DOI: 10.5061/dryad.dncjsxkw2
title: Ecological mechanisms explaining interactions within plant-hummingbird networks: morphological matching increases towards lower latitudes

description: Interactions between species are influenced by different ecological
 mechanisms, such as morphological matching, phenological overlap, and
 species abundances. How these mechanisms explain interaction frequencies
 across environmental gradients remains poorly understood. Consequently, we
 also know little about the mechanisms that drive the geographical patterns
 in network structure, such as complementary specialization and modularity.
 Here, we use data on morphologies, phenologies and abundances to explain
 interaction frequencies between hummingbirds and plants at a large
 geographic scale. For 24 quantitative networks sampled throughout the
 Americas, we found that the tendency of species to interact with
 morphologically matching partners contributed to specialized and modular
 network structures. Morphological matching best explained interaction
 frequencies in networks found closer to the equator and in areas with low
 temperature seasonality. When comparing the three ecological mechanisms
 within networks, we found that both morphological matching and
 phenological overlap generally outperformed abundances in the explanation
 of interaction frequencies. Together, these findings provide insights into
 the ecological mechanisms that underlie geographical patterns in resource
 specialization. Notably, our results highlight morphological constraints
 on interactions as a potential explanation for increasing resource
 specialization towards lower latitudes.
description: Data consisting of 24
 quantitative interaction networks sampled throughout the Americas, in
 areas mostly or entirely covered with native vegetation (Table ESM2). The
 networks comprise 106 hummingbird species, 31% of all described
 hummingbird species in the world according to the IOC World Bird List
 v.7.3 (ESM3), and 450 plant species belonging to 57 plant families (ESM4,
 see ESM5a for additional details on sampling). The abundance of plant species
 was measured as the number of flowers produced per species in each
 community throughout the entire sampling period. Flowers were counted in
 plots or transects estimated regularly throughout the sampling period. The
 abundance of hummingbirds within sites was measured in the field by
 counting the number of visual and aural detections of individuals across
 transects (n=12 networks) or point counts (n=4 networks), or the number of
 individuals captured by mist-netting (n=8 networks; ESM5a). Because the
 abundance sampling protocols were not standardized among networks, we
 treated the data as relative abundance, i.e. for all species we calculate
 their abundance as the proportion of the total number of individuals
 within a given community. Still, we note that mist nets may be especially
 efficient for surveying elusive understory species, such as traplining
 hummingbirds, whereas transects and point counts may be better at
 surveying species at higher vegetation strata [34]. We recognize that the
 caveat inherent in using different sampling schemes across networks may
 influence the outcome of our analyses. However, as we used relative
 abundances to model interaction frequencies within networks (not between
 networks), we believe that the different sampling schemes had a minimal
 influence on our results. The phenology of each plant and
 hummingbird species in each network was determined as the presence-absence
 of, respectively, open flowers and individuals at each sampling period
 (usually months). Flower morphology was characterized by the effective
 corolla length, measured as the distance from the nectary to the corolla
 opening. The effective corolla length reflects the minimum length of
 mouthparts required for pollinators to access the nectar legitimately.
 Hummingbird bill morphology was measured as the length of the exposed
 culmen from captured hummingbird individuals (see ESM5b further details on
 sampling).