Global decline of bumblebees is phylogenetically structured and inversely related to species range size and pathogen incidence

Abstract

Conservation biology can profit greatly from incorporating a phylogenetic perspective into analyses of patterns and drivers of species extinction risk. We applied such an approach to analyse patterns of bumblebee (Bombus) decline. We assembled a database representing approximately 43% of the circa 260 globally known species, which included species extinction risk assessments following the International Union fo Conservation of Nature Red List categories and criteria, and information on species traits presumably associated with bumblebee decline. We quantified the strength of phylogenetic signal in decline, range size, tongue length and parasite presence. Overall, about one-third of the assessed bumblebees are declining and declining species are not randomly distributed across the Bombus phylogeny. Susceptible species were over-represented in the subgenus Thoracobombus (approx. 64%) and under-represented in the subgenus Pyrobombus (approx. 6%). Phylogenetic logistic regressions revealed that species with small geographical ranges and those in which none of three internal parasites were reported (i.e. Crithidia bombi, Nosema spp. or Locustacarus buchneri) were particularly vulnerable. Bumblebee evolutionary history will be deeply eroded if most species from threatened clades, particularly those stemming from basal nodes, become finally extinct. The habitat of species with restricted distribution should be protected and the importance of pathogen tolerance/resistance as mechanisms to deal with pathogens needs urgent research.

Keywords: Bombus; Crithidia bombi; IUCN Red List; Locustacarus buchneri; Nosema spp.; pollinator decline.

Figure 1.

Phylogenetic relationships among Bombus species and decline status (binary-coded) for each species. The topology and divergence times were extracted from Cameron et al. [32] and Hines [33]. Declining and non-declining species are represented with closed and open symbols, respectively. Subgenera are indicated with different colours.

Figure 2.

Proportion of species declining in each subgenus. Error bars represent the standard error of the proportion. The dashed vertical line represents the mean proportion of species declining for the whole genus (0.34). The number of species analysed is indicated between parentheses. The subgenus Orientalibombus was not included because of a complete lack of information on the conservation status for any of its species.