Network Indices and Diversity Follow Distinct Patterns for Bee-Plant Networks Along a Neotropical Elevational Gradient

A critical goal of ecology is to quantify mutualistic network structure across all ecosystems and organisms, as these networks often represent essential ecosystem services and therefore network metrics are often used as conservation targets. Some major gaps still exist, one of which is Neotropical bees, a group expected to contribute exceptional insight to ecological network theory, owing to the extreme diversity of mutualistic interactions characteristic of the Neotropics, as well as the unique global distribution patterns of bees. Elevational gradients are a useful tool for quantifying change in network structure, with results having direct implications for predicting resilience in ecosystem service provisioning. In this study, we compared mutualistic network properties of 28 Neotropical bee-plant networks, along an elevational gradient from 200 to 1500 m elevation. Bee species richness peaked at mid-elevation while network specialization and modularity decreased with increasing elevation. Connectance and nestedness showed a statistically significant increase by elevation. Combining indicator analysis and the number of bee species collected from only one elevational band, the majority of bee species, 101 of 157 (64%), showed association with specific altitudes, and each elevational band harbored a unique assemblage of bee species, with 95% of beta-diversity explained by species replacement. The highest elevation band studied represents the top elevation of these short tropical mountains, emphasizing an urgent conservation need for these bee communities throughout the tropical realms as temperatures rise. Our finding of high specialization in lowland mutualistic networks of the tropics suggests that previously documented patterns of higher generalism toward lower latitudes may not have included bee-plant networks from Neotropical lowland and mainland ecosystems; resolving this debate will have important implications for conservation and restoration action.