Case study: the invasion of European bumble bees introduced for crop pollination in southern South America
The southern tip of South America (Argentina and Chile) is inhabited by a single native bumble bee species, Bombus dahlbomii, whose key role in plant-pollinator webs and in the pollination of native plant species has been recognized. This region has been invaded by the European bumble bee B. ruderatusin 1993 (RoigAlsina and Aizen, 1996) and B. terrestris in 2006 (Torretta et al., 2006), following their introduction for crop pollination into Chile in 1982 and 1997, respectively.
Three independent studies have shown that both introduced bumble bee species have spread widely in the region, invading new habitats (Montalva et al., 2011; Morales et al., 2013; Schmid-Hempel et al., 2014). More specifically, a recent large-scale survey of bumble bee fauna across the eastern slopes of the southern Andes in Argentina revealed that B. terrestris was by far the most widespread and abundant species, one order of magnitude more abundant than B. dahlbomii and B. ruderatus. Meanwhile, B. dahlbomii had disappeared from a large part of its historical range (Morales et al., 2013). B. dahlbomii closely interacts with the native endemic plant “amancay” (Alstroemeria aurea), related to a variety of commercial hybrid lilies. A 20-year survey of pollinators of amancay in an old growth forest whose understory is dominated by this flowering plant revealed that first B. ruderatus, and later B. terrestris, replaced B. dahlbomii, formerly the most abundant pollinator (Morales et al., 2013).
What are the mechanisms underlying displacement of native bumble bees by invasive ones? In the case of B. ruderatus, mechanisms behind its initial, partial displacement of B. dahlbomii on the local level remain unknown, and the hypothesis of competition for resources has received little support (Aizen et al., 2011). In the case of B. terrestris, its wide range and long-lasting displacement of B. dahlbomii has been hypothesized to be the result of an interplay between competition for resources and pathogen spillover. B. terrestris is a highly generalist species, foraging on many types of flowers – even those classified as anemophilous or ornithophilous (see Chapter 1). Furthermore, its colonies are larger and they begin their activity earlier in the spring than do colonies of B. dahlbomii; this likely provides it with a competitive advantage. Recent studies provide evidence that populations of B. terrestris in southern South America carry Apicystis bombi, a highly pathogenic parasite new to this region (Plischuk and Lange, 2009) that seems to have been introduced along with it and transmitted in situ to B. dahlbomii and B. ruderatus (Arbetman et al., 2013). This pathogen also infects honey bees (Apis mellifera). Moreover, the fact that infected honey bees have been detected in a region of southern Argentina invaded with B. terrestris but not in regions free of this invasive bumble bee (Plischuk et al., 2011), and that infected B. terrestris, B. ruderatus and A. mellifera from this region share the same Apicystis haplotypes (Maharramov et al., 2013), supports the theory of a common origin of this pathogen in all three species, and suggests a probable spillover from B. terrestris to these species, though this remains to be confirmed.
The impacts of these invasions on plant pollinator interactions and plant pollination range from disruption of local plant-pollinator webs (Aizen et al., 2011) to reduced weight and quality of raspberries along a gradient of increasing B. terrestris invasion (Sáez et al., 2014) due to their overabundance.