BCT Bats and Woodland Symposium

The BCT Bats and Woodlands symposium was organised in conjunction with the Warwickshire Barbastelle Project and was held in Rugby, Warwickshire on 18th November 2014. The outline below is a brief summary of the talks and topics covered:

woodland in spring

Bats and Woodland Overview (Helen Miller, BCT)

The first talk was from Helen Miller – BCT’s Woodland Officer. A wide range of bats utilise woodland habitats for activities including roosting, foraging and commuting. National Biodiversity Network (NBN) data shows that there is a positive association between bats and the extent and proximity of broadleaf woodland whilst other analysis show that roosts of 5-6 of our bat species are significantly closer to woodland than would be expected by chance. The peripheries of woodlands are frequently used by edge specialists such as pipistrelle species, noctule and Daubenton’s bats, whilst the interior is favoured by other specialists such as the brown long-eared, Natterer’s and Bechstein’s bats. The importance of woodlands to bats are reflected by the number of documents which are available to offer guidance on the management of woodlands for the benefit of bats including the Woodland Management for Bats, Development of Good Practice Guidelines for Woodland Management for Bats, and the UK Forestry Standard.

A history of British Woodlands (Oliver Rackham)

The next talk was by Oliver Rackham whose books on the history of the British countryside – especially woodlands and wood pasture – represent the authoritative texts on the subject. The fabled ‘wildwood’ land cover of the UK historically is likely to have been a complex and dynamic grassland/woodland mosaic which cycled on a roughly 1000 year basis. Factors such as drainage, fertility and the actions of herbivors would have prevented the comprehensive woodland cover which most people imagine. This habitat is still found in wood pasture and parkland habitats where the trees and grassland co-exist and these habitats are where the ancient, gnarled trees are most often found. Closed woodland is not conducive to the development of ancient trees as they are out-competed over time whereas they can survive as standards in parkland, and the grassland/woodland mosaic of the wildwood is likely to have naturally created these trees. The management of ancient woodlands has, for hundreds of years, been for the benefit of the locals and foresters. This meant that trees were coppiced and pollarded to allow them to be sustainably harvested – the development of large trees would be outside of the ability of the foresters to deal with, before the advent of power tools. Oliver Rackham suggested that this may mean that woodlands of the past may have been less useful to bats as these larger ancient trees were not present.

Henry Andrews, who has done much work recording and collating records of tree roosts, pointed out that this assumption that bats are associated with ancient trees is often false as roosts frequently occur in many much younger and smaller trees which are more likely to have been a component of these managed ancient woodlands. Henry Andrews’ Bat Tree Habitat Key page on facebook has many videos of the much younger trees which develop roosting features. Oliver Rackham suggested nonetheless that perhaps ‘bats have never had it so good’ in terms of roost availability with extensive roosting opportunities associated with more larger trees in less managed woodlands. There do, from personal experience, seem to be many features apparently suitable for roosting bats in woodlands where I have climbed and inspected. This would suggest that it is perhaps the loss of foraging habitat, the mechanisation and intensification of agriculture and the impact of these on the insect food source of bats which is responsible for their huge decline across the 20th century.

Branching out: understanding the importance of woodland to the barbastelle (Matt Zeale, University of Bristol)

Ian Davidson-Watts gave two talks back-to-back, first covering Matt Zeale’s talk on the importance of woodland to barbastelle bats before moving on to his own topic. Barbastelles are a rare species in the UK – sparsely but widely distributed. They are mainly tree roosting and specialise in foraging on hearing moths – that is those which are able to hear bat echolocation and take evasive action. This ability has developed in an evolutionary arms race with the result that barbastelle echolocation is 10-100x quieter than similar species. They are a stealth predator and they may therefore be under-recorded by acoustic survey methodologies. The barbastelle bats radiotracked in the study spent the first 1-2h in their roost woodland but would often forage 6-7km from their roost throughout the night with some individuals travelling 12-17km. Many of the barbastelles studied are highly associated with foraging habitats over water, although studies in 2014 on a Lincolnshire colony found that these individuals did not appear to exhibit this habitat selection. The bats roosted most frequently under loose bark with 80% of their roosts being found in these features although splits were also used. The nature of these roosts is transient as they are often in dead or dying trees – only 22 of the 36 roosts found during one year were still standing and suitable the following year indicating a high turnover of roosts. For this reason it was argued that the woodland, rather than the individual trees, should be considered as the roost.

