of the rationale will often result in useful changes to implementation plans.

ARC’s proposal to reintroduce the pool frog to Thompson Common in Norfolk is an example of good practice in reintroduction planning which has resulted in the establishment of a population (see Baker & Foster, 2015). Reporting on the outcome of reintroduction efforts, even when those efforts fail to reach their objectives, is an important part of reintroduction practice. ARC documents and communicates its reintroduction work in various ways, including through publications such as the IUCN Global Reintroduction Perspectives series.

Useful scientific overviews of reintroductions and translocations in general, and for amphibians and reptiles in particular, include: Berger-Tal et al 2020, Griffiths & Pavajeau 2008, Harding et al 2016, Smith and Sutherland 2014, Meredith et al 2018, Ewen at al 2014, Carter et al 2017.

Who should do reintroductions?

Reintroductions undertaken well require technical expertise, engagement with stakeholders, resources, strong governance, and long-term commitment. This tends to mean that professional nature conservation organisations are best suited to undertaking reintroductions, but other organisations with related aims may also implement reintroductions effectively. If organisations have additional objectives aside from conservation, there should be clear measures in place to ensure those objectives do not compromise the planning and implementation of reintroductions to good practice. This can arise for example when organisations are involved in scientific research, sale of animals, or public display of animals.

Are reintroductions always a good idea?

Whilst reintroductions can help species recovery, they are not always a prudent option. There are several key reasons for this. Firstly, reintroductions are often simply not the best way to achieve a desired goal; it may be more appropriate to achieve the same result through other routes such as habitat creation, allowing natural colonisation, or alleviating downward pressures on existing populations. Secondly, reintroductions can distract attention and resources from pressing conservation issues. Releasing lots of animals is, understandably, often seen as a solution, but this may not be the best way to achieve species recovery, and can divert efforts to address serious pressures. Linked to this, captive breeding and releases tend readily to garner public and media attention, and risk giving a false impression of the underlying problems facing wildlife and what society should do to address them.

Thirdly, releases of animals into the wild can harm existing wildlife, for example through altering predator-prey dynamics, altering genetic identity, or introducing disease. Whilst good reintroduction practice should minimise these risks, the consequences are so severe that extreme caution is merited. These are not simply theoretical risks; for example, a release from a captive collection has recently resulted in a serious outbreak of disease in wild amphibians in Spain (Martel et al 2020).

Fourthly, there is a balance to be struck between creating new populations through reintroductions, and conserving existing populations. Although this is a complex topic, generally speaking, where there is evidence of widespread ongoing decline, it is more sensible to focus efforts on reversing those declines, rather than generating new populations which will then likely be to subject to the same pressures. In the UK, all amphibians and reptiles are facing pressures to some extent, and reintroductions are just one tool to be used where appropriate alongside other efforts to protect and enhance populations. Lastly, if there is a chance that a reintroduction would be undertaken with inadequate standards, it would typically be best not to do it. As well as failing to reach their own aims, poorly implemented reintroductions can compromise the efforts of others wishing to do reintroductions to good practice, for example by affecting landowner relations.

What about reintroducing nationally extinct species?

Sometimes it is appropriate to reintroduce species that have gone extinct at national levels. In the UK, for example, the sand lizard and natterjack toad were both reintroduced to Wales, having gone extinct in that country, and the northern pool frog was reintroduced to Norfolk having gone extinct in England; in all these cases the species was returned within decades of the extinction. Guidance, including that issued by UK conservation agencies and the IUCN, help to determine whether it is appropriate and desirable to do this sort of project. The case for reintroducing species that went extinct hundreds or thousands of years ago is generally less compelling, largely since the landscape and other factors affecting species persistence have changed so much. Nonetheless, there is value in exploring the rationale for reintroducing species in this category, such as the moor frog Rana arvalis. The implications of climate change are important when considering reintroducing nationally extinct species (see below).

Should species with uncertain native status be reintroduced?

