Do reptiles live on floodplains? It is a question I have often asked myself, and usually concluded that the answer is largely 'no.' The obvious exception is the grass snake, a highy mobile species, at home in agricultural and managed landscapes, and a good swimmer.
I wouldn't expect to see any reptile in the vicinity of a lowland river or stream, except a grass snake. Grass snakes would be the only reptile I would normally expect to see on riverbanks and neighbouring floodplain land, however suitable the habitat seems.
As for other reptile species, I think it is unlikely for them to live in active river floodplains. They are all able to swim if necessary, but during their winter hibernation phase they need dry, well-drained ground voids. Such places are usually in elevated positions, safe from winter flooding.
However, I don't think the question of reptiles on floodplains is quite so clear-cut.
Floodplains do not necessarily flood every year in nature. Extensive flooding might only occur every ten years, and extreme events tend to occur only every hundred years or so. Yet any inundation event during cold winter weather would be disastrous for reptile populations. Hence reptiles tend not to colonise low-lying land prone to inundation.
So that's the theory, but does it stand up in practice? In the modern managed landscape, many river floodplains are no longer allowed to flood; or they are compartmentalised, such that only some areas are allowed to flood. Floodplain land occupied by important infrastructure and sites such as power stations is permanently well-drained and generally safe from flooding.
Reptile habitats are often well-developed in these former floodplain environments. Grassy meadows, scrubby verges and hedgerows offer large extents and extensive reptile-friendly corridors. Yet they are rarely occupied by reptiles; a simple anachronistic circumstance.
In those cases where reptile colonisation has occurred on floodplains, it is the result of anthropogenic management and drainage, where flooding has been prevented for long enough to allow colonisation. This could be natural, from neighbouring higher ground, or assisted by humans.
Some examples to illustrate reptile presence and absence on floodplains:
- Presence - On the River Avon floodplain in Christchurch, Dorset, adders (and presumably other reptiles) occupy manmade road embankments of the A35 as it crosses an area of floodplain called Purewell Meadows. The adders have presumably dispersed along the linear corridor of road embankments, and found winter refuge in this flood-free, elevated land. Up to five adders have been found basking together under one fallen road sign.
- Absence - Detailed survey of roadside verges on an 11km of the A338 near Bournemouth, Dorset, found reptiles along the entire route, except for a 3km stretch crossing the River Blackwater/Stour floodplain. Thousands of reptiles occupied the elevated stretches, but along the floodplain corridor, only a single grass snake and slow-worm were found.
- Absence - NARRS surveys of two 1km squares near Bournemouth found no reptiles in the survey areas. Both areas are former floodplain, but are no longer allowed to flood. One square is adjacent to Bournemouth Airport, and the other includes the Wessex Water sewage works and the Royal Bournemouth Hospital. Both were seemingly devoid of reptiles, or more accurately: reptiles were not detected, although grass snakes are probably present in both areas.
The potential value of former floodplain sites for reptiles is worthy of further consideration. Land that is occupied by strategically-important sites such as airports and utility works are unlikely to be allowed to flood, yet these landscapes are probably rarely occupied by reptiles. Could they be of conservation value?
Natural colonisation could involve very long timescales if several kilometres of floodplain are to be crossed. Passive assistance such as elevated road or rail embankments mean that the process may have been going on for a long time already in some cases.
On the other hand, deliberate introduction would arguably be a valid technique to consider in commercial mitigation, provided that the land is no longer prone to flooding.
In areas like Dorset, where it is difficult to find reptile-free land, former-floodplains constitute an extensive 'habitat bank', and reptile introductions there could prove a valuable conservation tool.
By Chris Gleed-Owen, CGO Ecology Ltd.