The data from the ten traps in each transect were pooled to
generate activity-densities (number of individuals captured per
ten traps per sampling event, AD) and yearly means were calculated
using these batches for each species. Means were calculated
in two different ways; (a) including all sampling events, counting
as zeroes when the traps were operating but the particular species
was not present (all mean, AM) and (b) including only sampling
events between the first and last date when the species was present
(limited mean, LM). AM is more useful and the most commonly
used measure in ecological studies when investigating the changes
in abundances of populations from a general aspect. However, with
AM being sensitive to the length of activity period, it would not be
possible, using this measure alone, to determine whether changes
were caused solely by a shortened activity period or by a general
decrease in AD over the entire sampling period. LM, on the other
hand, can detect changes in AD even when the length of the activity
period is changing.