PDF《Review article》

Else, 2000), they will still need to be monitored. The time course for the declines of some British species can be better resolved from the specimen data compiled by the Bees Wasps and Ants Recording Society (BWARS), be- cause their regional ranges can be plotted for successive 20-year time periods (Fig. 2). Al- though the sampling e?ort is not uniform, this Bumblebee conservation

369 Figure 1. Maps of regional species richness for British bumblebees (excluding Psithyrus, which have a di?erent, parasitic ecology) among

50 *

50 km grid cells from the BDMS data (Alford, 1980) used to examine range reductions, for: (a) '

former'

richness (pre

1960 and

1960 onwards records combined);

(b) '

present'

richness (1960 onwards records);

(c) declines in richness (pre

1960 records only). Equal-frequency grey scale classes (left of each map) representing relative species richness are used to maximize di?erentiation among re- gions. Reproduced from Williams P.H. (2005).

370 P.H. Williams, J.L. Osborne Figure 2. Trends in regional range size within Britain by 20-year time slices for the more regionally- restricted (Fig. 1) bumblebee species as counts of occupied

50 *

50 km grid cells from the BWARS data (2008). is less of a problem when measuring regional ranges (Williams P.H., 2005). For B. humilis, B. muscorum, B. ruderarius, B. ruderatus and B. soroeensis, there is little evidence of decline in range extent. Their broad trend towards ap- parent gradual increase is likely to be due more to increasing numbers of recorders and increasing recorder mobility with cars, rather than to increasing bumblebee range sizes. Sim- ilarly, the apparent dip during the 1940C59 pe- riod for all plotted species is likely to be due to the second world war and the weakened economy in its immediate aftermath temporar- ily constraining recording e?ort. However, in........