Linking Isotopes and Panmixia: High Within-Colony Variation in Feather δ2H, δ13C, and δ15N across the Range of the American White Pelican

Abstract

Complete panmixia across the entire range of a species is a relatively rare phenomenon; however, this pattern may be found in species that have limited philopatry and frequent dispersal. American white pelicans (Pelecanus erythrorhyncos) provide a unique opportunity to examine the role of long-distance dispersal in facilitating gene flow in a species recently reported as panmictic across its broad breeding range. This species is also undergoing a range expansion, with new colonies arising hundreds of kilometers outside previous range boundaries. In this study, we use a multiple stable isotope (δ2H, δ13C, δ15N) approach to examine feather isotopic structuring at 19 pelican colonies across North America, with the goal of establishing an isotopic basemap that could be used for assigning individuals at newly established breeding sites to source colonies. Within-colony isotopic variation was extremely high, exceeding 100‰ in δ2H within some colonies (with relatively high variation also observed for δ13C and δ15N). The high degree of within-site variation greatly limited the utility of assignment-based approaches (42% cross-validation success rate; range: 0-90% success). Furthermore, clustering algorithms identified four likely isotopic clusters; however, those clusters were generally unrelated to geographic location. Taken together, the high degree of within-site isotopic variation and lack of geographically-defined isotopic clusters preclude the establishment of an isotopic basemap for American white pelicans, but may indicate that a high incidence of long-distance dispersal is facilitating gene flow, leading to genetic panmixia.

Fig 1. Map of Sampling Locations.

Circles represent all sampled colonies; approximate main breeding range is shown in light gray, non-breeding range in medium gray, and year-round range in dark gray. Full colony names and location information available in S1 Table. The range map data were provided by NatureServe in collaboration with Robert Ridgely, James Zook, The Nature Conservancy, Conservation International, World Wildlife Fund, and Environment Canada [62].

Fig 2. Stable Isotope Basemaps.

Interpolated basemaps of isotopic variation in A. Hydrogen (δ²H), B. Carbon (δ¹³C), and C. Nitrogen (δ¹⁵N) obtained using ordinary kriging. Maps are based on the mean values of pelican feathers from 19 established American white pelican colonies.

Fig 3. Cluster Membership Map.

Proportion of feather samples from pelicans at 17 breeding colonies that fall into one of four clusters, based on k-means cluster analysis. The four colours (grey, red, blue, green) represent the proportion of samples assigned to each of the four different clusters defined by the clustering algorithm. Some colony locations have been slightly offset for better visibility.