The influence of biogeographical and evolutionary histories on morphological trait-matching and resource specialization in mutualistic hummingbird-plant networks

Functional traits can determine pairwise species interactions, such as
those between plants and pollinators. However, the effects of biogeography
and evolutionary history on trait-matching and trait-mediated resource
specialization remain poorly understood. We compiled a database of 93
mutualistic hummingbird-plant networks (including 181 hummingbird and
1,256 plant species), complemented by morphological measures of
hummingbird bill and floral corolla length. We divided the hummingbirds
into their principal clades and used knowledge on hummingbird biogeography
to divide the networks into four biogeographical regions: Lowland South
America, Andes, North & Central America, and the Caribbean
islands. We then tested: (i) whether hummingbird clades and
biogeographical regions differ in hummingbird bill length, corolla length
of visited flowers and resource specialization, and (ii) whether
hummingbirds’ bill length correlates with the corolla length of their food
plants and with their level of resource specialization. Hummingbird clades
dominated by long-billed species generally visited longer flowers and were
the most exclusive in their resource use. Bill and corolla length and the
degree of resource specialization were similar across mainland regions,
but the Caribbean islands had shorter flowers and hummingbirds with more
generalized interaction niches. Bill and corolla length correlated in all
regions and most clades, i.e. trait-matching was a recurrent phenomenon in
hummingbird-plant associations. In contrast, bill length did not generally
mediate resource specialization, as bill length was only weakly correlated
with resource specialization within one hummingbird clade (Brilliants) and
in the regions of Lowland South America and the Andes in which plants and
hummingbirds have a long co-evolutionary history. Supplementary analyses
including bill curvature confirmed that bill morphology (length and
curvature) does not in general predict resource specialization. These
results demonstrate how biogeographical and evolutionary histories can
modulate the effects of functional traits on species interactions, and
that traits better predict functional groups of interaction partners (i.e.
trait-matching) than resource specialization. These findings reveal that
functional traits have great potential, but also key limitations, as a
tool for developing more mechanistic approaches in community ecology.