Roos, S., Johnston, A. & Noble, D.
BTO Research Report 598
In order to assess the feasibility of generating national and regional population trends for hedgehogs, we obtained data from seven national surveys. Following extensive data clean-up and handling, we carried out analyses to estimate trends in hedgehogs (in practice – occupancy rates) and determine the statistical power of each survey to detect population declines between 10% and 50% over periods of 10 and 25 years, the latter being a commonly-used time period for assessing population status for red-listing species.
Five surveys were conducted using a sufficiently repeatable sampling protocol over enough years to generate a national (UK) population trend. Additionally, for two of these surveys, we were able to carry out additional analyses with another measure of hedgehog presence or abundance derived from the same survey. Of the seven measures assessed, five measures of hedgehog presence or abundance (from four surveys) showed a significant decline over the period of sampling, which varied from four to 14 years between 1996 and 2010. The other two measures declined but not significantly so. Hence, surveys of the wider countryside as well as for those focused on gardens and other human-dominated areas, showed evidence of declines.
Two additional surveys also targeted mainly at gardens and human dwellings (RSPB’s Making Your Nature Count in 2009 and 2010, and PTES/BHPS’s Hogwatch, conducted largely between 2005 and 2007) although involving a large number of volunteers, do not currently employ a sufficiently repeatable sampling design and/or protocol to reliably assess change in the same way. Tests for differences between years suggested very small increases in occupancy rates of hedgehogs, but the participant-driven sampling protocols may have resulted in slight bias towards increases.
We carried out power analyses for the seven measures (from five surveys) for which there were repeat visits to sites. All surveys had sufficient power (78 or greater) to detect red-level declines over 25 years (equivalent to red-listing for birds) or ten years but the WBBS and BBS records of dead hedgehogs only, were insufficient for reliably detecting 25% declines over 10 or 25 years. Two surveys (BTO Garden BirdWatch, and Mammals on Roads) had >80% power to detect changes of 10% over 10 or 25 years, whereas Living with Mammals and BBS (all records) had slightly less power (70% to 80%) to detect population changes of 10%.
Power analyses were undertaken to determine the annual sample of Living with Mammals or Mammals on Roads sites required to detect modest levels of change (5% or 10%) over shorter time periods (five and ten years) with a power of at least 80%.The Living with Mammals results suggest that 300 sites (well within current scope) would have sufficient power to detect a 10% decline as long as these were resurveys of the same sites each year. To achieve this power if different sites were surveyed each year, almost five times the sample would be required, i.e. in excess of 1300 sites and considerably more than have been monitored in the last few years. The Mammals on Roads results suggest that 200-250 sites would provide enough power to detect a national decline of 10% over 5-10 years, if the same routes are revisited. More than 800 routes would need to be surveyed if the routes differed each year.