Do conservation units correspond to the six recognized subspecies?
A few months ago, I wrote about the unusually low genetic diversity of the Red-billed Tropicbird (Phaethon aethereus), which renders this species vulnerable to extinction (see this blog post for the whole story). Another tropicbird species of conservation concern is the White-tailed Tropicbird (P. lepturus). Despite its wide distribution across the Atlantic, Pacific, and Indian Oceans, several breeding colonies are threatened due to predation by invasive species and habitat destruction from human activities or tropical storms. To implement proper conservation measures it is important to identify the number of conservation units within the White-tailed Tropicbird. Some colonies might be part of a larger metapopulation that needs to be protected as a whole, whereas other colonies are isolated and require specific conservation efforts.
Currently, the White-tailed Tropicbird is classified into six subspecies. Three large subspecies breed in the western (lepturus) and eastern (fulvus) Indian Ocean and in the northwestern Atlantic Ocean (catesbyi). Three small subspecies breed in the Pacific (dorotheae), Indian (europae), and south Atlantic Oceans (ascensionis). To determine whether all these subspecies represent distinct conservation units, a recent study in the journal Ecology and Evolution studied the morphological and genetic patterns of 13 breeding colonies.
The genetic analyses – based on microsatellites and mtDNA – uncovered four separate clusters. Three of these clusters correspond to specific subspecies (catesbyi, europae and ascensionis), while the fourth genetic cluster collects the remaining three subspecies from the Indo-Pacific region (lepturus, fulvus and dorotheae). The morphological data did not add much in terms of differentiation, since it only discriminated between the largest (catesbyi) and smallest (europae) subspecies, with all the other subspecies forming an intermediate group.
More detailed analyses of the Indo-Pacific subspecies revealed that the population on Christmas Island might represent a differentiated genetic pool and could be considered as a distinct conservation unit. All in all, the researchers suggest the recognition of five conservation units: (1) Bermuda (and all populations of the northwest Atlantic Ocean); (2) Ascension/Fernando de Noronha (and all populations of the southern tropical Atlantic Ocean); (3) Europa; (4) Christmas Island; and (5) the other Indo-Pacific colonies. Patterns of genetic diversity indicated that the first three conservation units are clearly vulnerable (catesbyi), endangered (ascensionis) or critically endangered (europae). They are thus in need of urgent conservation measures.
Humeau et al. (2020). Genetic structuring among colonies of a pantropical seabird: Implication for subspecies validation and conservation. Ecology and Evolution, 10(21), 11886-11905.
Feature image: White-tailed Tropicbird (Phaeton lepturus) © HarmonyonPlanetEarth | Wikimedia Commons