Specialization in plant-hummingbird networks is associated with species richness, contemporary precipitation and quaternary climate-change velocity

Abstract

Large-scale geographical patterns of biotic specialization and the underlying drivers are poorly understood, but it is widely believed that climate plays an important role in determining specialization. As climate-driven range dynamics should diminish local adaptations and favor generalization, one hypothesis is that contemporary biotic specialization is determined by the degree of past climatic instability, primarily Quaternary climate-change velocity. Other prominent hypotheses predict that either contemporary climate or species richness affect biotic specialization. To gain insight into geographical patterns of contemporary biotic specialization and its drivers, we use network analysis to determine the degree of specialization in plant-hummingbird mutualistic networks sampled at 31 localities, spanning a wide range of climate regimes across the Americas. We found greater biotic specialization at lower latitudes, with latitude explaining 20-22% of the spatial variation in plant-hummingbird specialization. Potential drivers of specialization--contemporary climate, Quaternary climate-change velocity, and species richness--had superior explanatory power, together explaining 53-64% of the variation in specialization. Notably, our data provides empirical evidence for the hypothesized roles of species richness, contemporary precipitation and Quaternary climate-change velocity as key predictors of biotic specialization, whereas contemporary temperature and seasonality seem unimportant in determining specialization. These results suggest that both ecological and evolutionary processes at Quaternary time scales can be important in driving large-scale geographical patterns of contemporary biotic specialization, at least for co-evolved systems such as plant-hummingbird networks.

Figure 1. Geographical patterns of contemporary plant-hummingbird specialization.

Map of the Americas showing degree of specialization (H) in native plant-hummingbird networks. Arrows indicate studies that are difficult to see due to low specialization. The network to the left depicts an extremely specialized network (H = 0.78, P<0.05) from the Costa Rican highlands at latitude 9°N. The red nodes to the left illustrate plant species, and the green nodes to the right hummingbird species. The widths of links are scaled to interaction frequency, and node sizes to total interaction frequency. It illustrates how low Quaternary climate-change velocity, high contemporary precipitation and high species richness may cause strong contemporary biotic specialization. See Table S1 for specialization estimates for networks containing both native and introduced species.

Figure 2. Relationship of contemporary specialization with latitude and underlying drivers.

(A) Linear relationship between latitude and specialization in plant-hummingbird networks (H: n = 31, R² = 0.22, Dutilleul's spatially corrected P<0.05). (B) Linear relationship between Log-transformed network size and specialization in plant-hummingbird networks (H: n = 31, R² = 0.28, Dutilleul's spatially corrected P<0.01). (C) Linear relationship between mean annual precipitation and specialization in plant-hummingbird networks (H: n = 31, R² = 0.31, Dutilleul's spatially corrected P<0.01). (D) Linear relationship between Log-transformed Quaternary climate-change velocity and specialization in plant-hummingbird networks (H: n = 31, R² = 0.25, Dutilleul's spatially corrected P<0.05). Each symbol represents a native plant-hummingbird network. See Tables 1 and S2 for predictor estimates in ordinary-least-squares (OLS) multiple regression models. Likewise, see Tables 1 and S3 for predictor estimates in plant-hummingbird networks containing both native and introduced species.