Life on the edge – the importance of woodland to pipistrelles and other ariel hawking bats (Ian Davidson-Watts)

Pipistrelle bats are often thought of as ‘edge’ or ‘generalist’ species reflecting their use of this semi-cluttered environment such as woodland edges, gardens and hedgerows as opposed to a dense woodland or open grassland. Ian argued that this is too simplistic – 43% of bats captured in woodland interior in trapping surveys were common or soprano pipistrelles and a bat logger found that 50% of calls within the woodland belonged to these two species showing that they do forage within the interior of woodland and are one of the most common species encountered there. Pipistrelles are often seen foraging around the canopy of woodlands even before sunset and Ian suggests that up at this level, there is a lot of ‘edge’ between tree canopies and the air, if we consider the woodland in a 3D manner rather than from our own perspective on the ground. Common pipistrelles showed a selection for deciduous woodland whilst soprano pipistrelles show a significant selection for riparian woodland habitat.

What appears a cluttered woodland interior from the ground could be much more open and 'edge' in character in the canopy - we must consider habitats from a bat's viewpoint rather than our own.
What appears a cluttered woodland interior from the ground could be much more open and ‘edge’ in character in the canopy – we must consider habitats from a bat’s viewpoint rather than our own.

Brown long-eared bat woodland ecology (Stephanie Murphy)

This study radiotracked 38 brown long-eared bats in 18 different woodland sites and monitored their movements over 3-6 days. The researchers identified a core foraging area for each individual which was an average of 2ha along with further peripheral foraging habitat where the chance of encountering the bat was around 50%. There was an increase in range size as the active season advanced as well as an increased use of hedgerows for foraging in July and August (although the latter did not account for the former). The key difference between the core and peripheral foraging area seemed to be a more diverse understorey – hawthorn, alder and hazel were more prevalent in the core areas. Bats which were caught together were often found to overlap their foraging areas but did not interact. They were however more likely to be found roosting together.

All female bats monitored roosted in, or adjacent to, the woodlands in which they were captured. 46 roosts were identified – 14 of these were in buildings whilst the remainder were in trees. All of the tree roosts were in oaks with the exception of one ash. The trees tended to be amongst the largest trees within the 50 x 50m quadrant they are in, and 21% of the roosting features used were not visible from the ground. The majority of tree roosts were more than 50m from the woodland edge. Around 50% of the bats used only a single roost during the trapping period whilst 25% switched once and the remaining 25% switched 2-7 times. Colony sizes were larger in building roosts than in trees and switching of roosts was more likely in trees than in buildings.

16 years of ringing Bechstein’s bats – what it has told us (Colin Morris)

The Bechstein’s bat is one of the rarest in the UK – it is a large myotis species with distinctive large ears which is a woodland specialist. Ten bats were radiotracked over two summers and were found to use foraging habitat ranges of 7-50ha at a maximum distance of 300-900m from their roost. There was a strong preference for closed canopy with understory – water and pasture foraging areas were the second and third most favoured habitats. 

The Bechstein’s bats in the Brackett’s Coppice in Dorset make use of Schwegler bat boxes erected by the Vincent Wildlife Trust. Both the large 1FW and the smaller 2FN boxes are used by the bats – the large 1FW boxes are used as maternity roosts with a strong preference shown for these in June and a strong preference for the smaller 2FN boxes in May and September before and after the maternity season during the ‘transitional periods’. The bats using the boxes are monitored and ringed and this has allowed their population dynamics to be observed over a long time frame. The number of babies varies significantly, between 10 and 50 over the 16 year study period. There is significant inter-year variation in the sex ratio of the young but this balances out to almost precisely 50/50 over the time frame. Rainfall is identified as the most significant factor affecting juvenile mortality. One of the first bats ringed in 2000 has subsequently been re-captured 47 times, had 10 babies and is at least 14 years old. 25% of Bechstein’s bats bred in their first year; 38% in their second; 28% in their third; 7% in their forth; and 2% in their fifth.

This talk highlighted the value of ringing studies – there is no other technique which would allow this data on longevity, breeding success and population dynamics to be gathered.

A standardised method for survey and monitoring of woodland bats (John Altringham)

John Altringham has developed a protocol for surveying woodland bat species. The hope is that this methodology, which uses Petterson detectors and a piece of software developed by the team, can be used by volunteers up and down the country to identify the occupancy of woodland bats including rarer species such as Bechstein’s and barbastelle. Sixty woodlands would need to be surveyed three times throughout the year to allow the populations of these species to be monitored. It is hoped that the scheme could be rolled out as part of the BCT’s National Bat Monitoring Programme (NBMP) in the future.

