Decline of a native Bombus population linked to recovery of feral Apis mellifera, drought effects on key food plant
A growing number of studies have documented bee declines around the world, heightening concerns about a potential crisis in pollination services. Yet much about the extent and causes of declines remains unknown, with one major source of uncertainty the scarcity of long term time series data for bees. Concerns have primarily focused on two key groups: the European honey bee, Apis mellifera, and bumble bees (Bombus spp.). For A. mellifera, the available data are mostly for managed hives, while little is known about feral populations. In the case of Bombus declines, most evidence of declines relies on comparisons of recent collections with historic museum specimens. I monitored densities of foraging Apis mellifera and native Bombus spp. in 11 patches of California coastal scrub over 15 years, starting in 1999 when feral A. mellifera populations were low due to the Varroa destructor mite; this period included both the appearance of colony collapse disorder (CCD) in North America and a major drought. I then used general linear mixed models to test for trends in feral A. mellifera and Bombus, as well as for links between weather variation, floral resource availability and the abundances of both bee groups.
Forager abundances of A. mellifera increased by more than five times over the study, except for a decline between 2006-2011 that coincided with the spread of CCD in North America. In contrast, native Bombus abundances at this site fell over the study period. Higher feral A. mellifera densities in the previous year were strongly associated with lower abundances for Bombus. Increases in A. mellifera correlated with shifts in plant use by Bombus that reduced niche overlap between the two, supporting the conclusion that resource competition with A. mellifera limited Bombus. Reductions in a key forage plant, Scrophularia californica, were also important in explaining Bombus declines, and this loss of floral resources significantly related to lower spring rainfall associated with drought. In contrast, A. mellifera densities were significantly higher in years with lower spring rainfall, and Eriophyllum staechadifolium, the plant most visited by A. mellifera, was better buffered against dry years than species preferred by Bombus. These results suggest that some feral A. mellifera populations are healthier than previously thought, consistent with hypotheses that they may be more resistant than managed colonies. However, A. mellifera recovery appears to be interacting with climate variation to drive declines of Bombus in the community.