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Robustness of monitoring

The robustness of the data obtained from the different survey methods used in the NBMP will vary depending on how the data are collected. In general, random samples and surveys are considered to be most robust and best representative of the total population compared to surveys using self-selected samples.

Overall, the Field and Waterway Surveys are considered to provide the most robust trend information, followed by the Hibernation Survey and then the Roost Counts.

Field and Waterway Surveys

Field Survey volunteers are assigned a random-stratified 1km OS grid square to survey. Field Survey sites are straified by ITE landclass to ensure a representative sample of UK land classes. Waterway Survey sites are randomly selected from the network of River Habitat Survey sites. The random selection process includes sites where the species of interest may not occur at present but has the potential to do so in the future. This provides a means of assessing change in distribution as a result of population expansion as well as change in relative abundance.

Potential challenges to address include volunteers using different types of equipment and changes in prevalence of different models of equipment over time, volunteer turnover and volunteer experience. These are included as covariates in the data analyses to assess potential influence on the results.

Hibernation Survey

For the Hibernation Survey volunteers select a known or potential hibernation site to survey. This includes sites that are suitable for hibernating bats but have not previously found to be occupied. This survey can therefore detect instances of bats moving into sites where they have not previously been recorded. A potential issue with counts of bats carried out in hibernation sites is the relationship between the numbers of bats observed and the actual numbers of bats present. Bats can hide in cracks and crevices and there is evidence that large numbers of bats can be present even when few are actually observed. However, if the proportion of bats seen/not seen remains constant over time at any given site, population trend conclusions will remain valid.

Roost Count

Roost Count sites are known roosts that are self-selected by surveyors. Roost Counts are most effective for monitoring population change when a high proportion of existing roosts are counted and when the species tends to be faithful to their roost site.

Many bat species will move between roost sites, either individually, in groups or as an entire roost. Bats may abandon a roost temporarily for several weeks, months or years before reoccupying it, or they may abandon it permanently. This is known as 'roost switching'. Roost switching may negatively bias Roost Count trends if monitoring ceases before a roost is reoccupied or if the roost is permanently abandoned, as this results in a zero count being the final value entered into trend analysis for that roost and therefore a negative site trend being included in trend analysis. It is also possible for a roost to 'split' into two or more smaller roosts if it outgrows it's original site, which will negatively bias the trend if bats occupying the new site(s) are not included in the count.

Species which switch or split roosts more frequently, in greater numbers or for longer periods of time are likely to have more biased Roost Count trends. Common and soprano pipistrelle show a high degree of roost switching and therefore Roost Count trends for these species should be treated with caution. However species that are highly faithful to their roost sites, such as greater and lesser horseshoe bat, and likely to have less biased trends. For these species Roost Count trends appear robust and are supported by evidence from other surveys.


Continue to Population trend analysis

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