Behavioral responses by a bumble bee to competition with a niche-constructing congener

While feeding, foragers can alter their environment. Such alteration
constitutes ecological niche construction (ENC) if it enables future
benefits for the constructor and conspecific individuals. The
environmental modification may also affect non-constructing, bystander
species, especially if they share resources with constructor species. If
so, ENC could confer the constructor species a competitive advantage by
both enhancing its foraging returns and reducing those of bystander
species. Expectations – (E1) ENC frequency should vary positively with the
recent and current density of the constructor species, and (E2)
constructors should use modifications disproportionately. In contrast,
bystanders should (E3) experience intensified competition for the affected
resource, and (E4) exhibit diverse, possibly mitigating, responses to ENC,
depending on opportunity and relative benefits. We investigated these
expectations in Argentina for competition for Fuchsia magellanica nectar
between an invasive bumble bee, Bombus terrestris (terr: putative
constructor), that often bites holes at the bases of floral tubes to rob
nectar, and native B. dahlbomii (dahl: bystander), which normally accesses
Fuchsia nectar through the flower mouth (front visits). Robbing holes
constitute ENC, as they persist until the 7-day flowers wilt. The dynamics
of the incidence of robbed flowers, abundance of both bees, and the number
and types of their flower visits (front or robbing) were characterised by
alternate-day surveys of plants during 2.5 months. After initially
accessing Fuchsia nectar via front visits, terr switched to robbing and
its abundance on Fuchsia increased 20-fold within 10 days (E2).
Correspondingly, the incidence of robbed flowers varied positively with
recent and past terr abundance (E1).  In contrast, dahl abundance
remained low and varied negatively with the incidence of robbed flowers
(E3). When terr ceased visiting Fuchsia, dahl abundance increased six-fold
within 10 days (E3), possibly because many dahl previously had avoided
competition with terr by feeding on other plant species (E4). While terr
was present, dahl on Fuchsia used front visits (tolerance) or used
existing robbing holes (adoption: E4). The diverse dahl responses suggest
partial compensation for competition with terr. ENC alters competitive
asymmetry, favouring constructor species. However, bystander responses can
partially offset this advantage, perhaps facilitating coexistence.