Stable isotopes and diet uncover trophic-niche divergence and ecological diversification processes of endemic reptiles on Socotra Island

Abstract

Ecological diversification on islands typically results in divergence of ecological niches. As diet is a majorcomponent of species niches, we hypothesize that sister species within island monophyletic groupsdiversify in their dietary preferences. We have examined this hypothesis in two Haemodracon and fourHemidactylus species endemic reptiles of from Socotra Island (Yemen), corresponding to two indepen-dent colonization events. Convergence i.e., similar dietary patterns of phylogenetically unrelated species,was also examined. Trophic niches were studied by the analysis of carbon and nitrogen stable isotopescombined with faecal samples. We collected tail tips (for isotopes) and faecal pellets during two visitsin 2013 and 2014 to Socotra. Specific trophic niche widths inferred from stable isotopes were estimatedfrom ellipse-based metrics, whereas interspecific differences were compared by linear mixed models andexamined in a phylogenetic framework. From faecal samples, diet variation among species was quanti-fied by the Bray-Curtis index. Isotope and dietary interspecific divergence was compared with Manteltests. For both isotopes, models detected interspecific differences between sister species i.e., trophic nichedivergence and also interspecific similarities of distant lineages that use similar microhabitats i.e., ecolog-ical convergence. We did not find any phylogenetic signal neither in the interspecific differences in 13Cnor in 15N isotopic values; thus species phylogenetically more closely related did not have more similarisotopic niches. The Mantel test demonstrated similar interspecific divergence using isotopes and faecalsamples. In a phylogenetic context, trophic-niche interspecific comparisons highlight some mechanismsthat are driving ecological diversification and speciation of Socotra Island.