We are used to hearing about a species only having a handful of individuals left before the decision is made to capture as many as possible to enter a captive breeding program. Sometimes these programs are successful such as the case with the black-footed ferret (Mustela nigripes) or the Mauritian kestrel (Falco punctatus). However more often than not, these programs fail to return animals to the wild in any quantity to qualify as a success. As a conservationist specialising in amphibians and reptiles I can easily understand that it is never as easy as it seems, especially if the husbandry and ecology of the species isn’t particularly well known. It has always puzzled me why we often take so long to act, why not act sooner when there are still a reasonable number of individuals left and a large representation of the genetic diversity?
It isn’t always possible to collect enough individuals to ensure a minimal viable population size (which varies between species) and so these species slip away into the clutches of extinction. Species monitored to extinction include the golden toad (Incilius periglenes) and the Yangtze River dolphin (Lipotes vexillifer) to name some well-known examples.1 If interventions had been undertaken sooner, those species may still be around today. This is not always practical due to political or financial influences. Despite many seeing these instances as failures, we can use them as examples to champion species conservation as a lesson. We have seen first-hand countless times the consequences of taking too long to reach a decision. What if you could prevent such occurrences from happening the first place and act upon them before ever reaching the outer fringes of the danger zone?
Go with the toad
Enter the common toad (Bufo bufo), which in my eyes is a model species for a radical method of conservation we should all be actively undertaking: the captive breeding and head-starting of widespread species. Whilst common toads are extremely widespread throughout Europe and listed as Least Concern by the IUCN Red List, the same cannot be said for their distribution in Great Britain. The common toad has declined by 68% over the past 30 years in the UK.2 There is a lack of knowledge on the most part as to why this decline has occurred. As we are an island it is impossible for toads from the continent to repopulate those areas where they have declined or been lost. In fact, if the common toad were to be listed on a regional red list using IUCN criteria, they could be classified as either Vulnerable or Endangered. The final outcome would be decided upon based on the most recent data available. Therefore, I think the common toad should now be considered the uncommon toad, this is a sign that it is time to act before it is too late.
Through the various outreach activities that I have taken part in over the course of my career so far, there are two main audiences I am interested in engaging with. The first of course is school children. It is more important than ever to ensure they are connected with nature and understand its importance. Not all of them have to become conservationists but as long as they know what they are fighting for and its value, that is the main thing. The other group is the children’s grandparents. Now this may surprise some of you but grandparents were of a generation that spent a large amount of time outside. This means they have vivid memories of the natural world as it used to be, before urbanisation went into overdrive following the Second World War. Through subsequent generations of attrition, our countryside has become devoid of wildlife, but in our minds it has remained static through shifting baseline syndrome. Thankfully those who are currently taking life easy in retirement are the key to setting the bar for the levels of wildlife and biodiversity we should aim for, as a minimum.
With this in mind, common toads readily breed in a number of environments including ponds, scrapes and even lakes that have been stocked with fish. Whilst other amphibians would perish due to predation, toads, their spawn and their eggs are toxic. This affords them protection from most predators except the wittiest of mammals and corvids but not newts. In order to boost a population of toads, a couple of methods could be used. The first involves collecting a sample of spawn from a breeding pond, rearing the tadpoles in a controlled environment and releasing them back at that site just before metamorphosis. This approach is quite basic and can easily be reproduced by anyone from school children to amphibian hobbyists. It also has the advantage that it avoids the risk of spreading disease to a new population as the animals you are rearing originated from that very pond.
The second approach is to collect the spawn from a breeding pond, rear it to a stage that is ready for release and then translocate it to an area where toads have been lost. This throws up a couple of issues that we need to overcome in order to make the effort worthwhile. First, the initial cause of decline in toads needs to be identified. Not all sites where they have been lost from will be suitable for reintroduction, they may have become too small and fragmented to support a healthy metapopulation. Other sites may just need some light habitat management work or a corridor created to rejuvenate that pond and make it viable again. It is going to take time to conduct the investigations and the solutions won’t always be easy but they will be worth it. There are a number of potential factors affecting the success of toads,3 existing literature should be consulted to inform decisions based on the circumstances of the ponds.
The toad less travelled
Time should also be taken to decide which stage in development it suitable for reintroduction or translocation to a site following head-starting. In those areas where large amounts of toads are available in the population, a number could be collected and reared in outdoor naturalistic enclosures to produce spawn and tadpoles for translocation. In this manner, you can generate spawn for reintroduction in large volumes, if it is decided that it is beneficial to that site to boost a site with that particular life-stage. To be able to decide which life-stage is optimal for a site with some reproducibility, a standardised evaluation method is required. This may consider such factors as the distance to the next viable breeding pond, the duration of time toads have been absent from the site and breeding success of the local metapopulation.
