There are advantages and disadvantages to translocating reptiles to sites that are located nearby and sites that are more distantly located.
In attempting to recommend appropriate translocation distances I will take a closer look at the previously identified negative factors that may influence the location of a proposed receptor site. Since this is likely to be rather a long blog post I will split the article into two sections. This is Part 1. Part 2 will follow in due course.
Availability of Receptor Sites
The more land area that is available for review, the greater the choice of suitable receptor sites that are likely to be found. If distance restrictions are too severe the availability of high quality receptor sites may be compromised.
Some reptile species have well defined home ranges. Detailed studies on the home range size of lizards are rather limited, but available information suggests that most are relatively small and can probably be defined by low hundreds of square meters (Beebee and Griffiths, 2000). Dispersing individuals (especially juveniles) may move over greater distances. Species that may appear to be rather sedentary (e.g. slow-worm) may actually display seasonally influenced movement patterns (e.g. Riddell, 2000), although the extent of such movements are likely to be site specific. Viviparous lizards can attempt to vacate adjacent receptor areas and return to their place of capture, even climbing over so-called reptile exclusion fencing in the process (pers. obs). Similar behaviour has been observed in grass snakes (pers. obs.) and adder (Whiting and Booth, 2012).
British snakes have larger home ranges than lizards. Some published studies have suggested that grass snakes can move over large areas with home ranges calculated as several hectares (e.g. Madsen, 1983). Like slow-worms, movement of grass snakes can be influenced by season (e.g. Reading and Jofre, 2009). Female grass snakes lay eggs and may display extensive movements before and after oviposition (Madsen, 1983). Adder also display seasonal movements, with snakes moving between winter hibernacula and summer foraging sites (e.g. Presett, 1971; Andersson, 2003). Such movements may occur across a kilometre or more, even through apparently unsuitable habitat (e.g. through mature woodland).
Human-wildlife conflicts following development may be an important consideration if species such as adder are translocated into nearby areas. In these situations, consideration must be given to ensuring that such conflicts are minimised (e.g. through installation of permanent exclusion fences and appropriate habitat management work). In some cases it may be necessary to translocate adder to sites that are located sufficiently far away to prevent homing so as to minimise potential risks to both people and snakes. Such situations may arise during the construction of housing developments, schools, hospitals etc. Although the actual risk of a person being bitten by a translocated adder is probably very low, the perceived risk may be considered much higher by the public. When stories about snakes and children hit the press, significant negative publicity that undermines conservation actions can result. These issues need to be carefully considered.
Increased dispersal has been found to be a significant factor affecting the welfare of translocated animals (Harrington et. al. 2013). If animals disperse away from a remote receptor site to such an extent that they experience increased mortality or are unable to form a viable breeding population, the translocation should be considered a failure - even if the work meets statutory requirements in preventing animals being killed during development. Problems associated with dispersal and homing behaviour can be minimised through the installation of fencing that restricts animals to previously prepared habitat areas (e.g. Whiting and Booth, 2012). Translocation of reptiles (especially snakes) to more distant receptor sites should be accompanied by increased monitoring effort and this will be discussed in a future blog post.
Loss of Local Biodiversity Interest
The loss of 'local' biodiversity interest may mean different things to different people. A site manager may consider the movement of animals out of her reserve to be a local loss. If the animals are moved into a different local authority area, the planning officer may consider it to be a local loss. Recorders may be concerned about animals being moved between recording areas etc.
The key thing here is to define what we mean by local.
The Oxford Dictionary defines local as:
1. relating or restricted to a particular area or one’s neighbourhood...
2. (in technical use) relating to a particular region or part, or to each of any number of these...
In effect then, local has no fixed spatial definition and can mean almost anything provided it is described in the context of neighbourhood or region.
At the risk of sticking my head above the parapet, I will propose a definition that is appropriate in conservation terms for UK reptiles:
Local relates to a predefined area of land that is situated within a specified landscape area within a recognised vice county.
i.e. A receptor site is local to a development site because they are both situated in the same character area and the same vice county.
Definitions for character area are dependent upon country. In England, national character areas have been developed by Natural England and appear to replace the previous natural areas, but as far as I can tell use the same boundaries and definitions. Countryside Council for Wales has published a Landscape Character Map (pdf download) and Scottish National Heritage has produced a Landscape Character Assessment. In Northern Ireland, Environment and Heritage Service has published Landscape Character Areas (pdf download).
Vice counties have fixed boundaries that are not influenced by changing political boundaries.
Note that my proposed definition is not limited by distance. Providing the criteria are met, a local site could actually be situated several kilometres away. This may not necessarily be a good thing and other factors are likely to come into play.
Inappropriate Mixing of Genes
I have already discussed the issue of subspecies and race, at least in the context of sand lizard. However, defining the maximum distance that translocations of more widespread species is problematic. Not only are the animals by definition more widely distributed but available research is very limited.
