Bombus terrestris is one of the most widespread commercially introduced bumblebees. It commonly escapes agricultural settings and establishes colonies in native habitats.
Studies show rapid declines in native pollinators in conjunction with the introduction of B. terrestris. Here, we report a long-term study of a native bumblebee, Bombus dahlbomii, and wild Apis mellifera before and after the introduction of B. terrestris on Chiloé Island, Chile. Our objective is to document population changes and ask if B. dahlbomii and A. mellifera populations can recover after contact with B. terrestris. We recorded visitation frequency of the native and introduced bumblebee and Apis mellifera visiting flowers of a common tree, Eucryphia cordifolia on Chiloé Island over 18 years (2001-2018). We observed (approx. 40,000 minutes of observation) floral visits on 16 Eucryphia cordifolia trees. Secondarily, we tracked abundance of B. terrestris and B. dahlbomii on 14 -17 Fuchsia magellanica shrubs, on mainland Chile and Chiloé Island over two years (2009 and 2018) to contrast mainland and island differences.
Results/Conclusions
In the long-term study, we found bimodal increases and decreases A. mellifera and B. dahlbomii populations before B. terrestris contact. After B. terrestris arrived in 2008, relative abundance of B. dahlbomii and A. mellifera decreased 98% and 39.7%, respectively. After that, both species are recovering, but B. dahlbomii is recovering more slowly than A. mellifera. In our second study, we found a low abundance of B. dahlbomii on the mainland in 2009 and 2018; N=4 and N= 10, respectively, and a high abundance of B. terrestris; N= 78 and N= 161, respectively. In contrast, we fond only two B. terrestris on Chiloé, while finding a high abundance of B. dalhbomii on F. magellanica in 2009 and 2018; N=65 and N=69, respectively. Overall, we find different responses of the effect of the invasion depending of plant species studied. We found a population recovery of B. dalhbomii and A. mellifera, however it is uncertain due to the continuous arrival of new populations of B. terrestris that may harbor pathogens that may spillover to these hymenoptera. Increases in B. terrestris on the island versus the mainland is likely slower because of the oceanic barrier. We discuss the unintended consequences of continued introductions of B. terrestris into Chile, management and legal measures taken by the scientific community.