Ancient biogeography of generalist predators on remote oceanic islands

Abstract

Aim Remote islands are known for providing spectacular examples of adaptive radiation, with ecological divergence across a lineage giving rise to multiple species, each specialized for a particular niche. These isolated environments also provide some of the best examples of recent colonization, often human-mediated, with a single generalist taxon distributed over vast regions. Highlighted by this dichotomy is a tendency of taxa to become more specialized over time. The current study focuses on two spider taxa (family Uloboridae) each occupying a broad range of habitats: Tangaroa tahitiensis in the Austral and Society islands of French Polynesia and Daramulunia gibbosa in Fiji. We ask whether the generalist tendency of each is associated with recent colonization of widespread species or, rather, maintained over extended evolutionary time. Location South Pacific: Fiji (18° S, 175° E), Society (18° S, 150° W), and Austral (23° S, 150° W) archipelagoes. Methods Mitochondrial COI and nuclear 18S rDNA were sequenced and analysed using maximum likelihood (RAxML) and Bayesian approaches with concurrent divergence dating calibrated with a general arthropod molecular clock. Ancestral ranges were reconstructed independently for French Polynesia and Fiji (RASP). Results While both T. tahitiensis and D. gibbosa occur broadly from sea-level to cloud forest, each has a separate and ancient origin within their respective archipelagoes, the pattern of colonization matching the geographical arrangement of the islands. Main conclusions Populations of two spider species, T. tahitiensis in the Australs and Societies and D. gibbosa in Fiji, are extreme generalists with broad physiological tolerances throughout their respective ranges. Yet, every population displays single island endemism and deep genetic divergences between islands indicating a long history on individual landmasses, a finding that contradicts the tendency for organisms to show increasing specialization associated with endemism.