Urban environments increase generalization of hummingbird-plant networks across climate gradients

Abstract

Urbanization has reshaped the distribution of biodiversity on Earth, but we are only beginning to understand its effects on ecological communities. While urbanization may have homogenization effects strong enough to blur the large-scale patterns in interaction networks, urban community patterns may still be associated with climate gradients reflecting large-scale biogeographical processes. Using 103 hummingbird-plant mutualistic networks across continental Americas, including 176 hummingbird and 1,180 plant species, we asked how urbanization affects species interactions over large climate gradients. Urban networks were more generalized, exhibiting greater interaction overlap. Higher generalization was also associated with lower precipitation in both urban and natural areas, indicating that climate affects networks irrespective of habitat type. Urban habitats also showed lower hummingbird functional trait diversity and over/underrepresentation of specific clades. From the plant side, urban communities had a higher prevalence of nonnative nectar plants, which were more frequently visited by the hummingbird species occurring in both urban and natural areas. Therefore, urbanization affected hummingbird-plant interactions through both the composition of species and traits, as well as floral resource availability. Taken together, we show that urbanization consistently modifies ecological communities and their interactions, but climate still plays a role in affecting the structure of these novel communities over the scale of continents.

Keywords: Latin America; Neotropics; functional diversity; pollination; urbanization.

Fig. 1.

Location and the characterization of the 103 hummingbird-plant networks sampled in natural (n = 67) and urban (n = 36) habitats in this study. (A–D), Map showing the distribution of the networks, with gray and green indicating urban and natural sites, respectively (A). Point size in the map refers to the network size, i.e., species richness of the hummingbird–plant network sampled in each location. The illustration on the Left depicts the Swallow-tailed Hummingbird E. macroura (Emerald clade), a large-bodied and mostly territorial hummingbird that visits both nonnative and native plants, such as the Brazilian native Erythrina speciosa (Fabaceae) shown here (artist: Natanael N. Santos). An illustrative urban hummingbird–plant network from Belo Horizonte, Brazil (B; urban network ID#12). Sampling effort expressing the ratio between observed and expected diversity of interactions—sampling completeness (C), and network size expressing the richness of species (D) between urban and natural habitat networks. n.s. indicates nonsignificant results. Empty points in the plots show raw data.

Fig. 2.

Effects of urbanization and precipitation on hummingbird–plant network indices (A–F). Results from models testing the effects of habitat type, mean annual precipitation–MAP, and their interaction on network indices including nestedness (ΔwNODF; A and B), specialization (H₂'; C and D), and modularity (ΔQw; E and F). Precipitation (in mm) was rescaled in the two latter models to improve their convergence. Points and line segments indicate predicted probabilities and 95% CIs, respectively. Asterisk (*) indicates significance of the interaction term. Empty points show raw data.

Fig. 3.

Results from models testing the effects of habitat type on hummingbird functional diversity indices including CWM values of body mass (A), bill length (B), and bill curvature (C), and functional dispersion FDis (D). n.s. indicates nonsignificant results. Points and line segments indicate predicted probabilities and 95% CIs, respectively (drawn only when significant). Empty points show raw data.

Fig. 4.

Results from models testing the effects of habitat type on species level hummingbird–plant network indices, including partner diversity (A), specialization d' (B), and the proportion of nonnative plants used by hummingbird species (C). Analyses were restricted to those hummingbirds occurring in both urban and natural habitats (42 species). Points and line segments indicate predicted probabilities and 95% CIs, respectively. Empty points show raw data.

Fig. 5.

Clade-specific differences in specialization d' (A and B) and partner diversity (C and D) between urban and natural environments for hummingbird species (along with the 95% CI). The average differences (∆) derive from regression coefficients along with their CI. Coefficients based on all species within each clade derive from linear models (A and C) and coefficients based only on the species sampled in both natural and urban habitats derive from linear mixed effect models, with species identity as a random effect (B and D). Species from Brilliants and Coquette clades were poorly represented in urban networks and were therefore excluded from these analyses. The clades are sorted in the panels from Left to Right based on the CI’s distance from zero, which corresponds to no significant difference between urban and natural environments when it does overlap with zero. Hummingbird illustrations highlight examples from the Emerald (Chionomesa fimbriata) and the Hermit clades (Phaethornis guy) at the Top and Bottom, respectively (artist: Katrine Hansen).