Replication of a historical survey of bumblebee assemblages in subalpine areas in Norway

A comprehensive inventory of Scandinavian bumblebees carried out
between 1957 and 1961 (Løken 1973)
served as a basis for historical data of bumblebee communities. We selected 18
of these sites to be re-surveyed in 2012. We selected sites so that
they covered a large latitudinal and longitudinal gradient (Lat.:
60.42°–62.62°, ca. 240 km; long.: 7.24°–10.61°, ca. 180 km), and to form 9
pairs of sites where each pair consisted of one site at a high altitude –
approximately 1000 m.a.s.l. (min. = 730 m, max. = 1000 m) and one site at a low
altitude – approximately 500 m.a.s.l. (min. = 440 m, max. = 765 m) in each pair. The maximum distance
between two sites in a pair was 44.4 km, and the minimum distance was 2.6 km
(median = 12.3 km). The high-altitude sites were located within the upper part
of the subalpine forest zone, close to the forest line which is around 1000
m.a.s.l. in this region.
A further criterion for selection was the presence of the plant Aconitum septentrionale, which was the
subject of a parallel study and occurs mainly in nutrient-rich, wet meadows and
sub-alpine forests with high soil pH. However, since we surveyed bumblebees
within a large area with a 1 km radius, this specific criterion is
unlikely to have any larger impact on the bumblebee species composition. We
identified the locations and checked the suitability of sites to be re-surveyed
in the field in July 2011.

In some cases, it
was difficult to identify the exact location of the site for the historical
records, which typically consisted of the name of a farm or village, rather
than any exact coordinates. The identification of sites was however facilitated
by the fact that the altitude was often stated. To account for this
uncertainty, we took a conservative approach, and surveyed bumblebees in
multiple flower-rich habitats within a circular area with a radius of 1 km around
the most likely location for the historical records. This means that we are
more likely to over-estimate than under-estimate current bumblebee species
richness. Prior to the field visits, potentially flower-rich habitats were
identified from aerial photos and by traveling along roads within the 1 km
circle by car. In the field, two experienced bumblebee experts visited as many
different flower-rich habitats within the 1 km circle as possible during 2
hours and recorded the abundance of all bumblebee species encountered. Most
species could be identified in the field, but in a few cases bumblebee
specimens were collected and identified later in the laboratory. Each site was
visited twice between July 6 and July 27 2012. To allow comparison with historical
data, we discarded abundances in further analyses and used species occurrences
only.

Reference cited:
Løken, A., 1973. Studies on Scandinavian bumble bees (Hymentoptera, Apidae). Norsk Entomologisk Tidsskrift 20, 1-218.