Pollinator declines: reconciling scales and implications for ecosystem services

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There is widespread concern regarding the fate of pollinators and the ecosystem services they deliver¹. However, the information we have is still limited and at times appears contradictory. Four recent articles, three from Science and one from PNAS, highlight this point^2–5. Burkle et al.² show that 50% of the bee species in one locality in the Midwestern USA became locally extinct during the last century, which in combination with recent evidence that wild pollinators are critical to global crop pollination³, has led some to conclude that we might face an imminent collapse of crop pollination⁴. In contrast, Bartomeus et al.⁵ explored bee declines over a similar time scale but at a regional scale (the northeastern USA) and reported only a 15%, non-significant decline in bee species richness. Here we present new analyses that help to reconcile this apparent contradiction in the magnitude of bee declines, while also suggesting that any effects on crop pollination might be less than previously thought.

First, we used the 67 bee species included in both the regional-scale⁵ and the local-scale² analyses (see data file below) to show that the two studies in fact found broadly consistent results: the locally extinct species of Burkle² tend to be declining regionally, whereas the locally persistent species tend to be increasing regionally (Figure 1A, ANOVA: F = 5.89, df = 1,65, P = 0.01). Second, we used data from Garibaldi et al.³ on the bee species that provide ecosystem services to four crops in the region covered by Bartomeus et al.⁵ to show that these ecosystem service providers tend to have increasing population trends compared to non-ecosystem service providers (Figure 1B, F = 7.12, df = 2,184, P = 0.001). All analyses were conducted in R⁶.

Figure 1. Trend in bee species' relative abundance in northeastern North American calculated over the period 1870–2011.

A) For species that either became locally extinct or persisted in Carleville, Illinois. B) For species that either are not ecosystem-service providers to crops (non-ESP), are at least occasionally ecosystem-service providers to crops (ESP), or are among the species cumulatively responsible for 90% of the pollinator visitation to at least one crop (main ESP). Regional data from Bartomeus et al.⁵, local data from Burkle et al.² and crop pollinator data from Garibaldi et al.³.

Thus, our analyses demonstrate that, as one would expect, local-scale extinctions do not imply regional-scale extinctions; and that bee species that are important crop pollinators are less likely to be declining at the regional scale. It is important to remember that all bee species may well be crucial to providing ecosystem functions in natural systems and therefore merit conservation attention.