Museum specimens reveal loss of pollen host plants as key factor driving wild bee decline in The Netherlands

Abstract

Evidence for declining populations of both wild and managed bees has raised concern about a potential global pollination crisis. Strategies to mitigate bee loss generally aim to enhance floral resources. However, we do not really know whether loss of preferred floral resources is the key driver of bee decline because accurate assessment of host plant preferences is difficult, particularly for species that have become rare. Here we examine whether population trends of wild bees in The Netherlands can be explained by trends in host plants, and how this relates to other factors such as climate change. We determined host plant preference of bee species using pollen loads on specimens in entomological collections that were collected before the onset of their decline, and used atlas data to quantify population trends of bee species and their host plants. We show that decline of preferred host plant species was one of two main factors associated with bee decline. Bee body size, the other main factor, was negatively related to population trend, which, because larger bee species have larger pollen requirements than smaller species, may also point toward food limitation as a key factor driving wild bee loss. Diet breadth and other potential factors such as length of flight period or climate change sensitivity were not important in explaining twentieth century bee population trends. These results highlight the species-specific nature of wild bee decline and indicate that mitigation strategies will only be effective if they target the specific host plants of declining species.

Keywords: bee decline; crop pollination; floral resources; land use change; pollen preference.

Fig. 1.

Relationship between bee population trends and body size, change index of pollen host plants, and initial rarity of bee species. Partial regression plots based on the best model in the candidate set for (A) log-transformed weighted mean change index of pollen host plants in bee species’ pollen diets, (B) body size measured as the intertegular distance (ITD), and (C) log-transformed number of occupied 5 × 5 km grid cells before 1950. Note that a bee change index of 1 indicates no change. Plotted points represent partial residuals. Shaded areas indicate 95% confidence bands.

Fig. 2.

Seasonal pattern of pollen host plant change and floral resource availability. (A) Relationship between the beginning of the flight period of bee species (number of days after 31 December) and the log-transformed weighted mean change index of pollen host plants in their pollen diets. (B) Mean estimated spring and summer floral resource availability (square root transformed flower cover) in 16 agricultural landscapes (1 km radius) in The Netherlands in 2012. Error bars represent SEM adjusted for paired observations.

Fig. 3.

Weighted mean population trends of bee species visiting the most frequently used host plant families. Mean population trends, weighted by the percentage pollen contribution of the host plant families to bee species’ diets, are based on bee species that relied for at least 10% of their pollen diet on a particular plant family. Results are shown for plant families that made up at least 10% of the pollen diets of at least 10 bee species. Error bars indicate weighted 95% confidence intervals. Numbers above bars indicate sample size.