Advanced survey techniques for woodland bat species (Daniel Whitby)

Acoustic lures were first developed by Frank Greenaway in 2001. These are effectively speaker systems which emit high frequency calls – either recordings of bats social calls, or a digitally recreated version of these. There are now a variety available for researchers to use to increase the capture rate when trapping bats to identify, tag or ring. There is some discussion on how these lures work – studies of brown long-eared bat suggest that the lures may work be eliciting a territorial response as the capture rate was much higher in the core foraging area than the peripheral foraging area or outside of a bat’s foraging area. There are many positives to using lures. They increase the trapping rate allowing more information to be gathered in a shorter period of time and therefore causing less disturbance to local bat populations. This also makes trapping cheaper in terms of surveyor man hours. There is an increased accuracy of recording species and identifying the species assemblage present, as well as the ability to capture specific species as in the Bechstein Bat Project in the south-east. The lures also help to capture foraging bats as well as the commuting bats which are more frequently captured by mist nets and harp traps. Negatives are also to be taken into account – there is much variation in the way in which different lures work and which calls are played which may make standardisation and comparison between studies more difficult. Some lures may be limited to particular frequencies and so may miss higher frequency components of a sopcial call for example. There is a poor understanding of precisely how the lures work and the use of this technique requires more care, expertise and training than standard trapping techniques.

Reliable tree-roost indicators (Henry Andrews)

Henry Andrews, author and compilor of the Bat Tree Habitat Key ran through his top three indicators that a potential roost feature (PRF) is a roost.

The presence of actual bats came in at #3. This is because the bat may only be present for a night or two and therefore the chances of encounter are not necessarily that high. The best field sign is the longest lasting field sign and therefore, counter-intuatively, the very definite evidence of an actual bat is not number one! Care must be taken when using an endoscope with actual bats involved – a Level 2 licence (as opposed to the basic Level 1) is required to use this equipment. Henry’s number one rule is that you should never touch the bat. Henry offered the guidance that an inspection of a PRF should take 30 seconds – 1 minute and that the bat should remain lit for no more than 10 seconds.

In at #2 is the presence of droppings as these often remain when the bat has left. DNA analysis of droppings can give you a confident identification of the species for a very cheap price – around £45 from Warwick University – and therefore a dropping can be as good as a bat. Look for droppings in spiders webs (which often preserve dropping intact), on leaves, branches or any surface below the roost entrance. Sycamores are particularly good as their leaves are sticky!

The #1 indicator is substrate condition because bats modify a PRF in ways which no other species do. Smooth, bobbled, bumpy, waxy, blackened and polished surfaces inside the PRF indicate the presence of bats. These characters are best described with photographs to aid understanding – there are many example photographs and more information on these features in the Bat Tree Habitat Key – a free download.

The Warwickshire Barbastelle Project (Lois Browne)

Lois Browne gave an inspiring talk on the Warwickshire Barbastelle Project which has been running in Warwickshire for the last two years. This project studied the behaviour and habits of one of the rarest bat species in the UK through trapping and radio-tracking as well as traditional acoustic survey techniques. The study colony roosts in an ancient woodland site but forages further afield, leaving the woods and foraging in a valley to the south as well as around lakes and woodland fragments within 5km of the roost. Non-breeding bats were found to travel further to feed; up to 7km from the roost. Some bats were found to have very traditional routes which they stuck to each evening when they left the wood to reach their foraging grounds. Boxes erected as part of the project were used by the barbastelles, including use by maternity colonies. They found the Colin Morris design to be used preferentially. The project also carried out targeted work to enhance the local habitat for barbastelles, informed by the findings of their study. So far they have planted 800m of hedgerows and standards to improve connectivity, are developing three new wildflower meadows and have built a new pond.

The ecological effects of woodland management (Keith Kirby)

Oliver Rackham’s talk illuminated the way in which all of our woodlands were historically managed and that this human interaction has shaped their structure for centuries. Keith Kirby provided an overview of the ways in which management affects the ecological character of a woodland.

Planting of trees affects species composition which has a significant effect on the associated specialist species, lichens, the abundance of flora below the trees, the nature of leaf litter, deadwood and shade.

