Bipartite interaction-site network
We constructed a bipartite interaction-site network by adapting
the species-habitat networks approach proposed by Marini et al.(2019). Specifically, we produced an interaction-site network
from the insect and pollen data using two steps. First, we listed
pollen-insect pairs based on the pollen grains collected from individual
insects. Each of these distinct pollen-insect pairs was assigned a
unique identifier, i.e. Syrphidae-Poaceae (syrphid flies carrying
Poaceae pollen) had a separate identifier to Syrphidae-Myrtaceae
(syrphid flies carrying Myrtaceae pollen). These unique identifiers were
upper level nodes in the network matrix. Second, we linked each unique
pollen-insect pair to the lower level nodes, which were the sites
corresponding to the collection of each pollen-insect pair. Thus, values
in cells of this weighted network were the frequency (based on the sum
number of pollen grains for each pollen taxa counted on each insect
taxa) that each plant-pollinator interaction occurred at each site.
To identify how land-use intensity alters plant-pollinator interactions
and to identify the most important sites for conserving interactions, we
calculated several metrics from our interaction-site network. First, we
computed the richness of plant-pollinator interactions at each site
(typically referred to as species degree in traditional bipartite
networks, Dormann et al. 2009) by summing the number of unique
pollen-insect pairs recorded at each site. Second, to identify the
importance of each land-use for maintaining unique plant-pollinator
interactions, we quantified the number of pollen-insect pairs that only
occurred at each site. Thus, interaction uniqueness contributes to the
importance of each site for maintaining interaction richness across our
study region. Third, we calculated the strength of interaction-site
dependencies, which is a qualitative extension of species degree
(sensu Bascompte, Jordano, & Olesen 2006), using the
“strength” function in the bipartite package (Dormann et
al. 2019). In traditional bipartite plant-pollinator networks, strength
is defined as the sum of dependencies of pollinators on their plant
partners or vice versa. However, in our interaction-site network, a
site’s strength value is the sum of dependencies of pollen-insect pairs
on that site. Finally, to determine if frequently occurring
pollen-insect interactions (those that occurred ≥ 20 times in our
network) were specialised to particular land-uses, we calculated the
Paired Difference Index (PDI) (Poisot et al. 2011) with the
“PDI” function in the bipartite package (Dormann et al.2019). PDI values are constrained between zero and one, wherein a value
of one represents a perfect specialist and zero represents a perfect
generalist. Specifically, the PDI analysis allowed us to determine if a
given plant-pollinator interaction occurred in the land-use with which
it was most strongly linked more frequently than other land-uses in the
landscape.