Research conducted using the pollen loads found on bees in museum collections shows that the loss of flowers could be one of the main reasons for the decline in wild bee species. Although this has been suspected for some time, there was no evidence for it until now; this evidence has now been found using museum collections.
Photo: Andrena chrysosceles
The decline in wild bee populations is often attributed to biodiversity loss: fewer flowering plants means that there is less food for the bees to find. Alterra researcher Jeroen Scheper and colleagues from Wageningen, Leiden, Nijmegen and other universities investigated the pollen loads found on bees in museum collections. Based on this, they were able to determine which plants were used by the ‘historical’ bees (from before 1950) as host plants. They investigated 57 different bee species.
“Our research showed a clear relationship between the prevalence of bee species and their host plants,” said Jeroen Scheper about the research that has just been published in the scientific journal PNAS. “The population trend of host plants determined to a large extent the population trend of the corresponding bee species. The preferred host plants of the declining bee species also declined over time, while the preferred host plants of the species that are increasing also increased. Bee species that use crop plant families are doing relatively well, while species that mainly make use of wild plants are in decline. This fits the trend that we found exactly.”
Bee species that are active early in the season do relatively better than species that become active later in the year. This is probably because the floral resources in agricultural areas in the Netherlands show less decline in the spring compared with later on in the season, from July onwards. The body size of the bees also seems to play a role. Jeroen Scheper: “Larger species show more decline than smaller species. This is probably because they require more food and therefore need more flowers.”The decline in the number of bees will not be halted by sowing standard flower mixes, currently often applied. “This only helps a few Dutch bee species.” According to the authors, specific measures need to be taken to promote the abundance of the preferred host plants of the different species found in the Netherlands.