Nature Blog Network

absence of bats or absence of surveyors?

My Daubenton’s bat waterway survey from last summer has been incorporated into the Bat Conservation Trust database for 2014. My data is the blob just above Manchester. Interestingly there seems to be another surveyor in the Blackburn area and another somewhere around Bury.

Of the BCT’s nine official bat surveys the Daubenton’s waterways surgery was the only one to return counts in this area. Now that is no surprise for the greater and lesser Horseshoe bat surveys as the results suggest that these are localised to SW England and SW England + Wales respectively. Is that because that is where they are or because that is where people looked for them? The BCT does report negative results and none are recorded for these two species but they may well be geographically localised.

The roost surveys for pipistrelles and soprano pips shows no roosts in the area. Mmm, there are plenty of pips about so they must roost somewhere. This suggests that their absence on the distribution maps is under reporting rather than lack of specimens.

It is a reminder to bear this in mind when looking at distribution records for any species really. Positive results are useful, corroborated ones more so (were some of my Daubenton’s really pips?). Negative results are useful as evidence of absence. But absence of evidence isn’t evidence of absence.