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

Ecological diversification on islands typically results in divergence of ecological niches. As diet is a major component of species niches, we hypothesize that sister species within island monophyletic groups diversify in their dietary preferences. We have examined this hypothesis in two Haemodracon and four Hemidactylus species endemic reptiles of from Socotra Island (Yemen), corresponding to two independent 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 isotopes combined with faecal samples. We collected tail tips (for isotopes) and faecal pellets during two visits in 2013 and 2014 to Socotra. Specific trophic niche widths inferred from stable isotopes were estimated from ellipse-based metrics, whereas interspecific differences were compared by linear mixed models and examined in a phylogenetic framework. From faecal samples, diet variation among species was quantified by the Bray-Curtis index. Isotope and dietary interspecific divergence was compared with Mantel tests. For both isotopes, models detected interspecific differences between sister species i.e., trophic niche divergence and also interspecific similarities of distant lineages that use similar microhabitats i.e., ecological convergence. We did not find any phylogenetic signal neither in the interspecific differences in δ13C nor in δ15N isotopic values; thus species phylogenetically more closely related did not have more similar isotopic niches. The Mantel test demonstrated similar interspecific divergence using isotopes and faecal samples. In a phylogenetic context, trophic-niche interspecific comparisons highlight some mechanisms that are driving ecological diversification and speciation of Socotra Island.