Drastic historic shifts in bumble-bee community composition in Sweden

Abstract

The species richness of flower-visiting insects has declined in past decades, raising concerns that the ecosystem service they provide by pollinating crops and wild plants is threatened. The relative commonness of different species with shared ecological traits can play a pervasive role in determining ecosystem functioning, but information on changes in abundances of pollinators over time is lacking. We gathered data on relative abundances of bumble-bee species in Swedish red clover fields during three periods in the last 70 years (1940s, 1960s and present), and on clover seed yields since 1921. We found drastic decreases in bumble-bee community evenness, with potential consequences for level and stability of red clover seed yield. The relative abundances of two short-tongued bumble-bees have increased from 40 per cent in the 1940s to entirely dominate present communities with 89 per cent. Average seed yield declined in recent years and variation in yield doubled, suggesting that the current dependence on few species for pollination has been especially detrimental to stability in seed yield. Our results suggest a need to develop management schemes that promote not only species-rich but also more evenly composed communities of service-providing organisms.

Figure 1.

Map of visited sites and detected proportional shifts in bumble-bee community composition in red clover seed fields in the last 70 years. Blue circles, all three periods; green circles, 1940s and present; blue squares, 1940s; yellow squares, 1960s; green squares, present. Proportion of bumble-bee abundance for the different species is presented as cumulative proportions for the communities averaged among sites and years within each period. Averages and standard error variation for the data on which the figure is based are presented in electronic supplementary material, table S1.

Figure 2.

Changes between periods in bumble-bee (a) observed species richness and (b) community evenness measured with the Simpson's index. The box-plots show average (point) and median (horizontal line in the box) value per clover field per year in three time periods in the last 70 years. The bottom and top of the box are the first and third quartiles. The whiskers show either the maximum value, or 1.5 times the difference between the first and third quartiles, corresponding roughly to two standard deviations. Observations outside the range of the whiskers are plotted individually.

Figure 3.

Trends in red clover seed yields in the last 90 years. (a) Yearly statistic of yield per hectare. (b) Variability in yield measured presented as the coefficient of variation calculated from 5 year moving average (with minimum four values).

Figure 4.

Relative frequencies of bumble-bee tongue lengths in red clover fields across Sweden in the (a) 1940s, (b) 1960s and (c) present. Ten thousand individuals were distributed among species according to national average relative abundance in each time period. Tongue lengths were then drawn from a distribution based on available mean and standard deviation in tongue length for each species, such that cumulative proportion in the community within each 0.20 mm tongue length interval was obtained. Tongue length estimates were obtained from Goulson et al. [36] for B. subterraneus and from Goulson et al. [38] for all other species.