Following publication of a Natural England report highlighting the national decline of adder (Baker et. al. 2004), several projects have been announced that attempt to review the conservation impacts of restricted gene flow. One such project has been organised by Zoological Society of London (ZSL) another by University of Sunderland. However, rather than discouraging translocations these projects may actually collect data that supports the earlier work of Madsen (1999), who found that an inbred population could be restored (at least temporarily) by introduction of a single male adder. Moving animals may actually benefit isolated populations by introducing new genes and therefore increasing reproductive fitness.
After reviewing distribution data for adder in Kent, I believe that genetically distinct populations may be found in different areas (i.e. North Downs and High Weald). Whilst no research is currently planned that will investigate such claims, the movement of animals between different landscape areas could jeopardise future research projects. If the movement of animals follows the previous definition of local, such problems can be minimised. Limiting translocations to local receptor sites should not increase the risk of inbreeding depression.
Differences in Habitat/Geology
Some species have very specific habitat requirements. These may be quite obvious for rare reptiles such as sand lizard, but widespread species can display their own preferences. At least one study in North America has shown that a reptile's prior experience can be an important factor in determining translocation success (Roe et. al., 2010). Slow-worm are sub-fossorial lizards that are frequently encountered in proposed development sites. Animals that have become accustomed to sites with disturbed ground (e.g. old allotments) may have difficulty in establishing populations in receptor sites characterised by different ground conditions (e.g. chalk grassland). The greater the distance that animals are translocated, the more likely it is that receptor sites will be characterised by a different geology. Chalk woodland and clay woodland habitats are very different. Adder favours one, but Kent populations are almost completely absent from the other.
Problems associated with habitat and geology can be minimised if receptor sites are kept local.
Displacement of Individuals from Home Range
Work with some snake species has shown that translocated animals display greater individual movements than undisturbed individuals (e.g. Roe et. al., 2010). These increased movements can result in higher mortality (e.g. through increased predation, movement into unsuitable habitat or roadkill). A large-scale translocation project described by Kyek et. al. (2007) ensured that animals were able to move from receptor areas through suitable habitat corridors back to enhanced habitat areas around the completed development. Although individual movement during this project may still have been high, released reptiles were less likely to disperse into unsuitable habitat.
Whilst the number of UK projects that publish post translocation monitoring results are limited, available reports tend to suggest relatively low numbers of observations of released animals. Whilst some of these results may be due to sample effort (e.g. Lyle, 2008) other reports have suggested low detectability in a high quality but complex habitat (e.g. Cresswell et. al., 2012). It is possible that low encounter rates of post translocation snakes are caused by dispersal of animals away from release areas. At sites where the species has not previously been recorded, such dispersal could result in failure of the translocation project due to animals dispersing away from each other across a large area. In these situations the apparent health of individual animals may remain high, but breeding success becomes less successful over time. Whether observed patterns of apparent population expansion are simply due to individual dispersal away from release areas or a genuine increase in population size remains unclear.
On one hand translocating reptiles to sites within or close to their existing home range may increase homing behaviour that results in conflicts with the residents of new developments (and their pets!). On the other, releasing animals at receptor sites located outside of their home range may increase the risk of translocation failure. It is important to consider that a local site may still be located beyond the home range of a translocated animals.
I warned you it was going to be a long post!
Part 2 will address issues relating to stress, disease and potential impacts on other species.
Andeersson, S. (2003) Hibernation, habitat and seasonal activity in the adder, Vipera berus, north of the Arctic Circle in Sweden. Amphibia-Reptilia, 24, 449-457.
Baker, J., Suckling, J. and Carey, R. (2004) Status of the adder Vipera berus and slow-worm Anguis fragilis in England. Natural England.
Harrington, L. A., Möhrenschlager, A., Gelling, M., Atkinsone, R. P., Hughes, J. and Macdonald, D.W. (2013) Conflicting and complementary ethics of animal welfare considerations in reintroductions. Conservation Biology, 27, 486-500.
Madsen, T. (1983) Movements, Home Range Size and Habitat Use of Radio-Tracked Grass Snakes (Natrix natrix) in Southern Sweden. Copeia, 1984, 3, 707-713.
Madsen, T. (1999) Restoration of an inbred adder population. Nature, 402, 34-35.
Presett, I. (1971) AN ecological study of the viper Vipera berus in southern Britain. Journal of Zoology, 164, 373-418.
Reading, C. and Jofre, G.M. (2009) Habitat selection and range size if grass snakes Natrix matrix in an agricultural landscape in southern England. Amphibian-Reptilia, 30, 379-388.
Riddell, A. (2000) The special ecology and ranging behaviour of the slow worm Anguis fragilis. MSc thesis, University of Kent.
Roe, J.H., Frank, M.R., Gibson, S.E., Attum, O. and Kingsbury, B.A (2010) No place like home: an experimental comparison of reintroduction strategies using snakes. Journal of Applied Ecology, 47, 1253-1261.
Whiting, C. and Booth, H. J. (2012) Adder Vipera berus hibernacula construction as part of a mitigation scheme, Norfolk England. Conservation Evidence, 9, 9-16.