Management affects the age structure of the wood through selective felling and other management techniques such as coppicing. It also affects the spatial structure, by which parcels of the woodland are managed, and the vertical structure which affects the species composition which uses them.

Natural deadwood levels on the forest floor are around 100 cubic metres/ha whereas a worked coppice has <10 cubic metres/ha and minimal intervention areas typically have 30-50 cubic metres/ha. This significantly affects the invertebrate and saprophytic communities which are present.

New forest woodland

Management of herbivores also affects the woodland structure and character. Since we have effectively wiped out any large predators in the UK, humans are left as the only effective managers of species such as deer which have hugely complex impacts and interactions on a woodland.

Finally, all of these management decisions interact with other factors, such as climate change and nutrient levels, as well as each other. This whistle-stop tour through the ecological impacts of our management reflects the importance of taking the requirements of bats into account when considering how to maintain a woodland.

The principles of ancient woodland restoration (Jeremy Evans)

Planted Ancient Woodland Sites (PAWS) are current plantations – such as coniferous forestry – which occupy the footprints of ancient woodlands which are defined as those which have maintained continuous woodland cover since 1600. The Woodland Trust takes the view that these are ‘damaged but now written off’ and Jeremy Evans talked about the strategy which the Trust takes to restoring these sites. First they identify remnant ancient woodland features which might include ancient woodland flora, old pre-plantation deadwood, archaeological remains or mature oak coppices. The level and immanency of the threat which these features are under is then identified and actions prioritised to take the most at-risk features out of immediate danger. This might include releasing old coppices from out-shading by halo thinning, or selective thinning to bolster the woodland flora. Once these remnant features are secure, Phase 2 is the movement towards a semi-natural composition through thinning, small-scale selective fells and restoration with native trees. The activities are planned, observed and recorded throughout so that the success can be monitored and feed into future restorations.

The Countryside Stewardship Scheme, woodland management and bats (Mike Render and Carol Williams)

The main priority of the Countryside Stewardship Scheme will be to deliver the objectives of Biodiversity 2020 where the success or failures of different species will be the litmus test for success. The scheme will have a ‘two tier’ approach to delivery conservation benefits for species. All UK bat species will be included within the Mosaic Approach which aims to provide broad, habitat scale enhancements which would benefit a wide range of species. Grants will be available within the scheme for woodland creation and management as well as measures to address tree health issues and all of these interventions may serve to benefit bat species falling within the mosaic approach.

Species which are of higher conservation concern would also be catered for through the Bespoke approach – this would involve the development of prescriptions for habitat creation and management specifically designed to benefit these rarer species. The prescriptions will be developed by NE species specialists with input from the BCT whose roost data will be used to identify hotspots for site specific and landscape level interventions. Species falling within this Bespoke approach would include greater horseshoe, lesser horseshoe, grey long-eared and Bechstein’s bats – there are ongoing discussions as to whether barbastelle bats may also qualify for the Bespoke interventions.

Management risk: We must open our eyes and make significant changes to accepted and widely practiced forest management in the UK – diversification of tree species and genetics within species (John Weir)

Two thirds of woodland in the UK is broadleaf woodland and 70% of the canopy cover of these woodlands are made up of just five species. Oak represents 32% of our trees; ash and beech cover 14% each; sycamore is 11% and birch is 6%. Sweet chestnut, alder, hazel and willow are the next four species in line. This dominance of a small number of species means that diseases and pests could lead to dramatic impacts upon our woodlands – Dutch elm showed that an entire species can be all but wiped out by disease. One of the reasons why the English elm was so susceptible was that its ecology and past management meant that many trees in an area were genetically identical and so there was less scope for resistance than would be expected – this issue is relevant today with mass plantations of nursery stock trees, often imported from the continent leading to the globalisation of tree disease. The graphs showing incidences of pests and diseases in recent years is truly terrifying.

Increasing incidence of tree disease and pathogens - image from John Weir's powerpoint talk
Increasing incidence of tree disease and pathogens – image from John Weir’s powerpoint talk

Climate change is another issue which is destined to affect all aspects of ecology within the UK over the next few decades and woodland is no exception. John posited the assisted migration of native trees as a way to help woodlands to adapt to this change. This would involve the supplementing of trees of local provenance with trees originating from 2-5 degrees south which will have developed in similar climatic conditions to those which will prevail in the future. They will be better adapted to the warmer climate and the likely increases in droughts which will accompany it.

