Monday, 22 April 2013

Gene flow ...

Arundel Wetlands

The beautiful water vole, Arvicola amphibius, was once a common site along the banks of the Arun Valley waterways. However, the modification of landscapes for agriculture and urbanisation, and the colonisation of the American mink, Neovison vison, led to their dramatic decline both locally and nationwide. In Sussex alone, we have sadly lost over 90% of our historic populations.

The Arun Valley water vole project is studying the gene flow within and between water vole populations, and investigating the effect of different habitats and landscapes in Sussex. The research is being undertaken during 2012 and 2013 and involves both field and laboratory techniques.

With the help of volunteers, numerous ditches were surveyed over a three month period, and evidence of breeding water voles was found on 13% of the waterways surveyed. This was an encouraging sign but, for the most part, only a few latrines were recorded. However, it was on an agricultural drainage ditch at Houghton Bridge, near Amberley, that signs of recolonisation were evident. Large concentrations of water vole latrines, feeding stations and burrows were present, indicating a potentially viable breeding population. This was great news as it suggested that water voles were recolonising northwards up the valley.

This raised several questions:

Had this newly found population come from Arundel Wildfowl & Wetlands Trust (AWWT), which is a reintroduction site located approximately 6.5km downstream?

Was the population too isolated and unable to rely on new individuals that would provide demographic and genetic rescue?

Can farmland ditches, not managed for water voles, hold viable populations?

Over the last two years Rowenna Baker has set out to answer these questions by studying the genetic structure and demographics of water voles at Houghton Bridge and AWWT through live trapping individuals each year, PIT (passive integrated transponder) Tagging, and by taking hair samples to obtain DNA for genetic mapping. Encouragingly, the abundance of water voles at Houghton Bridge is comparable to other natural colonies occurring along similar habitats nationwide, with estimates ranging from 20 to 50 individuals along 1km of bank. The population is small in comparison to  AWWT (135 individuals in 2010), where they are not affected by the external pressures of grazing and predation that commonly constrain habitat availability and density in unprotected sites. Both populations remain genetically diverse, however dispersal between populations is low, which is unsurprising as the average dispersal of water voles is 1-2km.

Despite the importance of genetic fitness, little is currently known about the genetic structure of water vole populations and what factors facilitate or impede gene flow within and between populations. Other studies have found that increasing distance between populations, land use and habitat design can all influence gene flow in animal populations.

The Arundel population is clearly a vital source for the recolonisation of water voles within the valley, however it is crucial that landscape connectivity is improved to aid dispersal between newly established populations. With additional colonies being found as far north as Pulborough Brooks, Rowenna now uses remote hair tubes to obtain genetic samples from all occupied sites in order to investigate gene flow and factors that facilitate and impede dispersal. This information will help to guide management to strengthen isolated populations by creating habitat networks that will hopefully promote water vole dispersal and recolonisation.

An important project regarding an excellent flagship species and one which I was privileged to witness first hand.

My thanks to Rowenna ...

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