It is important that there is a sound basis for a species being considered native before a reintroduction is planned. There are published approaches for assessing native status, and where there is uncertainty these approaches should be applied through research, allowing peer review of conclusions. The pool frog is a useful case study in this regard; for many years it was typically considered to be introduced, but a compelling case for native status emerged via co-ordinated research projects (see Beebee et al, 2015). The grass snake Natrix helvetica is an interesting case: whilst certainly native to Wales and England, there is equivocal evidence for its native status in Scotland; further research might prove persuasive and - if so - it is feasible that reintroduction would be warranted if the causes of decline could be remedied.

It is possible that in the future, climate change considerations would mean species with uncertain native status (and indeed even confirmed non-native species) might be deemed suitable for introduction (see below).

Captive breeding, translocation and genetic considerations

To re-establish a population in the wild, generally speaking animals can be sourced from captivity, or they can be moved (translocated) from other areas of the wild. In practice there are slight variations to this, for example “head starting” involves movement of wild animals into captivity for a short period of time before release. Captive breeding is often favoured when large numbers of animals are required to found new populations, or when it is inappropriate to translocate. However, captive breeding has a number of disadvantages, such as ongoing resource commitment, genetic impoverishment, and disease transmission risks. Translocation of wild animals can be effective but also has disadvantages such as the need to take animals from existing populations. Which approach is best will depend on the species concerned and the reintroduction objectives. ARC has used a variety of approaches in its reintroduction projects.

The genetic identity of animals used in reintroduction programmes is critical, and there are various reasons for this. For example, animals can develop local adaptations, which reintroduction programmes should try to preserve. The northern pool frog, for instance, is known to have particular adaptations to living at higher latitudes, and so the reintroduction programme has used carefully selected stock to ensure these traits are preserved. Captive breeding programmes must take particular care in sourcing animals, especially when there is uncertainty over captive history. When planning releases, the genetic consequences should be considered, and periodic genetic monitoring may be advisable (e.g. see Phillips et al 2020). As researchers discover more about the genetic relationships between wild populations, these will need to be fed into conservation programmes. Examples include the recent finding that common toads on mainland Britain are a different species to those on Jersey, or that grass snakes in Britain are part of a distinct western European species.

Where wild populations suffer from serious genetic problems, there can be a case for intervening, in particular via an approach called genetic rescue. However, deliberately altering the genetic nature of wild populations through releases of animals can also cause harm, and is only warranted in particular circumstances. ARC and its partners are assessing the value of genetic interventions for UK amphibians and reptiles.

Re-wilding, landscape-scale approaches, and reintroductions

Definitions of re-wilding vary, but tend to centre around projects operating over a large spatial scale and involving very low intervention, aiming to restore natural processes. Re-wilding is effectively one type of landscape-scale approach to nature conservation, and has attracted significant attention in recent years. Re-wildling holds much promise for amphibians and reptiles. It will create large areas of suitable habitat, and species may respond through natural colonisation. There is good evidence from early efforts that, for example, common frogs respond quickly to areas restored by the release of beavers. Reintroductions for amphibians and reptiles should be considered in re-wilding projects. In some cases reintroductions will be appropriate, in others natural colonisation may be preferable once the costs and benefits are weighed. ARC has suggested that there is more discussion among stakeholders over the role of reintroductions in re-wilding projects, in particular for the smaller species such as reptiles and amphibians. Discussions among re-wilding practitioners and researchers show that that there are a range of issues that need addressing at policy level (e.g. see Seddon et al 2019).

Climate change implications for reintroductions

Climate change is affecting the thinking behind many areas of nature conservation, and reintroductions are no exception. For example, there is increasing interest in undertaking reintroductions outside species recent natural range to account for shifts in climate, an approach known as assisted colonisation. This is a contentious topic and there are acknowledged risks in moving species to wholly new areas, but the serious impacts of climate change are prompting a re-evaluation of conventions. More controversially, there are suggestions that species threatened by climate change in other parts of the world might be considered “climate refugees,” and considered for introducing to the UK. There is currently no wide agreement on this but ARC is exploring the issues with others working in nature conservation. Even if these more radical approaches are not taken up, the impacts of climate change on our existing wildlife will mean that reintroduction planning for native species will increasingly need to take this factor into account.


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Banner image by Mary Braddock