The susceptibility of our woodlands is something which we need to acknowledge and begin to address to minimise the risk and mitigate the impacts which will have huge knock-on effects to the species which depend upon them, including bats. The powerpoint of John’s talk is available here.

The use of woodlands by bats in anthropogenic landscapes – implications for policy and practice (Kirsty Park)

During the summer, female bats band together in maternity colonies to bring up their young. Previous studies have found that Daubenton’s bats segregate during the maternity season so that female bats access the better quality foraging habitats whilst the males occupy less optimal habitats. One of the studies conducted at Stirling University looked at the sex ratio of bats in urban woodlands – females were found in better quality habitats with mature trees, an open story, a more compact shape and better connections with other woodlands whilst males were found in a much wider range of woodlands. Kirsty’s talk also touched on the use of plantation woodlands by bats – these are generally considered to be less favoured but initial results of an ongoing study found a 30% greater number of echolocation calls recorded in plantation compared with broadleaf woodland sites – primarily soprano pipistrelles including lactating females. This indicates that our view of the relative value of woodland types for bats may need to be re-assessed with increased research.

Wood pasture and woodland bats – restoring the first without losing the second. Lessons from Croft Castle, Herefordshire. (David Bullock)

Oliver Rackham’s talk earlier in the day highlighted the prevalence of ancient trees outside of woodlands and subsequent talks expanded on this to identify the development of woodland around ancient trees as a threat which can lead to their decline and death through competition and shading. At Croft Castle, the National Trust are aiming to fell the coniferous plantation around retained ancient and veteran trees to restore the habitat to a wood pasture habitat with mature trees and grassland which supports many rare lower plant species and invertebrates. This conservation objective would serve a range of species but there would be impacts upon the existing assemblage of bat species – including rare species such as the lesser horseshoe and barbastelle – which favour the woodland habitat which is on site at present. The aim of the National Trust scheme is to retain habitat and linkages for the bat species, through the retention of large western hemlock corridors through the re-created parkland to ensure continuity of woodland habitat. The Trust will track the distribution and abundance of bats in 2015 and beyond in order to monitor and assess the success of this strategy.

Integrating bat conservation into multi-objective woodland management – a case study focussing on Swanton Novers Woods (Ash Murray)

Ash Murray presented a case study on woodland management for bats focussing on Swanton Novers Woods in Norfolk. Different management creates woodlands with varying benefits for bats – active coppicing provides good foraging habitat but poor roosts whilst neglected coppices are the opposite. High forest can provide both good foraging and good roosting habitat but this is not viable as cover across a managed, working woodland. The strategy taken to address this is managing the woodland in varying ways throughout the site including short, medium and long-term coppice, high forest, minimum intervention and Planted Ancient Woodland restoration. The Great Wood is managed on a commercial basis to support the conservation objectives of the wood (designated as a National Nature Reserve) and this multi-objective mosaic approach to management ensures continuity of resource for roosting and foraging bats as well as the vast number of other species which depend upon the woodland.

Social structure of bats in Wytham Woods

This final talk was a deviation from the published programme but was a fascinating way to end the day. Wytham Woods in Oxfordshire is owned by Oxford University and is said to be the most studied woodland in the UK. There are 1273 Schwegler bird boxes within the woodland which are monitored for the nesting bird species. But after the birds have finished nesting and vacate the boxes, the bats move in for the summer– 1051 roosts have been recorded representing 82% of the boxes within the woodland. Those not used tend to be those along edges with boxes in other habitats – ranging from ancient woodland, plantation and rides – being frequently occupied. From extensive surveys, the researchers identified that 13 checks were required to confirm a roost using a saturation curve assessment.

Natterer’s, Daubenton’s and brown long-eared bats are the three key species with a further five species also recorded using the boxes. The three key species have different social structures, as revealed by ringing surveys. The ~600 Natterer’s bats exist in seven different colonies within the woodland and both males and females very rarely mix with other colonies. The ~600 Daubenton’s bats have 5-6 female colonies which rarely mix, although the males move between the different colonies and sometimes form large bachelor roosts of up to 62 bats. The ~400 brown long-eared bats have 21 different colonies through the woodland and they don’t mix at all.

This complex social structure needs to be understood if the impacts of management are to be truly understood – the coppicing of one area of woodland may remove the core foraging habitat for an entire colony and this was found to happen during the monitoring and the colony had to move to a different area of the wood.

Common Pipistrelle

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