This has been the topic of discussion between many UK based ecologists, home owners and developers. Perhaps a project was delayed and requires updated ecology surveys, however, for various reason you suspect that a previously identified bat roost is abandoned. You ask your ecologist: ‘how long can a bat roost be disused before it is considered abandoned?’ A client recently asked me this question; and I have had previous projects centred around this same question. For various reasons, it can be a difficult one to answer. Below, we will go through several examples in order to help you understand the question. If you need a quick answer, then scroll to The final word.
First, lets answer the basic premise of the question; bats can indeed abandon their roosts. Our issue with the above question is how we can prove that the roost is abandoned, to the Council or Natural England.
What the guidance says
Unfortunately, there is no formal guidance on the above question, either from good practice guidelines or from Natural England. That’s useful! Actually, when you look deeper into the subject, the reason becomes clear.
The reason there is no formal guidance or answer to the question, is simply, because there really is no biological answer to the question; bats can roost in a suitable structure at any time. Therefore, any time scale attributed to this question would be arbitrary and would make assumptions. Assumptions are the enemy of evidence! It would assume, for example, that the roost has been continually monitored for an extensive time period (only special roosts, usually as part of scientific experiments get such treatment). Only with continual monitoring of a roost would you be able to confidently prove that the roost is abandoned to the relevant governmental bodies.
In summary, it is all about evidence of bats — or a lack there of. A council, or Natural England will base planning decisions on up to date survey data, which they deem to contain sufficient evidence to prove presence (or likely absence) of bats. As a rough guide, bat surveys usually go out of date after one year.
But what if there is no new evidence of bats?
If there is existing evidence of bats, for example an old pile of bat droppings, it can still be difficult to prove that they are old. Droppings can have an ‘old’ appearance and texture – i.e. drier and fluffier – however rates of decomposition will differ for a range of environmental reasons. As a general rule, if bat droppings are present in a structure, and the bat surveys are out of date then further surveys will most likely be required.
What if the structure has radically changed since the first bat survey?
Another scenario may be a change in use of the structure, or a structural change. These can be black and white in terms of bat mitigation. I once had a site where the building roof burnt down; needless to say, we did not recommend further bat surveys!
More subtle change of usage can also cause bats to abandon their roosts, for example a change of lighting or a structural collapse. This may prompt a client to ask an ecologist if further bat surveys or a bat license from Natural England is required for the works. The answer will usually be an updated Preliminary Roost Assessment will be required. Even if bats abandon a particular roost for some reason, it doesn’t automatically mean that the space is unsuitable.
The final word…..
Earlier in the article I wrote that there is no formal guidance on the question. However, Natural England have issued some informal guidance to myself and my colleagues. The somewhat generic advice can be paraphrased as: It is down to the ecologist to determine whether or not they think the roost is abandoned.
So, a previously identified roost will most likely need an updated Potential Roost Assessment as a bare minimum. Then after that, it is up to the project ecologist to decide the best course of action, always depending upon the evidence found. Lets look at some made up scenarios and outcomes with the above in mind.
Scenario 1: A bat roost was identified in a barn 3 years ago, with a small pile of pipistrelle droppings found. However, the barn roof has since collapsed. Will I need a licence, or more surveys to get planning to convert it?
Potential Outcome: An updated Preliminary Roost Assessment finds no new droppings, but low potential for bats within crevices between wooden beams – undertake emergence/re-entry survey – if no bats found emerging – explain in the report that building is most likely no longer used by bats.
License needed? Unlikely
Scenario 2: A care home had a brown long-eared maternity roost identified in the roof, in a survey 2 years ago. Updated surveys found no evidence of bats currently using the home, however bat droppings and bat access is still present. The care home has since been abandoned.
Potential Outcome: An updated Preliminary Roost Assessment finds dropping in broadly the same area as before, and all are confirmed to be brown long-eared. The droppings seem old, and their locations match the photos of the old surveys. Three updated bat surveys are undertaken between May and September as slipped tiles provide access into the roof, remote detector surveys are also implemented. No signs of bats using the roost were found, so a hibernation bat survey was also undertaken.
License needed? Possibly. This is one where you want your project ecologist to call Natural England and chat to their licensing team, it could go either way in terms of requiring a mitigation license.
Scenario 3: An outhouse was found to have a common pipistrelle roosting within an external wall cavity, the roost was described as a day roost. It has been over 1 year and the planning application is rejected as it required an updated bat survey.
Potential Outcome: Three updated bat emergence and re-entry surveys are undertaken and no bats are recorded.
License needed? Probably. pipistrelles can have many day roosts and switch between them often. Even with no recordings after three surveys (remember, that’s just 3 days out of months) this is not enough evidence to say the roost is unused. The easiest option is to go straight to a low-impact license; these are relatively painless and inexpensive these days.
Ecologists are never trying to be difficult – we are often forced to be vague as we must walk a tight rope between client expectations, unpredictable survey results, the law and how we expect the council or Natural England to react to the evidence. The bottom of the bottom line is, bats don’t read the textbooks. Thanks for reading, especially if you got this far!