All of these will influence the success of a translocation/reintroduction and therefore govern the best method to be used. For example, a site where toads have successfully bred recently can be the receptor site of spawn, as there is direct evidence that larvae can successfully develop and metamorphose there. For a site where toads have been lost for some time, it may be best to use a combination of life stages to maximise the likelihood of establishment. When choosing a location for reintroduction or translocation a viable metapopulation should be nearby so that there is sufficient genetic flow. If this cannot be guaranteed then a different location should be chosen. Conserving as much of the genetic diversity available should be seen as one of the main goals, before returning toads to those areas where they have declined. This is for a number of reasons including preventing extinction vortexes and conserving as many of the genes for adaptability as possible, especially in light of the unknown consequences of climate change.
One risk to be taken seriously is that of biosecurity. If individuals are breeding and releasing toads, then they should ensure that the individuals are quarantined and tested for the presence of disease if they start to show any worrying clinical signs. This is particularly true if other amphibians are also housed nearby or within the same facility, as there is the potential for novel pathogens to spillover into wild populations. Whilst our native amphibian species appear to be quite robust to diseases such as the amphibian chytrid fungus, we don’t know what the future holds.4 Climate change may lead to common toads (or other amphibian species) becoming more susceptible. Therefore, it is in our interests to ensure that the breeding and release of toads contributes to the minimum spread of disease as possible.
Before they croak it
Picture this, you have successfully released some toads to a site – how do you know a population has been established? Common toads take a couple of years to reach sexual maturity so it is unlikely you will have any spawn the following spring, unless you released adults or near sexual-mature sub-adults. Like most amphibians, there is also a high mortality rate so it may take a few years of constant effort to establish a population and see the results. Therefore, ongoing monitoring of the pond is vital to ensure success. This includes counting the other amphibians that are present, such as newts and frogs. Other monitoring should focus on the aquatic invertebrates as a proxy of water quality as well as the hydrology of the pond. The last thing you want is for the toads to be ready to breed, but return to a desiccated bowl.
Another element of monitoring is post-release habitat management. It may be necessary to modify the habitat every year or so to ensure the pond is not encroached by a succession of scrub. Thankfully the material removed during habitat works can often be used to create hibernacula and other features that benefit wildlife – nothing goes to waste. When completing work you should consider all life stages as well as other wildlife, steep sides may accidentally trap metamorph toads as well as animals such as hedgehogs. By taking this more holistic approach, you can benefit as many species as possible whilst targeting your focal species.
The toad to success
In order for this to be beneficial to toads and other subsequent species such as the viviparous lizard (Zootoca vivipara), it is important to involve some responsibility and teamwork. Organisations should work together before undertaking such work and could produce guidance for such activities. Selecting suitable potential receptor sites is the key to success. There are large bodies of scientific evidence that should be used to help inform decision making throughout the process. Perhaps in the future a conservation organisation will take this project under their wing and see if it is feasible.
Pool frogs (Pelophylax lessonae) have been returned to the UK using a similar method5, together we can make regional extinction a thing of the past. If you build it, nature will come. As well as being perfect examples for this model, amphibians are also towards the base of the food chain in many ecosystems. If you restore the amphibian community to a level it was once at, there will be positive cascade effects. Amphibians are not the first candidates that come to mind when people think about rewilding, but they are in fact ideal. It is my vision that this method of boosting the populations of widespread species will be used not just for toads but for all taxonomic groups, where the species could use a helping hand due to our geographic isolation. By protecting the vulnerable species of today, we are preventing the extinction of species tomorrow.
1. Turvey, S. (2009). Witness to extinction: how we failed to save the Yangtze River dolphin. Oxford University Press, USA.
2. Petrovan, S. O. & Schmidt, B. R. (2016). Volunteer conservation action data reveals large-scale and long-term negative population trends of a widespread amphibian, the common toad (Bufo bufo). PLoS One, 11(10), e0161943.
3. Baker, J. M. & Halliday, T. R. (1999). Amphibian colonization of new ponds in an agricultural landscape. Herpetological Journal, 9, 55-64.
4. Allain, S. J. R. & Duffus, A. L. J. (2019). Emerging infectious disease threats to European herpetofauna. Herpetological Journal, 29(4), 189-206.
5. Baker, J. M. (2018). A head-starting trial for the reintroduction of the pool frog Pelophylax lessonae to England. Herpetological Bulletin, 143, 7-11.