Ecological mechanisms explaining interactions within plant-hummingbird networks: morphological matching increases towards lower latitudes

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.